Probing Solvation Dynamics around Aromatic and Biological

Review pubs.acs.org/CR

Probing Solvation Dynamics around Aromatic and Biological Molecules at the Single-Molecular Level Otto Dopfer*,† and Masaaki Fujii*,‡ †

Institut für Optik und Atomare Physik, Technische Universität Berlin, Hardenbergstrasse 36, 10623 Berlin, Germany Laboratory for Chemistry and Life Science, Tokyo Institute of Technology, Yokohama 226-8503, Japan

‡

ABSTRACT: Solvation processes play a crucial role in chemical reactions and biomolecular recognition phenomena. Although solvation dynamics of interfacial or biological water has been studied extensively in aqueous solution, the results are generally averaged over several solvation layers and the motion of individual solvent molecules is difficult to capture. This review describes the development and application of a new experimental approach, namely, picosecond time-resolved pump−probe infrared spectroscopy of size- and isomer-selected aromatic clusters, in which for the first time the dynamics of a single individual solvent molecule can be followed in real time. The intermolecular isomerization reaction is triggered by resonant photoionization (pump), and infrared photodissociation (probe) at variable delay generates the spectroscopic signature of salient properties of the reaction, including rates, yields, pathways, branching ratios of competing reactions, existence of reaction intermediates, occurrence of back reactions, and time scales of energy relaxation processes. It is shown that this relevant information can reliably be decoded from the experimental spectra by sophisticated molecular dynamics simulations. This review covers a description of the experimental strategies and spectroscopic methods along with all applications to date, which range from aromatic clusters with nonpolar solvent molecules to aromatic monohydrated biomolecules.

CONTENTS 1. Introduction 2. Experimental Strategies 2.1. Nanosecond Static and Picosecond TimeResolved REMPI-IR Spectroscopy 2.2. Nanosecond Static EI-IR Spectroscopy 3. Applications 3.1. Aromatic Clusters with Nonpolar Solvent 3.1.1. PhOH−Kr Dimer 3.1.2. PhOH−Ar2 Trimer 3.1.3. PhOH−Ar3 Tetramer 3.1.4. PhOH−CH4 Dimer 3.2. Aromatic Monohydrated Clusters 3.2.1. trans-Acetanilide−H2O 3.2.2. p-Aminobenzonitrile−H2O 4. Concluding Remarks Author Information Corresponding Authors Notes Biographies Acknowledgments References

impact on both the energetics and dynamics of chemical and biological reactions in solution.1−3 Rearrangement of the solvent network (solvation dynamics) is the initial step in solution-phase chemistry. Moreover, aromatic molecules play a fundamental role in biological and chemical recognition.4−12 In aqueous solution, the physical and chemical properties of the solute are strongly influenced by their interaction with the adjacent solvation layer, often called interfacial water or biological water. For example, it is well-known that the structure and function of proteins and enzymes are strongly affected by their surrounding hydration layers.13−42 In addition, solvation dynamics plays an important role in the initial steps of protein folding, for which hydrophobic interactions are also relevant.43−49 Owing to its fundamental importance, solvation dynamics has been studied extensively for many years in the liquid phase by sophisticated experimental and computational techniques. Experimental methods14,22,23,25 include vibrational spectroscopy,50−57 NMR spectroscopy,15,18,21,22,31,32,35,58 fluorescence spectroscopy (dynamic Stokes shift),28−30,59 terahertz spectroscopy,26,27 X-ray spectroscopy,39,57 and neutron scattering.22,23,25 Experimental results are then interpreted by theoretical modeling approaches, which nowadays are mostly based on molecular dynamics (MD) simulations.20,25,33,34,36−38,40−42,48,60 While computational techniques

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1. INTRODUCTION Solvation of aromatic and other solute molecules in hydrophilic and hydrophobic environments is a fundamental process in chemistry and biology. Solute−solvent interactions have a strong © 2016 American Chemical Society

Special Issue: Noncovalent Interactions Received: October 13, 2015 Published: April 7, 2016 5432

DOI: 10.1021/acs.chemrev.5b00610 Chem. Rev. 2016, 116, 5432−5463

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ment reaction is probed in real time by recording IR spectra at variable delay from the photoionization event. This laser spectroscopic technique allows for selecting the size, conformation, and quantum state of the aromatic cluster. The excess charge in the cation cluster generated by REMPI gives rise to additional electrostatic and induction forces, which for most aromatic clusters drastically change the topology of the potential energy surface describing the intermolecular interaction, with respect to both interaction strength and geometries of local and global minima.77,81 Hence, ionization causes a sudden change in the solute−solvent interaction, which is the driving force for solvent rearrangement. Figure 1 displays schematically three

often allow us to follow the motion of individual solvent molecules, the experimental data measured in the liquid phase are highly averaged over many solvent molecules present in the various solvation layers. Hence, probing the dynamics of an individual single solvent molecule is experimentally challenging if not impossible in the liquid phase. However, such singlemolecule-sensitive experimental data are highly desirable in order to characterize the solvation process and its dynamics at the single-molecular level from an experimental point of view. In addition, such experiments are highly requested in order to test and validate the computational approaches used to analyze experimental observations in the solution phase. Here, we review the first applications of a recently developed experimental technique, which has been able to directly probe for the first time the dynamics of individual single solvent molecules in real time: picosecond time-resolved infrared (ps-TRIR) spectroscopy of size- and isomer-selected aromatic clusters generated in molecular beams.61−68 Molecular clusters isolated in the gas phase may be considered as a nanodroplet or a snapshot of solution and thus serve as a finite model to probe reaction and solvation dynamics. Molecular clusters composed of solute and solvent molecules are held together by a variety of intermolecular forces, ranging from hydrogen bonding to π-stacking. They can readily be generated in high abundance in cold molecular beams with essentially any desired composition. Combination with mass spectrometry and multiple resonance laser spectroscopy allows for precisely selecting the size, conformation, and excitation (quantum) state of the cluster. Thus, the type and degree of solvation, the binding sites of solvent molecules attached to the solute, and the internal energy involved in the dynamical solvation process can be precisely adjusted, including the primary solvation by a single solvent molecule. Cooling the cluster in a molecular beam freezes it in a certain structural configuration; that is, the position of the solvent before the reaction is accurately determined and can be varied to some extent in a controlled fashion. In addition, the finite size of the clusters allows for application of sophisticated high-level quantum chemical approaches to reliably determine their geometric, vibrational, electronic, and energetic properties with high accuracy. The combined application of quantum chemical calculations with laser spectroscopy and mass spectrometry to molecular clusters and their intermolecular interactions, meanwhile, is a well-established approach to determine their static properties.6−8,65,69−98 However, until recently, there has not been any experimental investigation of the solvation dynamics in such molecular clusters. This is mainly due to the substantial experimental difficulties arising from the challenging laser technology required for such time-resolved studies. These involve widely tunable multicolor high-power picosecond lasers, which became available only recently, and their coupling to a molecular beam setup for spectroscopic pump−probe experiments.65,99 In this work, we utilize picosecond time-resolved three-color tunable UV−UV′−IR pump− probe spectroscopy to monitor the dynamics of aromatic clusters produced in molecular beams.61−68,100,101 The principal experimental strategy employed for these timeresolved experiments relies on triggering solvent rearrangement reactions by resonant photoionization of cold neutral aromatic molecular clusters generated in a molecular beam.65 Ionization is achieved from the neutral electronic ground state (S0) into the cationic electronic ground state (D0) via an intermediate excited electronic state (S1) via resonance-enhanced two-photon UV− UV(′) ionization (1 + 1(′) REMPI). Subsequently, the rearrange-

Figure 1. Sketch of potential energy diagrams for ionization-induced solvent rearrangement reactions in clusters using isomer-selective resonant two-photon ionization (REMPI) from the neutral ground electronic state (S0) to the ionic electronic ground state (D0) via the first excited neutral singlet state (S1): (a) no reaction will occur; (b) reaction will occur only for excitation above the reaction barrier; (c) reaction will always occur with 100% yield.

representative cases relevant for this review, which illustrate how the potential energy surface may qualitatively change upon D0 ← S0 ionization. In case a, the cluster has a single deep minimum in both charge states, so that ionization does not significantly change the solvent binding configuration (but merely the interaction strength). In case b, the global minimum of the neutral cluster in the S0 state is only a local minimum in the cationic D0 state, and the global minimum on the D0 potential energy surface is rather different from that on the S0 potential. Finally, in case c, a global (or local) minimum configuration in the S0 state becomes unstable and repulsive in the D0 state. Thus, D0 ← S0 ionization of the cluster will certainly induce a solvent rearrangement reaction in case c. In case b, ionization may trigger such a solvent migration reaction toward the most stable structure in the D0 state only for excitation above the isomerization barrier (tunneling can be neglected on the considered time scales),102 while the cluster may be trapped in the local minimum for soft ionization below the reaction barrier. The available ionization excess energy may be adjusted by variation of the energy of the second UV photon used for the 1 + 1′ REMPI process. For case a, no major solvent rearrangement occurs upon ionization. An illustrative example for case a is the phenol−water cluster (PhOH−H2O), for which ionization does not change the OH··· O H-bonding configuration, and consequently no solvent rearrangement dynamics occurs.71,89,103−111 A similar situation applies to the related H-bonded PhOH−L dimers with L = CH3OH,111,112 C2H5OH,113 (CH3)2O,114 and PhOH,111,115 although in these clusters the solvent undergoes some minor conformational changes in the OH···O H-bond orientation by 5433


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molecule in the neutral S0 state. This configuration becomes repulsive in the ionic D0 state, in which all stable configurations have a structure with favorable charge−dipole orientation (i.e., the dipole of H2O aligns with the positive charge of the solute molecule), often resulting in a H-bonded configuration with H2O acting as a proton acceptor. In these clusters, ionization triggers a solvent rearrangement, in which H2O changes its role from a proton donor to a proton acceptor, which is often connected to a change in the ligand binding site. Examples include the ionization-induced π → CH site switch in benzene−H2O (Figure 2b),77,141−144 which changes its structure from the archetypal π H-bond in the neutral S0 state83,145 to bifurcated CH···OH2 bonding in the cationic D0 state. In the biologically relevant clusters of aromatic amides (e.g., formanilide and acetanilide, Figure 2c), ionization triggers a CO → NH shuttling reaction for the polar solvent molecules H2O and CH3OH, which move from the CO to the NH site of the -CO-NH- peptide linkage.66,67,146−151 A CN → NH site switching migration has also been observed for 4-aminobenzonitrile−H2O, in which H2O moves from the cyano to the amino group upon ionization (Figure 2d).68,152,153 In the monohydrated neurotransmitter tryptamine−H2O, a NH → NH′ shuttling motion of H2O from the amino group to the indolic NH′ group can be triggered by ionization (Figure 2e), a reaction that is accompanied by intramolecular reorientation of the flexible ethylamino side chain.154−156 In the monohydrated amino acid phenylglycine− H2O, ionization first triggers a solvent motion from the C to the N terminus, before decarboxylation occurs.157 Further solvent rearrangements upon ionization are inferred for the most stable structures of halogenated benzene−H2O clusters containing fluorobenzene or p-dichlorobenzene.158−160 Interestingly, an ionization-induced OH → NH migration has been observed for highly excited 4-aminophenol+−H2O clusters that is suppressed at low ionization excess energy, while no reaction is observed for the related 3-aminophenol+−H2O dimer.161,162 The latter result has been rationalized by different isomerization path lengths and subtle differences in intermolecular bond strengths in the S0 and D0 states. Actually, the 4-aminophenol+−H2O cluster is closer to case b in Figure 1, because it has two deep wells for both reactant and product structures in the D0 state, which are separated by high barriers.161 Out of the various H2O shuttling reactions inferred from static experiments, the dynamics of only two of them have been probed so far by ps-TRIR spectroscopy, namely, CO → NH shuttling in acetanilide−H2O and CN → NH site switching in 4-aminobenzonitrile−H2O.66−68 Both reactions are described in detail in section 3.2. This review discusses all time-resolved experiments conducted so far for probing the dynamics of ionization-induced intermolecular solvent isomerization reactions in isolated clusters in real time. This research direction started about 15 years ago,65 and the first results were published in 2005 for PhOH−Ar2.61 Meanwhile, five clusters with increasing complexity and biological relevance have been characterized, namely, π → H site-switching reactions in PhOH−Kr,64 PhOH−Ar2,61,62 and PhOH−Ar3;63 CN → NH site switching in 4-aminobenzonitrile−H2O;68 and CO → NH site switching in acetanilide− H2O.66,67 Significantly, these experiments represent the first time-resolved studies of solvent rearrangement reactions in molecular clusters and thus provide for the first time benchmark data for solvation dynamics at the single solvent molecular level in general. Similar to the experimental studies, there are only very limited theoretical studies available for simulating the ionizationinduced solvent migration reactions. They mostly rely on MD

ionization. Examples for case b include the large and fundamental class of clusters composed of nonpolar ligand atoms and molecules [L = rare gas (Rg) atoms, N2, O2, CH4, etc.] and acidic aromatic solute molecules with OH and NH functional groups (e.g., phenols, anilines, indoles).63,65,116−140 In these clusters, the π-bonded global minimum in the S0 state is only a local minimum in the D0 state, in which the OH···L or NH···L H-bonded structure is most stable. While the interaction in the neutral cluster is dominated by dispersion (preferring π-stacking), it is dominated by induction in the ionic cluster (preferring Hbonding), leading to an ionization-induced π → H switch in the preferred ion−ligand recognition motif (Figure 2a).77 Indeed,

Figure 2. Selected examples for ionization-induced site-switching reactions observed in solvated aromatic clusters: (a) π → H (here π → OH) site switch in phenol−Rg dimers; (b) π → CH site switch in benzene−H2O; (c) CO → NH site switch in trans-acetanilide−H2O; (d) CN → NH site switch in 4-aminobenzonitrile−H2O; (e) NH → NH′ site switch in tryptamine−H2O.

the initial ps-TRIR spectroscopic studies of solvent rearrangement have measured the dynamics of hydrophobic → hydrophilic (π → H) site switching reactions in PhOH−Ln clusters with L = Ar and Kr triggered by ionization, as described in detail in section 3.1.61−65 Important examples for case c include the large class of monohydrated aromatic clusters, in which H2O binds as a proton donor in an H-bond to the (aromatic) solute 5434


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simulations67,144,148,151 or on wavepacket approaches limited to low dimensionality.102 A priori, it is unclear how reliable the predictions from such simulations are, because they involve many approximations. Hence, the new experimental data described in this review provide very valuable benchmark data for testing their predictive power. This review does not cover the very few recent dynamical studies on solvent isomerization in neutral clusters triggered by electronic excitation, namely, slow H2O migration in p-cyanophenylpentamethyldisilane−(H2O)n clusters163,164 and formation of the excimer state in benzene dimer.100 Of course, ionization of a molecular cluster may not only induce solvent rearrangement, possibly accompanied by intramolecular structural changes of flexible bonds, side chains, and functional groups, but may also trigger real chemical reactions (such as SN2 reactions or proton transfer);8,111,165−167 however , those are also not considered here. Similarly, we do not cover the elegant laserinduced solvent shuttling reactions observed by static spectroscopy in the neutral ground state (S0).168−170 The structure of this review is as follows. First, in section 2 we describe the experimental approaches employed for probing the solvation dynamics in molecular aromatic clusters in real time. Then the first results of the application of this new technique are discussed in terms of the two prototypical types of clusters studied so far. The first type of clusters (section 3.1) involves the solvation dynamics of nonpolar solvent molecules in phenol+− Rgn/CH4 clusters describing hydrophobic → hydrophilic site switching. The second type (section 3.2) is monohydrated aromatic cluster ions, to infer solvation dynamics of the strongly dipolar hydrophilic water molecule. Finally, in section 4, we summarize the salient results obtained and suggest future directions.

clusters to allow for convenient application of REMPI for state-, isomer-, and size-selective ionization by use of standard UV laser spectroscopy, because of their strong UV absorptions and their low ionization energies arising from ππ* excitation and subsequent removal of one aromatic π electron. Picosecond time-resolved infrared (ps-TRIR) spectroscopy is highly appropriate for probing solvation dynamics. First, the time resolution is fast enough to resolve the dynamics of intermolecular rearrangement, and second, the spectral resolution is high enough to still resolve the IR spectroscopic differences arising from this isomerization reaction. The laser system employed so far has a typical spectral and time resolution of Δν ≤ 15 cm−1 and Δt ≤ 3 ps, respectively. The isomerization coordinate is generally related to intermolecular vibrational and internal rotational motions, possibly accompanied by largeamplitude low-frequency intramolecular relaxation, which all occur on the picosecond time scale and thus can be readily resolved by ps-TRIR spectroscopy. The second approach (section 2.2)77 involves nanosecond infrared photodissociation (IRPD) of mass-selected cluster ions generated in an electron ionization plasma expansion (EI-IR). The important advantage of this technique over the REMPI approach is that it detects predominantly the most stable structure of the cluster cations in the D0 state, independent of the most stable neutral structure in S0/S1. This is in contrast to the REMPI scheme, which is governed by the Franck−Condon principle. Thus, vertical ionization of the neutral structure often populates only local minima or repulsive configurations in the cation ground state, which usually triggers a structural rearrangement reaction to more stable geometries in the D0 state. However, it is a priori unclear whether such solvent rearrangement traps the ionized cluster in local minima on the cation potential energy surface by possible barriers or whether the global minimum is reached as the final reaction product. To this end, comparison between the static EI-IR and IR-REMPI spectra is crucial for addressing this issue and determining the most stable cation structure in the D0 state. Analysis of static IR spectra is usually carried out by standard density functional theory (DFT) and/or ab initio calculations, whereby the measured spectra are compared to those calculated for individual isomers to achieve structural assignments. Quantum chemical calculations not only provide spectral assignments but also provide additional information not always readily available from experiment, such as detailed structures and binding energies of various isomers, barriers toward isomerization, charge distributions, shape of molecular orbitals, and type of intermolecular bonding. For the highly relevant cation ground state, MP2 calculations mostly fail to properly describe radical cations because of spin contamination.121 In contrast, dispersion-corrected DFT approaches appear to more reliably describe electronic and vibrational properties as well as intermolecular interactions. Time-resolved IR spectra are typically analyzed by simple classical rate equation models and/or by more advanced molecular dynamics (MD) simulations (vide infra).

2. EXPERIMENTAL STRATEGIES In this section, we briefly summarize the spectroscopic techniques used to investigate the solvation dynamics induced in molecular aromatic clusters by ionization. In general, the clusters are generated at low temperature and high yield in molecular beams, which are coupled to mass spectrometers in order to select specific cluster sizes and allow for high-resolution spectroscopy. A combination of IR and UV laser spectroscopy is employed for resonant and isomer-selective electronic and vibrational excitation, as both the electronic and vibrational response are highly characteristic of the structure of the solvated clusters. So far, IR spectroscopy has been preferred over Raman techniques for probing the dynamics of the vibrational fingerprint of clusters, due its higher sensitivity and somewhat easier experimental implementation. In particular, the 3 μm range is advantageous because the strongly IR-active C−H, N− H, and O−H stretch fundamentals (νCH/NH/OH) are highly sensitive to solvation structure. Two complementary techniques for generating cluster ions are employed to characterize their structure in the cation electronic ground state (D0) via static and dynamic IR spectroscopy.65,77 The first approach (section 2.1)65 utilizes REMPI of neutral aromatic molecular clusters generated in their ground electronic state (S0) for isomer-selective ionization into their cation electronic ground state (D0) via their first excited singlet state (S1) in order to trigger the isomerization dynamics in the D0 state. The geometry of these clusters before (S0, S1) and after (D0) ionization is probed by IR ion dip spectroscopy (REMPIIR) with either nanosecond (ns) or picosecond (ps) laser systems, yielding static or time-resolved IR spectra, respectively. Aromatic chromophores are implemented in the molecular

2.1. Nanosecond Static and Picosecond Time-Resolved REMPI-IR Spectroscopy

The principal strategy of nanosecond and picosecond timeresolved IR ion dip spectroscopy by use of the three-color tunable UV−UV′−IR approach is illustrated in Figure 3 and has been described in detail elsewhere.65 Briefly, cold neutral clusters are generated in a supersonic beam in their ground electronic 5435


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The produced cation clusters are detected in a quadrupole or time-of-flight mass spectrometer. Static nanosecond IR-REMPI spectra in the S0, S1, and D0 states are then obtained by firing an IR laser in a IR−UV−UV(′) (S0), UV−IR−UV(′) (S1), or UV− UV(′)−IR (D0) excitation scheme, with appropriate delays between the individual laser pulses (typically 5−100 ns), and monitoring the 1 + 1(′) REMPI signal of the selected cluster. If νIR is resonant with a vibrational transition in the considered electronic state, depopulation of the states involved in the REMPI process by vibrational excitation and/or vibrational predissociation produces a dip in the parent ion current. Tunable photons for the nanosecond experiments are produced by frequency doubling of dye lasers pumped by Q-switched nanosecond Nd:YAG lasers (UV(′)) and appropriate difference frequency mixing using these lasers (IR). The resulting static REMPI-IR ion dip spectra recorded in the S0 (S1) state give the starting geometry of the ionization-induced isomerization reaction in the D0 state by use of the 1 + 1 (1 + 1′) REMPI scheme. The corresponding REMPI-IR spectrum in the D0 state corresponds to the structure of the reaction product(s) long after the reaction, which occurs on the picosecond time scale. In addition to the reactant and product of the reaction, the spectrum of the D0 state contains valuable information about yield of the reaction and internal energy of the cluster resulting from ionization excess energy and exothermicity of the reaction. The picosecond three-color pump−probe time-resolved IR experiments rely essentially on the same strategy as the nanosecond REMPI-IR experiments, except that all nanosecond lasers are replaced by picosecond lasers (Figure 4). Details of the unique and challenging UV−UV(′)−IR laser system with high intensity required for gas-phase cluster spectroscopy are discussed in detail elsewhere61,62,65 and shall not be repeated here. The relevant characteristics are a typical time resolution of Δt = 3 ps and a spectral resolution better than 15 cm−1. In every case, it is carefully assured that the reduced spectral resolution resulting from shorter laser pulses does not affect isomer

Figure 3. Strategy of nanosecond static and picosecond time-resolved REMPI-IR spectroscopy performed by three-color tunable UV−UV′− IR pump−probe ion dip spectroscopy. For size-, isomer-, and stateselective ionization (pump pulse, νUV, νUV′) of a neutral cluster from the S0 to the D0 state via the neutral excited S1 state (typically via the S1 origin), 1 + 1′ REMPI is employed. The initially prepared nascent Franck−Condon state (FC+) undergoes some dynamical process involving solvent rearrangement toward the energetically more stable reaction product (P+), possibly via an intermediate (I+). The reaction dynamics is probed by an IR laser (probe pulse, νIR) fired at an adjustable delay Δt after the ionization event, which induces resonant vibrational predissociation, resulting in a dip in current of the parent ion cluster. Adapted with permission from ref 66. Copyright 2012 Wiley.

state (S0) and are resonantly ionized by two-photon one-color (1 + 1, UV + UV) or two-color (1 + 1′, UV + UV′) REMPI via their characteristic S1 transitions. As described above, the REMPI process is used to trigger the isomerization reaction, which occurs on the picosecond time scale. In most cases, isomerselective ionization via the intense vibrationless S1 origin transition is employed for efficient ionization and the selection of vibrationally cold (T = 0 K) clusters. By use of 1 + 1′ REMPI, the ionization excess energy (Eexc) can continuously be adjusted from soft to hard ionization by tuning the UV′ photon frequency.

Figure 4. Experimental setup used for picosecond time-resolved three-color tunable UV−UV′−IR (ps-TRIR, REMPI-IR) spectroscopy.65 A femtosecond mode-locked Ti:sapphire laser (800 nm) pumped by the second harmonic of a Nd:YVO4 laser is regeneratively amplified at 10 Hz and stretched to 3 ps, leading to pulse energies of 11−12 mJ. Of this pulse, 40% is frequency-doubled and used to pump two optical parametric generator/ amplifiers (OPA). The signal outputs of OPA1 and OPA2 are frequency-doubled to generate νUV and νUV′. A third OPA (OPA3) is pumped by 20% of the 800 nm pulse from the regenerative amplifier. The frequency-doubled idler output of OPA3 and the remaining 40% of the 800 nm light from the regenerative amplifier are differentially mixed in a KTiOAsO4 crystal to generate tunable IR laser light in the 3 μm range (νIR). The IR pulse energy is about 100 μJ/pulse. The two UV lasers and the IR laser are combined coaxially by beam combiners and focused by a CaF2 lens with 250 mm focal length into the molecular beam. Ions are detected in a linear time-of-flight (TOF) mass spectrometer. Reprinted with permission from ref 66. Copyright 2012 Wiley. 5436


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Figure 5. Experimental setup employed for nanosecond EI-IR spectroscopy of cluster cations by means of infrared photodissociation (IRPD) in a tandem quadrupole mass spectrometer, illustrated for A+−H2O clusters generated by electron ionization (EI).76,77 Ionic clusters are produced in a supersonic plasma cluster ion source, which combines electron and/or chemical ionization with a pulsed and skimmed supersonic expansion. The A+− H2O parent clusters under investigation are selected by the first quadrupole mass filter (QP I), deflected by 90°, and injected in an octopole ion guide, where the IRPD process is driven by infrared radiation from a pulsed optical parametric oscillator laser system (IR-OPO) pumped by a Q-switched nanosecond Nd:YAG laser. The generated A+ fragment ions are selected by the second quadrupole mass filter (QP II) and monitored with a Daly ion detector as a function of laser frequency (νIR) to obtain the IR-action spectrum of A+−H2O (denoted EI-IR spectrum). Adapted with permission from ref 65. Copyright 2012 Taylor & Francis.

be shown below, simple MD simulations148,151 may fail to catch even salient aspects of the reaction,66 and therefore, validation of reliable sophisticated MD approaches by comparison to the detailed experimental ps-TRIR data is mandatory.67 One major issue of both static and time-resolved REMPI-IR approaches is related to the question whether the final product of the isomerization reaction triggered by ionization corresponds to the global minimum of the D0 potential energy surface or merely to a local minimum separated by possibly high potential barriers from the most stable structure. To this end, the static REMPI-IR spectra must always be compared to complementary EI-IR spectra, which predominantly correspond to spectra of the global minimum in the D0 state.

selectivity in the REMPI process. Time-resolved IR spectra of the isomerization reaction in the D0 state are measured by changing the delay time Δt between the REMPI process and IR excitation. Hence, for each delay time Δt, a full IR dip spectrum is recorded, providing essentially a multidimensional spectroscopic probe of the isomerization reaction and its structural evolution (which would not be available from femtosecond approaches). In addition, the time evolution of individual resonances in the psTRIR spectra can be monitored by changing the delay time Δt and monitoring the IR dip signal for fixed νIR. As such, ps-TRIR spectra contain a wealth of additional but very fundamental information not available from static REMPI-IR spectra, including dynamics of the reaction (rates), occurrence of competing reaction pathways (and their branching ratio), appearance of transient short-lived intermediates (and their lifetime), and observation of back reactions, as well as dynamics of intracluster vibrational energy redistribution (IVR) describing the removal of internal energy of the nascent cluster ion from the reaction coordinate. All these parameters, which completely escape the static experiments, may strongly depend on the ionization excess energy (Eexc), which again is sensitively encoded in the corresponding ps-TRIR spectra. Significantly, these ps-TRIR spectra provide much more detailed information about the dynamics of the solvent rearrangement process than simple kinetic studies. Actually, it is a formidable task to extract all this information from the rich ps-TRIR spectra. The initial and quite preliminary analysis is usually carried out by solving classical rate equations developed for the simplest reaction scheme suggested from the resonances observed in the ps-TRIR spectra and their time evolution. Although such a simplified classical analysis often provides already a rough picture of the dynamics of the reaction and may reasonably well reproduce all main observed spectral characteristics, it is often not trivial or even impossible to extract unambiguously finer details of the reaction, in particular for more complex reaction schemes. In such cases, reliable MD simulations may provide an improved view of the reaction, and this has indeed been nicely illustrated for the acetanilide+−H2O case (section 3.2.1).67 However, as will

2.2. Nanosecond Static EI-IR Spectroscopy

The setup used for recording EI-IR spectra of mass-selected cluster ions generated in an electron impact (EI) source is shown in Figure 5. It consists of a quadrupole tandem mass spectrometer coupled to an octopole ion trap and has been described previously.76,77 Briefly, cold aromatic cluster ions in the cation ground electronic state (D0) are produced in the EI supersonic plasma expansion by expanding a suitable gas mixture containing vapor of aromatic solute (A) and solvent molecules (L) adiabatically from high pressure through a pulsed nozzle into vacuum. Initial electron (and/or chemical) ionization of the aromatic solute is followed by three-body aggregation reactions in the high-pressure region of the expansion, according to A + e → A+ + 2e

(1)

A+−Ln + L + X → A+−Ln + 1 + X

(2)

where X = A, L, or carrier gas. This reaction sequence ensures the predominant production of the most stable A+−Ln cluster ions in the D0 state,77,116 becauseunlike the REMPI approachthe EI approach for cluster cation generation is not limited by the Franck−Condon principle. The generation of less stable local minima on the D0 potential can (often) be completely suppressed by adjusting the ion source conditions.117 The central and coldest part of the plasma expansion is extracted 5437


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through a skimmer into the first quadrupole, which is tuned to the mass of the parent cluster ion under investigation. This massselected parent ion beam is then overlapped in the adjacent octopole ion trap with a tunable IR laser pulse emitted from an IR optical parametric oscillator laser system. Resonant singlephoton absorption excites vibrational resonances of the parent cluster. For excitation levels above the lowest dissociation threshold, evaporation of neutral ligands in the octopole produces fragment ions, which are in turn mass-filtered by the second quadrupole and sensed with a Daly ion detector. IRPD spectra are obtained by monitoring the fragment ion intensity as a function of the IR laser frequency (νIR). In the case of weakly bound ligands, such as L = Rg or CH4, the (zero-point-corrected) binding energies (D0 < 1500 cm−1) are well below vibrational resonances in the mid-IR range (νCH/NH/OH = 2500−4000 cm−1), and single-photon IRPD from the ground vibrational state of the cluster is feasible. However, in the case of strongly bound ligands such as H2O, the dissociation energies (D0 ≤ 10 000 cm−1) often exceed the frequencies of the vibrational fundamentals, preventing the detection of single-photon IRPD of cold cluster ions. Hence, only warm clusters with substantial internal energy prior to IR excitation are detected in single-photon IRPD spectra via the excitation of sequence hot bands, leading to broadening and shifting of the transitions because of vibrational anharmonic cross-coupling.77 Such vibrational band shifts and broadening are particularly pronounced for the excitation of proton donor stretch vibrations involved in hydrogen bonding.77,116,122,149,153,156,171−181 In order to suppress these effects, the cluster ions may be tagged with a weakly bound ligand, typically a rare gas atom, H2, or N2, which has two fundamental consequences.77,78,97,149,153,156,182−191 First, the tagged clusters are colder than the bare untagged clusters because the maximum possible internal energy in the cluster is generally given by the dissociation energy of the most weakly bound ligand. Second, the strongly reduced effective dissociation energy of the tagged cluster readily allows for single-photon IRPD from the ground vibrational state because D0 ≪ νIR. As an example, Figure 6 compares the EI-IR spectra of formanilide−H2O (FA+−H2O) and its Ar-tagged cluster ions (FA+−H2O−Ar) with REMPI-IR spectra obtained by one-color isomer-selective ionization of trans-FA−H2O.140,148 Although all spectra are due to the same NH-bound t-FA+−H2O(NH) isomer, the EI-IR spectrum of FA+−H2O−Ar exhibits by far the best spectral resolution due to the lowest cluster temperature. Application of the tagging technique to aromatic cluster ions, such as the A+−H2O monohydrated clusters investigated here, safely ensures the detection of global minima on the D0 potential energy surface by EI-IR spectroscopy. Even in cases in which EI-IR and REMPI-IR spectra of the same untagged species are directly compared (see Figure 6 for FA+−H2O), the EI-IR spectra often appear to be somewhat colder, because the internal energy of the A+−H2O reaction product generated by REMPI, resulting from high ionization excess energy and exothermicity of the isomerization reaction, is larger than the average internal energy of the corresponding cluster ions produced in the EI source. In general, although the EI-IR approach is double-mass-selective (Figure 5), it is usually not isomer-selective due to the one-color singlephoton absorption process. Nonetheless, the reaction sequence detailed above (eqs 1 and 2) ensures the predominant detection of the most stable structure in the D0 state, independent of the most stable structure of the neutral cluster, particularly when used in combination with the tagging approach. A further difference between REMPI-IR and EI-IR spectra is that the

Figure 6. IR dip spectra of t-FA+−H2O clusters, generated by isomerselective resonant photoionization of NH- and CO-bound isomers, tFA−H2O(CO) and t-FA−H2O(NH), via one-color two-photon (1 + 1) REMPI through their S1 origins (REMPI-IR, panels a and b),148 are compared to IRPD spectra of FA+−H2O and FA+−H2O−Ar generated in the electron ionization source (EI-IR, panels c and d).149 Significant effects of the different internal cluster temperature on the appearance of the IR spectrum are evident with respect to band positions, intensities, and widths. Clearly, the FA+−H2O−Ar spectrum corresponds to clusters with the lowest temperature. The computed structure of FA+− H2O−Ar, with intermolecular bond lengths in angstroms, is shown as well. The binding energies of H2O and Ar are D0 = 4856 and 418 cm−1, respectively (ωB97X-D/aug-cc-pVTZ).149 Adapted with permission from ref 149. Copyright 2015 American Chemical Society.

former are measured as depletion of the parent ion signal, while the latter are recorded as appearance of the fragment signal (Figure 6). Hence, the EI-IR spectra display often better signalto-noise ratios.

3. APPLICATIONS 3.1. Aromatic Clusters with Nonpolar Solvent

This section reviews the dynamical studies of isomerization reactions in PhOH+−Rgn clusters triggered by photoionization. Such studies have so far been performed for Rg = Ar (n = 2− 3)61−63 and Kr (n = 1);64 that is, they cover different degrees of solvation and Rg atoms with different mass and interaction strength, factors that both affect the dynamical process.65 The interaction of PhOH with Rg atoms represents the prototypical interaction of an acidic aromatic solute molecule with a nonpolar ligand. As a consequence, PhOH−Rgn are benchmark clusters to investigate the subtle interplay between two different but fundamental interaction motifs: dispersive π-stacking to the aromatic phenyl ring and H-bonding to the acidic OH group.77 These two prototypical binding motifs are frequently referred to as hydrophobic and hydrophilic interactions. The subtle competition between the two interactions in acidic aromatic molecules interacting with Rg ligands can be controlled by their charge or electronic excitation state, substitution of functional groups, and degree of solvation. For example, numerous spectroscopic studies of PhOH−Rg dimers reveal a π-bound global minimum, denoted (10),225 in the neutral ground 5438


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from its complexation-induced red shift of the S100 origin, ΔS1 = −55 cm−1, and from analysis of the observed intermolecular bending and stretching vibrations.194,199 Similarly, its REMPI-IR spectrum in the O−H stretch range exhibits only a single transition at νOH = 3657 cm−1, essentially unshifted from the corresponding transition of bare PhOH (3658 cm−1),63 consistent with a (10) structure. No spectroscopic signature has been identified for the less stable H-bound isomer. Similarly, an early REMPI-IR207 spectrum of cationic PhOH+−Kr obtained by REMPI of the neutral π-bonded precursor has also been interpreted with a single π-bonded structure in the D0 state, because its O−H stretch band at νOH = 3535 ± 2 cm−1 is again unshifted from that of bare PhOH+ at 3534 cm−1 .209 Subsequently, the ZEKE spectra measured via REMPI of πbonded PhOH−Kr have exclusively been attributed to the πbonded (10)+ isomer in the D0 state, with respect to both complexation-induced red shift in the adiabatic ionization energy, ΔIE = −234 cm−1, and analysis of the three intermolecular frequencies.199 This IE shift demonstrates that the binding energy of the intermolecular π-bond in PhOH−Kr increases by 234 cm−1 upon ionization due to the additional polarization forces arising from the excess charge.199 Intense progressions assigned to the intermolecular bending mode bx indicate that the equilibrium structure of (10)+ shifts slightly toward the OH group in the D0 state.199 In marked contrast to the early REMPI-IR207 and ZEKE199 spectra, which have solely been interpreted by the (10)+ structure, the EI-IR spectrum of PhOH+−Kr shown in Figure 7c is largely dominated by the broad O−H stretch band of the H-bonded (H00)+ isomer at νOH = 3411 cm−1.122 Its red shift of ΔνOH = −123 cm−1 induced by Hbonding is in good agreement with the calculated value of −124 cm −1 , and the blue-shaded band contour confirms its interpretation as proton donor stretch mode. The EI-IR spectrum displays only a very weak free νOH band at 3535 cm−1, clearly demonstrating that in the cationic D0 state the Hbonded (H00)+ isomer is substantially more stable than the two less stable equivalent π-bonded local (10)+ and (01)+ minima, in line with theoretical predictions of around 400 cm−1 for the energy difference between the two types of isomers (MP2/augcc-pVTZ).65 The population of the (10)+ isomer in the supersonic plasma expansion is estimated to be less than 10% of the abundance of the (H00)+ isomer. Indeed, careful nanosecond REMPI-IR spectra recorded via the S1 origin of πbonded PhOH−Kr also show the O−H stretch bands of both isomers (Figure 7b) even for a quite low ionization excess energy of Eexc = 7 cm−1,64,122 suggesting that π → H isomerization occurs in the ionic D0 state on the picosecond time scale. The same result is observed for the excitation of high-n Rydberg states by autoionization-detected infrared (ADIR) spectroscopy, indicating that the Rydberg electron is insensitive to the dynamics of the ion core.126 Recent MATI-IR spectra of PhOH+−Ar indicate that even direct excitation of the H-bound structure in the D0 state from the π-bound structure in the S1 state is possible, although with low probability due to small but nonvanishing Franck− Condon factors.127 The H-bonded isomer has escaped detection in the early REMPI-IR spectrum of PhOH+−Kr because of the insufficient scanning range.207 The νOH band of (H00)+ observed in the REMPI-IR spectrum at 3452 cm−1 is substantially blueshifted from that in the EI-IR spectrum (3411 cm−1), because of different temperatures of the clusters in the two experiments. While the PhOH+−Kr dimers probed in the EI-IR spectrum are rather cold, those detected in the REMPI-IR spectrum arise from rather exothermic π → H isomerization, which results in an

electronic state (S0), because dispersion interactions between the Rg atom and the highly polarizable aromatic π electrons dominate the attraction.62−64,72,90,192−201 So far, there is no experimental evidence for a less stable H-bound PhOH−Rg isomer, denoted (H00),225 in S0, and it is currently unclear whether the planar H-bound isomer is a shallow local minimum or a transition state between two equivalent global π-bound minima. This view from spectroscopy has been fully confirmed by high-level quantum chemical calculations on PhOH−Rg dimers.121,202−204 In contrast to neutral PhOH−Rg, the calculations predict for the ground electronic state of the PhOH+−Rg cations (D0) that the H-bound structure (H00)+ is substantially more stable than the π isomer (10)+,225 because additional polarization interactions induced by the positive charge favor H-bonding over π-stacking.116,117,119,121,122,193,205 This prediction has been confirmed by EI-IR spectra in the range of the O−H stretch fundamental (νOH), which are clearly dominated by red-shifted νOH bands characteristic of the Hbonded isomers.116−118,122,206 The νOH bands of the less stable π isomers are much weaker and disappear completely under cold conditions,117 leaving no doubt about the relative stability of the two types of isomers. Moreover, the magnitude of ΔνOH red shifts upon H-bonding increases with proton affinity of the Rg atoms, consistent with the H-bonded OH···Rg configuration (H00)+. Hence, ionization of PhOH−Rg dimers switches the preferred recognition motif from π-stacking to H-bonding (Figure 2a). Early photoionization efficiency (PIE),192 REMPIIR,207,208 and subsequent zero-kinetic-energy photoelectron ( Z E K E ) a n d m a ss -a n a l y z e d t h r e s h o l d io n i za t i o n (MATI)72,123,196−199 spectra of the (10) isomers have, however, been interpreted with the spectroscopic signatures of the local πbonded (10)+ minima; that is, they missed detection of the more stable H-bound (H00)+ global minima due to unfavorable Franck−Condon factors. This observation highlights the importance of the EI-IR technique, which always detects predominantly the global minimum. Nonetheless, the ionization-induced π → H switching process has been confirmed in resonant photoionization studies to occur not only in PhOH− Rgn clusters with n = 1 but also in larger clusters with n ≥ 2 (so far up to n = 4) by nanosecond static REMPI-IR spectroscopy in the D0 state62,63,122 and thermochemical analysis of MATI/PIE fragmentation spectra.63,124,125 Although this ionization-induced π → H switch in the preferred recognition motif between an acidic aromatic molecule and a nonpolar solvent was first detected for PhOH−Ar,116 it has since been established as a quite general phenomenon77 for a large number of aromatic molecules with acidic NH and OH functional groups, including phenols,63,65,116−130 anilines,131−135 indoles,136,137 imidazoles,138 and amides.139,140 The charge-induced π → H switch in PhOH−Rgn clusters, inferred from static spectroscopy,62,63,122,124,125 gives rise to interesting isomerization dynamics triggered by photoionization, which has been investigated in real time for Rg = Kr (n = 1)64 and Ar (n = 2, 3)61−63 by ps-TRIR spectroscopy. The salient results will be discussed in the following sections as a function of cluster size n, which has a drastic impact on the type and time scale of the reaction mechanism.65 Finally, comparison will be made to PhOH−CH4 to illustrate the qualitative impact of changing an atomic ligand to a polyatomic solvent molecule on the reaction mechanism.128 3.1.1. PhOH−Kr Dimer. The structure of the neutral PhOH−Kr dimer has convincingly been assigned to a π-bonded equilibrium structure from its S1 ← S0 REMPI spectrum, both 5439


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Figure 7. continued spike and not related to formation of the H-bound cluster. The profile of the spike corresponds to the cross-correlation function between UV′ and IR laser pulses and confirms the overall time resolution of 3 ps. This effect is included in the fitting procedure. Spectra in panels b and c have the same frequency axis. Adapted with permission from ref 64, copyright 2011 PCCP Owner Societies, and ref 122, copyright 2007 Elsevier.

internally hot (H00)+ population. As a result of the high internal energy, this proton donor stretch vibration is substantially blueshifted in the REMPI-IR spectrum.122 To explore the dynamics of π → H isomerization in PhOH+− Kr triggered by ionization of neutral π-bound PhOH-Kr via its S1 origin, ps-TRIR spectroscopy has been applied with an ionization excess energy of Eexc = 60 cm−1.64 The relevant potential energy surface in the cationic D0 state of PhOH+−Kr along the isomerization coordinate, emerging from static spectroscopy and quantum chemical calculations, is depicted in Figure 7a. It exhibits a single planar deep H-bound global minimum and two equivalent shallow π-bound local minima separated by barriers. The measured pump−probe three-color UV−UV′−IR ps-TRIR spectra are shown in Figure 7b for delay times ranging from Δt = −7 to +48 ps, along with the static REMPI-IR spectrum recorded at Δt = +20 ns (Eexc ≤ 10 cm−1). The ps-TRIR spectra at negative delay do not show any absorption in the depicted frequency range (3360−3600 cm−1) because the νOH bands of neutral πbonded PhOH−Kr in the S0/S1 states occur at higher frequency (3657 cm−1 in S0). The ps-TRIR spectra at Δt ≥ +8 ps are already similar in appearance to the one at Δt = +20 ns, with the main difference that the nanosecond spectrum exhibits higher resolution due to the longer laser pulses employed. These spectra show the νOH bands of H-bonded (H00)+ and π-bonded (10)+ structures at νOH = 3452 and 3537 cm−1, respectively. Time evolutions of both resonances as a function of Δt are compared in Figure 7d. The νOH(π) transition arises immediately after the ionization event at Δt = 0 ps and decreases gradually to a constant level, with substantial intensity even at Δt = +100 ps. The νOH(H) band is not visible at Δt = 0 ps but starts around Δt = +2 ps and rises monotonically before converging to a constant level at around Δt = +20 ps. These time evolutions are fully consistent with the reaction scheme depicted in Figure 7a. At Δt = 0 ps, the neutral (10) isomer is ionized into the D0 state, and the nascent ionic (10)+ population isomerizes subsequently toward the ionic H-bound (H00)+ structure, with a rate constant k+ assumed for this π → H forward reaction. If the π → H isomerization were an elementary single-step forward reaction, the ν OH(π) band should decay to zero intensity in a monoexponential fashion and not to a constant level. Consequently, the signal of the νOH(π) band at long delay has to be explained by the H → π back reaction with a rate constant k− to either of the two equivalent π minima (10)+ and (01)+, which eventually leads to an equilibrium population for both types of isomers described as π ↔ H. Fitting the time evolutions to this simple rate equation model yields rates of k+ = 0.051 ps−1 and k− = 0.042 ps−1, corresponding to lifetimes of τ+ = 20 ps and τ− = 24 ps, respectively, which reproduce the measured time evolutions well (Figure 7d). In the classical description, ionization of PhOH−Kr initiates a large-amplitude pendular motion, in which Kr moves from one π-bound local minimum to the other one located on the other side of the phenyl ring via the H-bound planar global minimum. In the quantum mechanical picture, resonant photoionization of neutral PhOH−Kr

Figure 7. (a) Potential energy diagram of PhOH+−Kr along the reaction coordinate for the ionization-induced π ↔ H site-switching reaction. The H-bound structure (H00)+ is the global minimum of the potential curve, whereas the two π-bound structures (10)+ and (01)+, in which Kr is attached to opposite sides of the aromatic ring, are local minima. Resonant photoionization from the S1 state of the π-bound phenol−Kr structure (10) generates the π-bound cation (10)+, due to vertical transitions according to the Franck−Condon principle, and triggers the π → H forward reaction. Rate constants for the forward and backward reactions used in the simple model are indicated. (b) Picosecond TRIR spectra of PhOH−Kr for isomer-selective 1 + 1′ REMPI of the π-bonded (10) isomer of PhOH−Kr with Eexc = 60 cm−1 as a function of Δt, ranging from −7 to +48 ps.64 The static REMPI-IR spectrum recorded with Eexc ≤ 10 cm−1 at Δt = 20 ns is also shown. (c) Static EI-IR spectrum of cold PhOH+−Kr cations in the D0 state.122 (d) Experimental time evolution of νOH(H) and νOH(π) transitions assigned to the shown (H00)+ and (10)+ structures, compared to fitted time evolutions derived from the rate equation model (the reaction model indicated in panel a is assumed).64 In the time evolution of the νOH(H) band, the peak near Δt = 0 marked by an asterisk is due to a coherent 5440


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generates a wavepacket on the potential of the D0 state, which is initially localized at the π-bound local minimum at Δt = 0. It quickly propagates to the center of the potential before spreading out by further dephasing over the whole region between the repulsive walls, thereby covering both H- and π-bound structures. The fact that there is a finite equilibrium population of the less stable π-bound local minimum at long delay results from the lack of (efficient) IVR in this dimer. As the isomerization reaction involves all three available intermolecular degrees of freedom, there remain no intermolecular bath modes for IVR. In addition, coupling of the intermolecular reaction coordinates to low-frequency intramolecular skeletal vibrations is also very weak because of small cross-anharmonicities between inter- and intramolecular modes. As a result of the lack of IVR, internal energy on the order of 500 cm−1 remains in the reaction coordinate and enables an efficient H → π back reaction, leading to the observed finite π ↔ H equilibrium. The same reaction mechanism is inferred for PhOH+−Ar from nanosecond REMPIIR spectra62,63,126,127 as well as preliminary unpublished ps-TRIR spectra. 3.1.2. PhOH−Ar2 Trimer. Historically, the picosecond timeresolved IR spectroscopic approach was first applied to the PhOH−Ar2 trimer.61,62 The complexity of the conformational landscape in PhOH−Rgn clusters increases rapidly with cluster size n.121 The most stable (11) isomer of the PhOH−Ar2 trimer is generally observed in large abundance in the molecular beam, and its two equivalent π-bonded ligands located on opposite sides of the aromatic ring show little interaction with each other.121,192,193 The less stable (20) isomer has both π-bonded ligands attached on the same side of the phenyl ring121 and is only rarely detected due to its usually low population.63 Both isomers are readily identified by their characteristic S1 band origin shifts,63,192,196 which closely follow an empirical additivity rule,63 fully supported by quantum chemical calculations.121 The structure of the (11) isomer has been unambiguously determined by UV spectroscopy at the level of rotational resolution193 and is consistent with analysis of the intermolecular modes observed in REMPI121,196,201,210 and hole-burning201 spectra of the S1 state, as well as the REMPI-IR spectrum in the S0 state.63 Initial information about the geometry of the PhOH+−Ar2 cation in the D0 state came from PIE spectra via the S1 origin of the π-bonded (11) isomer, and the observed additive ΔIE shift clearly demonstrates nascent formation of the (11)+ isomer.192 The corresponding MATI spectrum obtained via REMPI of the neutral (11) isomer exhibits extensive vibrational progressions with spacings of 10 cm−1, which have been assigned to intermolecular bending modes of the (11)+ structure.123 This view is consistent with the shift of the Ar atoms toward the OH group upon ionization predicted by calculations.121 The EI-IR spectrum of PhOH+−Ar2 shown in Figure 8d, however, exhibits only a single transition in the O−H stretch range at νOH = 3467 cm−1,118 and its red shift from bare PhOH+ of ΔνOH = −67 cm−1 leads to clear assignment to an isomer with at least one H-bonded ligand. Recent calculations suggest two possible structures with similar total (equilibrium) dissociation energies of De = 1359 and 1384 cm−1: the (H10)+ isomer, with one H-bonded and one πbonded ligand, and the (11)H+ isomer,225 featuring bifurcated nonlinear H-bonds of the OH group to both equivalent Ar ligands.121 Since the two isomers cannot be distinguished by their IR spectra,121 for simplicity we will consider in the following only the (H10)+ isomer (Figure 8). Clearly, the much lower dissociation energy of De = 1097 cm−1 calculated for the (11)+ isomer,121 which agrees well with the value measured by MATI

Figure 8. Static REMPI-IR spectra of PhOH-Ar2 in the neutral S0 state (a) and ionic D0 state (c), obtained by REMPI of the (11) isomer via its S1 origin,63 compared to static EI-IR spectrum of PhOH+−Ar2 in the D0 state (d).118 (b) Picosecond TRIR spectra of PhOH−Ar2 for isomerselective 1 + 1′ REMPI of the (11) isomer of PhOH−Ar2 with Eexc = 550 cm−1 as a function of Δt ranging from −4 to +56 ps.61,62 (e) Experimental time evolutions of νOH(H) and νOH(π) transitions at 3467 and 3537 cm−1 for Eexc = 550 cm−1 assigned to the depicted (H10)+ and (11)+ structures, compared to fitted time evolution derived from the rate equation model, where a single-step one-way π → H forward reaction is assumed (τ = 7.2 ± 0.7 ps).61,62 Spectra in panels a−d have the same 5441


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π-bonded Ar ligand switches its binding site from above the aromatic ring to the OH group of PhOH+ in a π → H forward reaction, but no H → π back reaction is observed. Thus, the reaction mechanism of ionization-induced π → H isomerization in PhOH+−Ar2 is fundamentally different from that observed for PhOH+−Kr, indicating that attachment of the second Rg atom has a profound impact on the mechanism.65 Indeed, the major difference between PhOH+−Rgn clusters with n = 1 and with n ≥ 2 is availability of intermolecular bath modes to facilitate efficient IVR. As discussed before for PhOH+−Kr, in the n = 1 case no such bath modes are available, and as a consequence of the lack of IVR, the internal energy remains in the reaction coordinate, allowing H → π back reaction to occur and leading to the finite π ↔ H equilibrium at long delay. In contrast, for n ≥ 2 clusters, such intermolecular bath modes are available from the nonreactive Rg spectator ligands, so that internal energy can be efficiently removed from the reaction coordinate, which prevents any H → π back reaction and traps the cluster in the potential well of the H-bound global minimum. Experimental time evolutions of both transitions involved in the π → H reaction in PhOH+−Ar2 can readily be fitted by a single reaction rate constant. The best fits are obtained for τ = 7.2 ± 0.7 ps (at Eexc = 550 cm−1)62 and reproduce both experimental time evolutions well (Figure 8e), clearly confirming the elementary character of the single-step π → H forward reaction. Interestingly, time evolutions recorded at lower and higher ionization excess energy (Eexc = 190 and 860 cm−1) yield, within the error bars, the same time constants (τ = 6.7 ± 0.6 and 6.7 ± 0.7 ps), demonstrating that the dynamics of the π → H siteswitching reaction is quite independent of Eexc in this energy interval.62 This result, at first glance surprising, can be rationalized in the following way. Increasing Eexc will mainly change the distribution of the excitation of intramolecular skeletal modes of PhOH+ in the nascent (11)+ isomer formed by REMPI of (11), which is given by the corresponding Franck− Condon factors between the S100 origin and vibrational excitations in the D0 state. Initial excitation of the intermolecular modes, which are relevant for the isomerization dynamics, is relatively independent of Eexc.125 Apparently, the coupling between inter- and intramolecular vibrational degrees of freedom is small in PhOH+−Rgn, which prevents efficient vibrational energy transfer from intramolecular modes into the intermolecular reaction coordinate.65 The time constant determined for the π → H forward reaction in PhOH+−Kr (20 ps) is longer than that extracted for PhOH+−Ar2 (7 ps), and this difference has mainly been attributed to the different (effective) masses of the Rg atoms involved in the isomerization reaction.65 3.1.3. PhOH−Ar3 Tetramer. The PhOH−Ar3 tetramer is the largest and most complex cluster system considered so far for probing solvation dynamics in real time.63 Consequently, far less spectroscopic and theoretical data are available for this cluster size. As no high-resolution spectrum has been measured for this cluster, structural information is based on vibrationally resolved spectra and calculations and thus is less reliable. Hole-burning spectroscopy201 reveals that the vibronic bands observed in the REMPI spectrum63,124,196,201,211 of neutral PhOH−Ar3 can mostly be attributed to a single isomer, which can be assigned by additivity rules63 with strong confidence to the (30) isomer, in which all three Ar atoms are π-bonded on the same side of the aromatic phenyl ring. Thermochemical analysis of the photoionization and photofragmentation spectra63,124 clearly demonstrates that all three Ar ligands are π-bonded, a conclusion consistent with the nanosecond REMPI-IR spectra recorded in

Figure 8. continued frequency axis. Adapted with permission from ref 62. Copyright 2007 American Institute of Physics.

spectroscopy, D0 ≈ 1115 cm−1,125 reveals that this π-bonded isomer is only a local minimum on the D0 potential of PhOH+− Ar2, and thus it is not observed at all in the EI-IR spectrum.118 Interestingly, the nanosecond REMPI-IR spectrum of PhOH+− Ar2 measured after isomer-selective REMPI of the (11) isomer (Figure 8c) does not show any free νOH band characteristic of the (11)+ isomer but only a single red-shifted band at 3474 cm−1, similar to the EI-IR spectrum assigned to (H10)+.62,63 This particular observation can be rationalized only by an ionizationinduced π → H isomerization with 100% yield. The nascent (11)+ structure, produced by REMPI of (11) in a vertical transition according to the Franck−Condon principle, isomerizes quantitatively toward the more stable (H10)+ structure and this reaction is complete on the nanosecond time scale. Significantly, the 100% yield observed for the π → H isomerization reaction mechanism in PhOH+−Ar262,63 is qualitatively different from that of PhOH+−Kr, for which less than 40% is converted at the π ↔ H equilibrium established at long delay (20 ns).122 The PIE and MATI spectra of PhOH+− Ar2, recorded in the parent and fragment channels for REMPI of the (11) isomer, confirm the ionization-induced π → H isomerization reaction mechanism and its thermochemical properties, particularly the low appearance threshold of 210 cm−1 observed for dissociation into PhOH+−Ar(H) and Ar.123 In the nascent (11)+ isomer of PhOH+−Ar2, both equivalent πbonded ligands are bound to the phenyl ring with a dissociation energy of around 535 cm−1,197 whereas that of an Ar ligand binding to the OH group of PhOH+ is around 870 cm−1.123 Hence, π → H isomerization of a single Ar ligand in the (11)+ → (H10)+ reaction releases about 335 cm−1 internal energy into the PhOH+−Ar2 cluster. This energy becomes available for evaporation of the remaining second π-bound Ar ligand and reduces its dissociation threshold from 535 to about 200 cm−1 for dissociation of (11)+ into (H00)+ and Ar. These thermochemical data derived from the MATI fragmentation spectra are also fully consistent with the appearance of REMPI-IR spectra recorded for 1 + 1′ REMPI of (11) at different ionization excess energies.62 To shed further light on the dynamics of π → H isomerization in PhOH+−Ar2 and to reveal details of the reaction mechanism and its dependence on ionization excess energy, ps-TRIR spectra have been recorded.61,62 Figure 8b shows ps-TRIR spectra obtained for REMPI of the (11) isomer at ionization excess energy Eexc = 550 cm−1 for delay times Δt ranging from −4 to +56 ps. As expected, at Δt = 0 only the νOH band of the nascent πbonded (11)+ isomer produced by REMPI of (11) is observed at 3537 cm−1. After the initial rise of this band, it monotonically decays until it is not discernible any more after around 9 ps. At the same time, the νOH band of the H-bonded (H10)+ isomer at 3467 cm−1 rises monotonically from zero intensity at Δt = 0 until it reaches a constant level at around 20 ps. The ps-TRIR spectrum at Δt = 56 ps is similar to the one obtained by nanosecond lasers at long delay (Figure 8c). The time evolution of both transitions reproduced in Figure 8e shows that the dynamics of both bands exhibit the same time constant. These data directly probe the π → H isomerization reaction induced by ionization, in which the nascent π-bonded (11)+ isomer reacts toward the (H10)+ isomer on the 10 ps time scale via a singlestep one-way reaction with 100% yield. In this reaction, a single 5442


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the νOH range.63 Further refinement of the structure relies on density functional calculations (M06-2X/aug-cc-pVTZ), which predict the particular (30) isomer depicted in Figure 9 to be by

reaction. The IVR rate in the n = 3 cluster is also higher than for n = 2 because the number of spectator ligands increases from one to two, thereby drastically increasing the density of states of lowfrequency intermolecular bath modes. Hence, obviously also for n = 3 no back reaction is expected, in agreement with experiment. 3.1.4. PhOH−CH4 Dimer. The studies on PhOH−Rgn clusters described above show that the π → H site-switching mechanism qualitatively changes from n = 1 (π ↔ H equilibrium), with no IVR but with back reaction, to n ≥ 2 (π → H forward reaction), with fast IVR and without back reaction. This difference has been rationalized by the fact in the n = 1 case no intermolecular bath modes are available for IVR because all three intermolecular degrees of freedom of the single Rg ligand are involved in the isomerization process, while in the n ≥ 2 case the intermolecular degrees of freedom of the (n − 1) atomic spectator ligands can act as accepting bath modes for IVR.65 In order to test this hypothesis, ionization-induced π → H siteswitching in the PhOH−CH4 cluster has been considered for the following reasons.128 First, although this cluster has only a single spherical top ligand (i.e., it belongs to the n = 1 case), the polyatomic CH4 molecule has additional internal degrees of freedom leading to six intermolecular modes in the n = 1 cluster rather than three for PhOH−Rg dimers with an atomic ligand. Hence, as the π → H isomerization reaction involves (at least) three intermolecular degrees of freedom, for PhOH+−CH4, intermolecular bath modes are available for IVR even in the n = 1 case. Consequently, according to the argument outlined, the π → H site-switching reaction mechanism occurring in PhOH+−CH4 (n = 1) should be similar to that of PhOH+−Rgn (n ≥ 2) but different from that of PhOH−Rg (n = 1). To this end, nanosecond REMPI-IR spectra in S0 and D0 states have been measured, because the yield of the π → H reaction at long delay is already indicative of the type of mechanism: 100% for a simple π → H forward reaction (without back reaction) and substantially less than 100% for the π ↔ H equilibrium (with back reaction). Second, similar to Rg ligands, CH4 interacts with neutral and ionic PhOH(+) mainly via its polarizability. As a consequence, the topology of the potential energy surfaces of PhOH(+)−CH4 and PhOH(+)−Rg is expected to be similar in both charge states (Figure 10a). The limited spectral information available for PhOH−CH4 (complexation-induced red shifts in S1 and IE energies,90,128,200 intermolecular vibrational structure in S1 state,128 and vibrational Raman200 and REMPI-IR128 spectra in fingerprint and νOH ranges in S0 state) are consistent with a π-bound (10) geometry in S0 and S1, in line with predictions from MP2 and M06-2X calculations.128 In contrast, EI-IR spectra of PhOH+−(CH4)n with n = 1, 2 have proven that the H-bound structure is the global minimum in the cationic D0 state,118 because no free νOH band is detected. The complexation-induced red shift of the H-bonded νOH of the (H00)+ isomer of PhOH+−CH4 observed at 3365 cm−1 (Figure 10d) from that of bare PhOH+ (3534 cm−1, Figure 10b),209 is substantially larger than those for L = Kr and Ar (−169 versus −123 and −67 cm−1), consistent with the stronger interaction arising from the larger proton affinity and polarizability. The REMPI-IR spectrum of PhOH+−CH4, generated by 1 + 1′ REMPI of the neutral π-bonded (10) isomer recorded 50 ns after the ionization event at ionization excess energy Eexc = 100 cm−1 (Figure 10c), also does not show any signal in the free νOH range of bare PhOH+ near 3534 cm−1 expected for (10)+.207,209,212 Instead, it exhibits a broad transition centered around 3390 cm−1, which is readily ascribed to the H-bonded νOH of hot H-bonded (H00)+ clusters. They are obviously

Figure 9. Calculated structures of PhOH−Ar3 isomers in S0 and D0 states (M06-2X/aug-cc-pVTZ)121 to illustrate possible pathways for ionization-induced π → H isomerization reaction of the (30) isomer. Adapted with permission from ref 65. Copyright 2012 Taylor & Francis.

far the most stable one.121 Its calculated total dissociation energy of De = 1083 cm−1 is in good agreement with the experimental value extracted from MATI spectra, D0 = 1179 ± 45 cm−1,124 and CC2 calculations predict for this isomer a ΔS1 origin shift and intermolecular vibrational structure in agreement with experiment.121 In this structure, a planar Ar3 trimer binds essentially parallel to the aromatic ring by dispersion forces, and two Ar ligands interact with the OH group by additional polarization forces. The EI-IR spectrum of PhOH+−Ar3 does not show any free νOH band, indicating that in the most stable structure of the cation at least one Ar ligand is H-bonded to the OH group, in line with the red shift of ΔνOH = −64 cm−1.118 The M06-2X calculations predict two low-energy isomers with similar stabilities, namely, (H20)+ and (21)H+ with De = 2043 and 2033 cm−1 (Figure 9), which have νOH frequencies consistent with those observed in the EI-IR spectrum. The (30)+ isomer produced by REMPI of the neutral (30) isomer is far less stable, with De = 1671 cm−1, close to the measured value of D0 = 1730 ± 30 cm−1.124 The static REMPI-IR spectrum of the PhOH+−Ar3 cation obtained after REMPI of the π-bonded (30) isomer also does not show any free νOH transition, suggesting that, similar to the (11) isomer of PhOH−Ar2, ionization of (30) triggers a π → H site-switching reaction with 100% yield and without any back reaction.63 Again, thermochemical analysis of the photoionization and photofragmentation spectra is fully consistent with this view63,65,153 that the nascent (30)+ isomer reacts in a single-step π → H forward reaction toward either (H20)+ and/or (21)H+.65 As both feasible reaction products have similar IR spectra and stabilization energies,121 they cannot be distinguished, and in the following we consider only the (H20)+ isomer for simplicity (Figure 9). The ps-TRIR spectra measured by use of REMPI of the (30) isomer of PhOH−Ar3 at Eexc = 160 cm−1 confirm that the π → H reaction mechanism for the (30)+ → (H20)+ forward reaction is indeed similar to that observed for the (11)+ → (H10)+ reaction of PhOH−Ar2, with the major difference that the reaction proceeds much faster in the former cluster (τ ≤ 3 ps versus τ = 7 ps).63,65 This is mainly attributed to the much shorter reaction pathway for the n = 3 (30)+ cluster as compared to the n = 2 (11)+ cluster. In the former structure, one of the π-bonded Ar ligands is already quite close to the OH binding site at the start of the 5443


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signal 50 ns after ionization implies that the π → H forward reaction is complete. This result is in marked contrast to the case of PhOH+−Kr (and also PhOH+−Ar), for which the reaction yield is only 40%. As we have discussed in detail, this difference arises from the polyatomic character of the CH4 ligand in PhOH+−CH4, whose additional intermolecular bath modes effectively remove internal energy from the reaction coordinate. Hence, the nascent hot (H00)+ population generated by the single-step π → H forward reaction cools down efficiently in the reaction coordinate via IVR and remains trapped in this (H00)+ configuration, thereby preventing any H → π back reaction (Figure 10a). So far, the time evolution of the π → H reaction has not been measured. However, we expect it to be somewhat faster than that measured for PhOH+−Ar2 (7 ps), due to the much lighter mass of the CH4 ligand (m/z 16 versus 40) and possibly the higher density of states leading to faster IVR. 3.2. Aromatic Monohydrated Clusters

Solvation dynamics of nonpolar atoms and molecular ligands around an aromatic molecule is reviewed in section 3.1. Clusters with rare-gas ligands provide a comprehensive view of solvation dynamics for hydrophobic → hydrophilic site switching at the molecular level. In these clusters, the competition between πstacking and H-bonding is the essential driving force for solvation dynamics triggered by photoionization. The effect of number of ligands and replacement of an atom by a polyatomic molecule reveals the role of IVR in the solvation dynamics and the observed reaction mechanism. What is the essential difference when the nonpolar solvent is replaced by a dipolar solvent? The electric dipole moment of the solvent molecule gives rise to much stronger electrostatic (and polarization) interactions and thus will react much more sensitively to photoionization (photoexcitation) of the solute molecule than a nonpolar solvent, leading to much larger rearrangements of the (H-bonded) solvent network. In addition, the solvent migration path will also be strongly dominated by the charge−dipole interaction. Hbonding with a polar solvent molecule is stronger than that with nonpolar ligands. If a polar solvent molecule has an acidic OH or NH group, it can act as a proton donor, which is not possible for a nonpolar solvent. All these aspects are characteristic features in the solvation dynamics of a polar solvent molecule. In this section, we focus on solvation dynamics of water in monohydrated aromatic clusters. Water is one of the most important polar solvents because almost all biological systems work in aqueous solution. The water molecule can act in Hbonds as both proton donor and proton acceptor. The partial positive and negative charges in the H and O atoms of the dipolar H2O molecule, respectively, are key factors to understand the solvation dynamics triggered by photoionization of (mono)hydrated clusters, because the sudden formation of the positive charge by ionization creates repulsion and/or additional attraction for the water dipole, which strongly depends on the binding site at the aromatic solute. Two examples for single solvent migration dynamics in aqueous monohydrated aromatic clusters have been studied so far by time-resolved spectroscopy: CO → NH water migration in trans-acetanilide+−H2O (AA+− H2O, Figure 2c) and CN → NH water migration in 4aminobenzonitrile+−H2O (4ABN+−H2O, Figure 2d). In both cases, H2O binds in the neutral cluster as a proton donor to the aromatic molecule, which becomes a repulsive configuration in the ionic D0 state. In this sense, both clusters belong essentially to case c in Figure 1.

Figure 10. (a) Sketch of potential energy diagram of PhOH+−CH4 along the reaction coordinate for ionization-induced site-switching reaction. The H-bound structure is the global minimum of the potential curve, whereas the two π-bound structures, in which CH4 is attached to opposite sides of the aromatic ring, are local minima. Resonant photoionization from the S1 state of the π-bound PhOH−CH4 structure generates the π-bound cation, due to vertical transitions according to the Franck−Condon principle, and triggers the π → H forward reaction. Rapid IVR of the hot H-bonded PhOH+−CH4 cluster structure removes internal energy out of the reaction coordinate and traps the cluster in the global minimum structure, preventing any H → π back reaction. (b) IR spectrum of bare PhOH+ in the D0 state; the free νOH band at 3534 cm−1 is indicated.209,212 (c) Static REMPI-IR (Eexc = 100 cm−1) and (d) EI-IR spectra of PhOH+−CH4 cation in the D0 state, showing the H-bonded νOH band.118,128 Spectra in panels b−d have the same frequency axis. Adapted with permission from ref 128, copyright 2014 PCCP Owner Societies, and ref 209, copyright 2000 Elsevier.

produced with 100% yield by rapid π → H site switching of the CH4 ligand of the nascent (10)+ cluster cations toward (H00)+ after the ionization event. According to M06-2X calculations, the (H00)+ global minimum with D0 = 1620 cm−1 is far more stable than the (10)+ local minimum with D0 = 647 cm−1; that is, the energy released by the exothermic π → H migration reaction is on the order of 1000 cm−1. As a result, the νOH transition of the hot (H00)+ reaction product probed by REMPI-IR is blueshifted and broadened compared to the νOH fundamental of cold (H00)+ clusters detected in the EI-IR spectrum. The lack of any 5444


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3.2.1. trans-Acetanilide−H2O. Acetanilide (AA) is one of the simplest aromatic amides with a -CO-NH- linkage, in this case connecting a phenyl with a methyl group (Figure 11). The

the solution phase, the various water molecules exist in different environments, and it is very difficult to distinguish a specific water molecule. In such inhomogeneous circumstances, it is not obvious that all the different methods detect the molecules in the same circumstance. Therefore, it is of crucial importance to measure the structural rearrangement dynamics of a specific individual water molecule in a well-defined model system, in which its environment is well-known and controlled. Although AA has two isomers with cis and trans configurations of the amide group, only the more stable trans isomer and its clusters are observed in molecular beam experiments. Two stable isomers of neutral AA−H2O are detected in the S0 state (Figure 11). In the more stable CO-bound isomer, H2O acts as a proton donor and forms a OH···O H-bond to the CO group of the amide. In the less stable NH-bound isomer, it acts as proton acceptor and forms a NH···O H-bond to the NH group of the amide. The energy difference is calculated as 221 cm−1 in favor of the CO isomer,150 which has a correspondingly shorter H-bond (1.87 versus 2.03 Å).213 Both AA−H2O isomers are detected by REMPI spectroscopy of the S1 ← S0 transition (Figure 12).66,146,214 The stronger S1 origin at 36 050 cm−1 is assigned

Figure 12. REMPI spectrum (1 + 1) of the S1 ← S0 transition of AA− H2O. The S100 origins of the NH and CO isomers at 35 697 and 36 050 cm−1 are indicated.

Figure 11. Energy-level diagram for NH- and CO-bound isomers of AA−H2O in the S0, S1, and D0 electronic states, derived from available spectroscopic data. Only the energy difference of 221 cm−1 in S0 state is taken from calculations (M06-2X/cc-pVDZ).150 Structures were optimized at the M06-2X/aug-cc-pVTZ level.213 Intermolecular bond lengths are given in angstroms. All energies are given in reciprocal centimeters. The water migration takes place after ionization into the ionic D0 state, while no such dynamics is observed in the neutral S1 state, because the barrier for isomerization lies above the S1 origin level used for the REMPI process.

to the more abundant CO isomer, while the weaker S1 origin at 35 697 cm−1 is attributed to the less stable NH isomer. The complexation-induced blue shift of +146 cm−1 and red shift of −207 cm−1 from the S1 origin of bare AA are consistent with H2O acting as a donor and acceptor in the CO and NH isomers, respectively. These shifts also indicate that S1 excitation weakens the OH···O H-bond while it strengthens the NH···O H-bond. As a result, the stability of the two isomers is reversed in the S1 state (Figure 11). These structural assignments are fully confirmed by isomer-selective static REMPI-IR spectra recorded via the two S1 origins.146 Figure 13 compares the nanosecond REMPI-IR spectra of AA−H2O(CO) and AA−H2O(NH) in the S0 state with that of the AA monomer recorded via the respective S1 origins for the REMPI process. The AA−H2O(CO) spectrum shows the H-bonded and free O−H stretching vibrations (νOHb and νOHf) of H2O together with the free N−H stretching band (νNHf). These spectral signatures indicate that H2O is H-bonded as a proton donor to the CO site of AA. The AA−H2O(NH) spectrum exhibits a red-shifted bound N−H stretching vibration (νNHb) and two weak free O−H stretching vibrations of H2O (νOHa, νOHs), directly proving that H2O is bound as a proton acceptor to the NH site. Spectroscopic information about AA+−H2O structures in the D0 state is available from ZEKE,214 EI-IR,66,213 and static REMPI-IR66,146 spectra. Analysis of the monohydration-induced

amide group attracts water molecules by H-bonding at both the CO acceptor and NH donor binding sites. Moreover, the -CONH- group is known as peptide linkage, which connects amino acids in peptides and proteins. Hence, the hydration dynamics in AA−H2O clusters can be considered as the simplest model system for hydrated proteins. Hydration dynamics of proteins is a fundamental issue to understand the function of proteins and thus has attracted much interest.13−17,19,22,25,27,28,30,31,33,44,49,60 For example, water molecules in the hydration shell of proteins have to be rearranged when proteins are folded. The water rearrangements are expected to be slower than in bulk water due to protein−water interactions. However, the time scale of water rearrangements has not been established because the reported time scales vary in a wide range from very slow (icelike behavior) to tens of picoseconds.14,15,22,27,28,30 One reason for the controversial results arises from the fact that these reports use different methods to measure hydration dynamics in aqueous solution such as NMR, X-ray, and time-resolved fluorescence. In 5445


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Figure 13. Isomer-selective static REMPI-IR spectra of AA and of AA− H2O(NH) and AA−H2O(CO) isomers in S0 state, recorded by 1 + 1 REMPI via their respective S1 origins, along with vibrational assignments.

IE shift and intermolecular modes observed in the wellstructured ZEKE spectrum demonstrates that ionization of the AA−H2O(NH) isomer produces a AA+−H2O(NH) cation with the same binding motif (Figure 11).214 The main difference arising from the excess positive charge is a substantial contraction of the intermolecular NH···O bond from 2.03 to 1.84 Å,213 which increases in strength by 2904 cm−1.214 No corresponding ZEKE spectrum of the AA−H2O(CO) isomer has been reported, probably because of the large geometry change induced by ionization of this isomer (Figure 11). Ionization of the CO isomer produces a cation local minimum structure, in which the H2O ligand rotates in order to reach the favorable charge−dipole orientation (the positive charge is mainly localized on the aromatic ring) and it also translates closer to the positively charged phenyl ring, thereby substantially changing its H-bond arrangement. Clearly, the energy difference between the CO and NH isomers further increases drastically by ionization from the S1 state. The EI-IR spectra of AA+−H2O and AA+−H2O−Ar shown in Figure 14 correspond to the IR spectrum of the most stable isomer of AA+−H2O and can fully be attributed to AA+− H2O(NH) by comparison to the calculated spectrum.66,213 The spectrum of the Ar-tagged species is much colder, leading to sharper transitions. Both spectra are dominated by a strongly redshifted N−H stretch transition (νNHb) occurring near 3200 cm−1, and there are two sharper free O−H stretch bands close to the symmetric and asymmetric modes (νOHs and νOHa) of free H2O. Significantly, these spectra lack any signal in the range of the free N−H stretch (νNHf) of AA+ near 3385 cm−1, as experimentally measured by the AA+−He spectrum also shown in Figure 14.139 Hence, all AA+−H2O clusters contributing to the EI-IR spectra have the H2O ligand binding to the NH group, forming a NH···O H-bond with favorable charge−dipole orientation, consistent with AA+−H2O(NH) being by far the most stable isomer in the D0 state. The static REMPI-IR spectrum of AA+−H2O(NH), measured by isomer-selective 1 + 1 REMPI of neutral AA− H2O(NH) via its S1 origin at 35 697 cm−1, is essentially the same as the EI-IR spectrum (Figure 14). The bands in the REMPI-IR spectrum are somewhat broader than those in the EI-IR spectra because of the high ionization excess energy of Eexc = 8761 cm−1 involved in the 1 + 1 REMPI process, giving rise to sequence hot band transitions that, particularly for proton-donor stretch vibrations, occur to the blue side of the fundamental transitions. Nonetheless, the REMPI-IR spectrum of AA+−H2O(NH) demonstrates that the H2O ligand remains at the NH binding

Figure 14. EI-IR spectra of AA+−He, AA+−H2O, and AA+−H2O−Ar in D0 state66,139,213 compared to static REMPI-IR spectra of AA+−H2O in D0 state, recorded by 1 + 1 REMPI of CO and NH isomers, and to linear IR absorption spectra of AA+, AA+−H2O(NH), and AA+−H2O(CO) calculated at the M06-2X/6-311++G(d,p) level with a scaling factor of 0.9516 to match the free N−H stretch of AA+ at νNHf = 3385 cm−1. Hot band transitions are indicated by asterisks. Adapted with permission from ref 66. Copyright 2012 Wiley.

site after photoionization. In contrast, the corresponding REMPI-IR spectrum of AA+−H2O(CO) is drastically different from the one predicted for the CO-bound isomer in the D0 state (Figure 14). It completely lacks the intense free N−H stretch (νNHf) near 3385 cm−1 characteristic for the AA+−H2O(CO) isomer. In addition, it closely resembles the spectrum measured for the AA+−H2O(NH) isomer, even though the ions are generated by isomer-selective 1 + 1 REMPI of neutral AA− H2O(CO) via its S1 origin at 36 050 cm−1. This observation indicates that the H2O ligand undergoes a CO → NH isomerization reaction in the D0 state of AA+−H2O triggered by ionization of the CO isomer, with a yield of 100%. As the adiabatic ionization energy and the dissociation energy of AA+− H2O(CO) have not experimentally been determined, estimation of the ionization excess energy available after 1 + 1 REMPI of AA−H2O(CO) relies barely on calculations and is on the order of Eexc ∼ 6000 cm−1.68,213 In addition, CO → NH water migration in the D0 state is calculated to be exothermic by ∼2500 cm−1.213 The rather large internal energy available after CO → NH reaction explains why the REMPI-IR spectrum of the CO 5446


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that is, the charge−dipole orientation is not favorable. Thus, this band A is the spectral signature of the initially formed nascent species by photoionization, which can be denoted as Franck− Condon state (FC+, Figure 16). The νOHb band becomes weaker

isomer in the cation state in Figure 14 is even hotter than that of the NH isomer. A similar CO → NH migration reaction is also observed for the related AA+−CH3OH and FA+−H2O clusters (Figure 6).148−150 Static nanosecond REMPI-IR spectra in the S0 and D0 states demonstrate the ionization-induced CO → NH isomerization of AA−H2O by measuring the spectra of reactant and product states. However, they do not contain any information about the dynamics of the reaction, such as migration rate, pathway, and intermediates. To this end, ps-TRIR spectra have been measured by utilizing 1 + 1 REMPI of AA−H2O(CO) for delay times Δt ranging from −5 to +17 ps (Figure 15).66 The top and bottom

Figure 16. Reaction scheme for ionization-induced CO → NH water migration in AA+−H2O. Isomer-selective photoionization of neutral AA−H2O(CO) (reactant R) produces the nascent AA+−H2O(CO) (Franck−Condon state, FC+). Analysis using the classical rate equation model assumes a two-step three-state reaction mechanism (FC+ → I+ → P+) via an intermediate I+ to reach the final AA+−H2O(CO) reaction product (P+).66 The MD simulations suggest two competing pathways: the slow indirect FC+ → I+ → P+ pathway with intermediate and the fast direct FC+ → P+ pathway without intermediate.67 Adapted with permission from ref 66. Copyright 2012 Wiley.

at Δt = 2 ps and disappears after Δt = 3 ps. This dynamics corresponds to release of H2O from the CO site. Instead, the broad νNHb transition (red band C) rises from Δt = 4 ps onward and monotonically grows in intensity with increasing delay. At Δt = 17 ps, the ps-TRIR spectrum almost reproduces the static spectrum (Δt = 50 ns), indicating that H2O has arrived at the NH site and forms a NH···O H-bond to the NH group of AA+. The sudden shift and disappearance of νOHb (band A) followed by the appearance of νNHb (band C) directly demonstrates the CO → NH forward reaction of H2O around the peptide linkage triggered by photoionization of the CO isomer, occurring with 100% yield without any back reaction. In addition to bands A and C, the ps-TRIR spectrum at Δt = 3 ps exhibits a significant transition (green-shaded band B) not observed in spectra with much shorter and longer delays. In this spectrum, band A (νOHb), the spectral signature of the initial FC+ structure, disappears but band C (νNHb), the signature of the final reaction product AA+−H2O(NH), is still not observed. Instead, band B at 3385 cm−1 is clearly detected and dominates the spectrum. From comparison to the EI-IR spectrum of AA+− He,66,139 this band is assigned to the free N−H stretching vibration of AA+ (νNHf). These observations indicate that H2O has been released from the initial CO site (FC+) but has not arrived at the NH-bound reaction product (P+). Thus, band B corresponds to an intermediate (I+) in which H2O is bound to neither the CO nor the NH site. Thus, the CO → NH water migration in AA+−H2O involves at least a two-step three-state reaction mechanism, FC+ → I+ → P+ (Figure 16, top). Another characteristic feature of the ps-TRIR spectra in Figure 15 is that the initially prepared AA+−H2O(CO) species is converted to the final AA+−H2O(NH) product with 100% yield and no back reaction is observed. As discussed in section 3.1, the 100% reaction yield means that fast IVR in the final reaction

Figure 15. Picosecond TRIR spectra of AA+−H2O for isomer-selective 1 + 1 REMPI of AA−H2O(CO) isomer with Eexc ≈ 6000 cm−1 as a function of Δt.66 For comparison, static nanosecond IR spectra measured for S0 and D0 states at Δt = −50 and +50 ns are shown as well. Time evolutions have been measured for bands A−C (Figure 17). Adapted with permission from ref 66. Copyright 2012 Wiley.

spectra, recorded at Δt = −50 ns and +50 ns with nanosecond lasers, correspond to static spectra in the S0 and D0 states shown in Figures 13 and 14, respectively. The ps-TRIR spectra for negative delay times are essentially the same as the IR spectrum in the S0 state, except for broadening of the bands due to lower resolution of the picosecond laser system. The ps-TRIR spectra change in appearance immediately after ionization (Δt = 1 and 3 ps). The νOHb band of neutral AA−H2O(CO) is shifted to higher frequency by 28 cm−1 in the cation (blue band A), which cannot be detected in the static REMPI-IR spectrum. The blue shift suggests that the H-bond between H2O and the CO site becomes weaker despite the positive charge. This observation is reasonable because in the initial structure the proton-donating H2O points with one H atom directly toward the benzene ring, which bears most of the positive charge generated by ionization; 5447


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understanding of CO → NH water migration in AA+−H2O, namely, a FC+ → I+ → P+ mechanism with an overall reaction time of around 4−5 ps.

product prevents the back reaction and stabilizes the cluster in that structure. In the AA+−H2O cluster, there is only a single ligand and the situation is analogous to the PhOH+−CH4 cluster described in section 3.1.4. Due to high ionization excess energy and the rather exothermic CO → NH reaction (Figure 11), accepting bath modes for IVR can be not only those intermolecular vibrational modes not involved in the migration reaction but also intramolecular modes of AA+. The reaction coordinate can be described by a linear combination of intermolecular modes. The AA+−H2O cluster has six intermolecular vibrational coordinates arising from three translational and three rotational degrees of freedom of H2O. The reaction coordinate requires inclusion of all three stretching modes originating from the translations. As H2O also rotates when going from the CO to the NH site (Figure 11), at least one of the bending and/or torsional modes originating from the rotations must be involved in the reaction coordinate. The remaining intermolecular bending modes, together with particularly lowfrequency intramolecular modes, can act as bath mode to accept vibrational energy in the IVR process from the reaction coordinate. Time evolutions of bands A (νOHb, 3495 cm−1), B (νNHf, 3385 cm−1), and C (νNHb, 3185 cm−1) in the ps-TRIR spectra in Figure 15, which correspond to initially prepared ionic cluster (FC+), intermediate (I+), and final reaction product (P+), are shown in Figure 17.66 Time evolution of band A mainly reflects the intensity of νOHb of FC+ but also includes the decay of AA− H2O(CO) because of spectral overlap. Time evolution of band B mainly corresponds to I+ but also contains the response of initially prepared AA+−H2O(CO) (FC+) and neutral AA− H2O(CO) (R). Time evolution of band C directly reflects the population of P+. These time evolutions have been simulated by considering the simple sequential two-step three-state reaction mechanism FC+ → I+ → P+ shown in the upper reaction path in Figure 16 (and taking care of the spectral overlaps). The corresponding rate equations are as follows:66 d[FC+]/dt = −k1[FC+]

(3)

d[I+]/dt = k1[FC+] − k 2[I+]

(4)

d[P+]/dt = k 2[I+]

(5)

Figure 17. Time evolution of bands A (νOHb, blue, 3495 cm−1), B (νNHf, green, 3385 cm−1), and C (νNHb, red, 3185 cm−1) observed in ps-TRIR spectra in Figure 15 (corrected for spectral overlap), which are characteristic of initially prepared ionic cluster (FC+), intermediate (I+), and final reaction product (P+).66,67 These experimental time evolutions are compared to (a) time evolutions fitted to solutions of the classical rate constant model, with the assumption of a simple two-step threestate FC+ → I+ → P+ reaction (Figure 16), and to (b) time evolutions obtained directly from DFT MD simulations (without any parameter fits).67 Adapted with permission from ref 66, copyright 2012 Wiley, and ref 67, copyright 2014 Wiley.

Here [X] indicates populations of X, and k1 and k2 are rate constants (ki = 1/τi) for the first and second steps of the reaction (Figure 16). The rate equations (eqs 3−5) are solved as follows. [R] = 1 − u(t )

(6)

[FC+] = exp(−k1t )u(t )

(7)

[I+] =

k1 [exp( −k1t ) − exp( −k 2t )]u(t ) k 2 − k1

[P+] =

1 {k 2[1 − exp(− k1t )] − k1[1 − exp(− k 2t )]}u(t ) k 2 − k1 (9)

As is demonstrated in Figure 17a, the simple rate equation analysis reasonably well reproduces the time evolution of the ionization-induced CO → NH water migration reaction in AA+− H2O. However, it should be noted that time constants obtained by the rate equation analysis correspond to the average of reaction rates for different initial internal energies. The AA− H2O(CO) cluster is ionized by REMPI via S1. In this process, the internal energy of the nascent AA+−H2O(CO) population is distributed in a certain energy range given by Franck−Condon factors between the D0 and S1 states. As the reaction rate should depend on the excess energy of the cluster, the analysis based on simple rate equations provides the picture averaged over all clusters having various initial energies. Indeed, preliminary psTRIR spectra employing two-color 1 + 1′ REMPI for ionization with reduced Eexc illustrate that the reaction speed is substantially slowed down at reduced internal energy. A second problem of rate equation analysis is that it strongly depends on the reaction model employed. We consider above the minimal two-step three-state reaction model suggested by the experimental data; however, other more complex models may be applicable. For example, if the reaction path is not unique, we may be able to add

(8)

Here u(t) is the unit step function, and the time profiles of the picosecond lasers determined by cross-correlation measurements correspond to Gaussians with a width of 2.8 ps. The measured time evolutions are consistently fit by eqs 6−9 with 1/ k1 = τ1 = 0.3 ± 0.1 ps and 1/k2 = τ2 = 4.0 ± 0.2 ps. The best fits from the simulations are included in Figure 17a as solid curves. This elementary rate constant model provides the simplest 5448


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Figure 18. (a) Picosecond TRIR spectra66 of the AA+−H2O for isomer-selective 1 + 1 REMPI of AA−H2O(CO) isomer with Eexc ≈ 6000 cm−1 as a function of Δt, compared to corresponding ps-TRIR spectra of AA+−H2O obtained from MD simulations for all 50 trajectories (b) and for the trajectories corresponding to fast (c) and slow (d) reaction channels.67 The S0 (top) and D0 (bottom) spectra are static IR spectra of AA−H2O(CO) reactant and AA+−H2O(NH) reaction product determined by nanosecond lasers (a) and by DFT harmonic frequency calculations for the optimized structures (b). Shaded areas indicate the signals uniquely attributed to NH-bonded isomer (red, P+), intermediate structure (green, I+), and CO-bound geometry (blue, R and FC+). Note that spectra in panels b−d are normalized to the same maximum value. Adapted with permission from ref 67. Copyright 2014 Wiley.

is 1 order of magnitude faster than the dynamics measured for AA+−H2O. The second point is that MD simulations for FA+− H2O show the NH → CO back reaction from final product P+ to initially prepared state FC+, which is also not detected in the experimental ps-TRIR spectra of AA+−H2O. The final point is that the MD simulations for FA+−H2O do not give any evidence for the presence of an intermediate I+, which is clearly detected in ps-TRIR spectra of AA+−H2O. Indeed, initial ps-TRIR experiments for FA+−H2O show similar reaction dynamics as AA+− H2O, confirming that these initial MD simulations largely fail to even qualitatively describe the dynamics of such molecular clusters. This fundamental inconsistency between predicted and observed dynamics strongly emphasizes the importance of experimental calibration and verification of the approach used for MD simulations. One of the best ways to calibrate and validate the computational approach used for calculations of molecules and clusters is to compare their theoretical IR spectra with observed ones. As the vibrational transitions sensitively change with the structure and environment of molecules, reproduction of the measured IR spectrum by the computed one provides substantial confidence that the simulations are realistic and provide reliable structures and energies. The calibration of theoretical approaches by comparing observed and calculated IR spectra is now one of the standard techniques to determine the structures of molecular

further independent reaction paths such as the direct reaction without intermediate (Figure 16). The results of fitting with such a model are not different enough to select one of them. Thus, to extract the maximum information from the ps-TRIR spectra, a theoretical analysis, which can account for the initial distribution of internal energy for the reaction, is required. Inhomogeneous energy distributions in the reactions can be accounted for in molecular dynamics (MD) simulations. Traditionally, a trajectory calculation was made on an analytically fitted potential energy surface (PES), but such an approach is not feasible for larger polyatomic molecules because the number of degrees of freedom is simply too large for the simulation. The onthe-fly simulation solves this problem and allows application of MD simulations to polyatomic molecular systems like AA− H2O.67,215 Initial ab initio and DFT MD simulations have been applied to the closely related FA−H2O cluster,148,151 in which the methyl group in AA is replaced by a H atom. Static REMPIIR148 and EI-IR149 spectra in Figure 6 illustrate that FA−H2O undergoes CO → NH water migration after ionization, similar to that observed for AA−H2O. Consequently, similar reaction dynamics may be expected. Nevertheless, the MD simulated dynamics in FA+−H2O148,151 is qualitatively different from that in AA+−H2O observed by ps-TRIR spectroscopy.66 The first point is that, according to these MD simulations, the water migration reaction in FA+−H2O is complete within 0.5 ps, which 5449


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clusters. Thus, to simulate the ps-TRIR spectra together with the structural dynamics, on-the-fly MD simulations have been performed in the framework of density functional theory and have been combined with the Wigner phase-space approach for the simulation of pump−probe spectra.66,215 Briefly, the time evolution of the phase-space density in the classical limit is described by the Liouville equation, dρ/dt = {H, ρ}, where the Hamilton function H = H0(p, q) − μ(q)ε(t) is composed of the field-free Hamiltonian H0 and the interaction of dipole moment μ(q) with laser field ε(t).216 From the rate of energy absorption dE/dt, the total absorption of energy for a given frequency ω and time delay Δt, corresponding to the experimentally measured transient ps-TRIR spectrum, has been calculated as67 ∞

⟨Sprobe(ω , Δt )⟩ =

∫−∞ dt dd⟨Et⟩

∞

=

∫−∞ dt ∫ ∫ dq dp H0{H , ρ} Ntraj

=

∑∫ i=1

∞

−∞

dt

dμ[qi(t )] dt

⎡ (t − Δt )2 ⎤ ε0 exp⎢ − ⎥ cos ωt ⎣ 2σ 2 ⎦

Figure 19. (a) Top and (b) side views of 50 trajectories resulting from MD simulations of ionization-induced CO → NH water migration in AA+−H2O.67 Trajectories of the fast channel over the methyl group are shown in red, while those of the slow channel via the intermediate above the phenyl ring are shown in blue. Note that the orientation of AA+− H2O differs in the top and side views. Adapted with permission from ref 67. Copyright 2014 Wiley.

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The phase-space density is represented by a discrete set of classical trajectories, which are propagated in the neutral (S0) and cationic (D0) states. The electric field of the probe laser pulse is assumed to have a Gaussian shape with width σ. To also take the experimental pump pulse width into account, the transient psTRIR spectra are convoluted by the change in population of the cationic state dP(t)/dt, which is derived from experiment, leading to the following expression for the ps-TRIR signal: ∞

⟨Spump − probe(ω , Δt )⟩ =

∫−∞ dt ⟨Sprobe(ω , Δt − t )⟩ dPd(tt ) (11)

Figure 18 compares observed ps-TRIR spectra (panel a) of AA+-H2O with simulated IR spectra (panel b) resulting from 50 trajectories of DFT-based MD simulations shown in Figure 19.67 S0 and D0 spectra, shown at the top and bottom in Figure 18a,b, are static IR spectra. The shift of νOHb by ionization (blue band A, R and FC+), appearance and disappearance of νNHf (green band B, I+), and rise of νNHb (red band C, P+) are well reproduced. To simulate the ps-TRIR spectra, a time step of 0.1 fs and a reasonably high quantum chemical level (dispersion- and gradient-corrected RI-PBE-D3/6-311G**) have been employed.67 Surprisingly, these simulations found two competing reaction pathways for the experimentally probed ionizationinduced CO → NH water migration in AA+−H2O, and all 50 trajectories are shown in Figure 19. Some 70% of the trajectories follow an essentially barrierless pathway across the methyl group with a reaction time of around 1 ps (fast channel, Figures 19 and 20). The remaining 30% of the trajectories follow a pathway via a local minimum above the phenyl ring and the reaction time is around 5 ps (slow channel, Figures 19 and 20). Calculated minimum energy pathways for both channels shown in Figure 20 illustrate that the CO → NH reaction is indeed highly exothermic. Simulated ps-TRIR spectra for fast and slow channels are separated out in Figure 18 panels c and d, respectively. In the fast channel, νNHf, the signature of I+ (green band B) does not appear. On the other hand, the slow channel shows the green band B clearly with significant intensity, and it is still visible at Δt = 5 ps. This result means that only a mixture of fast and slow channels can reproduce the observed ps-TRIR

Figure 20. Minimum energy paths for (a) fast and (b) slow reaction channels calculated for ionization-induced CO → NH water migration in AA+−H2O (RI-PBE-D3/6-311G**).67 Adapted with permission from ref 67. Copyright 2014 Wiley.

spectra in Figure 18a. Figure 17b shows that the time evolutions of transitions A−C derived from MD simulations agree well with the measured ones and with those derived from the simple twostep three-state reaction mechanism FC+ → I+ → P+ shown in Figure 17a. Significantly, these sophisticated MD simulations with two reaction channels reproduce all experimental observations to satisfactory accuracy: ps-TRIR spectra, reaction time constants, existence of an intermediate, lack of any back reaction, and time evolution profiles. They thus provide a great improvement over the picture derived from the simple rate equation analysis, because the latter did not reveal the competition of the two reaction channels and suggested only 5450


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the slow channel, which according to MD simulations is the minor (30%) channel. As such, this example nicely illustrates the drastic limitations of classical rate constant models, which are relatively insensitive to complex reaction mechanisms and may reveal only part of the reaction scheme. In addition, it appears that previous MD simulations for FA+−H2O148,151 with wrong predictions as outlined above may suffer severely from the low theoretical level and/or large step sizes used in the simulations. This observation emphasizes the importance of experimental data to validate the accuracy and reliability of the theoretical approach. In summary, ionization-induced CO → NH water migration in AA+−H2O involves both slow and fast reaction pathways, and the major channel is the fast one. To decode such detailed information from the spectroscopically rich ps-TRIR spectra, high-level MD simulations, which are able to simulate these spectra, are necessary, while application of a classical reaction rate analysis fails to capture the major reaction channel.66,67 As demonstrated here for the first time with AA+−H2O as an example, such a combination can be the standard technique to study reaction dynamics of large polyatomic molecules and clusters at the single-molecular level and may pave the way for reaction control. 3.2.2. p-Aminobenzonitrile−H2O. The p-aminobenzonitrile−H2O (4ABN−H2O) dimer is the second monohydrated aromatic cluster for which ionization-induced water migration has been characterized in real time by ps-TRIR spectroscopy.68 4ABN is a benzene derivative with an electron-donating NH2 group and an electron-withdrawing CN group. It thus offers several attractive competing binding sites for H2O ligands, and the interaction with the aromatic ring and the two functional groups can readily be modified by electronic excitation and ionization. Here, we describe the migration of H2O from the CN to the NH2 group triggered by isomer-selective REMPI (Figure 2d). Three stable isomers of neutral 4ABN−H2O have been detected in molecular beams by means of REMPI, laser-induced fluorescence (LIF), and hole-burning spectroscopy,152,217−222 and their structural assignment has been established by their isomer-selective REMPI-IR and LIF-IR spectra recorded in the S0 state,219,220 rotational band contour analysis of UV transitions,217,218 and comparison with quantum chemical calculations.218−220 In the 4ABN−H2O(NH) isomer, H2O as a proton acceptor forms a linear H-bond to one of the NH protons of the amino group (Figure 21). In the two other 4ABN− H2O(CN)-type isomers, H2O binds as proton donor to the CN group in either a linear or bent configuration. The linear 4ABN− H2O(CN) isomer can only be detected by LIF and not with REMPI217,220 and is thus not relevant for the REMPI-based studies discussed here. These three neutral 4ABN−H2O isomers are of quite similar stability and can readily be distinguished (and selected) by their UV and IR spectra. Our calculations predict the NH isomer to be slightly more stable than the bent CN isomer, as shown in the energy-level diagram presented in Figure 21.152,213 Electronic ππ* excitation into the S1 state leads to substantial stabilization of the NH isomer (by 532 cm−1) and simultaneous destabilization of the CN isomer (by 196 cm−1), so that the energy gap between both isomers increases by 728 cm−1, from 75 cm−1 in S0 (calculated at the M06-2X/aug-cc-pVTZ level) to 803 cm−1. These changes in intermolecular bond strengths are clearly reflected by the change in static IR spectra upon S1 ← S0 excitation,152 which also indicate that no isomerization occurs upon electronic excitation to the S1 origins of both isomers.

Figure 21. Energy-level diagram for NH- and CN-bound isomers of 4ABN−H2O in S0, S1, and D0 electronic states, derived from spectroscopic data.152 Only the energy difference of 75 cm−1 in the S0 state is taken from calculations (M06-2X/aug-cc-pVTZ).153,213 Structures were optimized at the M06-2X/aug-cc-pVTZ level.153 Intermolecular bond lengths are given in angstroms. All energies are given in reciprocal centimeters. Water migration takes place after ionization into the ionic D0 state, while no such dynamics is observed in the neutral S1 state, because the barrier for isomerization lies above the S1 origin level used for the REMPI process.

Ionization into the D0 cation ground state further amplifies the energy difference between the two isomers to 2225 cm−1. Spectroscopic information about the structure of NH and CN isomers in the D0 state of 4ABN+−H2O is available from ZEKE,223 PIE,152,218 REMPI-IR,152 and EI-IR spectroscopy,152,153 along with corresponding quantum chemical calculations. Analysis of the hydration-induced IE shift and inter- and intramolecular vibrational patterns observed in the wellstructured ZEKE and PIE spectra of the 4ABN−H2O(NH) isomer is consistent with a NH-bound structure also in the cation, which has a similar structure as the neutral isomer, with the main exception of a much stronger and shorter NH···O Hbond in the charged cluster (Figure 21).223 On the other hand, no resolved ZEKE spectrum could be obtained for the 4ABN− H2O(CN) isomer.223 In addition, its PIE spectrum shows only a flat onset at the adiabatic ionization energy (denoted IE0), with a slowly rising ion current toward higher energy, in marked contrast to the sharp steps observed in the PIE spectrum of 4ABN−H2O(NH) (Figure 22).152 Both observations provide a first strong indication for a large geometry change of 4ABN− H2O(CN) induced by ionization; that is, the nascent 4ABN+− 5451


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Figure 22. Photoionization efficiency (PIE) spectra of (a) 4ABN− H2O(CN) and (b) 4ABN−H2O(NH), recorded via their S1 origins.152 The lowest energy signal is taken here as the adiabatic ionization energy (IE0). The 4ABN−H2O(NH) spectrum shows sharp steps according to a progression in the intermolecular stretch mode (nσ). The arrows in panel a indicate positions for ionization excess energies used for nanosecond REMPI-IR spectra (Eexc = 296, 500, 707 cm−1) shown in Figure 25. The sharp depletions in the PIE spectra are attributed to stimulated emission down to vibrational levels in the S0 state. Adapted with permission from ref 152. Copyright 2014 Wiley.

H2O(CN) cations generated by vertical ionization have a quite different geometry from that of possible minima on the D0 state and significant amount of internal vibrational energy. The salient parts of the intermolecular potential energy surface of the 4ABN+−H2O cation in the D0 state are detailed in Figure 23, which displays the minimum energy paths for H2O migrating around 4ABN+ in the aromatic plane or in the plane perpendicular to it, as well as the structures of all minima along these paths.153 Clearly, by far the most stable structure is the 4ABN+−H2O(NH) global minimum with an equilibrium dissociation energy of De = 5644 cm−1 (θ = ±26°, D0 = 4992 cm−1). The linear NH···O H-bond contracts from 2.04 to 1.77 Å upon ionization, consistent with long progressions of the intermolecular stretching vibration (σ) in the ZEKE and PIE spectra (Figure 22b).152,223 The two equivalent NH-bound minima with Cs symmetry are separated by a rather large barrier of 1237 cm−1 at a transition state with C2v symmetry (θ = 0°), preventing facile interconversion by tunneling or modest thermal excitation. Significantly, the four other nonequivalent local minima on the 4ABN+−H2O potential have much smaller binding energies than the NH-bound global minimum, with De = 3312 cm−1 (CN, θ = 122°), 3345 cm−1 (CH, θ = 70°), 3115 cm−1 (π, θ = 52°), and 3064 cm−1 (π*, θ = 103°). In addition, their barriers for mutual interconversion and isomerization toward the NH global minima are rather low ( H Isomerization. J. Chem. Phys. 2010, 133, 154308. (126) Miyazaki, M.; Tanaka, S.; Ishiuchi, S.; Dopfer, O.; Fujii, M. Isomerization Reaction in High-n Rydberg States of Phenol−Ar/Kr Clusters Measured by Autoionization Detected Infrared Spectroscopy. Chem. Phys. Lett. 2011, 513, 208−211. (127) Miyazaki, M.; Yoshikawa, S.; Michels, F.; Misawa, K.; Ishiuchi, S.; Sakai, M.; Dopfer, O.; Müller-Dethlefs, K.; Fujii, M. Mass Analyzed Threshold Ionization Detected Infrared Spectroscopy: Isomerization Activity of the Phenol-Ar Cluster near the Ionization Threshold. Phys. Chem. Chem. Phys. 2015, 17, 2494−2503. (128) Miyazaki, M.; Takeda, A.; Schmies, M.; Sakai, M.; Misawa, K.; Ishiuchi, S.; Michels, F.; Müller-Dethlefs, K.; Dopfer, O.; Fujii, M. Ionization-Induced π → H Site-Switching in Phenol-CH4 Complexes Studied Using IR Dip Spectroscopy. Phys. Chem. Chem. Phys. 2014, 16, 110−116. (129) Patzer, A.; Langer, J.; Knorke, H.; Neitsch, H.; Dopfer, O.; Miyazaki, M.; Hattori, K.; Takeda, A.; Ishiuchi, S.; Fujii, M. IR Spectra of Resorcinol+-Arn Cluster Cations (n = 1, 2): Evidence for Photoionization-Induced π → H Isomerization. Chem. Phys. Lett. 2009, 474, 7−12. (130) Andrei, H. S.; Solca, N.; Dopfer, O. Ionization-Induced Switch in Aromatic Molecule-Nonpolar Ligand Recognition: Acidity of 1Naphthol+ (1-Np+) Rotamers Probed by IR Spectra of 1-Np+-Ln Complexes (L = Ar/N2, n ≤ 5). Phys. Chem. Chem. Phys. 2004, 6, 3801−3810. (131) Solcà, N.; Dopfer, O. Interaction between Aromatic Amine Cations and Nonpolar Solvents: Infrared Spectra of Isomeric Aniline+− Arn (n = 1, 2) Complexes. Eur. Phys. J. D 2002, 20, 469−480. (132) Solcà, N.; Dopfer, O. Interaction between Aromatic Amine Cations and Quadrupolar Ligands: Infrared Spectra of Aniline+-(N2)n (n = 1−5) Complexes. J. Phys. Chem. A 2002, 106, 7261−7270. (133) Gu, Q. L.; Knee, J. L. Binding Energies and Dissociation Pathways in the Aniline-Ar2 Cation Complex. J. Chem. Phys. 2008, 128, 064311. (134) Schmies, M.; Patzer, A.; Kruppe, S.; Miyazaki, M.; Ishiuchi, S.; Fujii, M.; Dopfer, O. Microsolvation of the 4-Aminobenzonitrile Cation (ABN+) in a Nonpolar Solvent: IR Spectra of ABN+-Ln (L = Ar and N2, n ≤ 4). ChemPhysChem 2013, 14, 728−740. (135) Nakamura, T.; Miyazaki, M.; Ishiuchi, S.; Weiler, M.; Schmies, M.; Dopfer, O.; Fujii, M. IR Spectroscopy of the 4-AminobenzonitrileAr Cluster in the S0, S1 Neutral and D0 Cationic States. ChemPhysChem 2013, 14, 741−745. (136) Solcà, N.; Dopfer, O. Microsolvation of the Indole Cation (In+) in a Nonpolar Environment: IR Spectra of In+-Ln Complexes (L = Ar and N2, n ≤ 8). Phys. Chem. Chem. Phys. 2004, 6, 2732−2741. (137) Sakota, K.; Schütz, M.; Schmies, M.; Moritz, R.; Bouchet, A.; Ikeda, T.; Kouno, Y.; Sekiya, H.; Dopfer, O. Weak Hydrogen Bonding Motifs of Ethylamino Neurotransmitter Radical Cations in a Hydrophobic Environment: Infrared Spectra of Tryptamine+−(N2)n Clusters (n ≤ 6). Phys. Chem. Chem. Phys. 2014, 16, 3798−3806. (138) Andrei, H. S.; Solca, N.; Dopfer, O. Interaction of Ionic Biomolecular Building Blocks with Nonpolar Solvents: Acidity of the

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Probing Solvation Dynamics around Aromatic and Biological

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