Thermodynamics of Water Dimer Dissociation in the Primary Hydration Shell of the Iodide Ion with Temperature-Dependent Vibrational Predissociation Spectroscopy

Observation of the Infrared-Induced Structural Change in the Microscopic Hydrogen Bond Network of Phenol-Methanol Cluster Cations in a Cold-Ion Trap

To gain insight into microscopic hydrogen bond networks, we measured ultraviolet photodissociation (UVPD) spectra of the phenol-methanol 1:3 cluster cation, [PhOH(MeOH)3]+ trapped in a variable temperature ion trap. At low temperatures, an isomer with a ring-type hydrogen bond structure dominates, whereas at higher temperatures the chain-type isomers become dominant due to the flexibility of their hydrogen bond structures. We also found a clear temperature dependence of the spectral features, such as band position and width. In addition to the above measurement, we observed the infrared (IR) induced isomerization of [PhOH(MeOH)3]+ to study the dynamical aspects of hydrogen bond networks. We succeeded in observing IR-induced isomerization from the ring to chain forms of [PhOH(MeOH)3]+ at low temperature. The isomerization was clearly identified as a change in the UVPD spectra. The time evolution of the UVPD spectra after IR excitation indicated that the IR-induced isomerization occurs within a nanosecond. The chain-type isomers produced by the IR-induced isomerization are then converted back to the ring-type form by collisions with cold He buffer gas in the trap. This backward isomerization proceeds with a time constant of 100 μs under our experimental conditions. In this study, we evaluated the temperatures of the chain isomers during the backward isomerization on the basis of the spectral features.

Encapsulation of charged halogens by the 512 water cage

This study focuses on the encapsulation of the entire series of halides by the 512 cage of twenty water molecules and on the characterization of water to water and water to anion interactions. State-of-the-art computations are used to determine equilibrium geometries, energy related quantities, and thermal stability towards dissociation and to dissect the nature and strength of intermolecular interactions holding the clusters as stable units. Two types of structures are revealed: heavily deformed cages for F- indicating a preference for microsolvation, and slightly deformed cages for the remaining anions indicating a preference for encapsulation. The primary variable dictating the properties of the clusters is the charge density of the central halide, with the most severe effects observed for the F- case. For the remaining halides, the anion may be safely viewed as a sort of "big electron" with little local disruptive power, enough to affect the network of non-covalent hydrogen bonds in the cage, but not enough to break it. Gibbs energies for dissociation either into cavity and halide or into water molecules and halide suggest that, in a similar way as to methane clathrate, a more weakly bonded complex that has been detected in the gas phase, all halide containing clathrate-like structures should be amenable to experimental detection in the gas phase at moderate temperature and pressure conditions.

Near-infrared spectroscopy of H3O+⋯Xn (X = Ar, N2, and CO, n = 1-3)

Near-infrared (NIR) spectra of H3O+⋯Xn (X = Ar, N2, and CO, n = 1-3) in the first overtone region of OH-stretching vibrations (4800-7000 cm-1) were measured. Not only OH-stretching overtones but also several combination bands are major features in this region, and assignments of these observed bands are not obvious at a glance. High-precision anharmonic vibrational simulations based on the discrete variable representation approach were performed. The simulated spectra show good agreement with the observed ones and provide firm assignments of the observed bands, except in the case of X = CO, in which higher order vibrational mode couplings seem significant. This agreement demonstrates that the present system can be a benchmark for high precision anharmonic vibrational computations of NIR spectra. Band broadening in the observed spectra becomes remarkable with an increase of the interaction with the solvent molecule (X). The origin of the band broadening is explored by rare gas tagging experiments and anharmonic vibrational simulations of hot bands.

Observation of Slow Eigen-Zundel Interconversion in H+(H2O)6 Clusters upon Isomer-Selective Vibrational Excitation and Buffer Gas Cooling in a Cryogenic Ion Trap

The formation of isomers when trapping floppy cluster ions in a temperature-controlled ion trap is a generally observed phenomenon. This involves collisional quenching of the ions initially formed at high temperature by buffer gas cooling until their internal energies fall below the barriers in the potential energy surface that separate them. Here we explore the kinetics at play in the case of the two isomers adopted by the H⁺(H₂O)₆ cluster ion that differ in the proton accommodation motif. One of these is most like the Eigen cation with a tricoordinated hydronium motif (denoted E), and the other is most like the Zundel ion with the proton equally shared between two water molecules (denoted Z). After initial cooling to about 20 K in the radiofrequency (Paul) trap, the relative populations of these two spectroscopically distinct isomers are abruptly changed through isomer-selective photoexcitation of bands in the OH stretching region with a pulsed (∼6 ns) infrared laser while the ions are in the trap. We then monitor the relaxation of the vibrationally excited clusters and reformation of the two cold isomers by recording infrared photodissociation spectra with a second IR laser as a function of delay time from the initial excitation. The latter spectra are obtained after ejecting the trapped ions into a time-of-flight photofragmentation mass spectrometer, thus enabling long (∼0.1 s) delay times. Excitation of the Z isomer is observed to display long-lived vibrationally excited states that are collisionally cooled on a ms time scale, some of which quench into the E isomer. These excited E species then display spontaneous interconversion to the Z form on a ∼10 ms time scale. These qualitative observations set the stage for a series of experimental measurements that can provide quantitative benchmarks for theoretical simulations of cluster dynamics and the potential energy surfaces that underlie them.

Quantum molecular simulations of micro-hydrated halogen anions

We report the results of a detailed and accurate investigation focused on structures and energetics of poly-hydrated halides employing first-principles polarizable halide-water potentials to describe the underlying forces. Following a bottom-up data-driven potential approach, we initially looked into the classical behavior of higher-order X^-(H₂O)_N clusters. We have located several low-lying energies, such as global and local minima, structures for each cluster, with various water molecules (up to N = 8) surrounding the halide anion (X^- = F^-, Cl^-, Br^-, I^-), employing an evolutionary programming method. It is found that the F^--water clusters exhibit different structural configurations than the heavier halides, however independently of the halide anion, all clusters show in general a selective growth with the anion preferring to be connected to the outer shell of the water molecule arrangements. In turn, path-integral molecular dynamics simulations are performed to incorporate explicitly nuclear quantum and thermal effects in describing the nature of halide ion microsolvation in such prototypical model systems. Our data reveal that at low finite temperatures, nuclear quantum effects affect certain structural properties, such as weakening hydrogen bonding between the halide anion and water molecules, with minor distortions in the water network beyond the first hydration shell, indicating local structure rearrangements. Such structural characteristics and the promising cluster size trends observed in the single-ion solvation energies motivated us to draw connections of small size cluster data to the limits of continuum bulk values, toward the investigation of the challenging computational modeling of bulk single ion hydration.

Infrared spectroscopy and theoretical structure analyses of protonated fluoroalcohol clusters: the impact of fluorination on the hydrogen bond networks

To explore the impact of fluorination on the hydrogen bond networks of protonated alkylalcohols, infrared spectroscopy and theoretical computations of protonated 2,2,2-trifluoroethanol clusters, H⁺(TFE)_n, (n = 4-7), were performed. It has been demonstrated that the development of the hydrogen bond networks from a linear type to cyclic types occurs in this size region for the protonated alkylalcohol clusters. In contrast, infrared spectroscopy of H⁺(TFE)_n in the OH/CH stretch region clearly indicated that the linear type structures are held in the whole size range, irrespective of temperature of the clusters. The extensive stable isomer structure search of H⁺(TFE)_n based on our latest sampling approach supported the strong preference of the linear type hydrogen bond networks. Detailed analyses of the free OH stretching vibrational bands evidenced the intra- and intermolecular OH⋯FC interactions in the clusters. In addition, infrared spectra of protonated clusters of 2,2-difluoroethanol, 2,2-difluoropropanol, and 3,3,3-trifluoropropanol were measured for n = 4 and 5, and their spectra also indicated the effective inhibition of the cyclic hydrogen bond network formation by the fluorination.

A Benchmark Protocol for DFT Approaches and Data-Driven Models for Halide-Water Clusters

Dissolved ions in aqueous media are ubiquitous in many physicochemical processes, with a direct impact on research fields, such as chemistry, climate, biology, and industry. Ions play a crucial role in the structure of the surrounding network of water molecules as they can either weaken or strengthen it. Gaining a thorough understanding of the underlying forces from small clusters to bulk solutions is still challenging, which motivates further investigations. Through a systematic analysis of the interaction energies obtained from high-level electronic structure methodologies, we assessed various dispersion-corrected density functional approaches, as well as ab initio-based data-driven potential models for halide ion-water clusters. We introduced an active learning scheme to automate the generation of optimally weighted datasets, required for the development of efficient bottom-up anion-water models. Using an evolutionary programming procedure, we determined optimized and reference configurations for such polarizable and first-principles-based representation of the potentials, and we analyzed their structural characteristics and energetics in comparison with estimates from DF-MP2 and DFT+D quantum chemistry computations. Moreover, we presented new benchmark datasets, considering both equilibrium and non-equilibrium configurations of higher-order species with an increasing number of water molecules up to 54 for each F, Cl, Br, and I anions, and we proposed a validation protocol to cross-check methods and approaches. In this way, we aim to improve the predictive ability of future molecular computer simulations for determining the ongoing conflicting distribution of different ions in aqueous environments, as well as the transition from nanoscale clusters to macroscopic condensed phases.

How many methanol molecules effectively solvate an excess proton in the gas phase? Infrared spectroscopy of H+(methanol)n-benzene clusters

An excess proton in a hydrogen-bonded system enhances the strength of hydrogen bonds of the surrounding molecules. The extent of this influence can be a measure of the number of molecules effectively solvating the excess proton. Such extent in methanol has been discussed by the observation of the π-hydrogen-bonded OH stretch bands of the terminal sites of protonated methanol clusters, H⁺(methanol)_n, in benzene solutions, and it has been concluded that ∼8 molecules effectively solvate the excess proton (Stoyanov et al., Chem. Eur. J. 2008, 14, 3596-3604). In the present study, we performed infrared spectroscopy of H⁺(methanol)_n-benzene clusters in the gas phase. The cluster size and hydrogen-bonded network structure are identified by the tandem mass spectrometric technique and the comparison of the observed infrared spectra with density functional theory calculations. Though changes of the preferred hydrogen bond network type occur with the increase of cluster size in the gas phase clusters, the observed size dependence of the π-hydrogen bonded OH frequency agrees well with that in the benzene solutions. This means that the observations in both the gas and condensed phases catch the same physical essence of the excess proton solvation by methanol.

Competition between Solvent-Solvent and Solvent-Solute Interactions in the Microhydration of the Hexafluorophosphate Anion, PF6-(H2O)n=1,2

This study systematically examines the interactions of the hexafluorophosphate anion (PF₆^-) with one or two solvent water molecules (PF₆^-(H₂O)_n where n = 1, 2). Full geometry optimizations and subsequent harmonic vibrational frequency computations are performed on each stationary point using a variety of common density functional theory methods (B3LYP, B3LYP-D3, M06-2X, and ωB97XD) and the MP2 and CCSD(T) ab initio methods with a triple-ζ correlation consistent basis set augmented with diffuse functions on all non-hydrogen atoms (cc-pVTZ for H and aug-cc-pVTZ for P, O, and F; denoted as haTZ). Five new stationary points of PF₆^-(H₂O)₂ have been identified, one of which has an electronic energy of approximately 2 kcal mol^-1 lower than the only other dihydrate structure reported for this system. The CCSD(T) computations also reveal that the detailed interactions between PF₆^- and H₂O can be quite difficult to model reliably, with some methods struggling to correctly characterize stationary points for n = 1 or accurately reproduce the vibrational frequency shifts induced by the formation of the hydrated complex. Although the interactions between the solvent and ionic solute are quite strong (CCSD(T) electronic dissociation energy ≈10 kcal mol^-1 for the monohydrate minimum), the solvent-solvent interactions in the lowest-energy PF₆^-(H₂O)₂ minimum give rise to appreciable cooperative effects not observed in the other dihydrate minima. In addition, this newly identified structure exhibits the largest frequency shifts in the OH stretching vibrations for the waters of hydration (with Δω exceeding -100 cm^-1 relative to the values for an isolated H₂O molecule).

Probing the conformational landscape and thermochemistry of DNA dinucleotide anions via helium nanodroplet infrared action spectroscopy

Isolation of biomolecules in vacuum facilitates characterization of the intramolecular interactions that determine three-dimensional structure, but experimental quantification of conformer thermochemistry remains challenging. Infrared spectroscopy of molecules trapped in helium nanodroplets is a promising methodology for the measurement of thermochemical parameters. When molecules are captured in a helium nanodroplet, the rate of cooling to an equilibrium temperature of ca. 0.4 K is generally faster than the rate of isomerization, resulting in "shock-freezing" that kinetically traps molecules in local conformational minima. This unique property enables the study of temperature-dependent conformational equilibria via infrared spectroscopy at 0.4 K, thereby avoiding the deleterious effects of spectral broadening at higher temperatures. Herein, we demonstrate the first application of this approach to ionic species by coupling electrospray ionization mass spectrometry (ESI-MS) with helium nanodroplet infrared action spectroscopy to probe the structure and thermochemistry of deprotonated DNA dinucleotides. Dinucleotide anions were generated by ESI, confined in an ion trap at temperatures between 90 and 350 K, and entrained in traversing helium nanodroplets. The infrared action spectra of the entrained ions show a strong dependence on pre-pickup ion temperature, consistent with the preservation of conformer population upon cooling to 0.4 K. Non-negative matrix factorization was utilized to identify component conformer infrared spectra and determine temperature-dependent conformer populations. Relative enthalpies and entropies of conformers were subsequently obtained from a van't Hoff analysis. IR spectra and conformer thermochemistry are compared to results from ion mobility spectrometry (IMS) and electronic structure methods. The implementation of ESI-MS as a source of dopant molecules expands the diversity of molecules accessible for thermochemical measurements, enabling the study of larger, non-volatile species.

Spectroscopic Signatures of Mode-Dependent Tunnel Splitting in the Iodide-Water Binary Complex

The gas-phase vibrational spectrum of the isolated iodide-water cluster ion (I^-·H₂O), first reported in 1996, presents one of the most difficult, long-standing spectroscopic puzzles involving ion microhydration. Although the spectra of the smaller halides are well described in the context of an asymmetrical ground-state structure in which only one OH group is hydrogen-bonded to the ion, the I^-·H₂O spectrum displays multiplet structures with partially resolved rotational patterns that are additionally influenced by quantum nuclear spin statistics. In this study, this complex behavior is unraveled with a combination of experimental methods, including ion preparation in a temperature-controlled ion trap and spectral simplification through applications of tag-free, two-color IR-IR double-resonance spectroscopy. Analysis of the double-resonance spectra reveals a vibrational ground-state tunneling splitting of about 20 cm^-1, which is on the same order as the spacing between the peaks that comprise the multiplet structure. These findings are further supported by the results obtained from a fully coupled, six-dimensional calculation of the vibrational spectrum. The underlying level structure can then be understood as a consequence of experimentally measurable, vibrational mode-dependent tunneling splittings (which, in the case of the ground vibrational state, is comparable to the rotational energy spacing between levels with K_a = 0 and 1), as well as Fermi resonance interactions. The latter include the hydrogen-bonded OH stretches and combination bands that involve the HOH bend overtones and soft-mode excitations of frustrated translation and rotation displacements of the water molecule relative to the ion. These anharmonic couplings yield closely spaced bands that are activated in the IR by borrowing intensity from the OH stretch fundamentals.

Probing the Microsolvation Environment of the Green Fluorescent Protein Chromophore In Vacuo

We present vibrational and electronic photodissociation spectra of a model chromophore of the green fluorescent protein in complexes with up to two water molecules, prepared in a cryogenic ion trap at 160-180 K. We find the band origin of the singly hydrated chromophore at 20 985 cm^-1 (476.5 nm) and observe partially resolved vibrational signatures. While a single water molecule induces only a small shift of the S₁ electronic band of the chromophore, without significant change of the Franck-Condon envelope, the spectrum of the dihydrate shows significant broadening and a greater blue shift of the band edge. Comparison of the vibrational spectra with predicted infrared spectra from density functional theory indicates that water molecules can interact with the oxygen atom on the phenolate group or on the imidazole moiety, respectively.

Effects of solvent molecules on hemi-bonded (CH3SH)2+: infrared absorption of [(CH3SH)2-X]+ with X = H2O, (CH3)2CO, or NH3 and (CH3SH)n+ (n = 3-6)

Three-electron two-center (3e-2c) hemi-bonds play important roles in the oxidation and electron transport of proteins and are implicated to be involved in some neurodegenerative diseases. Our previous investigations on infrared (IR) spectra of (CH3SH)2+ using vacuum-ultraviolet photoionization, infrared dissociation, and time-of-flight detection have shown that (CH3SH)2+ is (3e-2c)-bonded. To investigate the influence of the solvent molecules on the (3e-2c)-bonded (CH3SH)2+ in a supersonic jet, we added H2O or (CH3)2CO or NH3 or (CH3SH)n (n = 1-4) to (CH3SH)2+ and investigated their IR action spectra. The (3e-2c)-bonded (CH3SH)2+ ion core was maintained when a molecule of H2O or (CH3)2CO or CH3SH binds, indicating that the ion core is more stable than the hydrogen bond, whereas the (3e-2c)-bond became broken by a NH3 molecule because the proton transfer led to a more stable hydrogen-bonded structure. The spectral features of the SH-stretching modes of (CH3SH)n+ (n = 3-6) indicate that the (3e-2c)-bonded (CH3SH)2+ ion core is maintained and the first two additional CH3SH are H-bonded to the free SH groups of the ion core. For larger clusters with n = 5 and 6, the additional solvent molecules likely bind to the first solvation shell. These results show also that the (3e-2c)-bonded S∴S structure is more stable than the S∴O and S∴N structures in [(CH3SH)2-X]+ with X = H2O or (CH3)2CO or CH3SH or NH3.

Specific Ion Effects on Hydrogen-Bond Rearrangements in the Halide-Dihydrate Complexes

Small aqueous ionic clusters represent ideal systems to investigate the microscopic hydrogen-bonding structure and dynamics in ion hydration shells. In this context, halide-dihydrate complexes are the smallest systems where the interplay between halide-water and water-water interactions can be studied simultaneously. Here, quantum molecular dynamics simulations unravel specific ion effects on the temperature-dependent structural transition in X^-(H₂O)₂ complexes (X = Cl, Br, and I), which is induced by the breaking of the water-water hydrogen bond. A systematic analysis of the hydrogen-bonding rearrangements at low temperature provides fundamental insights into the competition between halide-water and water-water interactions depending on the properties of the halide ion. While the halide-water hydrogen-bond strength decreases going from Cl^-(H₂O)₂ to I^-(H₂O)₂, the opposite trend in observed in the strength of the water-water hydrogen-bond, suggesting that nontrivial many-body effects may also be at play in the hydration shells of halide ions in solution, especially in frustrated systems (e.g., interfaces) where the water molecules can have dangling OH bonds.

Unmasking Rare, Large-Amplitude Motions in D2-Tagged I-·(H2O)2 Isotopomers with Two-Color, Infrared-Infrared Vibrational Predissociation Spectroscopy

We describe a two-color, isotopomer-selective infrared-infrared population-labeling method that can monitor very slow spectral diffusion of OH oscillators in H-bonded networks and apply it to the I^-·(HDO)·(D₂O) and I^-·(H₂O)·(D₂O) systems, which are cryogenically cooled and D₂-tagged at an ion trap temperature of 15 K. These measurements reveal very large (>400 cm^-1), spontaneous spectral shifts despite the fact that the predissociation spectra in the OH stretching region of both isotopologues are sharp and readily assigned to four fundamentals of largely decoupled OH oscillators held in a cyclic H-bonded network. This spectral diffusion is not observed in the untagged isotopologues of the dihydrate clusters that are generated under the same source conditions but does become apparent at about 75 K. These results are discussed in the context of the large-amplitude "jump" mechanism for H-bond relaxation dynamics advanced by Laage and Hynes in an experimental scenario where rare events can be captured by following the migration of OH groups among the four available positions in the quasi-rigid equilibrium structure.

Hydrogen bond network structures of protonated short-chain alcohol clusters

Protonated alcohol clusters enable extraction of the physical essence of the nature of hydrogen bond networks.

Influence of argon and D2 tagging on the hydrogen bond network in Cs+(H2O)3; kinetic trapping below 40 K

Tuning cluster ion conformations between 12 and 21 K.

Isolating the Isomeric Hydrogen Bonding Signatures of the Cyanide-Water Complex by Cryogenic Ion Trap Vibrational Spectroscopy

The vibrational spectroscopy of the cyanide-water complex and its fully deuterated isotopologue is studied in the spectral range from 800 to 3800 cm^-1. Infrared/infrared double-resonance population labeling spectroscopy of the cryogenically cooled, messenger-tagged complexes isolates the spectral signature of the two quasi-isoenergetic, singly hydrogen-bonded isomers HOH···NC^- and HOH···CN^-. The infrared photodissociation spectra are assigned based on a comparison to simulated anharmonic spectra. Infrared multiple photon dissociation spectra in the temperature range from 6 to 300 K confirm the stability of the two isomers at lower temperatures and provide evidence for a considerably more dynamic structure, also involving doubly hydrogen-bonded configurations, at higher internal energies. The observed red shifts Δν_OH of the hydrogen-bonded O-H stretches, 671 cm^-1 (HOH···NC^-) and 812 cm^-1 (HOH···CN^-), confirm the universal correlation of Δν_OH with the corresponding proton affinities.

Disentangling the Complex Vibrational Spectrum of the Protonated Water Trimer, H+(H2O)3, with Two-Color IR-IR Photodissociation of the Bare Ion and Anharmonic VSCF/VCI Theory

Vibrational spectroscopy of the protonated water trimer provides a stringent constraint on the details of the potential energy surface (PES) and vibrational dynamics governing excess proton motion far from equilibrium. Here we report the linear spectrum of the cold, bare H⁺(H₂O)₃ ion using a two-color, IR-IR photofragmentation technique and follow the evolution of the bands with increasing ion trap temperature. The key low-energy features are insensitive to both D₂ tagging and internal energy. The D₂-tagged D⁺(D₂O)₃ spectrum is reported for the first time, and the isotope dependence of the band pattern is surprisingly complex. These spectra are reproduced by large-scale vibrational configuration interaction calculations based on a new full-dimensional PES, which treat the anharmonic effects arising from large amplitude motion. The results indicate such extensive mode mixing in both isotopologues that one should be cautious about assigning even the strongest features to particular motions, especially for the absorptions that occur close to the intramolecular bending mode of the water molecule.

Spectroscopic snapshots of the proton-transfer mechanism in water

The Grotthuss mechanism explains the anomalously high proton mobility in water as a sequence of proton transfers along a hydrogen-bonded (H-bonded) network. However, the vibrational spectroscopic signatures of this process are masked by the diffuse nature of the key bands in bulk water. Here we report how the much simpler vibrational spectra of cold, composition-selected heavy water clusters, D⁺(D₂O)_n, can be exploited to capture clear markers that encode the collective reaction coordinate along the proton-transfer event. By complexing the solvated hydronium "Eigen" cluster [D₃O⁺(D₂O)₃] with increasingly strong H-bond acceptor molecules (D₂, N₂, CO, and D₂O), we are able to track the frequency of every O-D stretch vibration in the complex as the transferring hydron is incrementally pulled from the central hydronium to a neighboring water molecule.

Exploring the Gas-Phase Activation and Reactivity of a Ruthenium Transfer Hydrogenation Catalyst by Experiment and Theory in Concert

This study elucidates structures, activation barriers, and the gas-phase reactivity of cationic ruthenium transfer hydrogenation catalysts of the structural type [(η⁶-cym)RuX(pympyr)]⁺. In these complexes, the central ruthenium(+II) ion is coordinated to an η⁶-bound p-cymene (η⁶-cym), a bidentate 2-R-4-(2-pyridinyl)pyrimidine ligand (pympyr) with R = NH₂ or N(CH₃)₂, and an anion X = I^-, Br^-, Cl^-, or CF₃SO₃^-. We present infrared multiple-photon dissociation (IR-MPD) spectra of precursors (before HCl loss) and of activated complexes (after HCl loss), which elucidates C-H activation as the key step in the activation mechanism. A resonant two-color IR-MPD scheme serves to record several otherwise "dark" bands and enhances the validity of spectral assignments. We show that collision-induced dissociation (CID)-derived activation energies of the [(η⁶-cym)RuX(pympyr)]⁺ (R = N(CH₃)₂) complexes depend crucially on the anion X. The obtained activation energies for the HX loss correlate well with quantum chemical activation barriers and are in line with the HSAB concept. We further elucidate the reaction of the activated complexes with D₂ under single-collision conditions. Quantum mechanical simulations substantiate that the resulting species represent analogues for hydrido intermediates formed after abstraction of H⁺ and H^- from isopropanol, as postulated for the catalytic cycle of transfer hydrogenation by us before.

Quantitative Temperature Dependence of the Microscopic Hydration Structures Investigated by Ultraviolet Photodissociation Spectroscopy of Hydrated Phenol Cations

To discuss the temperature effect on microscopic hydration structures in clusters, relative populations of the isomers having different hydration structures at well-defined temperatures are quite important. In the present study, we measured ultraviolet photodissociation spectra of the temperature-controlled hydrated phenol cation [PhOH(H₂O)₅]⁺ trapped in the 22-pole ion trap. Two isomers having a distinct hydration motif with each other are identified in the spectra, and a clear change in the relative populations is observed in the temperature range from 30 to 150 K. This behavior is quantitatively interpreted by statistical mechanical estimation based on density functional theory calculations. A ring with tail-type hydration motif is dominant in cold conditions, whereas a chain-like motif is dominant in hot conditions. The present study provides very quantitative information about the temperature effect on the microscopic hydration structures.

Structural and electrostatic effects at the surfaces of size- and charge-selected aqueous nanodrops

The effects of ion charge, polarity and size on the surface morphology of size-selected aqueous nanodrops containing a single ion and up to 550 water molecules are investigated with infrared photodissociation (IRPD) spectroscopy and theory.

The effects of ion charge, polarity and size on the surface morphology of size-selected aqueous nanodrops containing a single ion and up to 550 water molecules are investigated with infrared photodissociation (IRPD) spectroscopy and theory. IRPD spectra of M(H₂O)_n where M = La³⁺, Ca²⁺, Na⁺, Li⁺, I^–, SO₄^2– and supporting molecular dynamics simulations indicate that strong interactions between multiply charged ions and water molecules can disrupt optimal hydrogen bonding (H-bonding) at the nanodrop surface. The IRPD spectra also reveal that “free” OH stretching frequencies of surface-bound water molecules are highly sensitive to the ion's identity and the OH bond's local H-bond environment. The measured frequency shifts are qualitatively reproduced by a computationally inexpensive point-charge model that shows the frequency shifts are consistent with a Stark shift from the ion's electric field. For multiply charged cations, pronounced Stark shifting is observed for clusters containing ∼100 or fewer water molecules. This is attributed to ion-induced solvent patterning that extends to the nanodrop surface, and serves as a spectroscopic signature for a cation's ability to influence the H-bond network of water located remotely from the ion. The Stark shifts measured for the larger nanodrops are extrapolated to infinite dilution to obtain the free OH stretching frequency of a surface-bound water molecule at the bulk air–water interface (3696.5–3701.0 cm^–1), well within the relatively wide range of values obtained from SFG measurements. These cluster measurements also indicate that surface curvature effects can influence the free OH stretching frequency, and that even nanodrops without an ion have a surface potential that depends on cluster size.

Modeling Molecular Interactions in Water: From Pairwise to Many-Body Potential Energy Functions

Almost 50 years have passed from the first computer simulations of water, and a large number of molecular models have been proposed since then to elucidate the unique behavior of water across different phases. In this article, we review the recent progress in the development of analytical potential energy functions that aim at correctly representing many-body effects. Starting from the many-body expansion of the interaction energy, specific focus is on different classes of potential energy functions built upon a hierarchy of approximations and on their ability to accurately reproduce reference data obtained from state-of-the-art electronic structure calculations and experimental measurements. We show that most recent potential energy functions, which include explicit short-range representations of two-body and three-body effects along with a physically correct description of many-body effects at all distances, predict the properties of water from the gas to the condensed phase with unprecedented accuracy, thus opening the door to the long-sought "universal model" capable of describing the behavior of water under different conditions and in different environments.

Hydrogen bond mediated stabilization of the salt bridge structure for the glycine dimer anion

The formation of a salt bridge in deprotonated glycine dimer anions in a solvent-free environment is investigated using both infrared multiple photon dissociation spectroscopy between 600 and 1800 cm(-1) and theory. The zwitterionic and nonzwitterionic forms of glycine in this complex are computed to be nearly iso-energetic, yet predominantly the zwitterionic form is observed experimentally. The zwitterion stability is attributed to both the Coulombic attraction and the high stabilization from intramolecular hydrogen bonding that drives the energetic cost of proton transfer in a solvent free environment. These results show that there is a fine balance between the stabilities of these two forms of the anion. Elucidating the role of intrinsic factors, such as hydrogen bonding, can lead to a better understanding of the stabilities of salt bridges in the interiors of large proteins or at protein interfaces.