Stepwise Microhydration of Aromatic Amide Cations: Formation of

Article pubs.acs.org/JPCB

Stepwise Microhydration of Aromatic Amide Cations: Formation of Water Solvation Network Revealed by Infrared Spectra of Formanilide+−(H2O)n Clusters (n ≤ 5) Johanna Klyne,† Matthias Schmies,† Masaaki Fujii,‡ and Otto Dopfer*,† †

Institut für Optik und Atomare Physik, Technische Universität Berlin, Hardenbergstrasse 36, 10623 Berlin, Germany Chemical Resources Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan

‡

S Supporting Information *

ABSTRACT: Hydration of peptides and proteins has a strong impact on their structure and function. Infrared photodissociation spectra (IRPD) of size-selected clusters of the formanilide cation, FA+−(H2O)n (n = 1−5), are analyzed by density functional theory calculations at the ωB97X-D/aug-cc-pVTZ level to determine the sequential microhydration of this prototypical aromatic amide cation. IRPD spectra are recorded in the hydride stretch and fingerprint ranges to probe the preferred interaction motifs and the cluster growth. IRPD spectra of cold Ar-tagged clusters, FA+−(H2O)n−Ar, reveal the important effects of temperature and entropy on the observed hydration motifs. At low temperature, the energetically most stable isomers are prominent, while at higher temperature less stable but more flexible isomers become increasingly populated because of entropy. In the most stable structures, the H2O ligands form a hydrogen-bonded solvent network attached to the acidic NH proton of the amide, which is stabilized by large cooperative effects arising from the excess positive charge. In larger clusters, hydration bridges the gap between the NH and CO groups (n ≥ 4) solvating the amide group rather than the more positively charged phenyl ring. Comparison with neutral FA−(H2O)n clusters reveals the strong impact of ionization on the acidity of the NH proton, the strength and topology of the interaction potential, and the structure of the hydration shell.

1. INTRODUCTION The amide group (-NH−CO-) serves as a linkage between amino acids in peptides and proteins and thus plays a key role in protein folding. In this process, the interaction of the flexible amide group with water and the structure and dynamics of the hydration shell(s) around this group are of fundamental importance, because dehydration of the amide groups followed by the formation of hydrogen bonds (H-bonds) between different amide moieties are important initial steps in allowing proteins to fold into their biologically relevant structures. In addition, the structure, dynamics, and function of proteins are strongly influenced by solvating water molecules (“interfacial” or “biological” water).1−10 Fragmentation and charge delocalization of radical cations of peptides, DNA, and their building blocks are relevant for sequencing and charge transport phenomena of these biological macromolecules.11 For a complete molecular-level understanding of protein folding and recognition, enzymatic processes, and the effects of an excess charge, detailed knowledge of the potential describing the interaction between the amide group with water and other surrounding molecules is required. It is well established that the combination of spectroscopy and quantum chemical calculations of molecular clusters isolated in the gas phase provides the most direct and detailed access to such interaction potentials involving aromatic and biological molecules.12−35 To this end, the stepwise microhydration of formanilide (FA, © 2014 American Chemical Society

N-phenylformamide, C7H7NO, Figure 1) is characterized in its cation ground electronic state by vibrational infrared (IR) spectroscopy of size-selected FA+−(H2O)n clusters generated in a molecular beam and density functional theory (DFT) calculations. Formanilide is one of the simplest aromatic amides, with a -NH−CO- peptide linkage between a phenyl group and a terminal hydrogen atom, and serves thus as a benchmark molecule to investigate the microhydration of aromatic peptides. Such aromatic amides exhibit several competing water binding sites at the amide and phenyl parts of the molecule, and their relative interaction strengths as well as the structure of the hydration shell depend sensitively on the charge state.36−41 The IR spectra of FA+−(H2O)n are measured by photodissociation (IRPD) of mass-selected cluster ions generated in an electron ionization (EI) source,28,42 which produces predominantly the most stable isomer of a given cluster ion.28,43 This approach has recently been employed in our laboratory to investigate the microhydration process of a variety of aromatic and biological cations.28,30,44−53 In the following, we briefly review the knowledge about FA(+)−Ln clusters with polar and nonpolar ligands L relevant for the present work. The geometric, vibrational, and electronic Received: November 14, 2014 Revised: December 20, 2014 Published: December 23, 2014 1388

DOI: 10.1021/jp511421h J. Phys. Chem. B 2015, 119, 1388−1406

Article

The Journal of Physical Chemistry B

Figure 1. Structures of t-/c-FA+, H2O, and t-/c-FA+-H2O clusters obtained at the ωB97X-D/aug-cc-pVTZ level. Relevant structural, vibrational, and energetic parameters are listed in Table 2. Relative energies (E0), binding energies (D0), and bond lengths are given in cm−1 and Å, respectively.

because dispersion forces with the aromatic π-electron system dominate the attraction.69 In contrast, for polar hydrophilic and protic ligands, such as H2O, the most attractive binding sites are offered by the amide group via H-bonding.36,40,60,62,63,70 In the most stable t-FA−H2O(NH) isomer, H2O binds as proton acceptor to the acidic NH group with a binding energy of D0 = 5.65 ± 0.30 kcal/mol (1976 ± 105 cm−1), whereas in the only marginally less stable t-FA−H2O(CO) isomer the H2O ligand binds as a proton donor to the CO group with D0 = 5.40 ± 0.28 kcal/mol (1889 ± 98 cm−1),40 as shown by their isomerselective IR spectra.36,60 The barriers for isomerization have been determined in elegant spectroscopic experiments as 892 ± 96 cm−1 (CO → NH) and 869 ± 119 cm−1 (NH → CO).70 Two isomers have been identified for t-FA−(H2O)2 by IR−UV hole burning and UV rotational contour spectroscopy.36,60,62 By comparison to calculations, their cyclic structures have been determined in which (H2O)2 dimers are attached either to the NH or CO site of t-FA and close an intermolecular H-bonded solvent ring to either the aromatic π-electron system or the amide CH group, respectively.36,60,62 The t-FA−(H2O)4 cluster appears as a quite stable single isomer in the molecular beam, with a H-bonded (H2O)4 chain bridging the gap between the NH and CO groups of t-FA.36,60 Surprisingly, no clusters in the size range n = 3 and n ≥ 5 have been identified in any of the molecular beam experiments.36,60,62,63 Ionization into the 2A″ ground electronic state (D0) of FA+ is accomplished by removal of a bonding π-electron from the HOMO, which is delocalized over the phenyl and amide groups.68,71 As a consequence, the t-FA+ and c-FA+ rotamers are planar, and their energy difference increases by 302 ± 7 cm−1.68 Zero-kinetic-energy (ZEKE) photoelectron spectra of t-/c-FA+ yield information on a few low-frequency modes of their amide group as well as on the amide torsion.68 The important amide I−II and hydride stretch frequencies (νCH/NH/OH), which are sensitive to protein folding, are available from recent IRPD spectra of cold FA+−Ln clusters

properties of isolated FA in its 1A′ ground electronic state (S0) have been characterized by electron diffraction54 and microwave (MW),55−58 IR,59,60 and ultraviolet (UV) spectroscopy.59,61−64 In combination with quantum chemical calculations, these studies reveal the detection of two stable FA rotamers, namely, the planar trans-FA isomer (t-FA, anti) and the slightly less stable nonplanar cis-FA isomer (c-FA, syn). In c-FA, the phenyl group is rotated by ∼35° with respect to the amide plane, and the two equivalent minima are separated by a barrier to planarity of 152 ± 2 cm−1.54,55,58 The c-FA and t-FA isomers are separated by a rather high barrier, measured as 7000 cm−1 in solution65 and calculated as 5700 cm−1 for the isolated molecule,56 which arises from the partial double bond character of the NH−CO bond.66 Thus, in the gas phase, interconversion between both isomers is prevented and both rotamers can be detected. However, the range of the observed t-/c-FA population ratio drastically depends on temperature and other experimental parameters. While most studies in cold molecular beams do not detect the c-FA isomer at all55,60 or at an abundance of less than 10%,61 studies at room temperature or in solution even report roughly equal abundance.54,56,67 At low temperature, t-FA is favored energetically, while c-FA benefits from entropy at higher temperature because of its two degenerate minima and its higher flexibility.56 The observed c-FA/t-FA abundance ratios have been rationalized by experimental estimates of their energy difference ranging from ΔD0trans−cis = 276 ± 8458 to 350 ± 15056 and ∼875 cm−1.61 These values are in accordance with the theoretical predictions.54,56,61,62,68 Isomer-selective IR spectra of t-/c-FA recorded in the hydride stretch (amide A, νNH, νCH) and fingerprint (amide I−II) ranges demonstrate the large structural differences of the amide groups in the two isomers.59,60 Interestingly, clusters of FA with polar (L = H2O) and nonpolar ligands (L = Ar) have so far only been observed for the more stable t-FA rotamer.36,37,40,41,60,62,63,69,70 Nonpolar ligands, such as Ar, prefer π-stacking to the phenyl ring of FA, 1389


Article

The Journal of Physical Chemistry B tagged with Ar and N2.72 ZEKE spectra of t-FA+−Ar(π) generated by resonance-enhanced multiphoton ionization (REMPI) of the neutral precursor have been interpreted with a π-bound cation structure, t-FA+−Ar(π),69 whereas IRPD spectra of FA+−Arn and FA+−(N2)n clusters generated by EI clearly unravel the preference of nonpolar and quadrupolar ligands to form H-bonds to the acidic NH proton of the amide, t-FA+−Ar/N2(NH).72 ZEKE and IR spectra of t-FA+−H2O clusters generated by REMPI show that the H2O ligand binds exclusively to the NH group of t-FA+, t-FA+−H2O(NH).36,37,41 The ZEKE spectrum of t-FA+−H2O(NH) provides a few lowfrequency intramolecular modes of the amide group and the amide torsion, as well as frequencies of the three in-plane intermolecular modes (β′ = 49 cm−1, γ′ = 307 cm−1, and σ = 174 cm−1).41 The binding energy of the H-bond drastically increases from 1976 ± 105 to 5158 ± 105 cm−1 upon ionization, illustrating the large contributions of electrostatic and induction forces of the positive excess charge to the H-bond interaction.41 As a consequence of the strong NH···O H-bond in t-FA+−H2O(NH), the N−H stretch frequency exhibits a large red shift, ΔνNH ∼ −186 cm−1.36 Ionization of t-FA−H2O(CO) clusters with H2O binding to the CO group in the neutral S0 state triggers a CO → NH isomerization reaction,36 in which the H2O ligand migrates with 100% efficiency to the NH binding site on the picosecond time scale, as suggested by molecular dynamics (MD) simulations.36,73 A similar CO → NH shuttling reaction has also been observed for the related monohydrated trans-acetanilide cation cluster, t-AA+−H2O,38 for which the H2O migration time was measured in real time as 5 ps by time-resolved pump−probe IR spectroscopy,30 in good agreement with recent MD simulations.46 The dynamical CO → NH isomerization process triggered by ionization of t-FA−H2O(CO) gives rise to broad features with unclear assignments in its ZEKE spectrum.41 Finally, 1 + 1 REMPI of t-FA−(H2O)4 with a cyclic structure60 produces a hot cation cluster and thus triggers a ring opening reaction of the H-bonded (H2O)4 network, which has been rationalized by entropic rather than energetic arguments.36 Here we report and systematically analyze IRPD spectra of size-selected FA+−(H2O)n clusters in the size range n = 1−5 to characterize in detail the stepwise microhydration of this prototypical aromatic amide radical cation. The clusters are generated in an EI source, which ensures the predominant production of the most stable cluster ions.28,43 All previous studies of FA+−(H2O)n employ REMPI of the neutral precursor for the cluster ion generation, which often populates only local minima on the cation potential surface so that the global minima can completely be missed.28,35,72,74−82 While previous studies of FA+−(H2O)n only consider the sizes n = 1 and n = 4, we cover here the whole range up to n ≤ 5. In addition to the hydride stretch range (2800−3800 cm−1), which covers the νNH, νCH, and νOH fundamentals and is thus very sensitive to the H-bonded (H2O)n solvation network, we also include for the first time the fingerprint range (1200−1900 cm−1), which covers the amide I and II modes and is thus sensitive to the solvation environment of the amide group. To investigate in detail the drastic impact of temperature and entropy on the preferred hydration structure and isomer population ratio of FA+−(H2O)n,37 we also analyze for the first time the much colder FA+−(H2O)n−Ar clusters, which freeze into their energetically most stable isomer(s). Although so far there is no information available on clusters of c-FA+, the properties of its clusters are also taken into account here, as

they may be produced in the employed EI source. One interesting question about the hydration structure in FA+− (H2O)n addresses the competition between interior ion solvation and the formation of a H-bonded solvent network, which depends on the subtle balance between noncooperative and cooperative three-body forces.28,50,83 As the proton affinity of (H2O)n clusters strongly increases with size, there is also the possibility for proton transfer from FA+ to the solvent network in FA+−(H2O)n clusters larger than a critical size (nc).47,50,84−88 The IRPD spectra are complemented by DFT calculations at the ωB97X-D/aug-cc-pVTZ level, which support the vibrational and isomer assignments and allow for a more detailed investigation of the intermolecular potential energy surface. Comparison of FA+−(H2O)n with the recently studied FA+−Ln clusters with nonpolar (L = Ar) and quadrupolar (L = N2) ligands72 illustrates the drastic impact of a dipolar and protic solvent (L = H2O) on the microsolvation of the aromatic amide cation, while comparison with the related AA+−(H2O)n clusters30,38 reveals the effects of methyl substitution of the amide group on its interaction with solvent molecules.

2. EXPERIMENTAL AND COMPUTATIONAL TECHNIQUES 2.1. Experimental Methods. The IRPD spectra of massselected FA+−(H2O)n clusters (n = 1−5) shown in Figure 2 are

Figure 2. IRPD spectra of FA+−(H2O)n with n = 1−5 recorded in the FA+−(H2O)n−1 fragment channel. The positions, widths, and vibrational and isomer assignments of the transitions observed (A− E) are listed in Table 1. The dashed lines and arrows indicate the experimental N−H and O−H stretch fundamentals of bare t-FA+ and H2O (νNH = 3380 cm−1, νsOH = 3657 cm−1, and νaOH = 3756 cm−1). For comparison, the IRPD spectrum of FA+−Ar recorded in the FA+ channel is included.

recorded in a quadrupole−octopole−quadrupole tandem mass spectrometer coupled to an electron ionization cluster ion source.28,42 In addition, IRPD spectra of cold FA+−(H2O)n−Ar clusters are measured to reduce the effective temperature and dissociation energy of the clusters leading to higher resolution spectra with narrower bands. The clusters are produced in a pulsed supersonic plasma expansion by electron and/or chemical ionization of FA followed by three-body aggregation reactions. Solid FA purchased from Sigma-Aldrich (>99.9%) is used without further purification. The expanding gas mixture is generated by passing Ar carrier gas (8−10 bar) through two 1390


Article


Table 1. Positions, Widths (FWHM, in Parentheses), and Suggested Vibrational and Isomer Assignments of the O−H and N−H Stretch Transitions Observed in the IRPD Spectra of FA+−(H2O)n and FA+−(H2O)n−Ar Compared to Frequencies of the Most Stable Isomers Calculated at the ωB97X-D/aug-cc-pVTZ Level exp (cm−1)

molecule/cluster H2O (H2O)2

FA+ FA+−H2O

FA+−H2O−Ar

FA+−(H2O)2

FA+−(H2O)2−Ar

FA+−(H2O)3

a

3756 3657a 3746a 3735a 3654a 3601a 3380a A 3718 (29) B 3637 (20) E 3030 (50) X 3150 (50) Y 2935 (broad) A 3717 (7) B 3632 (7) E 3020 (8) X 3155 (15) A 3736 (12) C 3694 (14) B 3645 (10) D 3380 (37) E 2788 (40) A 3736 (6) C 3694 (7) B 3647 (6) D 3361 (11) E 2782 (10) A 3738 (16) C 3700 (20) B 3645 (13) D1 3461 (22) D2 3423 (20) D3 3170 (broad) E ≤2670

FA+−(H2O)3−Ar

FA+−(H2O)4

FA+−(H2O)4−Ar

FA+−(H2O)5

A 3739 (5) B 3649 (6) D1 3455 (10) D2 3420 (10) E ≤2720 A 3731 (11) C3 3714 (11) C4 3703 (15) B 3649 (9) D1 3465 (40) D3 3250 (broad) C1 3729 (6) C2 3719 (6) C3 3715 (6) C4 3701 (6) D4 3629 (5) D1 3400 (100) C1 C2 C3 C4

3733 3722 3716 3707

(6) (7) (10) (20)

calc (cm−1)b

vibration ν OH νsOH νaOH νfOH νsOH νbOH νNH νaOH νsOH νNH 2βNH, FRc νCH, FRc νaOH νsOH νNH 2βNH, FRc νaOH νfOH νsOH νbOH νNH νaOH νfOH νsOH νbOH νNH νaOH νaOH νfOH νsOH νsOH νbOH νbOH νbOH νbOH νNH νNH νaOH νsOH νbOH νbOH νNH νaOH νfOH νfOH νsOH νbOH νbOH νfOH νfOH νfOH νfOH νbOH νbOH νNH νfOH νfOH νfOH νfOH a

3757 3656 3746 3724 3648 3539 3381 3723 3635 3085

(63) (5) (85) (88) (11) (332) (99) (132) (60) (1062)

3740 3701 3648 3325 2910 3740 3701 3648 3325 2910 3743 3743 3712 3650 3650 3439 3395 3395 3174 2817 2703 3743 3650 3439 3395 2703

(115) (103) (33) (937) (1736) (115) (103) (33) (937) (1736) (112) (210)/3743 (1) (89)/3705 (99) (26) (33)/3650 (17) (1102) (655) (538) (1312) (2120) (2310) (210)/3743 (1) (33)/3650 (17) (1102) (538) (2310)

t-FA+ t-FA+−H2O(NH) t-FA+−H2O(NH) t-FA+−H2O(NH) t-FA+−H2O(NH) t-FA+−H2O(NH) t-FA+−H2O(NH)−Ar(π) t-FA+−H2O(NH)−Ar(π) t-FA+−H2O(NH)−Ar(π) t-FA+−H2O(NH)−Ar(π) t-FA+−(H2O)2 t-FA+−(H2O)2 t-FA+−(H2O)2 t-FA+−(H2O)2 t-FA+−(H2O)2 t-FA+−(H2O)2−Ar(π) t-FA+−(H2O)2−Ar(π) t-FA+−(H2O)2−Ar(π) t-FA+−(H2O)2−Ar(π) t-FA+−(H2O)2−Ar(π) t-FA+−(H2O)3(II) t-FA+−(H2O)3(I) t-FA+−(H2O)3(II) t-FA+−(H2O)3(II) t-FA+−(H2O)3(I) t-FA+−(H2O)3(I) t-FA+−(H2O)3(II) t-FA+−(H2O)3(I) t-FA+−(H2O)3(II) t-FA+−(H2O)3(II) t-FA+−(H2O)3(I) t-FA+−(H2O)3(I) t-FA+−(H2O)3(I) t-FA+−(H2O)3(I) t-FA+−(H2O)3(I) t-FA+−(H2O)3(I)

3728 3717 3714 3707 3592 3383 2803

(222) (107) (117) (109) (204) (344)/3336 (775), 3230 (940) (1728)

t-FA+−(H2O)4(IV) t-FA+−(H2O)4(IV) t-FA+−(H2O)4(IV) t-FA+−(H2O)4(IV) t-FA+−(H2O)4(IV) t-FA+−(H2O)4(IV) t-FA+−(H2O)4(IV)

3729 (129) 3641 (58) 3089 (1032)

1391

isomer


Article

The Journal of Physical Chemistry B Table 1. continued exp (cm−1)

molecule/cluster

calc (cm−1)b

vibration

D1 3510 (broad) D2 ∼3430 (broad) D3 ∼3230 (broad)

isomer

ν OH νbOH νbOH b

a c

Frequencies measured for bare t-FA+, H2O, and (H2O)2 taken from refs 72, 90, and 112−114. bIR intensities (km/mol) are listed in parentheses. FR = Fermi resonance with νNH.

Table 2. Selected Geometrical Parameters (Distances, Å; Angles, deg), Vibrational Frequencies (cm−1), and Dissociation Energies (cm−1) of FA(+), H2O, and FA(+)−H2O Clusters (Figure 1) Calculated at the ωB97X-D/aug-cc-pVTZ Levela species H2O t-FA c-FA t-FA−H2O(NH) c-FA−H2O(NH) t-FA+ c-FA+ t-FA+−H2O(NH) c-FA+−H2O(NH) a

rNH

rCO

rOH

νNH

νCO

0.9574 1.0053 1.0076 1.0116 1.0168 1.0120 1.0153 1.0300 1.0386

1.2062 1.2064 1.2092 1.2178 1.1861 1.1856 1.1894 1.1891

0.9575 0.9565

0.9592 0.9598

3449 (26) 3428 (32) 3354 (324) 3281 (204) 3381 (99) 3348 (123) 3085 (1062) 2966 (1222)

1759 1757 1753 1728 1812 1802 1800 1790

(361) (702) (382) (707) (132) (243) (137) (226)

νsOH

νaOH

De

D0

RFA−L

θNH‑L

3752 (90) 3723 (80)

2125 3452

1761 2640

1.9888 2.0186

179 143

3723 (132) 3718 (128)

5535 5557

4897 4878

1.7805 1.7510

180 175

3656 (5)

3757 (63)

3653 (21) 3427 (405)

3635 (60) 3633 (60)

IR intensities (km/mol) are listed in parentheses.

successive reservoirs filled with H2O (room temperature) and FA (heated to 140 °C). In contrast to previous studies on the neutral clusters, it has been straightforward to generate and detect all cluster sizes in the size range n ≤ 10. The FA+− (H2O)n or FA+−(H2O)n−Ar parent clusters of interest are mass-selected by the first quadrupole and irradiated in the adjacent octopole by an IR laser pulse of a tunable optical parametric oscillator pumped by a Q-switched nanosecond Nd:YAG laser, with pulse energies of ∼0.3−1 and 2−5 mJ in the fingerprint and hydride stretch ranges, respectively, a repetition rate of 10 Hz, and a bandwidth of 1 cm−1. Calibration of the IR laser frequency accurate to better than 1 cm−1 is accomplished by a wavemeter. Resonant vibrational excitation of FA+−(H2O)n or FA+−(H2O)n−Ar leads to the evaporation of the most weakly bound ligand, i.e., H2O or Ar, respectively. The resulting fragment ions are mass-selected with the second quadrupole and recorded by a Daly detector as a function of the IR laser frequency to derive the IRPD spectrum of the FA+−(H2O)n(−Ar) parent clusters. IRPD spectra are normalized for laser intensity variations monitored by a pyroelectric detector. Several spectra in the hydride stretch range are composed of separate scans recorded in the O−H and N−H/C−H stretch ranges. They are connected such that the intensities of overlapping bands are adjusted. The observed peak widths in the IRPD spectra originate mainly from unresolved rotational substructure in combination with possible sequence hot band transitions involving low-frequency modes. Hence, the transition widths correlate with the binding energy of the most weakly bound ligand of the cluster, which determines the upper limit of the internal energy deposited in the cluster before leading to metastable decay on the way from the ion source to the octopole. Thus, the attachment of an Ar ligand to FA+−(H2O)n drastically reduces the widths of the observed IR bands. 2.2. Computational Techniques. Quantum chemical calculations at the ωB97X-D/aug-cc-pVTZ level of theory are carried out for cationic t-/c-FA+ and their t-/c-FA+−(H2O)n hydrates to determine their structural, energetic, and spectroscopic properties.89 As has been shown for the related

t-/c-FA+−Ln clusters with L = Ar and N2,72 the dispersioncorrected ωB97X-D functional accounts well for the electrostatic, inductive, and dispersion forces of the considered clusters and reproduces the experimental IR spectra and binding energies to satisfactory accuracy. Spin contamination, which is severe at the MP2 level,72 is negligible at the ωB97X-D level, with values of ⟨S2⟩ − 0.75 = 0.0256 and 0.004 for t-FA+ before and after annihilation, respectively. Interaction energies (De) are corrected for harmonic zero-point energy to provide binding energies (D0). Free energies (G) are evaluated at room temperature. Harmonic intramolecular vibrational frequencies are linearly scaled by factors of 0.969, 0.944, and 0.9427 in the fingerprint (1000 cm−1). The observed sequence hot band transitions may exhibit very different anharmonic couplings as compared to transitions from the vibrational ground state of cold FA+−H2O−Ar clusters, giving rise to the observed frequency shifts and intensity changes. Calculations for t-FA+−H2O(NH) and t-FA+−H2O(NH)−Ar(π) available in Figure S2 in the SI confirm that Ar tagging at the aromatic ring has essentially no impact on the (harmonic) frequencies and IR intensities of t-FA+−H2O(NH). Hence, the FA+−H2O−Ar spectrum is considered to exhibit the correct relative IR intensities of cold FA+−H2O clusters. Finally, we note that the IRPD spectrum of FA+−H2O−(N2)2 confirms the interpretation derived from the FA+−H2O(−Ar) spectra. In contrast to the Ar ligands, the νOH bands shift upon N2 complexation indicating that the N2 ligands prefer H-bonding to the H2O ligand over π-bonding to the aromatic ring (Figure S4 in the SI). The IRPD spectra of FA+−H2O and FA+−H2O−Ar generated in the EI source are compared in Figure 4 to previously reported IR dip spectra of t-FA+−H2O clusters generated by isomer-selective resonant photoionization of the

NH-bound and CO-bound isomers using one-color twophoton (1 + 1) REMPI via their S1 origins.36 Both IR dip spectra are similar in appearance to the IRPD spectrum of FA+−H2O generated by EI, indicating that all three IR spectra probe the t-FA+−H2O(NH) isomer, which is indeed the global minimum on the t-FA+−H2O cation potential energy surface, as demonstrated here unambiguously by the calculations and the IRPD spectra recorded for FA+−H2O(−Ar) generated by EI. The production of t-FA+−H2O(NH) by REMPI of neutral t-FA−H2O(NH) is expected because of the vertical Franck− Condon factors.36,41,40 However, the observation of the spectrum of the t-FA+−H2O(NH) isomer 20 ns after REMPI of neutral t-FA−H2O(CO) is not expected but has been explained by a rapid isomerization reaction of the CO isomer toward the NH isomer triggered by photoionization.36 This view is also supported by molecular dynamics simulations for the CO → NH isomerization reaction of t-FA+−H2O36,73 and recent time-resolved IR spectra for the corresponding reaction of the related t-AA+−H2O cluster,30,46 which demonstrate that this reaction occurs with 100% yield on the picosecond time scale. As a consequence of 1 + 1 REMPI of t-FA−H2O(CO) and t-FA−H2O(NH), the resulting t-FA+−H2O(NH) clusters are vibrationally highly excited due to corresponding Franck− Condon factors (NH isomer) and the substantial energy release resulting from the rather exothermic CO → NH reaction (∼3000 cm−1, CO isomer). Thus, the IR dip spectra of t-FA+− H2O(CO/NH) in Figure 4 probe relatively hot t-FA+− H2O(NH) ions. As a result, the transitions are quite broad, shifted, and modified in intensity. For example, transition X near 3200 cm−1 dominates the IR dip spectra and hence was attributed to the νNH transition.36 The IRPD spectrum of cold FA + −H 2 O−Ar, however, clearly suggests that the ν NH frequency of t-FA+−H2O(NH) is in fact near 3020 cm−1 (band E). Indeed, anharmonic calculations for t-FA+− H2O(NH) predict the νNH fundamental at 3031 cm−1 (using the cc-pVTZ basis set), i.e., very close to band E, supporting the reassignment. Moreover, the similar topology observed in the IRPD spectra of FA+−H2O(−Ar) and the isomer-selective IR dip spectrum of t-FA+−H2O(NH) confirms the preceding conclusion that contributions of the c-FA+−H2O(NH) isomer to the IRPD spectra are at most minor. The weak band Z at 3310 cm−1 occurs in all three IR spectra of FA+−H2O in Figure 4 and thus is assigned to a hot band transition characteristic of warm clusters of the t-FA+−H2O(NH) isomer, which is absent in the cold t-FA+−H2O−Ar spectrum. 3.3. FA+−(H2O)2. As there is no indication for the presence of c-FA+−H2O dimers in the molecular beam, larger c-FA+− (H2O)n clusters are not considered further. In the most stable t-FA+−(H2O)2 cluster, a H-bonded (H2O)2 dimer is attached to the acidic NH proton of t-FA+, leading to a total binding energy of D0 = 8432 cm−1 (Figure 5). There are two equivalent isomers separated by an appreciable barrier at a transition state with Cs symmetry. Due to cooperative effects arising from the substantially increased proton affinity of (H2O)2 as compared to H2O (808 versus 690 kJ/mol),92 the N−H···O H-bond is much stronger than in the t-FA+−H2O(NH) dimer (1.688 versus 1.780 Å). Thus, the N−H bond elongation and N−H stretch frequency red shifts are correspondingly larger (ΔrNH = +28 versus +18 mÅ; −ΔνNH = 471 versus 296 cm−1; ΔINH = 1654 versus 973%). Also, the charge transfer from t-FA+ to the solvent cluster increases (Δq = 55 versus 41 me). In turn, the positive charge on the t-FA+ cation has a drastic stabilizing effect on the H-bond in the (H2O)2 dimer, which contracts

Figure 4. IR dip spectra of t-FA+−H2O clusters generated by isomerselective resonant photoionization of the NH-bound and CO-bound isomers, t-FA-H2O(NH) and t-FA-H2O(CO), via one-color twophoton (1 + 1) REMPI through the S1 origin36 are compared to IRPD spectra of FA+−H2O and FA+−H2O−Ar generated in the EI source. 1395


Article


Figure 5. Structures of (H2O)2 and t-FA+−(H2O)n with n = 2−3 obtained at the ωB97X-D/aug-cc-pVTZ level. Binding energies (D0) and bond lengths are given in cm−1 and Å, respectively.

from 1.973 Å in the free dimer to 1.740 Å in t-FA+−(H2O)2. It gives rise to a characteristic intense bound O−H stretch fundamental at νbOH = 3325 cm−1 in t-FA+−(H2O)2, which is much lower in frequency than in bare (H2O)2, νbOH = 3539 cm−1. The corresponding free O−H stretch in t-FA+−(H2O)2 is predicted at νfOH = 3701 cm−1. The O−H stretch frequency red shifts of the terminal H2O ligand are smaller than those of the t-FA+−H2O dimer (−Δνs/aOH = 8/17 versus 21/34 cm−1), because the positive charge is further away. Other t-FA+− (H2O)2 isomers, in which two individual H2O ligands are separately attached to the various attractive binding sites of t-FA+ are calculated to be much less stable (Figure S5 in the SI). They are not considered further, because they are not observed experimentally. It is noted that the binding energy calculated for (H2O)2, D0 = 1007 cm−1, agrees well with the measured value, D0 = 1105 ± 10 cm−1,93 demonstrating that the chosen ωB97X-D/aug-cc-pVTZ level describes the H2O··· H2O interaction to sufficient accuracy. The IRPD spectrum of FA+−(H2O)2−Ar in the hydride stretch range is similar in appearance to the spectrum of bare FA+−(H2O)2, except for its higher spectral resolution and narrower transitions (Figure 6, Table 1). It reveals five intense transitions at 3736 (A), 3647 (B), 3694 (C), 3361 (D), and 2782 (E) cm−1, which can readily be assigned to the O−H/N−H stretch fundamentals of the most stable t-FA+− (H2O)2 isomer (Figure 5) predicted at νaOH = 3740, νsOH = 3648, νfOH = 3701, νbOH = 3325, and νNH = 2910 cm−1. The FA+−(H2O)2(−Ar) spectra do not exhibit any signal in the range of the νNH transition of the FA+−H2O(−Ar) clusters near 3020 cm−1 (band E), implying that attachment of the second H2O ligand has a profound impact on the νNH frequency. This observation can only be rationalized by a t-FA+−(H2O)2 structure, in which the second H2O binds to the first H2O ligand with a yield of unity (Figure S5 in the SI). There are weaker bands in the FA+−(H2O)2−Ar spectrum at 2876, 2929, 3083, 3161, 3229, and 3305 cm−1, which are again assigned to

Figure 6. IRPD spectra of FA+−(H2O)2 and FA+−(H2O)2−Ar in the hydride stretch range compared to the linear IR absorption spectra of t-FA+−(H2O)2 and (H2O)2 calculated at the ωB97X-D/aug-cc-pVTZ level. The (H2O)2 spectrum is vertically expanded by a factor of 5. The positions, widths, and vibrational and isomer assignments of the transitions observed are listed in Table 1.

C−H stretch fundamentals and/or overtone/combination bands of amides I/II and H2O bend vibrations, which gain intensity from anharmonic coupling with the intense νNH fundamental. 1396


Article

The Journal of Physical Chemistry B 3.4. FA+−(H2O)3. The two most stable t-FA+−(H2O)3 isomers I and II shown in Figure 5 have structures in which a water trimer chain is H-bonded to the acidic NH proton of t-FA+. The structures are derived from the most stable t-FA+− (H2O)2 cluster by adding the third H2O ligand to a free OH proton donor of the first or second H2O ligand. Their IR spectra in the hydride stretch range are compared in Figure 7.

the strength of the N−H···O H-bond as compared to isomer I (1.652 versus 1.615 Å). Hence, the total N−H bond elongation and N−H stretch frequency red shift are less pronounced (ΔrNH = +34 mÅ; −ΔνNH = 564 cm−1), as is the charge transfer from t-FA+ to the solvent cluster (Δq = 72 me). Both H-bonds in the (H2O)3 trimer solvent are not equivalent but still substantially stronger and shorter (1.668 and 1.779 Å) than in the free dimer (1.973 Å). They produce two isolated bound O−H stretch modes predicted at νbOH = 3174 and 3395 cm−1. The corresponding two free O−H stretch bands are similar in frequency, νfOH = 3705 and 3712 cm−1, and appear between the symmetric and antisymmetric free O−H stretch fundamentals of the terminal H2O ligand, νs/aOH = 3650/3743 cm−1. Clearly, the IR spectra predicted for isomers I and II are very different in the hydride stretch range so that they can readily be distinguished (Figure 7). While there is only a single isomer I global minimum on the potential, several equivalent and also slightly different isomer II type structures are possible, depending on the exact connectivity of the H2O ligands to t-FA+. As these are all similar in energy and have essentially the same IR spectrum, we consider only the one shown in Figure 5 as a representative isomer II structure. Further t-FA+−(H2O)3 isomers, in which for example the third H2O ligand binds separately to other binding sites of the most stable t-FA+− (H2O)2 dimer are calculated to be much less stable and not considered further, because they are not detected experimentally. The IRPD spectrum of FA+−(H2O)3−Ar is compared in Figure 7 to the IR spectra calculated for the two isomers I and II. It exhibits four sharp bands in the O−H stretch range at 3739 (A), 3649 (B), 3455 (D1), and 3420 (D2) cm−1, which can readily be assigned to the corresponding transitions of the most stable isomer I predicted at 3743, 3650, 3439, and 3395 cm−1, respectively. There is good correspondence between the measured and calculated spectra with respect to both the peak positions and relative intensities. There is no clear evidence for the presence of isomer II in the observed FA+−(H2O)3−Ar spectrum. In particular, the free and bound O−H stretch bands calculated at ∼3710 and 3174 cm−1, which are characteristic of isomer II, are not detected. The signal at the red end of the FA+−(H2O)3−Ar spectrum rises toward lower frequency and provides an upper limit for the νNH fundamental as 2670 cm−1, which is close to the predicted frequency of 2703 cm−1. In contrast to n = 1 and 2, Ar tagging has a strong impact on the appearance of the IRPD spectrum of FA+−(H2O)3, with respect to the number of transitions observed. The IRPD spectrum of bare FA+−(H2O)3 does not only show much broader transitions but shows also new additional bands (C and D3), which are completely absent in the FA+−(H2O)3−Ar spectrum. While the spectral signatures of the most stable isomer I (A, B, D1, D2, E) are also observed in the FA+− (H2O)3 spectrum with similar frequencies and IR intensities, the transitions C and D3 at 3700 and 3170 cm−1 can be assigned to the less stable isomer II. Apparently, the higher temperature of the generated FA+−(H2O)3 clusters as compared to FA+−(H2O)3−Ar allows for a significant population of the less stable isomer II. Comparison with the calculated IR spectra suggests a roughly equal population of isomers I and II in the FA+−(H2O)3 spectrum. Such a high population of the energetically less stable isomer II at elevated temperatures is also favored by entropic arguments. There are two equivalent binding sites for the third ligand in isomer II, whereas there is only one in isomer I. Moreover, there are two

Figure 7. IRPD spectra of FA+−(H2O)3 and FA+−(H2O)3−Ar in the hydride stretch range compared to the linear IR absorption spectra of two isomers of t-FA+−(H2O)3 calculated at the ωB97X-D/aug-ccpVTZ level. The positions, widths, and vibrational and isomer assignments of the transitions observed are listed in Table 1.

Isomer I with a total binding energy of 11646 cm−1 has Cs symmetry, and the first H2O ligand is symmetrically solvated by two H2O ligands in the second solvation shell. The attachment of the third solvent molecule further strengthens the N−H···O H-bond as compared to the t-FA+−(H2O)2 cluster (1.615 versus 1.688 Å). Hence, the total N−H bond elongation and N−H stretch frequency red shift are more pronounced (ΔrNH = +41 versus +28 mÅ; −ΔνNH = 678 versus 471 cm−1). Furthermore, the charge transfer from t-FA+ to the solvent cluster increases (Δq = 79 versus 55 me). Both H-bonds in the (H2O)3 trimer solvent are substantially stronger and shorter (1.790 Å) than in the free dimer (1.973 Å). The O−H proton donor bonds of the first ligand are elongated by 16 mÅ, giving rise to intense symmetric and antisymmetric bound O−H stretch modes at νbOH = 3395 and 3439 cm−1, i.e., largely redshifted from bare H2O (−ΔνbOH = 261 and 318 cm−1) but blue-shifted from the νbOH band of t-FA+−(H2O)2 at 3325 cm−1. The symmetric νbOH mode couples strongly to the νNH vibration. The two outer ligands are much less affected, with −Δνs/aOH = 6/14 cm−1. Isomer II with asymmetric solvation and a total binding energy of D0 = 11456 cm−1 is only slightly less stable than isomer I. Attachment of the third H-bonded solvent molecule to the terminal ligand of t-FA+−(H2O)2 has a smaller effect on 1397


Article


Figure 8. Structures of low-energy t-FA+−(H2O)4 isomers obtained at the ωB97X-D/aug-cc-pVTZ level. Binding energies (D0), relative free energies (G0), and bond lengths are given in cm−1 and Å, respectively.

structures are possible. For t-FA+−(H2O)4, a previous study predicted six isomers within 350 cm−1 above the most stable one,37 all having a structure in which a H-bonded water network is attached to the NH group. These structures are also calculated here at the ωB97X-D level (Figure 8), using the same notation (I−VI). Starting from n = 4, the hydration network can bridge the gap between the NH and CO groups of the amide via a four-membered linear (H2O)4 chain, and in agreement with the previous prediction37 this cyclic isomer IV is energetically the most stable one, with D0 = 14552 cm−1. H2O binds to the positive NH proton (q = 420 me) as acceptor and to the negative CO oxygen atom (q = −459 me) as proton donor in the respective H-bonds. The strengths of the H-bonds along the hydration chain decreases from the starting NH group to the terminal CO group, with bond lengths varying from 1.649 to 2.110 Å. In this structure, each H2O ligand has a

binding sites for the terminal (H2O)2 dimer in isomer II; i.e., there are in total four (nearly) equivalent structures for II, while there is only one for isomer I, corresponding to statistical weights of 4 and 1 for the two isomers. In addition, while isomer I has a more rigid compact structure, the open solvent chain in isomer II is extended and more flexible leading to higher entropy. Finally, we note that both the bound N−H and O−H stretch frequencies (E and D) change drastically by adding one H2O ligand to FA+−(H2O)2, clearly confirming that the third H2O ligand in t-FA+−(H2O)3 is attached to the solvent network and thus excluding any structures in which one or more individual H2O ligands separately bind to the FA+ cation. 3.5. FA+−(H2O)4/5. With increasing degree of hydration, the potential energy surface of t-FA+−(H2O)n becomes substantially more complex, and a larger number of low-energy 1398


Article


amide is quite different for the isomers, their IR spectra predicted in the N−H and O−H stretch ranges are different, too (Figure 9). The IRPD spectrum of cold FA+−(H2O)4−Ar clusters is compared in Figure 9 to the IR spectra calculated for the six considered t-FA+ −(H 2 O) 4 isomers I−VI (actually, the spectrum of isomer I is not shown because it is essentially the same as that of isomer II).37 The IRPD spectrum exhibits four sharp transitions in the free O−H stretch range at 3729 (C1), 3719 (C2), 3715 (C3), and 3701 (C4) cm−1, which can readily be assigned to the corresponding transitions of the most stable isomer IV predicted at νfOH = 3728, 3717, 3714, and 3707 cm−1, respectively, with an average deviation of 2.5 cm−1 well below the experimental resolution of 6 cm−1. Each ligand in isomer IV has an isolated bound and free O−H stretch mode, leading to the absence of coupled νa/sOH transitions labeled A/B, in good agreement with the experiment. Three out of the four isolated νbOH fundamentals of isomer IV correspond to strong O−H···OH2 bonds of the solvent network and exhibit rather large red shifts with frequencies between 3200 and 3400 cm−1. These correspond well with the broad signal of band D1 in the FA+−(H2O)4−Ar spectrum starting from 3450 cm−1 and extending to the red. The remaining fourth νbOH mode arises from the weaker O−H···OC bond to the amide group and produces a weaker high-frequency band at 3592 cm−1, which is assigned to the experimental transition D4 at 3629 cm−1. Thus, there is good correspondence between the measured FA+− (H2O)4−Ar spectrum and the one predicted for the most stable isomer IV of t-FA+−(H2O)4 with respect to the number of transitions, their positions, and their relative intensities (apart from a slight but consistent overestimation of the red shifts in all νbOH modes). As the IR spectra predicted for all higher energy t-FA+−(H2O)4 isomers are quite different in this spectral range, there is no evidence for any noticeable abundance of a second isomer contributing to the measured FA+−(H2O)4−Ar spectrum. For example, isomers I, II, and VI have predicted intense νaOH modes above 3740 cm−1, which are absent in the observed spectrum. Isomer III cannot reproduce the intense measured C2−C4 transitions between 3700 and 3720 cm−1. The spectrum of isomer V exhibits intense νbOH modes at 3508 and 3538 cm−1, a spectral range in which the recorded FA+−(H2O)4−Ar spectrum has no signal. The exclusive observation of the energetically most stable isomer IV in the IRPD spectrum of the cold Ar-tagged n = 4 clusters parallels the scenario for the n = 3 clusters and confirms that cooling via rare gas tagging leads predominantly to the production of the global minimum structure. Similar to n = 3, the IRPD spectra measured for Ar-tagged and bare FA + −(H 2O) 4 clusters are quite different in appearance. In addition to broader transitions observed in the spectrum of bare FA+−(H2O)4 due to warmer clusters, a new prominent transition emerges at 3465 cm−1, which is completely absent in the spectrum of the cold Ar-tagged clusters. Indeed, all higher energy isomers considered have intense νbOH fundamentals in this spectral range. In addition, transition B at 3649 cm−1 assigned to νsOH transitions appears newly in the spectrum of bare FA+−(H2O)4. Such transitions are not predicted for isomer IV but for all other considered isomers. Therefore, we conclude that, at elevated temperatures, the population of these higher energy isomers increases and can be probed in the IRPD spectrum of the warmer bare FA+− (H2O)4 clusters. This conclusion is supported by the evaluation of the free energies (G0) at 298 K rather than the energies (E0

bound and a free OH group, leading to a unique IR spectrum in the O−H stretch range (Figure 9). Because the proton affinity

Figure 9. IR dip spectrum of t-FA+−(H2O)4 clusters generated by isomer-selective resonant photoionization of the neutral cluster via one-color two-photon (1 + 1) REMPI through the S1 origin37 compared to IRPD spectra of FA+−(H2O)4 and FA+−(H2O)4−Ar generated in the EI source and linear IR absorption spectra of FA+− (H2O)4 isomers calculated at the ωB97X-D/aug-cc-pVTZ level. The spectrum of isomer I is not shown because it is essentially the same as that of isomer II. The positions, widths, and vibrational and isomer assignments of the transitions observed are listed in Table 1. For clarity, the subscript OH has been omitted in the assignment of the free OH stretch modes νa/f/sOH.

of the linear chain at the O terminus is smaller than for branched and more compact structures, the bond to the NH group is relatively long (1.649 Å) and the impact on the NH group (elongated to 1.047 Å) is similar to the one of isomer II of n = 3, with a calculated red shift of −ΔνNH = 578 cm−1. The charge transfer to the solvent is 76 me for this isomer. While isomer IV of n = 4 is derived from isomer II of n = 3, the other five low-energy structures considered are obtained by adding a further ligand to isomer I of n = 3. Structures I and II with D0 = 14350 and 14303 cm−1 represent open-chain isomers, which are derived by adding a H2O molecule to the terminal H2O ligand either pointing toward FA+ or away from it. Similar to IV, isomer III with D0 = 14182 cm−1 has a linear solvent chain but solvates the NH and two adjacent CH groups, rather than forming a bridge between the NH and CO groups. Isomers V and VI with D0 = 14419 and 13855 cm−1 have cyclic solvent structures with four- and three-membered H2O rings. Interestingly, the energetic order of the six considered isomers changes drastically depending on whether one compares the energies (E0) or the free energies evaluated at 298 K (G0).37 The relative energies of isomers I−VI are +47, +249, +370, 0, +133, and +697 cm−1 with respect to isomer IV, while the corresponding relative free energies are −637, −687, −503, 0, −443, +100 cm−1. Hence, while at T = 0 K isomer IV is clearly the most stable structure, at elevated temperature it becomes quite unfavorable with respect to the isomers I−III and V. Because the H-bonded network and their connection to the 1399


Article


Figure 10. IRPD spectrum of FA+−(H2O)n−Ar with n = 0−3 in the fingerprint range compared to linear IR absorption spectra of t-FA+−(H2O)n calculated at the ωB97X-D/aug-cc-pVTZ level. The positions and vibrational and isomer assignments of the transitions observed are listed in Table 3. The spectra for n = 0 are adapted with permission from ref 72. Copyright 2014 American Chemical Society. The arrows indicate the frequencies of bare H2O. Filled and open circles indicate the βOH and βNH modes, respectively.

The IRPD spectrum recorded for FA+−(H2O)5 in the O−H stretch range is included in Figure 2. The signals for this cluster size have been too weak to generate a sufficient amount of Artagged clusters required for recording an IRPD spectrum. Although the IRPD spectrum of FA+−(H2O)5 does not show as much structure as those of the smaller clusters, one can infer several conclusions. First, the lack of measurable signals in the range of the A/B transitions near 3650 and 3750 cm−1 implies the absence of free νa/sOH modes. Thus, the cluster population is dominated by structures, in which (nearly) all H2O ligands are engaged as single or double proton donors in the H-bonded network. The single proton donor ligands have free O−H stretch modes in the vicinity of 3700 cm−1, and the IRPD spectrum indeed exhibits a broad structured feature with (partially) resolved maxima at 3707, 3716, 3722, and 3733 cm−1 (C1−C4). The corresponding bound O−H stretch modes give rise to broad signals between 3100 and 3600 cm−1, with weak and broad maxima near 3510, 3430, and 3230 cm−1, respectively. Although no attempt has been made to calculate low-energy isomers of this cluster size, the appearance of the IRPD spectrum suggests that the solvent network of the FA+−(H2O)4 clusters is simply expanded by adding one more ligand either to the surface or the interior of the solvent network. Similar to the n = 4 spectrum, the n = 5 spectrum does not cover the νNH fundamental, which is shifted out of the spectral range investigated. 3.6. Fingerprint Range. The IRPD spectra of the FA+− (H2O)n−Ar clusters with n ≤ 3 recorded in the fingerprint range (1200−1900 cm−1) are compared in Figure 10 to IR spectra calculated for the bare t-FA+−(H2O)n clusters. The observed transitions are listed in Table 3, along with their suggested vibrational assignments. The spectra for n = 0 are taken from ref 72. Ar tagging has been necessary to obtain IRPD spectra in the fingerprint range, because in this range the available laser intensities are low and the photon energies are small in comparison to the binding energies of the H2O ligands. The fingerprint range covers the amide I and II bands, namely, νCO (amide I) and βNH coupled to various modes of the amide and phenyl groups (amide II), some ring skeletal modes of the phenyl ring, and the O−H bends of the water ligands (βOH). The notation employed for the aromatic modes follows the classification of Wilson.94 The IRPD spectrum of FA+−Ar has readily been assigned to the t-FA+−Ar(H/π) isomers and does not show any sign of the presence of c-FA+−Ar clusters, which

or D0), which changes the energetic order of the isomers. Even when neglecting statistical weights, isomer II becomes clearly the global minimum on the free energy potential, whereas isomer IV lies quite high in free energy (+687 cm−1). Isomer II with its noncyclic branched solvent network has by far the most flexible hydration structure, which is entropically favored at elevated temperatures. Indeed, isomer II has two strong νbOH resonances at 3424 and 3448 cm−1 close to the new band D1 near 3465 cm−1 in the FA+−(H2O)4 spectrum, and it also has the νsOH transitions at 3652 cm−1 required to explain band B at 3649 cm−1. Unfortunately, a more quantitative evaluation of the contribution of individual isomers to the FA +−(H 2 O) 4 spectrum is not warranted because of its insufficient spectral resolution. Significantly, the substantial change in the appearance of the IRPD spectra in the O−H stretch range when adding the fourth H2O ligand implies that it is implemented in the H2O solvent network rather than attached as a single ligand to the FA+ core of FA+−(H2O)3. The IRPD spectra of FA+−(H2O)4(−Ar) generated in the EI source are compared in Figure 9 to the previously reported IR dip spectra of t-FA+−(H2O)4 cation clusters generated by isomer-selective resonant 1 + 1 REMPI of the neutral precursor via its S1 origin.37 The neutral t-FA−(H2O)4 cluster has a structure similar to isomer IV of the cation, with a H-bonded (H2O)4 chain connecting the NH and CO groups of the amide group.37,60 Although the IR dip spectrum is similar in appearance to the IRPD spectra, the transitions are much broader in the former spectrum indicating much hotter species. The IR dip spectrum has been taken 800 ns after isomerselective 1 + 1 REMPI of the cyclic n = 4 isomer, with an ionization excess energy for vertical ionization of around 3300 cm−1.37 Comparison with calculated spectra has suggested an ionization-induced ring opening reaction driven by entropy.37 Comparison of the three experimental IR spectra of FA+− (H2O)4 in Figure 9 illustrates the drastic effect of temperature on the isomer population and the spectral appearance. While the FA+−(H2O)4−Ar spectrum is dominated by cold clusters with isomer IV cores, the contribution of open isomers increases with temperature in the IRPD spectrum of FA+− (H2O)4 and further in the IR dip spectrum of t-FA+−(H2O)4. This spectral comparison emphasizes once more the importance of generating cold clusters for the spectroscopic interrogation of the most stable isomers of hydrated aromatic cations. 1400


Article

The Journal of Physical Chemistry B have a quite different predicted IR spectrum.72 In addition, Ar complexation has only a minor effect on the IR spectra predicted for FA+−(H2O)n, as shown in ref 72 for n = 0 and in Figure S2 in the SI for n = 1. Hence, we compare the experimental IRPD spectra of FA+−(H2O)n−Ar in Figure 10 to those predicted for t-FA+−(H2O)n. Indeed, closer inspection of Figure 10 reveals that all intense transitions observed in the IRPD spectra of FA+−(H2O)n−Ar are well reproduced by the spectra calculated for the most stable FA+−(H2O)n isomers with respect to both their frequencies and relative IR intensities. Most of the skeletal modes in the fingerprint range are fairly insensitive to the cluster growth. However, as expected, the amide I and II modes as well as the H2O bending vibrations (βOH) are quite sensitive to the structure of the hydration shell. The intense transition F at around 1790 cm−1 is attributed to νCO, which is in strong Fermi resonance with an unidentified vibration (band G near 1770 cm−1).72 Attachment of (H2O)n to the NH group is calculated to reduce the νCO frequency of tFA+ by 12, 17, and 23 cm−1 for n = 1−3, because the CO bond elongates by 3.3, 4.6, and 5.9 mÅ. However, the experimental red shifts are smaller because of the Fermi interaction with vibration G. Bands H, I, and L between 1484 and 1601 cm−1 are attributed to amide II modes, which all carry substantial contributions of the in-plane N−H bend (βNH). As the attachment of (H2O)n to the NH group provides an additional retarding force to the βNH mode, these transitions experience corresponding blue shifts as n increases, because the intermolecular bond to the solvent network gets monotonically stronger due to the cooperativity of the solvent network. The largest shifts are observed for band H, which carries the highest N−H bend character, and the observed blue shifts of 34, 49, and 63 cm−1 are well reproduced by the calculated shifts of 37, 68, and 65 cm−1 for n = 1−3 (indicated by open circles in Figure 10), a clear indication that the H2O solvent network is connected to the NH group of the amide. As expected, the FA+−(H2O)n−Ar spectra show for n ≥ 1 additional bands arising from the O−H bending modes of the H2O ligands (βOH) and indicated by filled circles in Figure 10, which are absent in the n = 0 spectrum. For bare H2O, the calculations predict a frequency of 1584 cm−1, which is somewhat lower than the measured value of 1595 cm−1 (indicated by arrows in Figure 10).90 Monohydration at the NH site predicts a blue shift of ΔβOH = 19 cm−1, which is smaller than the measured shift (33 cm−1). This discrepancy arises partly from the strong coupling of βOH with βNH, whose frequency blue shift is much larger. Upon further hydration, the βOH transition is predicted to split into several subbands, but the ones with highest IR activity shift slightly to lower frequency, in agreement with the experiment. The mixing between βOH and βNH increases for the larger clusters. In general, the good agreement between the experimental IRPD spectra and those predicted for the most stable isomers confirms the cluster growth sequence derived from the IRPD spectra in the hydride stretch range.

solvent chain closes the gap to the CO group. Such solvation structures benefit from the large cooperativity of H-bonding of the solvent and the additional formation of C−H···O H-bonds of the CH group of the amide. In contrast, structures corresponding to interior aromatic ion solvation, in which individual H2O ligands bind to the various attractive binding sites of the FA+ cation core, are substantially less stable and not observed. At first glance, this result may be surprising, in particular in view of the NBO charge distribution, which predicts a larger positive charge of 68 me on the phenyl ring and a smaller charge on the amide group. However, the noncooperativity of interior ion solvation can explain this observation.28,47,83 The IRPD spectra of the cold Ar-tagged clusters can be rationalized by a single isomer assigned to the global minimum of each cluster size (n ≤ 4). In contrast, the bare FA+−(H2O)n clusters are warmer and their IRPD spectra exhibit the presence of energetically less stable isomers for the size range n ≥ 3. This observation is rationalized by entropic factors, which play a more prominent role for the preferred cluster structures at elevated temperature. The calculated incremental binding energies in the most stable t-FA+− (H2O)n clusters, ΔD0(n) = D0(n) − D0(n−1), decrease as ΔD0 = 4897, 3535, 3214, and 2906 cm−1 for n = 1−4, and the latter value for n = 4 is still larger than the binding energies of the CO, CN, CH, and ring binding sites of the dimer potential (2000 cm−1), and only the most stable isomer can be observed. In contrast, the energy differences are much smaller for the neutral dimer, leading to the detection of both the NH-bound and CO-bound dimers, which are separated by less than 100 cm−1.40 Photoionization of the CO-bound neutral dimer leads however to a repulsive configuration on the cation potential and triggers a CO → NH isomerization reaction terminating at the NH-bound global minimum, as described in section 3.2. Similar to n = 1, two isomers with comparable stabilities are detected for neutral t-FA−(H2O)2 in a molecular beam, and their structures are characterized by H-bonded (H2O)2 dimers attached to either the NH or CO groups of t-FA, leading to cyclic structures.36,60,62,63 Ionization drastically changes the charge distribution of FA+ such that the CO group of the peptide linkage is no longer an attractive binding site as proton acceptor for H-bonding to H2O ligands for all cluster sizes, and a single stable isomer is detected for t-FA+−(H2O)2. This structure has a linear water dimer chain attached to the NH group because the excess positive charge prevents the formation of cyclic solvation structures. Similar to n = 1, ionization of the CO-bound isomer of n = 2 will cause a drastic reorganization of the hydration structure, which thus is an interesting target for time-resolved experiments.30,46 Interest1402


Article


formyl H atom, the bulky CH3 group introduces steric hindrance and cannot participate in H-bonding to the solvent.

ingly, no experimental and theoretical data have been reported so far for the neutral n = 3 cluster,36,60,62,63 while the corresponding cation cluster can readily be observed, and its two isomers I and II have linear and branched hydration structures attached to the NH group. The largest neutral cluster observed, t-FA−(H2O)4, occurs as a single very stable isomer, with a (H2O)4 solvent chain bridging the gap from the NH donor to the CO acceptor group.37,60,62,63 Calculations suggest for the n = 4 cation clusters at least six low-energy isomers within 350 cm−1,37 with the most stable one having a similar cyclic structure as the neutral cluster (Figure 8). However, the IR spectrum measured after isomer-selective ionization of this cluster suggests an ionization-induced ring opening reaction and the subsequent population of higher energy noncyclic isomers with chain-like and branched water solvation networks. Such more flexible open structures are largely favored by entropic factors at high internal energies. Our IRPD spectra of Ar-tagged cold t-FA+−(H2O)4−Ar clusters are clearly consistent with the cyclic solvation ring, confirming that this hydration motif is energetically favored at low temperature. Finally, it is noted that similar to ionic FA+−(H2O)n clusters also for neutral FA−(H2O)n no clusters with a c-FA core have been observed so far. This result is particularly striking in view of the prediction that c-FA−H2O is by far more stable than t-FA−H2O because it can form two H-bonds rather than only one (D0= 2640 versus 1761 cm−1, Table 2), and neutral c-FA has also been readily detected in molecular beams. 4.2.3. Comparison to AA+−H2O. In an effort to quantify the effects of methyl substitution on the acidity of the amide group in t-FA+, the IRPD spectra of FA+−H2O(−Ar) are compared in the hydride stretch range to those of AA+−H2O(−Ar)30 in Figure S6 in the SI. For comparison, the IR spectra calculated for t-FA+−H2O(NH) and t-AA+−H2O(NH) are also shown. As noted in our previous study,72 H → CH3 substitution has the largest effect on the adjacent CO and C−N bonds, followed by weaker effects on other bonds of the amide group, whereas the more remote aromatic ring is only little affected. The electron-withdrawing CH3 group slightly strengthens the N−H bond,72 which becomes less acidic and thus forms weaker Hbonds. For example, the H-bond in t-FA+−H2O(NH) is characterized by D0 = 4897 cm−1 and R = 1.780 Å, while the corresponding values for t-AA+−H2O(NH) are D0 = 4513 cm−1 and R = 1.831 Å. As a result, the calculated νNH fundamental of t-AA+−H2O(NH) has a larger frequency as compared to that of t-FA+−H2O(NH), 3139 versus 3085 cm−1. This has drastic implications on the appearance of the IRPD spectra of t-FA+− H2O(−Ar) and t-AA+−H2O(−Ar) in the N−H stretch range, because the energetic order of the νNH fundamental and the 2βNH overtone, which are in Fermi resonance in both clusters, changes since the βNH frequency is essentially unaffected by H → CH3 substitution (1571 cm−1). While the 2βNH level lies above νNH in t-FA+−H2O(−Ar), the situation is reversed for t-AA+ −H 2 O(−Ar). Consequently, the experimental ν NH frequency of t-FA+−H2O(−Ar) is pushed downward by the Fermi resonance, while it is shifted upward for t-AA+− H2O(−Ar). H → CH3 substitution has no major principal impact on the monohydration motif; i.e., for both t-AA+−H2O and t-FA+−H2O only the NH-bound isomer is detected.30 Unfortunately, IRPD spectra of t-AA+−H2O in the fingerprint range and IR spectra of larger t-AA+−(H2O)n clusters have not been reported yet, preventing any further evaluation of the effects of H → CH3 substitution on sequential microhydration. Such effects must however exist because, in contrast to the

5. CONCLUDING REMARKS The initial steps of microhydration of a prototypical aromatic amide cation have been determined by IRPD spectra of sizeselected FA+−(H2O)n clusters with n ≤ 5 in the fingerprint and hydride stretch ranges and DFT calculations at the ωB97X-D/ aug-cc-pVTZ level. Tagging of FA+−(H2O)n with Ar produces colder clusters, which exhibit IRPD spectra at higher resolution facilitating an unambiguous structural assignment up to n = 4. Only clusters of the more stable t-FA+ rotamer are identified. The IRPD spectra of the Ar-tagged clusters can essentially be attributed to a single isomer for a given cluster size, which corresponds to the global minimum determined by the calculations. The untagged clusters are warmer, and less stable isomers are observed in addition to the global minima for cluster sizes with n ≥ 3, because they are entropically favored. Such variations in the relative population of isomers as a function of temperature have been reported previously.74,104−106 The preferred sequential cluster growth starts with hydration of the acidic NH proton of the amide group by the first H2O ligand (n = 1), which acts as a proton donor for H-bonds to two further H2O ligands (n = 2 and 3), leading to a H-bonded branched (H2O)3 solvent network. Starting from n = 4, cluster structures with a noncyclic H-bonded (H2O)4 chain bridging the gap from the NH to the CO groups are most stable. Hence, hydration of the amide group of t-FA+ is preferred over hydration of the phenyl ring, although the latter carries a higher positive charge. In general, strong cooperative effects of clusters with a H-bonded solvent network win over the strong charge−dipole forces of structures with interior ion solvation, in which individual H2O ligands are attached to the t-FA+ cation core. Although charge transfer from t-FA+ to the solvent network increases for t-FA+−(H2O)n with cluster size n, the positive charge remains largely on the aromatic amide in the size range n ≤ 5. Similarly, the N−H bonds are destabilized by stepwise microhydration, but no proton transfer to the solvent is observed for n ≤ 5. The cluster growth of t-FA+−(H2O)n is qualitatively different from that of t-FA+−Ln with L = Ar and N2, indicating the difference of microsolvation of aromatic amide cations in a protic polar (hydrophilic) and nonpolar/ quadrupolar (hydrophobic) solvent. Comparison between t-FA + −H 2 O and t-AA + −H 2 O reveals that H → CH 3 substitution reduces the acidity of the amide group leading to weaker H-bonds to the solvent. The additional interactions arising from the excess charge imply that the topology of the interaction potential in FA+− (H2O)n is rather different from that of the corresponding neutral FA−(H2O)n clusters with respect to both the strength of the interaction, the structures of the global and local minima, and the structure of the hydration shell. Thus, photoionization of certain FA−(H2O)n isomers triggers isomerization and rearrangement reactions, which have been deduced from static IR spectra already for the CO-bound isomer of FA−H2O and the global minimum structure of FA−(H2O)4.36,37 It will be interesting to study the dynamics and energetics of these solvent migration and rearrangement reactions in real time by picosecond time-resolved vibrational and electronic spectroscopy as a function of the ionization excess energy, the degree of hydration, and the isomeric structure.30,35,46,107−109 Such experiments are currently underway. 1403


Article


■

ASSOCIATED CONTENT

■

AUTHOR INFORMATION

(18) Müller-Dethlefs, K.; Hobza, P. Noncovalent Interactions: A Challenge for Experiment and Theory. Chem. Rev. 2000, 100, 143− 168. (19) Rizzo, T. R.; Stearns, J. A.; Boyarkin, O. V. Spectroscopic Studies of Cold, Gas-Phase Biomolecular Ions. Int. Rev. Phys. Chem. 2009, 28, 481−515. (20) Nagornova, N. S.; Rizzo, T. R.; Boyarkin, O. V. Exploring the Mechanism of IR-UV Double-Resonance for Quantitative Spectroscopy of Protonated Polypeptides and Proteins. Angew. Chem., Int. Ed. 2013, 52, 6002−6005. (21) Nagornova, N. S.; Rizzo, T. R.; Boyarkin, O. V. Interplay of Intra- and Intermolecular H-Bonding in a Progressively Solvated Macrocyclic Peptide. Science 2012, 336, 320−323. (22) Wolk, A. B.; Leavitt, C. M.; Garand, E.; Johnson, M. A. Cryogenic Ion Chemistry and Spectroscopy. Acc. Chem. Res. 2014, 47, 202−210. (23) Garand, E.; Kamrath, M. Z.; Jordan, P. A.; Wolk, A. B.; Leavitt, C. M.; McCoy, A. B.; Miller, S. J.; Johnson, M. A. Determination of Noncovalent Docking by Infrared Spectroscopy of Cold Gas-Phase Complexes. Science 2012, 335, 694−698. (24) Zwier, T. S. Probing Frozen Molecular Embraces. Science 2012, 335, 668−669. (25) Polfer, N. C.; Oomens, J. Vibrational Spectroscopy of Bare and Solvated Ionic Complexes of Biological Relevance. Mass Spectrom. Rev. 2009, 28, 468−494. (26) Carcabal, P.; Cocinero, E. J.; Simons, J. P. Binding Energies of Microhydrated Carbohydrates: Measurements and Interpretation. Chem. Sci. 2013, 4, 1830−1836. (27) Zwier, T. S. The Spectroscopy of Solvation in HydrogenBonded Aromatic Clusters. Annu. Rev. Phys. Chem. 1996, 47, 205−241. (28) Dopfer, O. IR Spectroscopy of Microsolvated Aromatic Cluster Ions: Ionization-Induced Switch in Aromatic MoleculeSolvent Recognition. Z. Phys. Chem. 2005, 219, 125−168. (29) Melandri, S. “Union is Strength”: How Weak Hydrogen Bonds Become Stronger. Phys. Chem. Chem. Phys. 2011, 13, 13901−13911. (30) Tanabe, K.; Miyazaki, M.; Schmies, M.; Patzer, A.; Schütz, M.; Sekiya, H.; Sakai, M.; Dopfer, O.; Fujii, M. Watching Water Migration Around a Peptide Bond. Angew. Chem., Int. Ed. 2012, 51, 6604−6607. (31) Müller-Dethlefs, K.; Dopfer, O.; Wright, T. G. ZEKE Spectroscopy of Complexes and Clusters. Chem. Rev. 1994, 94, 1845−1871. (32) Brutschy, B. The Structure of Microsolvated Benzene Derivates and the Role of Aromatic Substituents. Chem. Rev. 2000, 100, 3891− 3920. (33) Zwier, T. S. Laser Probes of Conformational Isomerization in Flexible Molecules and Complexes. J. Phys. Chem. A 2006, 110, 4133− 4150. (34) Bieske, E. J.; Dopfer, O. High Resolution Spectroscopy of Cluster Ions. Chem. Rev. 2000, 100, 3963−3998. (35) Fujii, M.; Dopfer, O. Ionisation-Induced Site Switching Dynamics in Solvated Aromatic Clusters: Phenol−(Rare Gas)n Clusters as Prototypical Example. Int. Rev. Phys. Chem. 2012, 31, 131−173. (36) Ikeda, T.; Sakota, K.; Kawashima, Y.; Shimazaki, Y.; Sekiya, H. Photoionization-Induced Water Migration in the Hydrated transFormanilide Cluster Cation Revealed by Gas-Phase Spectroscopy and Ab Initio Molecular Dynamics Simulation. J. Phys. Chem. A 2012, 116, 3816−3823. (37) Sakota, K.; Shimazaki, Y.; Sekiya, H. Entropy-Driven Rearrangement of the Water Network at the Hydrated Amide Group of the Trans-Formanilide-Water Cluster in the Gas Phase. Phys. Chem. Chem. Phys. 2011, 13, 6411−6415. (38) Sakota, K.; Harada, S.; Shimazaki, Y.; Sekiya, H. Photoionization-Induced Water Migration in the Amide Group of TransAcetanilide-H2O in the Gas Phase. J. Phys. Chem. A 2011, 115, 626− 630. (39) Nakamura, T.; Schmies, M.; Patzer, A.; Miyazaki, M.; Ishiuchi, S.; Weiler, M.; Dopfer, O.; Fujii, M. Solvent Migration in Microhydrated Aromatic Aggregates: Ionization-Induced Site Switch-

S Supporting Information *

Figures showing (1) structures of t-/c-FA+−H2O isomers, (2) structures and IR spectra of t-FA+−H2O(−Ar), (3) IR spectra of t-FA+−H2O isomers, (4) IRPD spectrum of FA+−H2O− (N2)2, (5) IR spectra of t-FA+−(H2O)2 isomers, (6) IR spectra of t-FA+−H2O and t-AA+−H2O. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*Fax: (+49) 30-31423018. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This study was supported by Deutsche Forschungsgemeinschaft (DO 729/4). M.S. is grateful for an Elsa Neumann fellowship. We thank Kenji Sakota and Hiroshi Sekiya for providing their REMPI-IR spectra of t-FA+−(H2O)n with n = 1 and 4 and fruitful discussions. M.F. is grateful for support from the Core-to-Core Program 22003 of the Japan Society for the Promotion of Science (JSPS).

■

REFERENCES

(1) Denisov, V. P.; Jonsson, B. H.; Halle, B. Hydration of Denatured and Molten Globule Proteins. Nat. Struct. Biol. 1999, 6, 253−260. (2) Pal, S. K.; Zewail, A. H. Dynamics of Water in Biological Recognition. Chem. Rev. 2004, 104, 2099−2123. (3) Levy, Y.; Onuchic, J. N. Water Mediation in Protein Folding and Molecular Recognition. Annu. Rev. Biophys. Biomol. Struct. 2006, 35, 389−415. (4) Zhang, L. Y.; Wang, L. J.; Kao, Y. T.; Qiu, W. H.; Yang, Y.; Okobiah, O.; Zhong, D. P. Mapping Hydration Dynamics Around a Protein Surface. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 18461− 18466. (5) Bagchi, B. Water Dynamics in the Hydration Layer Around Proteins and Micelles. Chem. Rev. 2005, 105, 3197−3219. (6) Chaplin, M. OpinionDo We Underestimate the Importance of Water in Cell Biology? Nat. Rev. Mol. Cell Biol. 2006, 7, 861−866. (7) Ball, P. Water as an Active Constituent in Cell Biology. Chem. Rev. 2008, 108, 74−108. (8) Ball, P. More Than a Bystander. Nature 2011, 478, 467−468. (9) Nucci, N. V.; Pometun, M. S.; Wand, A. J. Site-Resolved Measurement of Water-Protein Interactions by Solution NMR. Nat. Struct. Mol. Biol. 2011, 18, 245−249. (10) Otting, G.; Liepinsh, E.; Wuthrich, K. Protein Hydration in Aqueous Solution. Science 1991, 254, 974−980. (11) Turecek, F.; Julian, R. R. Peptide Radicals and Cation Radicals in the Gas Phase. Chem. Rev. 2013, 113, 6691−6733. (12) Schermann, J. P. Spectroscopy and Modelling of Biomolecular Building Blocks; Elsevier: Amsterdam, 2008. (13) Hobza, P.; Müller-Dethlefs, K. Non-Covalent Interactions; The Royal Society of Chemistry: Cambridge, U.K., 2010. (14) Nir, E.; Kleinermanns, K.; de Vries, M. S. Pairing of Isolated Nucleic-Acid Bases in the Absence of the DNA Backbone. Nature 2000, 408, 949−951. (15) de Vries, M. S.; Hobza, P. Gas-Phase Spectroscopy of Biomolecular Building Blocks. Annu. Rev. Phys. Chem. 2007, 58, 585−612. (16) Kleinermanns, K.; Nachtigallova, D.; de Vries, M. S. Excited State Dynamics of DNA Bases. Int. Rev. Phys. Chem. 2013, 32, 308− 342. (17) Cocinero, E. J.; Carcabal, P.; Vaden, T. D.; Simons, J. P.; Davis, B. G. Sensing the Anomeric Effect in a Solvent-Free Environment. Nature 2011, 469, 76−79. 1404


Article

The Journal of Physical Chemistry B ing in the 4-Aminobenzonitrile−Water Cluster. Chem.Eur. J. 2014, 20, 2031−2039. (40) Mons, M.; Dimicoli, I.; Tardivel, B.; Piuzzi, F.; Robertson, E. G.; Simons, J. P. Energetics of the Gas Phase Hydrates of TransFormanilide: A Microscopic Approach to the Hydration Sites of the Peptide Bond. J. Phys. Chem. A 2001, 105, 969−973. (41) Ullrich, S.; Tong, X.; Tarczay, G.; Dessent, C. E. H.; MüllerDethlefs, K. Hydration of a Cationic Amide Group: A ZEKE Spectroscopic Study of Trans-Formanilide-H2O. Phys. Chem. Chem. Phys. 2002, 4, 2897−2903. (42) Dopfer, O. Spectroscopic and Theoretical Studies of CH3+-Rgn clusters (Rg=He, Ne, Ar): From Weak Intermolecular Forces to Chemical Reaction Mechanisms. Int. Rev. Phys. Chem. 2003, 22, 437− 495. (43) Solcà, N.; Dopfer, O. Infrared Spectra of the Phenol-Ar and Phenol-N2 Cations: Proton-Bound Versus π-Bound Structures. Chem. Phys. Lett. 2000, 325, 354−359. (44) Chakraborty, S.; Patzer, A.; Lagutschenkov, A.; Langer, J.; Dopfer, O. Infrared and Electronic Spectra of Microhydrated ParaDichlorobenzene Cluster Cations. Chem. Phys. Lett. 2010, 485, 49−55. (45) Chakraborty, S.; Patzer, A.; Lagutschenkov, A.; Langer, J.; Dopfer, O. Infrared and Electronic Spectroscopy of p-C6H4Cl2+-Ln Clusters with L=Ar, N2, H2O, p-C6H4Cl2. Int. J. Mass Spectrom. 2010, 297, 85−95. (46) Wohlgemuth, M.; Miyazaki, M.; Weiler, M.; Sakai, M.; Dopfer, O.; Fujii, M.; Mitric, R. Single Water Solvation Dynamics Probed by Infrared SpectraTheory Meets Experiment. Angew. Chem., Int. Ed. 2014, 53, 14601−14604. (47) Dopfer, O.; Patzer, A.; Chakraborty, S.; Alata, I.; Omidyan, R.; Broquier, M.; Dedonder, C.; Jouvet, C. Electronic and Vibrational Spectra of Protonated Benzaldehyde-Water Clusters, [BZ(H2O)n≤5]H+: Evidence for Ground-State Proton Transfer to Solvent for n ≥ 3. J. Chem. Phys. 2014, 140, No. 124314. (48) Solcà, N.; Dopfer, O. IR Spectrum of the Benzene-Water Cation: Direct Evidence for a Hydrogen-Bonded Charge-Dipole Complex. Chem. Phys. Lett. 2001, 347, 59−64. (49) Lorenz, U.; Solcà, N.; Dopfer, O. Entrance Channel Complexes of Cationic Aromatic SN2 Reactions: IR Spectra of Fluorobenzene+− (H2O)n Clusters. Chem. Phys. Lett. 2005, 406, 321−326. (50) Solcà, N.; Dopfer, O. Prototype Microsolvation of Aromatic Hydrocarbon Cations by Polar Ligands: IR Spectra of Benzene+-Ln clusters (L = H2O, CH3OH). J. Phys. Chem. A 2003, 107, 4046−4055. (51) Alata, I.; Broquier, M.; Dedonder-Lardeux, C.; Jouvet, C.; Kim, M.; Sohn, W. Y.; Kim, S.; Kang, H.; Schütz, M.; Patzer, A.; Dopfer, O. Microhydration Effects on the Electronic Spectra of Protonated Polycyclic Aromatic Hydrocarbons: [Naphthalene-(H2O)n=1,2]H+. J. Chem. Phys. 2011, 134, No. 074307. (52) Patzer, A.; Chakraborty, S.; Dopfer, O. Infrared Spectra and Quantum Chemical Characterization of Weakly Bound Clusters of the Benzoyl Cation with Ar and H2O. Phys. Chem. Chem. Phys. 2010, 12, 15704−15714. (53) Andrei, H. S.; Solcà, N.; Dopfer, O. Microhydration of Protonated Biomolecular Building Blocks: IR Spectra of Protonated Imidazole-Watern Complexes. ChemPhysChem 2006, 7, 107−110. (54) Marochkin, I. I.; Dorofeeva, O. V. Molecular Structure and Relative Stability of Trans and Cis Isomers of Formanilide: Gas-Phase Electron Diffraction and Quantum Chemical Studies. Struct. Chem. 2013, 24, 233−242. (55) Ottaviani, P.; Melandri, S.; Maris, A.; Favero, P. G.; Caminati, W. Free-Jet Rotational Spectrum and Ab Initio Calculations of Formanilide. J. Mol. Spectrosc. 2001, 205, 173−176. (56) Blanco, S.; Lopez, J. C.; Lesarri, A.; Caminati, W.; Alonso, J. L. Conformational Equilibrium of Formanilide: Detection of the Pure Rotational Spectrum of the Tunnelling Cis Conformer. Mol. Phys. 2005, 103, 1473−1479. (57) Moreno, J. R. A.; Huet, T. R.; Petitprez, D. The Trans-Isomer of Formanilide Studied by Microwave Fourier Transform Spectroscopy. J. Mol. Struct. 2006, 780−81, 234−237.

(58) Moreno, J. R. A.; Petitprez, D.; Huet, T. R. The Conformational Flexibility in N-Phenylformamide: An Ab Initio Approach Supported by Microwave Spectroscopy. Chem. Phys. Lett. 2006, 419, 411−416. (59) Miyazaki, M.; Saikawa, J.; Ishizuki, H.; Taira, T.; Fujii, M. Isomer Selective Infrared Spectroscopy of Supersonically Cooled Cisand Trans-N-Phenylamides in the Region from the Amide Band to NH Stretching Vibration. Phys. Chem. Chem. Phys. 2009, 11, 6098− 6106. (60) Robertson, E. G. IR-UV Ion-Dip Spectroscopy of N-Phenyl Formamide and its Hydrated Clusters. Chem. Phys. Lett. 2000, 325, 299−307. (61) Manea, V. P.; Wilson, K. J.; Cable, J. R. Conformations and Relative Stabilities of the Cis and Trans Isomers in a Series of Isolated N-Phenylamides. J. Am. Chem. Soc. 1997, 119, 2033−2039. (62) Dickinson, J. A.; Hockridge, M. R.; Robertson, E. G.; Simons, J. P. Molecular and Supramolecular Structures of N-Phenyl Formamide and its Hydrated Clusters. J. Phys. Chem. A 1999, 103, 6938−6949. (63) Fedorov, A. V.; Cable, J. R. Spectroscopy of Hydrogen-Bonded Formanilide Clusters in a Supersonic Jet: Solvation of a Model Trans Amide. J. Phys. Chem. A 2000, 104, 4943−4952. (64) Pasquini, M.; Schiccheri, N.; Pietraperzia, G.; Becucci, M. TransFormanilide: On the Properties of S1 State from High Resolution Electronic Spectroscopy and Ab Initio Calculations. Chem. Phys. Lett. 2009, 475, 30−33. (65) Nakanishi, H.; Yamamoto, O. Nuclear Magnetic-Resonance Study of Exchanging Systems. 6. Rotational Barrier of Formanilide by 13C NMR Complete Line-Shape Analysis. Chem. Lett. 1974, 521−524. (66) Stewart, W. E.; Siddall, T. H. Nuclear Magnetic Resonance Studies of Amides. Chem. Rev. 1970, 70, 517−551. (67) Bourn, A. J. R.; Randall, E. W.; Gillies, D. G. Cis-Trans Isomerism in Formanilide. Tetrahedron 1964, 20, 1811−1818. (68) Ullrich, S.; Tarczay, G.; Tong, X.; Dessent, C. E. H.; MüllerDethlefs, K. ZEKE Photoelectron Spectroscopy of the Cis and Trans Isomers of Formanilide. Angew. Chem., Int. Ed. 2002, 41, 166−168. (69) Ullrich, S.; Tarczay, G.; Tong, X.; Ford, M. S.; Dessent, C. E. H.; Müller-Dethlefs, K. A REMPI and ZEKE Spectroscopic Study of the Trans-Formanilide-Ar van der Waals Cluster. Chem. Phys. Lett. 2002, 351, 121−127. (70) Clarkson, J. R.; Baquero, E.; Shubert, V. A.; Myshakin, E. M.; Jordan, K. D.; Zwier, T. S. Laser-Initiated Shuttling of a Water Molecule Between H-Bonding Sites. Science 2005, 307, 1443−1446. (71) Klasinc, L.; Novak, I.; Sabljic, A.; McGlynn, S. P. Photoelectron Spectroscopy of Biologically Active Molecules. 12. Benzene-Containing Amides. Int. J. Quantum Chem. 1986, 251−260. (72) Klyne, J.; Schmies, M.; Dopfer, O. Microsolvation of the Formanilide Cation (FA+) in a Nonpolar Solvent: IR Spectra of FA+Ln Clusters (L = Ar, N2; n ≤ 8). J. Phys. Chem. B 2014, 118, 3005− 3017. (73) Tachikawa, H.; Igarashi, M.; Ishibashi, T. Ionization Dynamics of trans-Formanilide-H2O Complexes: A Direct ab Initio Dynamics Study. J. Phys. Chem. A 2003, 107, 7505−7513. (74) Solcà, N.; Dopfer, O. Infrared Spectra of the H-bound and πbound Isomers of the Phenol-Ar Cation. J. Mol. Struct. 2001, 563/564, 241−244. (75) 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. (76) Nakamura, T.; Miyazaki, M.; Weiler, M.; Schmies, M.; Dopfer, O.; Fujii, M. IR Spectroscopy of the 4-Aminobenzonitrile-Ar Cluster in the S0, S1 Neutral and D0 Cationic States. ChemPhysChem 2013, 14, 741−745. (77) 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. (78) 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 1405


Article

The Journal of Physical Chemistry B Complexes (L = Ar/N2, n ≤ 5). Phys. Chem. Chem. Phys. 2004, 6, 3801−3810. (79) 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. (80) Takeda, A.; Andrei, H. S.; Miyazaki, M.; Ishiuchi, S. I.; Sakai, M.; Fujii, M.; Dopfer, O. IR Spectra of Phenol+−Krn Cluster Cations (n = 1,2): Evidence for Photoionization-Induced π → H Isomerization. Chem. Phys. Lett. 2007, 443, 227−231. (81) Patzer, A.; Langer, J.; Knorke, H.; Neitsch, H.; Dopfer, O.; Miyazaki, M.; Hattori, K.; Takeda, A.; Ishiuchi, S. I.; 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. (82) Schmies, M.; Patzer, A.; Schütz, M.; Miyazaki, M.; Fujii, M.; Dopfer, O. Microsolvation of the Acetanilide Cation (AA+) in a Nonpolar Solvent: IR Spectra of AA+-Ln clusters (L = He, Ar, N2; n ≤ 10). Phys. Chem. Chem. Phys. 2014, 16, 7980−7995. (83) Schmies, M.; Miyazaki, M.; Fujii, M.; Dopfer, O. Microhydrated Aromatic Cluster Cations: Binding Motifs of 4-Aminobenzonitrile(H2O)n Cluster Cations with n ≤ 4. J. Chem. Phys. 2014, 141, No. 214301. (84) Kleinermanns, K.; Janzen, C.; Spangenberg, D.; Gerhards, M. Infrared Spectroscopy of Resonantly Ionized (Phenol)(H2O)n+. J. Phys. Chem. A 1999, 103, 5232−5239. (85) Brutschy, B. Ion−Molecule Reactions within Molecular Clusters. Chem. Rev. 1992, 92, 1567−1587. (86) Ebata, T.; Fujii, A.; Mikami, N. Structures of Size-Selected Hydrogen-Bonded Phenol-(H2O)n clusters in S0, S1, and D0. Int. J. Mass Spectrom. Ion Processes 1996, 159, 111−124. (87) Miyazaki, M.; Fujii, A.; Ebata, T.; Mikami, N. Electronic Spectroscopy of Benzene−Water Cluster Cations, [C6H6−(H2O)n]+ (n = 1−6): Structural Changes Upon Photoionization and Proton Transfer Reactions. Chem. Phys. Lett. 2004, 399, 412−416. (88) Miyazaki, M.; Fujii, A.; Ebata, T.; Mikami, N. Infrared Spectroscopy of Hydrated Benzene-Water Cluster Cations, [C6H6− (H2O)n]+ (n = 1−4): Spectroscopic Evidence for Phenyl Radical Formation Through Size-Dependent Intracluster Proton Transfer Reactions. Phys. Chem. Chem. Phys. 2003, 5, 1137−1148. (89) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery , J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Rev. A.02; Gaussian: Wallingford, CT, USA, 2009. (90) Herzberg, G. Molecular Spectra and Molecular Structure. II. Infrared and Raman Spectra of Polyatomic Molecules; Krieger: Malabar, FL, USA, 1991. (91) Csaszar, A. G.; Czako, G.; Furtenbacher, T.; Tennyson, J.; Szalay, V.; Shirin, S. V.; Zobov, N. F.; Polyansky, O. L. On Equilibrium Structures of the Water Molecule. J. Chem. Phys. 2005, 122, No. 214305. (92) Goebbert, D. J.; Wenthold, P. G. Water Dimer Proton Affinity from the Kinetic Method: Dissociation Energy of the Water Dimer. Eur. J. Mass Spectrom. 2004, 10, 837−845. (93) Rocher-Casterline, B. E.; Ch’ng, L. C.; Mollner, A. K.; Reisler, H. Determination of the Bond Dissociation Energy (D0) of the Water Dimer, (H2O)2, by Velocity Map Imaging. J. Chem. Phys. 2011, 134, No. 211101.

(94) Wilson, E. B. The Normal Modes and Frequencies of Vibration of the Regular Plane Hexagon Model of the Benzene Molecule. Phys. Rev. 1934, 45, 706−714. (95) Alauddin, M.; Song, J. K.; Park, S. M. Infrared Photodissociation of Aniline(H2O)n+ (n=7−12) Clusters. Int. J. Mass Spectrom. 2012, 314, 49−56. (96) Belau, L.; Wilson, K. R.; Leone, S. R.; Ahmed, M. Vacuum Ultraviolet (VUV) Photoionization of Small Water Clusters. J. Phys. Chem. A 2007, 111, 10075−10083. (97) Tomoda, S.; Kimura, K. Ionization Energies and HydrogenBond Strength of the Water Clusters. Chem. Phys. Lett. 1983, 102, 560−564. (98) Fujii, A.; Ebata, T.; Mikami, N. Direct Observation of Weak Hydrogen Bonds in Microsolvated Phenol: Infrared Spectroscopy of OH Stretching Vibrations of Phenol−CO and −CO2 in S0 and D0. J. Phys. Chem. A 2002, 106, 10124−10129. (99) Nizkorodov, S. A.; Dopfer, O.; Meuwly, M.; Maier, J. P.; Bieske, E. J. Mid-Infrared Spectra of the Proton-Bound Complexes Nen-HCO+ (n=1,2). J. Chem. Phys. 1996, 105, 1770−1777. (100) Olkhov, R. V.; Dopfer, O. Spectroscopic and Ab Initio Studies of Ionic Hydrogen Bonds: The O-H Stretch Vibration of SiOH+-X Dimers (X=He, Ne, Ar, N2). Chem. Phys. Lett. 1999, 314, 215−222. (101) Solcà, N.; Dopfer, O. Microsolvation of the Phenol Cation (Ph+) in Nonpolar Environments: Infrared Spectra of Ph+-Ln (L = He, Ne, Ar, N2, CH4). J. Phys. Chem. A 2001, 105, 5637−5645. (102) Olkhov, R. V.; Nizkorodov, S. A.; Dopfer, O. Hindered Rotation in Ion-Neutral Molecular Complexes: The ν1 Vibration of H2-HCO+ and D2-DCO+. J. Chem. Phys. 1997, 107, 8229−8238. (103) Ullrich, S.; Tarczay, G.; Tong, X.; Dessent, C. E. H.; MüllerDethlefs, K. A ZEKE Photoelectron Spectroscopy and Ab Initio Study of the Cis- and Trans-Isomers of Formanilide: Characterizing the Cationic Amide Bond? Phys. Chem. Chem. Phys. 2001, 3, 5450−5458. (104) Mizuse, K.; Fujii, A. Infrared Photodissociation Spectroscopy of H+(H2O)6-Mm (M=Ne, Ar, Kr, Xe, H2, N2, and CH4): MessengerDependent Balance Between H3O+ and H5O2+ Core Isomers. Phys. Chem. Chem. Phys. 2011, 13, 7129−7135. (105) Wang, Y.-S.; Tsai, C.-H.; Lee, Y. T.; Chang, H.-C.; Jiang, J. C.; Asvany, O.; Schlemmer, S.; Gerlich, D. Investigation of Protonated and Deprotonted Water Clusters in a Low-Temperature 22-Pole Ion Trap. J. Phys. Chem. A 2003, 107, 4217−4225. (106) Hamashima, T.; Li, Y.-C.; Wu, M. C. H.; Mizuse, K.; Kobayashi, T.; Fujii, A.; Kuo, J.-L. Folding of the Hydrogen-Bonded Network of H+(CH3OH)7 with Rare Gas Tagging. J. Phys. Chem. A 2013, 117, 101−107. (107) Miyazaki, M.; Takeda, A.; Ishiuchi, S.; Sakai, M.; Dopfer, O.; Fujii, M. Photoionization-Induced Large-Amplitude Pendular Motion in Phenol+-Kr. Phys. Chem. Chem. Phys. 2011, 13, 2744−2747. (108) Ishiuchi, S. I.; Sakai, M.; Tsuchida, Y.; Takeda, A.; Kawashima, Y.; Dopfer, O.; Müller-Dethlefs, K.; Fujii, M. IR Signature of the Photoionization-Induced Hydrophobic → Hydrophilic Site Switching in Phenol-Arn Clusters. J. Chem. Phys. 2007, 127, No. 114307. (109) Ishiuchi, S.; Sakai, M.; Tsuchida, Y.; Takeda, A.; Kawashima, Y.; Fujii, M.; Dopfer, O.; Müller-Dethlefs, K. Real-Time Observation of Ionization-Induced Hydrophobic → Hydrophilic Switching. Angew. Chem., Int. Ed. 2005, 44, 6149−6151. (110) Hunter, E. P. L.; Lias, S. G. Evaluated Gas Phase Basicities and Proton Affinities of Molecules: An Update. J. Phys. Chem. Ref. Data 1998, 27, 413−656. (111) Goebbert, D. J.; Garand, E.; Wende, T.; Bergmann, R.; Meijer, G.; Asmis, K. R.; Neumark, D. M. Infrared Spectroscopy of the Microhydrated Nitrate Ions NO3−(H2O)1−6. J. Phys. Chem. A 2009, 113, 7584−7592. (112) Huisken, F.; Kaloudis, M.; Kulcke, A. Infrared Spectroscopy of Size-Selected Water Clusters. J. Chem. Phys. 1996, 104, 17−25. (113) Huang, Z. S.; Miller, R. E. High Resolution Near Infrared Spectroscopy of Water Dimer. J. Chem. Phys. 1989, 91, 6613−6631. (114) Kuyanov-Prozument, K.; Choi, M. Y.; Vilesov, A. F. Spectrum and Infrared Intensities of OH-Stretching Bands of Water Dimers. J. Chem. Phys. 2010, 132, No. 014304. 1406


Stepwise Microhydration of Aromatic Amide Cations: Formation of

Recommend Documents