High-Resolution Spectroscopic Studies of Complexes Formed by

Review pubs.acs.org/CR

High-Resolution Spectroscopic Studies of Complexes Formed by Medium-Size Organic Molecules Maurizio Becucci*,† and Sonia Melandri‡ †

Department of Chemistry “Ugo Schiff” and European Laboratory for Nonlinear Spectroscopy, University of Florence, Via della Lastruccia 3, 50019 Sesto Fiorentino, Firenze, Italy ‡ Department of Chemistry “Giacomo Ciamician”, University of Bologna, Via Francesco Selmi 2, 40126 Bologna, Italy ABSTRACT: A wealth of structural and dynamical information has been obtained in the last 30 years from the study of high-resolution spectra of molecular clusters generated in a cold supersonic expansion by means of highly resolved spectroscopic methods. The data obtained, generally lead to determination of the structures of stable conformations. In addition, in the case of weakly bound molecular complexes, it is usual to observe the effects of internal motions due to the shallowness of the potential energy surfaces involved and the flexibility of the systems. In the case of electronic excitation experiments, also the effect of electronic distribution changes on both equilibrium structures and internal motions becomes accessible. The structural and dynamical information that can be obtained by applying suitable theoretical models to the analysis of these unusually complex spectra allows the determination and understanding of the driving forces involved in formation of the molecular complex. In this way, many types of non-covalent interactions have been characterized, from pure van der Waals interactions in complexes of rare gases to moderate-strength and weak hydrogen bonds and to the most recent halogen bonds and n−π interactions. The aim of this review is to underline how the different experimental and theoretical methods converge in giving a detailed picture of weak interactions in small molecular adducts involving medium-size molecules. The conclusions regarding geometries and energies can contribute to understanding of the different driving forces involved in the dynamics of the processes and can be exploited in all fields of chemistry and biochemistry, from design of new materials with novel properties to rational design of drugs.

CONTENTS 1. Introduction 2. Intermolecular Forces and High-Resolution Spectroscopy 2.1. Structural Determination 2.2. Electric Dipole Moment 2.3. Excitonic Splitting 2.4. Proton Tunneling and Proton Transfer 2.5. Large-Amplitude Motions and Predissociation Dynamics 3. Complexes of Bioactive Molecules 4. Complexes of Chiral and Prochiral Molecules 4.1. Self-Recognition and Chiral Recognition 4.2. Transient Chirality 4.3. Solvation of Chiral Molecules 5. Toward Rational Design of New Materials and Drugs: A High-Resolution Spectroscopic View on Tuning of Molecular Properties 6. Conclusions Author Information Corresponding Author Author Contributions Notes Biographies Acknowledgments © 2016 American Chemical Society

Abbreviations References

5014 5016 5016 5017 5018 5019

5030 5030

1. INTRODUCTION The allure of high-resolution (HR) spectroscopy arises from the fact that the resulting spectra can be theoretically analyzed with a high degree of accuracy. Deduced parameters such as bond lengths, dipole moments, rotational constants, and quadrupole and centrifugal distortion parameters can be very well determined, both numerically and also in terms of their physical description. This “knowing a lot about very little things” is a rarity in science.1 However, the assignment of rotationally resolved spectra measured on samples at room temperature can be extremely difficult or even impossible for large molecular systems due to the high density of states. Thus, HR spectroscopy combined with jet-cooling techniques has proved essential for the study of such systems. Moreover, weakly bound molecular complexes can easily be isolated in the cold environment of a jet expansion. In this way, even if structure determination remains

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Special Issue: Noncovalent Interactions Received: August 31, 2015 Published: March 17, 2016 5014

DOI: 10.1021/acs.chemrev.5b00512 Chem. Rev. 2016, 116, 5014−5037

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different approaches. The optothermal spectroscopic technique was introduced at the University of Waterloo, Canada,22 and used to measure the energy content of the molecular beam after interaction with a narrow-line-width laser. After these pioneering studies, several groups have implemented different absorption techniques utilizing tunable diode lasers (TDL) for the infrared23−26 or far-infrared27,28 regions or Fourier transform infrared (FTIR) spectroscopy.29−31 All of them usually make use of slit expansions and multipass cells to enhance the intensity of the signal. Recent developments in this region concern combined instruments (such as the one developed in the Herman group in Bruxelles, Belgium, which presents FTIR, TDL, and mass spectrometer probes and also optional cavity ring-down facilities)32,33 or the use of synchrotron radiation as the infrared source.34 It must be noted that complete rotational resolution is reached in the farIR region for small molecular systems, the largest of which is the water octamer.35 In the IR region, the largest systems studied at high resolution are the class of substituted acetylene dimers33,36 while for larger systems such as oxirane hydrochloride only partial rotational resolution was achieved.34 In the UV−vis spectral range, most of the work on molecular clusters has been carried out by HR laser-induced fluorescence (LIF) experiments, due to the availability of suitable laser sources and the sensitivity of fluorescence detection. Rotationally resolved LIF experiments on jet-cooled molecules with narrow-band, widely tunable lasers were pioneered by Meerts and Majewski37,38 on both isolated molecules and van der Waals complexes. Frequency-stabilized dye lasers in combination with collimated molecular beams have enabled spectral resolution on the order of 10 MHz in the ultraviolet spectral region, limited by residual Doppler broadening. Improvements in molecular beam collimation allow for better spectral resolution (down to 1 MHz) with corresponding signal loss. Different groups have followed this approach, mostly focusing their work on simple aromatic molecules and their clusters.39−42 An alternative experimental approach was the rotational coherence method based on the use of ultrafast laser pulses.43,44 HR pulsed lasers were also combined with the technique of mass-selected resonance-enhanced two-photon ionization to measure rotationally resolved spectra of many clusters by Neusser and co-workers.45 The reported 1 MHz spectral resolution represents a physical limit for UV spectroscopy of aromatic molecules observed in molecular beams, given the natural lifetime of electronic transitions and residual Doppler broadening due to divergence of the molecular beam itself. Practical considerations, based on the available spectral resolution of UV spectroscopy and the spacing of rotational lines, suggest that this approach is suitable for the study of systems containing up to 20 heavy (C, N, O) atoms. In fact, the study of aromatic molecules’ dimers46−49 was shown to be a quite challenging matter due to spectral congestion of the rotational lines and the presence of vibrational hot bands or dynamical effects. Clearly MW spectroscopy experiments, with their inherent much higher absolute spectral resolution, are not limited by these physical boundaries. In this case both natural line width and Doppler broadening are much smaller due to the longer lifetime of the states involved and the linear dependency of the Doppler effect on excitation frequency, respectively. A high number of complexes involving biologically relevant molecules (adrenergic neurotransmitters,50 peptides,51 and carbohydrates52,53) has been studied by the group of Simons using LIF and UV−UV and UV−IR double-resonance

one of the main results of high-resolution studies, many other problems involving dynamics have been investigated in recent years: conformational and tautomeric equilibria, large-amplitude motions, intermolecular vibrational energy redistribution, and isomerization. Different molecular systems with multiple internal motions, larger complexes, aggregates, biomolecules, and transient species can be studied. The production of cold and controlled molecular samples has been very recently reviewed.2 In addition, during the past decade, the first developments regarding the cooling of molecules in traps have been presented, making it possible to design laser spectroscopy experiments at much higher spectral resolution in ultracold samples.3−5 High-resolution spectroscopic techniques have been developed in all frequency ranges, from the microwave (MW) region to the ultraviolet, and different optical experimental schemes, such as absorption, emission, and multiphoton processes, can be used. Pioneering work in the field of radio and MW spectroscopy applied to the study of weakly bound complexes has been performed by Klemperer and co-workers6 by means of molecular beam electric resonance spectroscopy. Extensive studies on a variety of different chemically significant small systems have been reported in important papers and reviews.7−10 A rather recent and thorough description of MW techniques and their applications can be found in reference texts.1,11 The molecular beam Fourier transform microwave (MB-FTMW) technique has been the most widely used since its introduction in 1981 by Balle and Flygare.12 It offers automation of cavity tuning13 and computer-controlled automatic recording, exploiting the coaxial arrangement of the MW and molecular beams, giving unprecedented resolution and sensitivity (coaxially oriented beam resonator arrangement (COBRA) in Hannover, Germany).14,15 The high sensitivity and resolution of FTMW (usually in the range up to 40 GHz) still remains unbeatable, but the narrow band feature of the pulsed technique constitutes a limit due to the long time required for scanning the spectral region. The advantage of using a Stark-modulated free jet absorption microwave (FJAMM) spectrometer at slightly higher frequencies (48−56 MHz) has been demonstrated at Monash University in Australia;16 the technique has been further developed and is still used by the microwave group in Bologna, Italy.17,18 Although the sensitivity and resolution of this kind of spectrometer are lower than FTMW, the broadband nature of the technique allows for a rather fast recording of the spectrum (typically days with FJAMM for tens of gigahertz, compared to weeks with FTMW). Recently, following the development of high-speed electronics, another breakthrough in MW spectroscopy has occurred: the chirp pulsed Fourier transform microwave (CP-FTMW) technique coupled to jet cooling was developed and demonstrated at the University of Virginia in the United States.19 To attain the highest sensitivity and ultra-broadband capabilities is economically very demanding, and several variations of this instrument have been proposed where “very” and not “ultra” broadband are achieved. One example is the surprisingly simple in-phase/quadrature-phase modulation passage-acquired coherence technique (IMPACT) which has been developed in Hannover and can achieve very high resolution, similar to that of the COBRA setup.20 Infrared spectroscopy of molecular clusters was reviewed by Nesbitt21 quite a few years ago. The techniques applied in this frequency range to study molecular complexes comprise 5015


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taken into account in this review is mostly related to the last 10 years. An effort is made to present to nonspecialist readers a broad view of the present research on molecular clusters based on high-resolution spectroscopic methods, together with a possible multifield approach to the problem. The first part of the review deals with information that can be obtained from high-resolution spectroscopic experiments, mostly focusing on the experimental observables: from the implications of structural determination to those that reach beyond. Subsequently, results obtained on specific classes of molecular clusters are presented (a complete overview of the extensive literature on complexes containing rare gas atoms is not given). The treated topics are limited to a summary of a few specific problems, emphasizing how these studies can be of general interest and their results can be transferred to different fields of research. For instance, complexes involving mediumsize molecules are relevant because they can be taken as models for the study of much larger molecular systems, comprising those biologically relevant or important for the development of new materials. Moreover, the study of larger homo- or heteroclusters can be considered as a step toward the understanding of bulk properties or solvation processes, respectively. The obtained accurate gas-phase data, with their many features and broad information content, may help to shed light on the interactions that are at the basis of the conformational and functional properties of larger systems.

techniques. In these experiments, only partial rotational resolution was generally obtained for the observed spectra, but the synergy between band contour analysis and independent electronic and vibrational information allowed for the assignment and detailed structural study of the observed conformers lying on the complex conformational potential energy surfaces. One of the limits of HR studies resides in the high density of populated states for larger molecules or complexes, which gives rise to high spectral density. It should be mentioned here that, together with the development of experimental techniques, which allowed for the study of larger and more complex systems, during the past decade several groups have worked on implementing methods for automatic assignment of rotationally resolved spectra. The graphical assignment of very complex spectra can be supported by software packages such as AABS, developed by Kisiel et al.,54 JB95, developed by Plusquellic,55,56 or PGopher, from the Bristol laser group.57 Proposed strategies for automatic assignment of spectra are based either on genetic algorithms, following Meerts and Schmitt,58 or on spectrum cross-correlation, as developed by Neusser and co-workers.59 Recently, prompted by the development of CP-FTMW spectroscopy, which allows the recording of rich and complex spectra, a new method utilizing a simple data analysis has been proposed by Pate and co-workers.60 This method, called AUTOFIT, is based on the search for a “triple” of transitions in the spectrum, which can fit an input of rotational constants and offers prediction-and-search of a new “triple”. This is iterated until a successful fit is reached. This method is particularly useful for the analysis of rich spectra where many species (such as different conformers or isotopologues) are present. In the case of highly resolved Rydberg spectra, automated computer analysis was proposed by Neusser and co-workers.61 These automatic methods can be particularly useful when one can use accurate ab initio predictions as input for the program, the interacting units are rigid, and the influence of centrifugal distortion is not very large. This might not be the case for weakly bound molecular complexes, where the presence of large amplitude motions can give rise to very complex spectra. Nevertheless, some of these methods have been successfully applied even for weakly bound systems.58 Several general reviews on HR studies of weakly bound molecular complexes have appeared: from the early ones of Legon62,63 on MW results and that of Nesbitt21 on infrared studies to those on MW, infrared, or UV results10,64,65 and also ionic clusters.66 More information on high-resolution studies of molecular adducts can be found in several chapters of two recently published books.1,11 Other reviews have been dedicated to specific processes highlighting the results obtained by HR studies of small adducts: the hydrogen bond,67,68 weak hydrogen bonds,69 or interaction with water molecules,70 while properties of larger clusters have been discussed mostly on the basis of low spectral resolution data or theoretical methods (see, for example, the special issue of Chemical Reviews published in 2000).71 In this review, we present and discuss studies of molecular adducts containing relatively large molecules (consisting of at least three heavy atoms) that generally present different possible sites of interaction. We also focus on spectroscopic signatures of these interactions and on molecular properties that can be derived from these HR spectroscopic studies. We will refer to studies based on different data sets ranging from purely rotational to electronic spectroscopy. The literature

2. INTERMOLECULAR FORCES AND HIGH-RESOLUTION SPECTROSCOPY 2.1. Structural Determination

Rotational spectroscopy is traditionally associated with the very accurate structural information that can be obtained from measured rotational constants. The observation of different isotopologues is essential for this purpose: either isotopically substituted samples are made or high-sensitivity methods like supersonic jet FTMW, which allow for the observation of several isotopologues in natural abundance, are used. Furthermore, rotational spectroscopy is the most accurate method for structure determination of gas-phase molecules and complexes. By use of very high resolution techniques, it is generally possible to resolve spectral fine (i.e. rotational) structures and hyperfine (i.e., nuclear quadrupole, spin− rotation, and spin−spin) couplings, which give some insight into the electronic structure of the molecular system. Quantum chemical methods can provide structural parameters with good accuracy when rather rigid molecules are considered,72 but when weak nonbonding interactions are present, either intra- or intermolecular, the results of ab initio or density functional calculations must be treated with caution. For weakly bound molecular complexes, the accuracy of theoretical calculations is still a highly debated subject, as can be seen from the number of reviews on the subject in this thematic issue. When several conformations close in energy are involved, precise structural information is needed to benchmark theoretical calculations, and indeed some recent rotational results focus on structure determination and on the information associated with the nature of weak interactions. For example, in an attempt to provide new insight into the intermolecular potential of water, which is one of the most thoroughly studied problems in chemistry, detailed and extensive structural analysis has been reported in recent papers. The water hexamer is 5016


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monomers, and (iii) calculation of the polarizability and quadrupole moments by ab initio methods, it was possible to evaluate the induced dipole moment upon cluster formation by using classical electrostatic formulas. From all this information combined in the above expression for the dipole moment of the complex, comparison of the calculated and experimental dipole moment of indole−H2O in both states was fully satisfactory without inclusion of any term related to charge transfer. In a later work, β-naphthol−NH3, an acid−base cluster, was studied.86 In this case, by a very similar approach, a small level electron transfer from ammonia to β-naphthol (∼0.09e in the ground state and 0.14e in the electronic excited state) was determined between the two adjacent heavy atoms. The overall consistency of the model was tested by calculation of the electronic stabilization energy of the complex relative to the bare molecules as

predicted to be the smallest water cluster for which a threedimensional structure is formed, but theoretical calculations performed with different methods and different basis sets do not agree on the relative stability of different conformations. The CP-FTMW rotational study performed on normal and enriched H218O species of the water hexamer73 allowed the three lowest-lying conformations to be identified by their oxygen frame: namely, the cage, book, and prism forms. Relative intensity measurements with different carrier gases, where the relaxation processes are different, allowed determination of the minimum-energy structure, which is the cage. The improved sensitivity of CP-FTMW reported in a later work74 has allowed determination of the full substitution structure of the water heptamer through analysis of all singly-substituted species (H218O; HOD) in enriched samples. In the case of weakly bound complexes, structural information can also reveal changes taking place within the individual moieties upon formation of the molecular adduct. Changes in monomer conformation upon complex formation were observed in aminoethanol−H2O (AE−H2O),75 aminoethanol−ammonia (AE−NH3),76 glycidol−water (GLD− H2O),77 and glycidol−ammonia (GLD−NH3).78 In other cases structural changes can be observed upon isotopic substitution: a typical example is shortening of the O···O distance upon H → D substitution, first observed by Ubbelohde and Gallagher79 in carboxylic acid dimers and then observed in many other experiments.80 Other examples can be found in the literature, such as that of the anisole−H2O dimer,81 where variation in the angles as large as 10° was observed upon passing from the normal to the water deuterated species, as discussed in section 2.4.

Ecompl,rel = Eμμ + Eαμ + ECT

where Eμμ represents purely dipolar interaction energy, Eαμ is the energy contribution due to polarization of β-naphthol by ammonia, and ECT is the interaction energy between the two calculated point charges. The difference in interaction energy between the ground and excited states corresponded well with the experimental shift of the electronic transition in the cluster with respect to that occurring in isolated β-naphthol. In the high-resolution electronic−Stark spectroscopic study of βnaphthol−H2O complex, two subbands produced by a tunneling motion of the water molecule were identified, in agreement with previous reports.87,88 The water molecule acts as a base, being hydrogen-bonded to β-naphthol, with its two H atoms lying in a plane perpendicular to that of β-naphthol. Tunneling of the water molecule occurs with a coupled torsion−inversion motion that exchanges the two H atoms, a motion with a barrier evaluated to be as large as 200 cm−1. Therefore, the simple static model of the dipole moment cannot be sufficient to describe the system. An accurate evaluation of the electric dipole modulation with electronic excitation can be obtained only if the internal dynamics of the cluster is properly taken into account. Finally, the indole−NH3 cluster was studied.89 The increase in strength of the hydrogen bond upon electronic excitation was observed through the decrease in intermolecular distance and increase in internal rotation barrier of ammonia. The measured value of the dipole moment is very large, even larger than that of the indole−H2O cluster, probably due to alignment of the dipole moments of the two units. Nevertheless, the vector composition of the dipole moments of the two units and polarization effects are not large enough to account for the experimental value of the dipole moment in the cluster. Therefore, a charge-transfer process is involved that was quantified as described above. However, electrostatic evaluation of the interaction energies in the two electronic states was not able to predict the observed frequency shift of the transition in the cluster with respect to the monomer. This was taken to be a signature for more significant covalent character of the hydrogen bond in this cluster. Stark measurements are not routinely carried out in MW spectroscopic experiments on molecular clusters. However, very recently an important series of papers was published on the benzene dimer, where Stark measurements were used both to determine the dipole moment of the cluster and to gain insights into its geometry.90−92 Although it was already known that the benzene dimer has a T-shaped conformation,93,94 the

2.2. Electric Dipole Moment

Measurement of the electric dipole moment in a molecular cluster gives direct access to the distribution of charges in the system. It has been shown how the dipole moment in bimolecular clusters can be evaluated as a result of the following (vector) equation: μab⃗ = μa⃗ + μ ⃗b + μind ⃗ + μCT ⃗

where μ⃗ a and μ⃗ b are related to the dipole moment of the two units in the corresponding states, μ⃗ind represents the induced dipole moment in the cluster due to the permanent dipole moment of the two units and their polarizability, and μ⃗CT is the dipole moment associated with the degree of charge transfer between the two units. The dipole moment in a molecular system can be determined by Stark spectroscopic experiments that measure the changes in line positions and intensities by effect of the electric field applied to the system. These changes depend on the values and orientations of both electric dipole moments (in both states involved in the transition), the transition dipole moment, and also the mutual orientation between excitation electric field polarization and the Stark electric field. Therefore, the most interesting results are obtained with Stark measurements applied to rotationally resolved spectroscopies, as in such experiments all the relevant molecular parameters can be directly determined. The Pratt and Neusser groups have presented a series of seminal papers on this subject. After the first studies on single-molecule properties,82−84 an analysis of the indole−H2O cluster was reported.85 Given (i) determination of the spatial arrangement of the two monomers in the cluster, (ii) measurement of the dipole moments of both 5017


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electronic coupling term (already present in the strong coupling model) is modulated by Franck−Condon factors for the relevant vibrational components of the transition. It can be shown99 that, in the case of strong coupling, the coupling term is purely electronic and is directly proportional to the transition dipole moment of the monomer unit:

Stark experiment provided a value of 0.58D value for its dipole moment, in good agreement with electrostatic calculations and ab initio modeling of the system.95 The different linear or quadratic shift of the K = 1 or K = 0 lines, respectively, as a function of the applied electric field, unambiguously demonstrated the symmetric-rotor character of the system, associated with nearly free rotation of the benzene ring located on top. In the study of β-propiolactone−H2O (BPL−H2O) clusters containing many (up to 5) water molecules, measurement of molecular dipole moments was also instrumental to support determination of the structures of different complexes.96

el V ab ∝ |μa |2

In the weak coupling regime, the coupling term can be calculated as el Vabvibr ∝ V ab |⟨χv ′(Q a)|χ0 (Q a)⟩|2

2.3. Excitonic Splitting

Excitonic splitting is a spectral feature observed in a few cases when dealing with the study of center-symmetric dimers formed by aromatic molecules. It concerns the nature of the electronic wave function in the excited state, and thus it is studied by electronic spectroscopy methods: in the case of two equivalent molecular systems bound in a center-symmetric cluster, the electronic systems of the two units can interact and the occurring coupling removes the degeneracy of the excitedstate levels. Hence, upon electronic excitation, two nonequivalent supramolecular excited states can be created. High(or intermediate-) resolution electronic spectroscopy is the technique of choice for studying these systems. Besides, on these dimers, where the symmetry is such that it leads to vanishing dipole moments, MW studies are not possible. Usually excitonic splitting is described in terms of weak or strong coupling of the electronic molecular wave functions, in the framework of exciton theory.97,98 In the strong coupling regime, where the resonance integral is large, the ground- and excited-state wave functions, ΨG,vw and Ψ±,vw, respectively, are expressed as

where the last term is the Franck−Condon factor for the electronic transition in the monomer unit. Earlier reports on excitonic coupling dealt with dimers formed by two equivalent aromatic units linked through a symmetric double hydrogen bond in a collinear arrangement. It is now recognized that excitonic splitting exists for all symmetric dimers studied so far [i.e., those formed by 7azaindole,100,101 2-pyridone (2PY),102,103 2-aminopyridine,104 benzoic acid,105−107 o-cyanophenol,99,108,109 and benzonitrile46,110,111]. The observed excitonic splittings range from 1 to ∼40 cm−1. Recently, excitonic splitting was reported for a dimer in a partial π-stacking configuration, the anisole dimer.48,112,113 In the case of hydrogen-bonded, coplanar, symmetric dimers, an intuitive explanation for the effectiveness of this coupling was presented. The electronic interaction was described as a consequence of coupling between the electronic wave functions mediated by a through-bond mechanism. According to this view, a simple through-space electronic coupling mechanism can be effective for the anisole dimer, given the much shorter distance and the partial overlap of the aromatic electronic system: no strong and specific (e.g., hydrogen bond) contact point is present between the two units, and thus the throughbond mechanism cannot be effective. In all these systems, effective coupling between the two units can be interpreted within the framework of the weak interaction regime. When the two molecules involved are completely equivalent (i.e., with even the same isotopic distribution on each atomic site) and coupling can be effective, two delocalized electronic excited states exist. Usually, the system symmetry renders only one of these states optically accessible for one-photon transitions originating from the ground state.48,100 It should be noted that the equivalence condition between the two units for excitonic coupling to occur is quite strict. Already in the case of isotopically nonequivalent dimers, symmetry is broken and it was clearly demonstrated that the difference in zero-point vibrational energy is usually large enough to effectively decouple the two molecular units: the origin bands of the two localized electronic excitations are both present and their relative frequency shift is strictly related to differences in the changes of vibrational energy with electronic excitation.112,114 Such information can be easily deduced from standard resonance-enhanced multiphoton ionization (REMPI) experiments at intermediate (∼1 cm−1) spectral resolution. In the case of the anisole dimer, from the observed frequency shift of the homodimer origin band of the electronic transition and the corresponding band of the same molecular unit in dimers with two units at different isotopic composition, it was possible to evaluate the magnitude of the effective perturbation term involved in the mixing of electronic wave functions to be as

ΨG,vw = ϕϕ χ (Q a) χw (Q b) a b v

Ψ±,vw =

1 [ϕ ′ϕ ± ϕϕ ′] χv ′(Q a) χw ′(Q b) a b 2 a b

where ϕi and χj are electronic and vibrational wavefunctions, respectively Qi are the normal coordinates; labels a and b refer to the different molecules and labels v and w refer to specific vibrational states involved, while the prime refers to electronic excited-state properties. It is clear that in this case the excited state corresponds to a complete mixing of excitations localized on both molecules. In the weak coupling case, the electronic excited state is instead described as Ψ±,vw = C1[ϕa′ϕb χv ′(Q a) χw (Q b)] + C2[ϕϕ ′χ (Q a) χw ′(Q b)] a b v

where C1 ≈ C2 ≈ 1/√2 for degenerate levels or C1 ≈ 1 and C2 ≈ 0 (or vice versa) for levels with energy separation much larger than the coupling. Quite recently, a more thorough discussion on the effective description of excitonic coupling in molecular dimers has been provided.99 First-order perturbation theory (within the Born− Oppenheimer approximation) recognizes strong dipolar interactions between the two units as responsible for the coupling. It was clearly demonstrated that this description overestimates the relevance of the process by 1 or 2 orders of magnitude, and only the weak coupling regime can deliver a quantitatively correct prediction of the excitonic splitting. The key factor is that in the weak coupling regime the purely 5018


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large as 14 cm−1.113 The large differences (∼2000 cm−1) in zero-point energies between the different isotopomers, according to perturbation theory, leads to a change in energy separation between the levels of the two units as small as 0.1 cm−1. A similar shift in frequency between the band centers cannot be studied systematically in a large series of isotopomers, as other factors (mostly anharmonicity) could possibly lead to larger random changes. Exceptions are represented by the 2PY dimer115 and the 7-azaindole dimer,116 where the spectroscopic sign of excitonic coupling (a single-origin vibronic band) was observed also for mixed dimers with one H/D isotopic substitution in one of the OH or NH groups directly involved in double and symmetric H-bond formation. Rotationally resolved electronic spectroscopy provides a stringent test of the effective coupling between two units. In the case of the 2PY dimer, the presence of different isotopic bands was resolved only with single-eigenstate−frequency-resolved experiments.115 In the study of the anisole dimer, all clusters with different deuterium substitution on the aromatic ring showed noncoupled excited states; however, complete excitonic coupling was revealed for the mixed dimer formed by one normal anisole molecule and one with full deuteration of the methyl group. In this case, the transition frequency of the two different homodimers is almost the same and the only origin band observed for the mixed cluster is still at the same frequency and exhibits a single set of rovibronic transitions, as revealed by rotationally resolved electronic spectroscopy. The relevant portion of the S1 ← S0 electronic excitation spectrum of the anisole dimer around the origin band is shown in Figure 1. This behavior can be explained only if we assume that the methyl group is completely decoupled from the electronic πelectron system, which is the chromophore group where the electronic excitation is localized.

2.4. Proton Tunneling and Proton Transfer

The level of detail in the HR rotationally resolved spectra is so high that subtle effects, such as the doubling of lines due to the existence of equivalent conformations for molecular clusters separated by high energy barriers, can be revealed. In this section we will focus on interconversion processes mediated by hydrogen atom exchange or the switching of hydrogen bonds. These are very fundamental interactions already studied in the condensed phase, where the presence of cyclic centersymmetric dimers in neat organic acids was demonstrated mainly on the basis of infrared vibrational absorption spectroscopy data. For instance, it was shown that, in condensed phase, benzoic acid is found as a dimer, formed by two units connected by a double hydrogen bond, in a coplanar arrangement with high binding energy (about 6000 cm−1).117,118 Other kinds of atomic displacements can lead to different molecular/cluster conformations characterized by an equivalent energy. This is the case of internal rotations, pseudorotation in five-membered aliphatic rings, or interconversion of T-shaped homodimers, such as (HF)2119 or (HCCH)2.120 All these processes have been extensively studied in MW or rovibrational spectroscopic experiments; hence, herein we will concentrate on larger systems. Gas-phase complexes formed by carboxylic acids have been studied for many years (some of these molecules and the formic acid dimer are shown in Figure 2). The first reports in

Figure 2. Structures of (a) formic acid, (b) acetic acid, (c) trifluoroacetic acid, (d) benzoic acid, and (e) formic acid dimer.

the 1960s concerned complexes formed by trifluoroacetic acid with the simplest aliphatic carboxylic acids.121,122 Since then, the planar configuration with a double hydrogen bond forming an eight-atom cyclic structure has been established. Much more recent is the detailed description of nuclear dynamics due to hydrogen exchange or internal rotations. The first carboxylic acid dimer studied in great detail in gas-phase experiments was the benzoic acid dimer.105 Molecular beam−rotationally resolved electronic excitation experiments were able to provide the structure of the complex and a measure of the change in tunneling splitting due to synchronous double-proton transfer with electronic excitation (about 1100 MHz). Later studies based on isotopically substituted molecules and including vibrationally excited bands in the S1 state were able to set absolute values of the tunneling splitting in both states: Δ(S0) = 1385.2(0.7) MHz and Δ(S1) = 271.2(0.7) MHz.106 This leads to determination of barrier heights for proton transfer by use of a semiclassical model and ab initio estimations for both the

Figure 1. Resonance-enhanced multiphoton ionization (REMPI) spectra of anisole dimers with different isotopic compositions in the region of the S1 ← S0 origin band. Different anisole isotopomers are indicated as H8 (no deuterium), D3 (full D/H exchange on the methyl group), D5 (full D/H exchange on the aromatic ring), and DOP (H/D exchange on the ortho and para positions of the aromatic ring). (a−c) Dimers containing (a) H8, (b) DOP, and (c) D5 bound to D5 (top, black), DOP (middle, blue), or H8 (bottom, red). (d) Dimers D3−D3 (top, black), H8−D3 (middle, blue), and H8−H8 (bottom, red). Adapted with permission from ref 113. Copyright 2011 Elsevier. 5019


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frequency of the involved vibration and the equilibrium structure.123 In this way, the barrier height was calculated to be 6224 cm−1 in the ground state and 6672 cm−1 in the S1 state. An intriguing issue is related to the excited-state properties of this complex. HR-LIF data indicate that it is not centersymmetric in the excited state, with in-plane tilting possibly associated with different lengths of the hydrogen bonds. This result suggests that the two units in the excited cluster are not equivalent. However, recent work by Ottiger and Leutwyler107 was able to show the existence of excitonic coupling between the two units with complete delocalization of the excitation. If excitonic splitting and proton tunneling splitting are converted, in a semiclassical view, to the corresponding time characteristics of the two processes, it can be shown that the exciton hopping frequency is about 40 times higher than that of proton transfer by tunneling. Thus, proton transfer effectively senses a symmetric double minimum potential. Another reference example for proton-transfer studies is that of the formic acid dimer (see Figure 2). Early studies by electron diffraction were able to establish its planar geometry with a center-symmetric, double-hydrogen-bond arrangement.124 High-resolution infrared spectroscopy work by Havenith and co-workers25 allowed determination of the tunneling splitting for ground and CO stretching vibrationally excited state levels to be 474(12) and 300(9) MHz, respectively. These results were also confirmed by more recent HR IR absorption experiments.125 This simple organic acid dimer has been taken as a reference system for many accurate calculations on the proton-transfer exchange process. This specific subject has already been thoroughly reviewed.68 More recently, some mixed organic acid clusters have also been able to be studied in great detail by MW spectroscopy, due to the lack of a center of symmetry of the adducts and their small permanent dipole moment. A particularly important study is that on the formic acid−acetic acid complex, a system exhibiting two tunneling motions: one related to the double proton transfer, the other to the internal rotation of the methyl group.126 It is noteworthy that the barrier height for methyl internal rotation is found to be about 110 cm−1, much smaller than for isolated acetic acid (165 cm−1), a signature of the more symmetrical distribution of charge around the adjacently bonded carbon. The barrier for proton exchange is predicted to be large (∼8000 cm−1), in line with previous determinations for similar systems. Recently Caminati and co-workers127,128 published a few papers on different dimers of carboxylic acids where, on the basis of MW spectroscopy data, proton-transfer dynamics were addressed. An alternative approach to describe the motion was used,129 and much lower values for the proton exchange barrier were determined (around 2500 cm−1), in very good agreement with ab initio data. Similar results were obtained also for other classes of molecular complexes. An example is the adduct formed in the gas phase by 2PY−2-hydroxypyridine (2HP).130 The two molecules interconvert by simple exchange of two protons, in a process very similar to the exchange observed in dimers of carboxylic acids. In homodimers of 2PY or 2HP, this exchange process is still possible but the chemical nature of the cluster would change drastically upon proton exchange and the potential energy surface for this motion will be very asymmetric. The structures of the three adducts are shown in Figure 3. The observed tunneling splitting of 520 MHz in the electronic excitation spectrum of the 2PY−2HP cluster is

Figure 3. Scheme of possible adducts of 2PY−2HP. Adapted with permission from ref 130. Copyright 2002 Elsevier.

attributed to the double proton transfer, and different models were presented in order to evaluate the energy-level splitting in both ground and excited states.130−132 The study of this and similar clusters has received much attention because they represent prototypical examples of base-pair analogues, as discussed in section 3.133,134 Exchange of equivalent hydrogen atoms takes place also in water-containing clusters. The water molecule in the anisole− H2O cluster is hydrogen-bonded to the oxygen atom on anisole, thus acting as an acid (see Figure 4).81,135−137 The

Figure 4. Structure of anisole−H2O complex and its change upon water deuteration. Adapted with permission from ref 138. Copyright 2010 PCCP Owner Societies.

minimum energy configurations are characterized by the O− H···O atoms in plane with respect to the aromatic ring, while the second H atom from water is positioned (equivalently) above or below the aromatic ring plane. The bifurcated structure, with Cs symmetry, is a transition state connecting two equivalent configurations of the cluster that can interchange upon rotation of the water molecule. This description is supported by the doubling of the rotational lines (∼730 MHz splitting) with 3:1 intensity ratio due to nuclear spin statistics. Another peculiar feature of this system is the large conformational change upon H/D isotopic exchange in the water molecule. This effect is related to the presence of a secondary interaction between the water oxygen atom and either aromatic CH or methyl CH groups: the change in O−H/D distance affects the relative stability of these two possible interactions and a large angular, in-plane displacement of water is observed with H/D isotopic substitution in water (see Figure 4). Another factor that must be taken into account is the water bending motion, which causes the water molecule to move closer to the methoxy group when heavy hydrogen is used; this occurs on a potential energy surface that is extremely flat and 5020


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2.5. Large-Amplitude Motions and Predissociation Dynamics

asymmetric. Thus, the different energy of ground vibrational levels for the two isotopomers leads to a different effective position along that coordinate.137 The opposite geometrical change (water moving closer to the methoxy group) occurs with electronic excitation. The electronic lone pairs at the methoxy group are more involved in the π-electronic system (n−π* electronic transition) and less available for ordinary hydrogen bonding, as revealed by the blue shift of the electronic transition for the complex with respect to the monomer and the larger intermolecular distance in the excited state. Secondary interaction at the methoxy becomes more important and the potential curve for the bending becomes more harmonic, causing the effective positions of the two isotopomers to become almost equivalent in the S1 state.137 Finally, detailed calculations have been made on the mechanism for interchange of equivalent H atoms, and the role of a tunneling mechanism was demonstrated by negligible splitting observed in the presence of D2O.81,137 The situation present in the 1,2-dimethoxybenzene−H2O complex is slightly different.139 The chromophore molecule is planar with the two methoxy groups oriented in a trans configuration, pointing away from each other. The bound water molecule acts as an acid with its protons pointing to the lone pair of the methoxy groups. The plane of the water molecule is tilted at approximately 45° with respect to the aromatic ring plane as shown in Figure 5. In this configuration, the water

Large-amplitude vibrational motions are a general characteristic of molecular clusters because of the shallow potential energy surface (PES) of the intermolecular bonds. Many studies have highlighted their importance in systems bound by either strong or weak interactions.69,138,142 Proton tunneling and proton transfer have been summarized in section 2.4. A more detailed discussion on the dynamics of clusters containing water hydrogen-bonded to different molecular moieties has been recently published.138 Tunneling effects are extremely important also in complexes where weak hydrogen bonds are present, such as those formed by the CF3H moiety, where the strong polarizing effect of fluorine atoms determines the binding properties of the molecule. In the case of clusters with a single contact point, such as CF3H−benzene143 or CF3H−NH3,144 interaction of the “activated” sp3 CH with aromatic electron density or n electrons stabilizes the cluster. MW spectroscopy shows that the two units are free to rotate with respect to each other about the interaction coordinate and that this leads to observation of spectra corresponding to those of a symmetric top because of the averaging in atomic coordinates due to internal rotation. Similar results were obtained in the case of the pyridine−CF3H dimer, where the principal interaction coordinate is that of sp3 CH with the electron lone pair of the pyridine aromatic nitrogen atom.145 A secondary interaction occurs between a fluorine atom from CF3H and an aromatic CH group of pyridine in position 2. However, the secondary interaction is rather weak and, in the rotational spectra, the signature of internal rotation of CF3H about its principal inertia axis is present. Three contact points are involved in stabilization of the CF3H−CH3F complex, where each fluorine atom of CF3H points toward an hydrogen atom of CH3F.146 As a result of the availability of multiple equivalent contact points, the two units can easily rotate with respect to each other, with the dynamics being controlled by the different inertia of the two rotors: the lighter CH3F seems to rotate freely, while the rotation of the heavier CF3H seems to be hindered. These dynamic properties result in an extremely large value of the A rotational constant of the cluster. A similar result, related to almost free rotation of the CF3 group, has been very recently reported, also for the CClF3−CH2O cluster, by MW spectroscopy.147 Other examples where the effects of almost free rotation of a part of the cluster can be observed in the rotational spectrum are those where CF4 is bound to the oxygen of water148 or the nitrogen atom of pyridine149 as a rotating cap and in terbutylalcohol−NH3150 and chlorotrifluoroethylene−NH3,151 where the ammonia molecule rotates freely about a single O− H···N and π···N interaction, respectively. The dimer quinuclidine−CF3H152 shows free internal rotation of the CF3 group with respect to the C−H···N bond. Free rotation has been observed also in halogen-bonded adducts such as CF3Cl−H2O,153 CF3Cl−dimethyl ether,154 and CF3Cl− CH3F.155 As cited, hindered rotation in complexes is usually observed even when more than one bond is present in the dimer. Examples include CF3H−CO2,156 GLD−NH3,78 AE−NH3,76 pyridine−CH3F,157 and ClH2F−formaldehyde.158 In some cases the combined effect of two different motions can be observed, such as in ethanol−NH3159 or CF3Cl−H2CO.147 From the splitting observed in the rotational spectrum, the barrier height for the motion can generally be inferred or

Figure 5. Structure of 1,2-dimethoxybenzene (left) and the two equivalent conformations of its adduct with water (right). Adapted with permission from ref 139. Copyright 2005 American Chemical Society.

molecule can flip so that the two hydrogen atoms will interact with the other oxygen atom in a new and identical arrangement. The hindered internal motion of the water generates a doubling of levels and a splitting of the rotational lines that gives rise to two subbands in a 3:1 intensity ratio separated by 0.04 cm−1. The barrier to internal rotation is ∼140(50) cm−1 in both ground and S1 electronic states. Line splitting with 3:1 intensity ratio is observed also in rotationally resolved spectra of the 1,4-difluorobenzene−H2O complex.140,141 Once again water acts primarily as an acid, being hydrogen-bonded to one of the fluorine atoms. A secondary interaction occurs between the water oxygen atom and the aromatic H atom adjacent to fluorine. In this system, the exchange of the two water hydrogen atoms is possible through different mechanisms. The most likely motion involved may be visualized as a combination of switching of the fluorine electronic lone pair for hydrogen bonding and internal rotation of water. The effective barrier for this combined motion was evaluated to be 360 cm−1 in the ground state and 230 cm−1 in the S1 excited state. The change in barrier height with electronic excitation is related to electronic charge redistribution that follows electronic excitation, a process that considerably reduces the electron density at the fluorine atom. 5021


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direct access to the vibrationally averaged structure of the complex, which reflects the effects of large-amplitude motions involved in the dissociation process. Moreover, the line width is related to the lifetime of the excited state, and the frequency of the transition provides information on the predissociation dynamics.167 Study of the aniline−neon cluster has demonstrated that there is a correlation between all these different parameters.168−171 The aniline−neon complex is able to form principally as a consequence of dispersion interactions between the neon atom and the aromatic system, with partial involvement of the electron lone pair at the nitrogen atom. It is noteworthy that the experiments enabled clusters containing either 20Ne or 22Ne to be distinguished. The two isotopes have a natural abundance ratio 20Ne:22Ne of 10:1, but the measured intensity ratios of the cluster spectra exhibit a 4:1 intensity ratio. The difference compared to the natural isotopic distribution can be attributed to different zero-point energies that, because of the small binding energy, favor stabilization of the heavier cluster. The system was characterized by studying different vibronic levels at rotational resolution in LIF experiments. Measured properties were compared to those of isolated aniline in the corresponding states.40,172,173 Results have also been discussed vis-à-vis dispersed fluorescence data in order to compare the fluorescence branching ratios on different product channels with the excited-state line widths.169 The aniline−neon system was studied in different vibronic levels, with energies up to 1200 cm−1 above the S1−S0 origin band. Most of the involved excited-state vibrations were related to in-plane deformation of the aniline frame (elongation of C− N bond, aromatic ring breathing, etc.). Excitation to the I2 state has a different interpretation, which involves two quanta in the amino group inversion vibration; during this out-of-plane motion, hybridization of the nitrogen atom changes from sp3 to sp2 as the molecule passes through the planar transition geometry that interconnects the two equivalent bent configurations. Clearly, this motion is much more anharmonic than any other in-plane vibration. It is possibly strongly coupled to the van der Waals coordinates due to modulation of the lone-pair electron density at the amino group resulting from the inversion vibration; the importance of this electron density for the cluster stabilization was discussed previously. The anharmonicity of the different vibrational states was evaluated by ab initio calculation.171 The dynamics of the aniline−neon complex in the excited states has been interpreted as being due to the presence of different relaxation channels, that is, those already available to the bare aniline (e.g., couplings between the different electronic states) and new ones associated with van der Waals vibrational degrees of freedom (e.g., vibrational predissociation). In rotationally resolved LIF spectra of aniline in different vibronic states, the measured line width was always the same (18 MHz), indicating that no new relaxation processes became active with the changes in available vibrational energy. The same line width was measured also in a high-resolution LIF spectrum of the aniline−neon origin band of the electronic transition, where an almost perfectly linear increase in line width as a function of available vibrational energy in the S1 state was observed. Only when the amino group inversion is excited does the line width suddenly show an increase (∼140 MHz vs 40 MHz measured for other states with comparable energy in other vibrational modes). Predissociation is responsible for the observed dynamics. This has been verified by analyzing large-amplitude

determined, and it can be related to the strength of the nonbonding interaction. For example, in the case of GLD− NH378 (see Figure 6), rotation of the NH3 moiety occurs about

Figure 6. Internal rotation of NH3 moiety in GLD−NH3.

its C3 axis; thus the V3 barrier determined for this motion represents the energy of the N−H···O interaction that must be overcome to reach the transition state of the motion. The effect of the hindered motion can also cause switching of the intermolecular bond. This kind of motion was already cited for proton tunneling, but there are other examples including heavier atoms, such as dimethyl ether−HCl160 and dimethyl ether−CS2,161 (where the hydrogen-bond moiety switches from one lone pair of dimethyl ether to the other) or CH2F2− CO2162 (where CH2F2 rotates, switching the H-bond from one oxygen atom to the other). The shape of the potential energy surface on which the largeamplitude motion takes place can be derived by calculations that are based on different approaches. Simple semiempirical models or ab initio calculations can be used to directly evaluate PES properties. The spacing of vibrational levels corresponding to hindered motions and the rotational constants on these levels can be used to refine empirical PES models by use of , for example, numerical methods as in the case of Meyer’s flexible model.129 A recent example and a partial review of this kind of application was presented by Evangelisti and Caminati,138 who applied flexible model analysis to the dimer of terbutylalcohol− H2O to determine the barrier for internal rotation of water. A two-dimensional example is the detailed analysis of the PES related to the weak hydrogen bond in phenol−H2O, based on ab initio data and flexible model calculation used to reproduce the observed MW experimental results.163 Binding energy for clusters bound along the a inertial axis can be derived from the centrifugal distortion constants by use of the so-called pseudodiatomic approximation and the assumption of a Lennard-Jones potential.164,165 If the intermolecular stretching coordinate is not coupled to other motions, the determined binding energy is in good agreement with electrostatic model calculations and accurate ab initio calculations.166 Rotationally resolved spectroscopy allows insight into the predissociation dynamics of van der Waals complexes in the excited states. In fact, measured rotational constants provide 5022


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generated by rotatable bonds and axes and the different shapes correspond to structures found at the PES minima. In general, the conformational landscape of biomolecules is governed by a delicate balance between intramolecular interactions, occurring through space between different parts of the molecule, and intermolecular interactions between the molecule and its surroundings. The role of water in biological processes is essential; thus the majority of high-resolution spectroscopic studies of complexes with bioactive molecules are those in which the effects produced by sequential addition of water (microsolvation) are studied. The questions usually addressed are: which is the preferred binding site, which type of nonbonding interactions are established, and whether any conformational change takes place in the monomers upon complexation. Answers to these questions allow insight into the solvation process at the molecular level, bridging the gap between gas-phase properties and those of aqueous solutions. A very important example is, of course, that of amino acids and peptides, which generally display extremely high flexibility of the backbone chain. Their structures are stabilized by intramolecular hydrogen bonds occurring between the amine and carboxyl groups or any other functional group on the amino acid side chain. The solvation process interferes with intramolecular interactions, weakening intramolecular bonds and triggering the transition from neutral forms found in neutral isolated molecules181 to charged zwitterionic forms present in solution.182,183 Moreover, different conformations provide more than one possible binding site for water molecules. The first high-resolution spectroscopic studies on molecular complexes of peptide bond model systems analyzed only 1:1 adducts involving a single water molecule, such as formamide− H2O (FA−H2O)184 or alaninamide−H2O (ALAm−H2O).185 In both of these studies, the most stable conformation of the 1:1 adduct was observed when the water moiety acts both as a proton donor to the oxygen atom of the carbonyl group and as a proton acceptor from the amino group, forming a sixmembered closed structure. In a subsequent study, two higherenergy conformations of FA−H2O (ΔEMP2/6311++G** greater than 10 kJ·mol−1) were observed.186 In these conformations the water molecule was bound either to the carbonyl or to the amino group of FA but not to both groups, such as in the most stable form of the dimer. In the same work, the most stable conformation of FA−(H2O)2 was also observed. In this case, the second water molecule inserts itself in the six-membered cycle and forms an eight-membered ring-like structure stabilized by a chain of hydrogen bonds. The conformational situation becomes more complicated when different substituents are present on the side chains. First, the monomer can have different stable conformations that interact differently with water. This is the case observed for Nmethylformamide−H2O (NMF−H2O),187 where different cis and trans species of the monomer are present and give rise to different forms of the complex. On the other hand, water can assume different positions with respect to the other moiety. This latter effect was already observed in FA−H2O,186 where three different arrangements of water were found, but also in trans-NMF−H2O and 2-azetidone−H2O,188 where different kinds of hydrogen bonds (also some weak C−H···O ones) were found to stabilize different structures of the complexes. The solvation of amino acids studied at a molecular level was investigated for the first time for glycine−H2O189 (GLY−H2O) by a combination of Fourier transform microwave spectroscopy

motion of the neon atom. The effective position of the neon atom with respect to the aniline frame can be easily calculated if rotational constants of both the cluster and isolated aniline in the corresponding state are known, under the assumption that the aniline structure is not changed upon complex formation. The inertia tensor of the system is modeled by the approach of Kraitchman,174 and it is possible to determine the effective position of the neon atom with respect to the aniline frame. As these effective coordinates are derived from moments of inertia of the cluster (through measured rotational constants), they can be taken as root-mean-square (RMS) values that are related to both equilibrium positions ⟨X⟩ and mean displacements ⟨ΔX⟩: ⟨X2⟩1/2 = (⟨X ⟩2 + ⟨ΔX ⟩2 )1/2

Disentanglement of the two contributions is possible only for the coordinate describing displacement of the neon atom out of the symmetry plane of the complex; in this case the average value of the coordinate is 0, and a direct measure of mean displacement of the neon atom is possible. The amplitude of this motion is 0.25 Å in the ground state and ranges from 0.20 (ground S1 state) up to 0.30 Å in different vibronic levels, but in the I2 S1 state it is ∼0.6 Å as a consequence of energy transfer from internal to van der Waals coordinates. A direct correlation has been demonstrated between changes in line widths and amplitude of the van der Waals motion. Furthermore, as suggested in a previous work,167 the square of the vibrational shift for different vibronic bands (that is, the shift of bands of the complex with respect to those of the chromophore) is approximately proportional to the predissociation rate.169 A final remark should be made regarding the behavior observed for both isotopomers of the complex, which is fully equivalent in all the different states with a single remarkable difference; in case of excitation to the S1 121 state, the aniline−20Ne complex shows a much larger line width for a few rotational lines, corresponding to levels with principal quantum number J′ = 0, 1, or 2.170 This behavior was attributed to an occasional resonance occurring for these specific quantum states. This occurrence definitively demonstrates the predissociative nature of relaxation dynamics occurring in all other cases of the complex. A wealth of detail on the properties of van der Waals complexes formed by aromatic molecules and rare gas atoms was provided by Neusser and Krause175 using high-spectral resolution photoionization experiments, which have been reviewed several years ago. The structure of clusters containing one or two rare gas atoms was elucidated and details on important interactions were obtained.176−178 For instance, in the case of phenylacetylene−argon, the argon atom was displaced toward the acetylenic system, suggesting direct involvement of the acetylene π-electrons in stabilization of the system.177 Furthermore, mass-analyzed threshold ionization (MATI) experiments provided precise determination of binding energies for different benzene−rare gas atom clusters.179 The hydrogen-like nature of the Rydberg states in van der Waals clusters was also demonstrated.180

3. COMPLEXES OF BIOACTIVE MOLECULES The selectivity and function of biomolecules is influenced by their shape. When flexible and complex molecular systems are studied, the word “shape” is a concept that must be related to the complicated conformational potential energy surface (PES) 5023


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population measurements in the gas phase.196 In contrast, in the first MW194 and LIF195 reports on the hydrated complex of 2HP−2PY only 2PY−H2O was observed, in agreement with the calculated greater stability of this form with respect to the hydrated enol tautomer. The identification of the hydrated enol form of 2HP in a subsequent study197 allowed determination of the population ratio 2HP−H2O/2PY−H2O as 0.6 ± 0.2. In the same study, the dihydrated pyridone tautomer was also observed, and the calculated binding energy gap between hydrated keto and enol forms was found to widen upon passage from mono- to dihydrated complex. Structural differences of mono- and dihydrated adducts of 2PY with respect to bare 2PY have been identified by rotationally resolved LIF spectra of several isotopic species.195 Overall, the data showed that the structure of the solvated molecule undergoes substantial changes, which can be related to an increase in zwitterionic character of the 2PY moiety, while the geometry of the water molecule remains basically unchanged. The interpretation of these results is fully confirmed by a subsequent study,198 where the geometry changes are discussed in terms of ab initio calculations. A general discussion on the complexes of 2PY with water or ammonia and its dimer can be found in the review by Pratt.39 Another class of small bioactive molecules that show a high degree of flexibility is that of the adrenergic neurotransmitters. This class includes compounds such as adrenaline, noradrenaline, dopamine, serotonin, and also synthetic drugs such as ephedrine. Many of these flexible bioactive compounds and some of their complexes with water have been studied by Robertson and Simons199 and Neusser and co-workers.200 In general, the conformational landscapes of the molecules give rise to many stable conformations, the most stable of which are folded ones showing intramolecular hydrogen bonds.199 In several cases the water molecule is attached to a single conformation, either inserting itself in the intramolecular hydrogen bond (insertion complex) or on the outer part of the structural isomer, giving rise to an addition complex.201 A rotational spectroscopy study regards phenylethylamine (PEA), which can be considered the prototype molecule for studying the conformational landscape of this class of compounds. In the two lowest-energy conformers of PEA, the ethylaminic side chain is folded and stabilized by a weak intramolecular N−H···π interaction, while in the three highestenergy forms, the side chain is extended and the aminic group is found in the anti position with respect to the ring. In a free jet absorption microwave spectroscopic study of the 1:1 adduct between PEA and water (PEA−H2O),202 it was found that the water molecule approaches PEA, creating an O−H···N bond with the aminic group and forming a secondary interaction between the oxygen atom and a C−H group on the ring (see Figure 8). In this case, the intramolecular weak N−H···π interaction remains untouched and the water binds externally to the folded monomer conformer of PEA. More interestingly, it is clear from theoretical calculations that the energy sequence for different conformations changes upon going from the monomer to the water complex. In particular, it is shown that, of the two almost isoenergetic folded conformations of the PEA monomer, the global minimum structure is much more stabilized with respect to the second conformation when PEA is bound to water. Water thus helps to “lock” different stable arrangements into a preferred conformation (in this case the global minimum). The preference for one conformation in a complicated conformational space upon microsolvation was

and laser ablation (LA-FTMW). In the isolated GLY monomer, two stable forms were detected by MW spectroscopy.190,191 In the global minimum structure, the oxygen and hydroxyl group are in the cis position while the amine group straddles the oxygen of the carboxyl group, giving rise to a bifurcated hydrogen bond. In the second, higher-energy conformation, the hydroxyl group acts as a proton donor to the lone pair of the aminic group. A third stable conformation similar to the previous one but with only one N−H···O could not be observed, probably due to relaxation processes in the jet. Upon interaction with water, the energy order of the GLY monomer conformations is retained, and the observed structure of the complex189 originates from the most stable structure of the monomer with the water moiety bound to the carbonyl terminus, thus retaining the intramolecular hydrogen bond. From the structure determined for GLY−H2O, it can be seen that water forms a network of intermolecular bonds where two new hydrogen bonds are formed between water and GLY. The O−H···Ow bond length and angle (1.806 Å, 161.8°) are shorter and closer to linearity than those for O−Hw···O (2.073 Å, 112.4°), indicating the weaker nature of the hydrogen bonds formed by water with respect to those formed by the acidic carboxylic group. In another study on hydrated GLY, it was found that, in the dihydrated complex of glycine [GLY−(H2O)2],192 the glycine monomer still retains its most stable arrangement and remains in the neutral form with two water molecules linked to the carboxyl terminus, forming an eight-membered ring structure. Thus, GLY does not change from its neutral form or its intramolecularly blocked geometry upon complexation with either one or two water molecules, as shown in Figure 7. Very

Figure 7. Most stable conformation of GLY−(H2O)2. Adapted with permission from ref 192. Copyright 2013 Royal Society of Chemistry.

similar results have been obtained in a more recent LA-FTMW study on mono- and dihydrated complexes of alanine (ALA),193 where one of the aliphatic hydrogen atoms is substituted by a methyl group. These results on aminoacids are different from those obtained for other hydrated complexes where the intramolecular bonds of the monomers are weaker. As already noted in the Introduction, changes in the monomer conformation upon complexation were observed for AE− H2O,75 AE−NH3,76 GLD−H2O,77 and GLD−NH3.78 A dramatic effect, caused by complexation with water, is observed in the relative population of the two tautomers 2hydroxypyridine (2HP) and 2-pyridone (2PY) upon formation of 1:1 adducts with water.194,195 The 2HP/2PY system is considered to be the prototype system for the study of keto− enol tautomerism, such as that present in nucleobases. Concerning the monomers, the enol form is more stable by 3.2(4) kJ·mol−1 as determined experimentally by relative 5024


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Figure 8. Most stable conformation of PEA−H2O complex.

Figure 9. Different water complexes with BPL. Adapted with permission from ref 96. Copyright 2014 John Wiley & Sons, Inc.

already observed by Schmitt et al.203 in a rotationally resolved LIF study of the water complex of tryptamine. Additional HR spectroscopic studies on the binding of water to bioactive molecules involve an anesthetic (isoflurane− H2O),204 a hydrated model system for antibiotics (Vince lactam−H2O),205 methyl lactate−H2O,206 and methyl lactate− ammonia.207 On the basis of these studies, it is clear that as the complexity of the monomers increases, it is even more interesting to probe the different binding sites, which reveals that water shows highly specific binding-site selectivity. In the rotational study on methyl lactate−ammonia,207 determination of the nitrogen nuclear quadrupole coupling constant shows the presence of a small charge transfer upon complexation with water. Cooperativity and analysis of water−water and water−solute interactions is the focus of a study on β-propiolactone−H2O complexes [BPL−(H2O)n, n = 1−5).96 The monomer itself is rigid and offers different binding sites for water, making it a good candidate to understand how a molecule forms its hydration shell. The structures found for clusters of BPL with up to five molecules of water can be compared to the results of high-resolution studies on water clusters.73,208,209 Detailed structural results, derived from analysis of all heavy-atom isotopologues detected in natural abundance, show that for each complex the water molecules form a hydrogen-bonded network that interacts at multiple sites with the BPL molecule, with the Ow−H···O carbonyl hydrogen bond being the dominant interaction. Other interactions, such as C−H···Ow and a Burgi−Dunitz n−π* interaction between the water oxygen lone pair and the carbonyl group of BPL, are also present. For n > 3, it is seen that water molecules interact among themselves, forming cycles that then will bind to BPL. These cycles resemble those observed for pure water clusters; however, significant differences are present due to the balance between water−water and solute−water interactions. As an example, different structures of BPL−(H2O) are shown in Figure 9. Another important class of compounds is nucleobases. Addition of water to isolated nucleobases was studied in a LA-FTMW study of uracil−H2O and thymine−H2O.210 Rotational spectroscopy proved to be particularly helpful to

shed light on the favored binding sites among the several present on the bare molecules. In nucleobases, for which the global minimum is the diketo form, there are various groups present that can bind the first water molecule, but only one conformation was observed for both uracil−H 2O and thymine−H2O. From analysis of several isotopically enriched species, the structure of the observed form for both complexes was determined and found to correspond to the global minimum diketo form, with the water molecule accepting an N−H···O bond from the nucleobase and forming an O−H···O bond with it. The preferred N−H group is that in the para position with respect to the carbonyl groups. Besides solvation, another focus of the HR studies on nucleobases is factors that can influence double-helix formation. The double-helix structure of a DNA strand results from a variety of different interactions that ultimately can lead to differences in the structure (e.g., number of residues per turn, angles between base planes and helix axis) and thus differences in functionality. Likewise, HR spectroscopy has helped in building up detailed knowledge of how the process of base pairing occurs. Several model systems were used for this purpose: 2HP−2PY,130 (2PY)2,103 and the adduct 2-aminopyridine−pyridinone133 were studied as model systems for biologically relevant interactions, such as the adenine−thymine base pair (see Figure 10). One of the aims of studies on bioactive model compounds is comprehension of the factors that can influence interaction of a bioactive molecule with its receptor. The sevoflurane− benzene211 complex can be considered a model of local hydrophobic interactions that may be present when the anesthetic sevoflurane binds to the receptor. Sevoflurane itself possesses different groups that can interact with the benzene molecule: oxygen lone pairs, C−F organic bonds that can act as hydrogen-bond acceptors (though weak), and C−H bonds that can act as hydrogen donors, since they are activated by the presence of fluorine or oxygen, while benzene itself can interact via its π-cloud or its C−H bonds. As all of these possible interactions are very weak, one can predict that the structure of the adduct is stabilized by multiple interactions occurring simultaneously, as is usually the case. Indeed the final structure, derived from observation of 46 isotopic species observed either 5025


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4.1. Self-Recognition and Chiral Recognition

When a 1:1 adduct is formed between two chiral partners, it can be either homo- or heterochiral, depending on whether the partners have the same or different absolute configurations. The homo- or heterocomplexes will be diasteroisomers and thus spatially different species. The first high-resolution study on chirality recognition is the FTMW study reported by King and Howard220 on dimers of butan-2-ol. Four structures of the dimer (two homodimers and two heterodimers) were predicted in which the two alcohol moieties are bound by a single O−H··· O interaction. In the cited study, only the rotational spectrum of the heterodimer representing the global minimum was observed, and the structure was rationalized in terms of steric hindrance. Different dimeric structures were observed in the selfrecognition study of methyloxirane (propylene oxide, PO), a small etheric cycle with a chiral center.221 Three homochiral and three heterochiral PO dimers were detected in the molecular jet, and their energy order was established and compared to ab initio calculations. All dimers are stabilized by four C−H···O interactions between the aliphatic hydrogen atoms of one moiety and the etheric oxygen of the other, forming six- or five-membered rings that efficiently bind the two partners, forming a cagelike structure. Moving toward increasing complexity, glycidol (GLD) is an interesting chiral probe that has been used in several studies. It provides two different sites for hydrogen-bond interactions: the etheric oxygen of oxirane and the hydroxyl group of the methanol side chain. For this reason, the monomer also displays two conformations stabilized by an intramolecular hydrogen bond that are rather close in energy and, thus, are well-suited for chiral recognition processes occurring in the gas phase. In the FTMW study on GLD dimers,222 of the 14 structures characterized theoretically by an electrostatic model calculation223 followed by standard ab initio optimizations, two were also characterized by their rotational spectra. The various conformations of (GLD)2 differ with regard to (i) formation of an eight- or five-membered ring, (ii) whether they involve one or the other conformation of GLD, and (iii) being either homoor heterochiral. There is no trivial explanation for the order of relative stability of the various geometries calculated for (GLD)2, but the experiment confirms the results of theoretical predictions since the two most stable structures, stabilized by formation of eight-membered ring structures, were those observed spectroscopically. The study also indicates that, besides thermodynamic stability, kinetic effects are important in the process of dimer formation; the dimer involving two GLD molecules in the least stable monomer conformation was not observed, even though its energy and dipole moment component values would have warranted its observation. The cagelike structures observed in the self-recognition study of PO are typical when weak hydrogen bonds are the only interactions stabilizing the dimers. In fact, no such structure was observed in the FTMW study of PO and ethanol,224 where the six observed structures of the dimer show a single O−H···O bond. The order of stability observed in the experiment (which is in poor agreement with the basis-set superposition errorcorrected ab initio results) shows that different structures of the dimer are stabilized by secondary interactions and the effects of steric hindrance. The study on the adduct between PO and ethanol is also used to elucidate Fischer’s lock-and-key principle observed on a microscopic scale. In this case, the rigid and

Figure 10. Observed structure of 2-aminonpyridine−pyridinone dimer. Adapted with permission from ref 133. Copyright 2003 National Academy of Sciences.

in natural abundance (including single-substituted 13C and 18O) or in isotopically enriched samples in a CP-FTMW experiment, shows that a weak C−H···π interaction is the dominant one, modulated by various C−H···F interactions between aromatic hydrogens of benzene and fluorine atoms of sevoflurane. The structure of the adduct also shows that the monomer geometry is not distorted upon complexation as expected for weak interactions. In addition, although rotation of the benzene ring around the C−H···π bond is hindered by the multiple bonds, it still shows tunneling splittings in the spectra.

4. COMPLEXES OF CHIRAL AND PROCHIRAL MOLECULES Chirality recognition, the process in which a chiral probe is able to recognize and bind in a preferential mode to an enantiomer of a chiral molecule, is an extremely important process in all fields of chemistry and biochemistry. Chirality recognition occurs through noncovalent interactions; thus the study of isolated molecular complexes involving chiral or prochiral molecules is particularly interesting since it provides direct characterization at the molecular level. This subject has been recently reviewed by Zehnacker and Suhm.212 Pioneering spectroscopic studies on this topic were made by Zehnaker-Rentien and co-workers, who described interactions between an aromatic chiral molecule and chiral alcohols using LIF213,214 and hole burning,215 and by Giardini-Guidoni and co-workers using resonance-enhanced multiphoton ionization (REMPI).216 Later, Suhm and co-workers used FTIR spectroscopy on free jet expansions to study the vibrational features of several chiral complexes, such as the dimer of GLD217 and other alcohols.218 One of the first chiral complexes characterized in the gas phase by mass- and isomer-selective laser techniques is the oxirane−phenol adduct.219 By use of an UV− UV hole burning experiment, which gives rise to the vibrational spectrum of a selected isomer, the authors showed that in the case of oxirane and phenol, which are two nonchiral partners, a single chiral adduct with C1 symmetry is formed. In all these cases, rotational resolution was not achieved; thus, although it was possible to identify the different complexes with a reasonable degree of accuracy, no direct structural information was obtained. Here we will focus on the detailed information that can be obtained from HR studies and their direct comparison with theoretical calculations. 5026


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chiral PO represents the lock while the flexible ethanol molecule represents the key. A variation on studies of the lock-and-key principle at the molecular level can be found in the works on GLD−H2O77 and GLD−NH3,78 where two different locks, represented by two conformations of the chiral GLD molecule, accept the same master key represented by water or ammonia. In Figure 11, the two observed conformations of GLD−NH3 generated by two different conformations of the monomer are shown.

The results obtained on these systems by Borho and Xu224,227 show that there is a strong preference for heterochiral dimers and that the presence of substituents enforces binding with additional through-space interactions. On the basis of pressuredependent measurements, it has been demonstrated that the population of different conformers agrees with calculated dissociation energies, thus suggesting that the process is thermodynamically controlled. A more dramatic effect of secondary interactions on the stabilization energies of dimers can be seen by comparing the possible arrangements of PO bound to ethanol224 or 2fluoroethanol (2FE).228 Introduction of the fluorine atom in 2FE offers an additional contact point with respect to ethanol and influences the number of possible conformers and their energy. Thus, whereas all six conformers of PO−ethanol lie within 0.7 kJ·mol−1,224 the 14 conformers of PO−2FE228 span about 10 kJ·mol−1, with the four lowest compact conformations lying about 6 kJ·mol−1 below the open forms, underlining the importance of very subtle difference in “tailoring” the partner molecules. Besides the formation of intermolecular complexes, intramolecular forces can also render transient chirality permanent. This is the case for 2FE, where the intramolecular O−H···F bond gives origin to two enantiomers that can then form homoor heterochiral dimers. The most stable forms of the 2FE dimer originate from the most stable conformation of the monomer; however, several structures, both homo- and heterochiral as well as insertion and addition structures, have been characterized by FTMW and ab initio calculations.229 Again, the presence of fluorine enhances the possibilities of conformational arrangements with respect to ethanol.225 A particular form of chiral recognition is chiral amplification, where a chiral partner induces chirality upon binding to an achiral molecule. This concept is outlined in a rotational study of PO−2,2,2-trifluoroethanol (TFE) where the effect of chiral induction is investigated at the molecular level in detail.230 In fact, TFE is known to be a solvent able to promote and induce chirality. It is used as a peptide cosolvent for protein structure investigations, and thus it is an important model system. Intermolecular interactions of the cosolvent with peptides can alter the folding process of the macromolecules. Upon binding to PO, a strong preference is shown for one of the forms of TFE, although to a lesser extent than that shown in formation of the TFE dimer, which shows the presence of only one form. Biomolecular macromolecules show a chirality or handedness also in their secondary structures. The incipient chirality is created when a planar base connects to a nonplanar one, and the base pair can then have a right- or left-handed sense. This has been shown in the previously cited model study on the base-pair analogues aminopyridine−pyridinone, where a small angle is formed between the pyridinone plane and the aromatic plane of aminopyridine (see Figure 10).133

Figure 11. Two conformations of GLD give rise to two different conformations of GLD−NH3 dimer. Adapted with permission from ref 78. Copyright 2008 John Wiley & Sons, Inc.

4.2. Transient Chirality

In the literature on chirality recognition, several studies involving ethanol are reported. Isolated ethanol is not a chiral molecule since it has rotatable bonds, allowing interconversion of the two possible gauche conformations of the hydroxyl group with respect to the methyl group, which are mirror images. In isolated ethanol these two forms will interchange rapidly due to quantum mechanical tunneling through the low potential barrier, but if the hydroxyl group is participating in hydrogen bonding, tunneling is suppressed and two different conformations can be observed experimentally. This phenomenon of transient chirality that becomes permanent was outlined initially by Hearn et al.225 in a FTMW study on dimers of ethanol and later in a study on dimers of 2-propanol.226 In fact, three different forms of hydrogen-bonded ethanol dimers were observed experimentally by FTMW, showing that the “outer” conformations are preferred to the “inner” ones because of steric hindrance. In the work on dimers of 2-propanol, several isomers very close in energy were characterized and their structures were rationalized in terms of the main O−H···O electrostatic interaction and the dispersive interaction between methyl groups. The importance of additional dispersive interaction is the main theme of a paper on ethanol complexes of oxirane and trans-2,3-dimethyloxirane.227 Structural formulas for oxirane and methyloxiranes are shown in Figure 12. This study, together with that on ethanol−PO,224 on a series of complexes of ethanol with the oxirane ring shows the changes that occur when one or two methyl groups are added.

4.3. Solvation of Chiral Molecules

Interaction of water with a chiral molecule is of great interest since it can influence the outcome of a reaction or the chiroptical properties of molecules. In an attempt to better understand such interactions, Su and Xu231,232 have characterized mono- and dihydrated complexes of PO by FTMW and theoretical calculations. They demonstrated that while in the monohydrate complex of PO the syn form is preferred, in the dihydrated complex the anti form prevails. Here syn and anti refer to the side of PO that is the same or opposite to the

Figure 12. Structures of oxirane, methyloxirane (propylene oxide, PO), and 2,3-trans-dimethyloxirane. 5027


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The picture is based on a series of studies performed by Caminati and co-workers, which show that the geometry of the 1:1 adduct between pyridine and its partner can change from a nondirectional π-type arrangement [when the partner moiety is a rare gas (He,237 Ne,238,239 Ar,18 Kr,240 or Xe241)] to a σ-type hydrogen-bond interaction with fluorine-substituted methanes (fluoromethane, 157 difluoromethane, 242 or trifluoromethane145). Finally, in the case of pyridine−tetrafluoromethane,149 it becomes a halogen-bonded interaction, resembling a σ-type bond. The structure of the pyridine−methane236 cluster presents a kind of π-type arrangement that shows some degree of directionality, with the aliphatic C−H bond directed toward the aromatic ring in a type of weak C−H···π one-legged bond. Figure 13 also shows the binding energy of the adducts as deduced from experimental data, with the assumption that the complex can be described by a pseudodiatomic model. As expected, the two types of complexes are distributed on two different curves and the reported data must be rationalized in two different ways. The linear trend for binding energy with increasing atomic number of the rare gas atom, in the case of πtype arrangements, is typical of van der Waals interactions and should thus be related to a linear dependence on the polarizability of the rare gas (RG). This is indeed shown in the previously cited case of pyridine−RG 1:1 adducts240 and has also been reported for propylene oxide−RG complexes.243 The behavior of the curve on which the σ-type hydrogen-bond adducts lie, follows the trend that bonding energy increases with increasing degree of substitution of the fluoromethane, which is related to the increase in acidic character of the C−H bond due to the fluorine electron-withdrawing effect. The “activation” of the C−H bond upon halogenation is a general concept that opens the possibility of observing C−H···X bonds, as has been previously reported and reviewed.69 An extended discussion on binding energy determination for different cluster classes, from both theoretical and experimental points of view, is presented in this thematic issue. From the structures depicted in Figure 13, it can also be seen that the arrangement of each complex with fluoromethanes is stabilized by a C−H···N and a C−H···F interaction. Evidence that organic fluorine can act as a hydrogen-bond acceptor was reported by Howard et al.244 in an analysis of data contained in the Cambridge Structural Database. In the same study, the authors also disregarded C−H···F interactions since their energy would be close to those of van der Waals bonds. Direct spectroscopic evidence on the capability of the fluorine atom to be a hydrogen-bond acceptor dates back to the late 1990s. The rotational spectrum of difluoromethane−H2O245 shows that the observed geometry is stabilized by an O−H···F hydrogen bond and a secondary C−H···F interaction. Subsequent studies on the difluoromethane dimer,246 trimer,247 and tetramer248 show that oligomers held only by C−H···F interactions can exist. Contextually, the energy of C−H···F was estimated to be about 2.2 kJ·mol−1, which is very close to the energies involved in van der Waals interactions; thus, the stability of the oligomers is related to the network of weak bonds. The different acceptor capabilities of fluorine and chlorine were assessed in a study of the 1:1 adduct of chlorofluoromethane with water.249 The observed structure of the dimer is essentially the same as in difluoromethane−H2O with the water hydrogen atom bound to chlorine, clearly showing the preference of the water molecule to bind to this atom.

methyl group. Furthermore, these gas-phase studies have shown that addition of more than one water molecule can strongly influence the stability of different conformations of the complex, in agreement with experiments and models which highlighted that the anti form is dominant in aqueous solutions.

5. TOWARD RATIONAL DESIGN OF NEW MATERIALS AND DRUGS: A HIGH-RESOLUTION SPECTROSCOPIC VIEW ON TUNING OF MOLECULAR PROPERTIES Tuning of molecular properties through understanding the effects of atom or molecular group substitutions is a very important research theme in chemistry. Since in many cases the properties of molecules in the condensed phase are affected by noncovalent interactions, such as those occurring with the solvent or with reacting partners or in the case of drugs with their receptors, study at the molecular level of how the substitution of atoms can change intermolecular interactions is particularly interesting. The number of examples available solely in the literature of HR spectroscopy is very large; thus, we will focus on particular themes and some recent examples. Halogen substitution, in particular fluorine substitution, has recently attracted great interest as a means to perform tuning of molecular properties. The importance of fluorinated compounds in science as well as in everyday life is well-recognized. Their use is increasing year by year in the biochemical field233,234 and in production of new materials,235 as the presence of fluorine can have important consequences on function and structure. In regard to noncovalent interactions, introduction of a halogen atom, such as fluorine, can dramatically alter the binding properties of a molecule. The halogen atom can induce changes in bonding properties through its electron-withdrawing effect up to the point of becoming itself a site for bonding. We will use here a set of examples where the precise structural data obtained from highresolution experiments will help us highlight these concepts. A series of MW studies on adducts of pyridine with several small fluorine-substituted methanes has shown that changes occur in the 1:1 intermolecular interaction. This is summarized and depicted in Figure 13.

Figure 13. Geometries of complexes of pyridine with fluoromethanes, methane, and rare gas atoms and dissociation energies deduced from experimental data by use of a pseudodiatomic model.164,165 CH4 behaves similarly to Kr and it can be considered as a pseudo-rare gas. Adapted with permission from ref 236. Copyright 2014 PCCP Owner Societies. 5028


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CH3F,155 NH3,261 dimethyl ether,154 H2O,153 formaldehyde,147 and the anesthetic isoflurane.204 Other cases cannot be discussed in such a general way and the effect of substitution is more subtle. This can happen, for example, when one of the moieties itself possesses a complex conformational surface related to possible intramolecular interactions. In the case of 1:1 adducts of o- and p-fluoro-2phenylethanol with water studied by high-resolution UV spectroscopy,262 it has been shown that no structural changes occur when water binds to the folded fluoro-2-phenylethanol with respect to the unsubstituted molecule263,264 and that the intramolecular bond prevails. A higher-energy structure of the ortho-substituted molecule was also observed, which is stabilized by the linkage with water, demonstrating that substitution in a particular position can have a marked effect.

The inductive effect of substitution with halogen atoms can drive the change from one kind of weak interaction to another. One of the studied prototypical systems is the benzene−H2O cluster, where it was shown that water is hydrogen-bonded to the aromatic cloud and has nearly free internal rotation.250,251 The weak O−H···π bond can remain the dominant interaction only in the case of unsubstituted benzene, since in pdifluorobenzene−H 2 O 140,141 or 1,2-dimethoxybenzene− H2O139 the water molecule is in the plane of the aromatic ring and attached via one or two hydrogen bonds. Another example is the interaction of ethylene with water, which was characterized as an O−H···π bond by molecular beam electric resonance252 and MW spectroscopy,.253 This interaction scheme is completely different from that occurring between chlorotrifluoroethylene and water254 or ammonia,151 which has been shown to be of the n···π−hole kind, with the lone pair of oxygen or nitrogen directed toward the center of the double bond. Recent results have shown that this can happen also when stronger hydrogen bonds are present. Such a case is exemplified by the water moiety hydrogen-bonded to the lone pair of pyridine in pyridine−H 2 O and in 2- or 3fluoropyridine−H2O clusters, while in pentafluoropyridine− H2O it is found to be above the plane of the ring with the oxygen atom directed toward the center of the aromatic ring, where it is bound by a lone pair π−hole interaction.255 The change in the chemical nature of the solvent is a good way to effectively explore the multiplicity of binding sites offered by a reference molecule. A considerable number of clusters formed by anisole with different solvents have been studied over the years. RG atoms are bound to anisole through its aromatic electron density.256 The anisole dimer48,112 exhibits a π-stacking configuration, while the anisole−phenol cluster49 is characterized by hydrogen bonding and partial π−π interaction. A nonpolar molecule like carbon dioxide bonds in-plane to anisole as a consequence of weak interaction of its oxygen atoms with both aromatic and aliphatic hydrogen atoms, while its carbon atom points to the lone pairs at the methoxy group.257 As already discussed, the anisole−H2O complex is hydrogen-bonded and the water oxygen atom lies in-plane with anisole.135 The anisole−methanol structure is also controlled by hydrogen bonding.258 Conversely, in the anisole−NH3 cluster the ammonia molecule is found above the plane, lying between the aromatic ring and the methoxy group as a result of the balance between different interactions (including interaction of the anisole methyl group with the ammonia lone pair and interaction of the ammonia hydrogen atoms with both the lone-pair electrons at the methoxy group and the aromatic system on anisole).259 Many of these complexes have been characterized in both ground and S1 electronic excited states. The geometric changes upon electronic excitation are related to displacement of the electron density. Halogen atoms can interact not only through hydrogen bonds but also through halogen bonds. The nature of the halogen bond is the subject of the review by Kolár ̌ and Hobza in this thematic issue, but many high-resolution studies have provided evidence of its existence in different clusters and determined their properties at a level of detail that represents a benchmark for computational chemistry studies. Since the early work of Legon260 on small halogen-bonded adducts, there has been a great body of work on larger systems. Recently, direct spectroscopic evidence of halogen-bond formation has been found by use of CF3Cl as a probe, forming 1:1 adducts with

6. CONCLUSIONS We have reviewed the most recent results in HR spectroscopy of weakly bound molecular complexes, focusing on those formed by medium-size molecules. This choice is motivated by the fact that such systems are small enough to be studied at the very detailed level typical of these techniques and that accurate theoretical calculations can also be performed on them. The level of detail allows unambiguous determination of the conformation of molecular systems and, in cases of multiple conformations, their relative stability. Fine effects can also be observed related to the orientation of groups of atoms or the presence of large-amplitude motions, together with other properties such as dipole moment orientation or charge-transfer effects. Whenever possible, we have tried to emphasize the complementarity of information obtainable from experiments in different spectral ranges or different experimental approaches. The large amount of information that can be rationalized and compared to theoretical calculations can give great insight into the driving forces holding the complexes together in particular arrangements and their flexibility. Besides this interesting and fundamental knowledge, such molecular complexes, although small, can often be used as models for larger systems, for which neither such informative experiments nor such accurate calculations can be performed. In the latter sections of the review we have focused on complexes that were chosen as model systems for biomolecular interactions or chiral recognition. Other themes that can be recognized throughout the whole review are study of the influence that solvent molecules have on molecular properties and how the substitution of an atom or groups of atoms can change the nature of the interaction, sometimes causing the molecules to behave like a new molecular system. We believe that the conclusions reached can help in rational design of more active drugs and more efficient materials. AUTHOR INFORMATION Corresponding Author

*E-mail maurizio.becucci@unifi.it. Author Contributions

The paper was written through contributions of both authors. Notes

The authors declare no competing financial interest. 5029


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Biographies

RG RMS TDL TFE UV VIS

Becucci Maurizio graduated in chemistry (1988) and defended his Ph.D. thesis in chemical sciences (1994) at the University of Florence (Italy). He was one of the first members of the European Laboratory for Nonlinear Spectroscopy (LENS) in Florence. There he set up the Molecular Beam Laser Spectroscopy facility, and in 2011 he became responsible for the Molecular Spectroscopy section. In 2000 he became a faculty member of the Department of Chemistry at the University of Florence. He has been involved in many national and international research projects and has been a visiting scientist at the University of California, San Diego. His current research interests span from fundamental research on isolated molecules and clusters to the application of molecular spectroscopy in material science and for the conservation of cultural heritage.

rare gas root mean square tunable diode laser 2,2,2-trifluoroethanol ultraviolet visible

REFERENCES (1) Frontiers of Molecular Spectroscopy; Laane, J., Ed.; Elsevier, 2009. (2) Jankunas, J.; Osterwalder, A. Cold and Controlled Molecular Beams: Production and Applications. Annu. Rev. Phys. Chem. 2015, 66, 241−262. (3) Bethlem, H.; Berden, G.; Crompvoets, F.; Jongma, R.; van Roij, A. J.; Meijer, G. Electrostatic Trapping of Ammonia Molecules. Nature 2000, 406 (6795), 491−494. (4) Zeppenfeld, M.; Motsch, M.; Pinkse, P. W. H.; Rempe, G. Optoelectrical Cooling of Polar Molecules. Phys. Rev. A: At., Mol., Opt. Phys. 2009, 80 (4), No. 041401. (5) Zeppenfeld, M.; Englert, B. G. U.; Glöckner, R.; Prehn, A.; Mielenz, M.; Sommer, C.; van Buuren, L. D.; Motsch, M.; Rempe, G. Sisyphus Cooling of Electrically Trapped Polyatomic Molecules. Nature 2012, 491 (7425), 570−573. (6) Novick, S. E.; Davies, P.; Harris, S. J.; Klemperer, W. Determination of the Structure of ArHCl. J. Chem. Phys. 1973, 59, 2273−2279. (7) Klemperer, W. Rotational Spectroscopy of van Der Waals Molecules. Faraday Discuss. Chem. Soc. 1977, 62, 179−184. (8) Nelson, D. D., Jr.; Fraser, G. T.; Klemperer, W. Does Ammonia Hydrogen Bond? Science (Washington, DC, U. S.) 1987, 238 (4834), 1670−1674. (9) Klemperer, W. Intermolecular Interactions. Science (Washington, DC, U. S.) 1992, 257 (5072), 887−888. (10) Leopold, K. R.; Fraser, G. T.; Novick, S. E.; Klemperer, W. Current Themes in Microwave and Infrared Spectroscopy of Weakly Bound Complexes. Chem. Rev. 1994, 94 (7), 1807−1827. (11) Handbook of High-Resolution Spectroscopy; Quack, M., Merkt, F., Eds.; John Wiley & Sons, Ltd.: 2011; DOI: 10.1002/9780470749593. (12) Balle, T. J.; Flygare, W. H. Fabry−Perot Cavity Pulsed Fourier Transform Microwave Spectrometer with a Pulsed Nozzle Particle Source. Rev. Sci. Instrum. 1981, 52 (1), 33−45. (13) Andresen, U.; Dreizler, H.; Grabow, J. U.; Stahl, W. An Automatic Molecular Beam Microwave Fourier Transform Spectrometer. Rev. Sci. Instrum. 1990, 61 (12), 3694−3699. (14) Grabow, J. U.; Stahl, W. A Pulsed Molecular Beam Microwave Fourier Transform Spectrometer with Parallel Molecular Beam and Resonator Axes. Z. Naturforsch., A: Phys. Sci. 1990, 45 (8), 1043−1044. (15) Grabow, J. U.; Stahl, W.; Dreizler, H. A Multioctave Coaxially Oriented Beam-Resonator Arrangement Fourier-Transform Microwave Spectrometer. Rev. Sci. Instrum. 1996, 67 (12), 4072−4084. (16) Brown, R. D.; Crofts, J. G.; Godfrey, P. D.; McNaughton, D.; Pierlot, A. P. A Stark-Modulated Supersonic Nozzle Spectrometer for Millimetre-Wave Spectroscopy of Larger Molecules of Low Volatility. J. Mol. Struct. 1988, 190 (C), 185−193. (17) Melandri, S.; Caminati, W.; Favero, L. B.; Millemaggi, A.; Favero, P. G. A Microwave Free Jet Absorption Spectrometer and Its First Applications. J. Mol. Struct. 1995, 352-353, 253−258. (18) Melandri, S.; Maccaferri, G.; Maris, A.; Millemaggi, A.; Caminati, W.; Favero, P. G. Observation of the Rotational Spectra of van Der Waals Complexes by Free Jet Absorption Millimeter Wave Spectroscopy: Pyridine-Argon. Chem. Phys. Lett. 1996, 261 (3), 267− 271. (19) Brown, G. G.; Dian, B. C.; Douglass, K. O.; Geyer, S. M.; Pate, B. H. The Rotational Spectrum of Epifluorohydrin Measured by Chirped-Pulse Fourier Transform Microwave Spectroscopy. J. Mol. Spectrosc. 2006, 238 (2), 200−212. (20) Grabow, J. U.; Mata, S.; Alonso, J. L.; Peña, I.; Blanco, S.; López, J. C.; Cabezas, C. Rapid Probe of the Nicotine Spectra by High-

Sonia Melandri received her degree in chemistry in 1990 and her Ph.D. in chemical sciences in 1994 from the University of Bologna, where she is now an associate professor. Since her Ph.D., under the supervision of Professor P. Favero, she has been involved in setting up the free jet MW spectroscopy laboratory in Bologna. Her research interest is the spectroscopic characterization of molecules and small clusters of chemical and biochemical interest, focused in particular on understanding the effects of nonbonding interactions. She has been involved in many national and international research projects and has been a visiting scientist at the Universities of Valladolid, Hannover, Coimbra, Bilbao, and at the Elettra Synchrotron.

ACKNOWLEDGMENTS We acknowledge the support of the Italian MIUR (PRIN Project 2010ERFKXL_001), the University of Bologna, the University of Florence, and LASERLAB-EUROPE (Grant Agreement 284464, EC’s Seventh Framework Programme). Also, we thank Dr. Barry Howes (UNIFI) for carefully reading the manuscript and much improving its readability. ABBREVIATIONS 2FE 2-fluoroethanol 2HP 2-hydroxypyridine 2PY 2-pyridone AE aminoethanol ALAm alaninamide ALA alanine BPL β-propiolactone COBRA coaxially oriented beam resonator arrangement CP chirped pulsed FA formamide FJAMMW free jet absorption millimeter wave FTIR Fourier transform infrared FTMW Fourier transform microwave GLD glycidol or oxiranemethanol GLY glycine HR high-resolution IMPACT in-phase/quadrature-phase modulation passageacquired coherence technique LA laser ablation LIF laser-induced fluorescence MB molecular beam MW microwave NMF N-methylformamide PEA phenylethylamine PES potential energy surface PO propylene oxide or methyloxirane REMPI resonance-enhanced multiphoton ionization 5030


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