On the structure of the water trimer. A matrix isolation study

Insight into the Binding of Argon to Cyclic Water Clusters from Symmetry-Adapted Perturbation Theory

This work systematically examines the interactions between a single argon atom and the edges and faces of cyclic H2O clusters containing three-five water molecules (Ar(H2O)n=3-5). Full geometry optimizations and subsequent harmonic vibrational frequency computations were performed using MP2 with a triple-ζ correlation consistent basis set augmented with diffuse functions on the heavy atoms (cc-pVTZ for H and aug-cc-pVTZ for O and Ar; denoted as haTZ). Optimized structures and harmonic vibrational frequencies were also obtained with the two-body-many-body (2b:Mb) and three-body-many-body (3b:Mb) techniques; here, high-level CCSD(T) computations capture up through the two-body or three-body contributions from the many-body expansion, respectively, while less demanding MP2 computations recover all higher-order contributions. Five unique stationary points have been identified in which Ar binds to the cyclic water trimer, along with four for (H2O)4 and three for (H2O)5. To the best of our knowledge, eleven of these twelve structures have been characterized here for the first time. Ar consistently binds more strongly to the faces than the edges of the cyclic (H2O)n clusters, by as much as a factor of two. The 3b:Mb electronic energies computed with the haTZ basis set indicate that Ar binds to the faces of the water clusters by at least 3 kJ mol-1 and by nearly 6 kJ mol-1 for one Ar(H2O)5 complex. An analysis of the interaction energies for the different binding motifs based on symmetry-adapted perturbation theory (SAPT) indicates that dispersion interactions are primarily responsible for the observed trends. The binding of a single Ar atom to a face of these cyclic water clusters can induce perturbations to the harmonic vibrational frequencies on the order of 5 cm-1 for some hydrogen-bonded OH stretching frequencies.

Assessing the Experimental Hydrogen Bonding Energy of the Cyclic Water Dimer Transition State with a Cyclooctatetraene-Based Molecular Balance

We have conducted an experimental and computational study of cyclooctatetraene-1,4/1,6-dimethanol (1,4 and 1,6) as a molecular balance with the goal in mind to determine the otherwise inaccessible hydrogen bonding energy (HBE) of the cyclic water dimer, which constitutes a transition state. The 1,4/1,6 folding equilibrium is governed by an intramolecular hydrogen bond in the folded 1,6-isomer, in which the OH groups adopt a cyclic planar geometry, akin to the structure of the cyclic water dimer transition state. We characterized hydrogen bonding in 1,6 and reference complexes utilizing SAPT2 + (3)δMP2/aug-cc-pVTZ and selected quantum theory of atoms in molecule descriptors at M06-2XD3(0)/ma-def2-TZVPP. Additionally, we computed HBEs at the DLPNO-CCSD(T)/aug-cc-pVQZ level of theory. We find that hydrogen bonding in 1,6 is very similar to the interaction in the C_i symmetric cyclic water dimer TS, both in magnitude and character. We experimentally determined the Gibbs free energy of the folding process (ΔG_eq) in a variety of organic solvents via nuclear magnetic resonance spectroscopy measurements at room temperature. By combining experimentally obtained ΔG_eq values with corrections derived from accurate computational methods, we provide estimates for the HBE of cyclic water dimers and the cyclic water dimer TS, as the most stable cyclic water dimer.

Raman study of water deposited in solid argon matrix

New Raman data are presented concerning H₂O and D₂O water aggregation in argon matrix having the ratio of number of argon atoms to water molecules close to 40:1. Experiments were conducted at temperatures from 8 K to 34 K allowing observation of OH and OD stretching vibrations of water monomers, dimers, trimers and higher multimers, as well as broad bands corresponding to solid amorphous water. Molecular dynamics simulations were performed for thirteen or sometimes fourteen water molecules dispersed among 500 argon atoms. Resulting final configurations included dimers, trimers, tetramers and pentamers, all in open chain configurations which upon optimization resulted in mostly cyclic conformations. Observed OH stretching vibrations were assigned by comparing calculated normal modes in harmonic approximation at the B3LYP/aug-cc-pVDZ and PBEPBE1/aug-cc-pVDZ level of theory with our data and previously observed bands from infrared matrix isolation studies and Raman jet cooled experiments. Raman bands assigned to water multimers in argon matrix are shifted 20 to 25 cm^-1 towards lower wavenumbers with respect to the positions of OH stretching vibrations of almost free water clusters.

Vibrational Spectroscopy of the Water Dimer at Jet-Cooled and Atmospheric Temperatures

The vibrational spectroscopy of the water dimer provides an understanding of basic hydrogen bonding in water clusters, and with about one water dimer for every 1,000 water molecules, it plays a critical role in atmospheric science. Here, we review how the experimental and theoretical progress of the past decades has improved our understanding of water dimer vibrational spectroscopy under both cold and warm conditions. We focus on the intramolecular OH-stretching transitions of the donor unit, because these are the ones mostly affected by dimer formation and because their assignment has proven a challenge. We review cold experimental results from early matrix isolation to recent mass-selected jet expansion techniques and, in parallel, the improvements in the theoretical anharmonic models. We discuss and illustrate changes in the vibrational spectra of complexes upon increasing temperature, and the difficulties in recording and calculating these spectra. In the atmosphere, water dimer spectra at ambient temperature are crucial. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 73 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The water trimer reaction OH + (H2O)3→ (H2O)2OH + H2O

All important stationary points on the potential energy surface (PES) for the reaction OH + (H₂O)₃→ (H₂O)₂OH + H₂O have been fully optimized using the "gold standard" CCSD(T) method with the large Dunning correlation-consistent cc-pVQZ basis sets. Three types of pathways were found. For the pathway without hydrogen abstraction, the barrier height of the transition state (TS1) is predicted to lie 5.9 kcal mol^-1 below the reactants. The two major complexes (H₂O)₃OH (CP1 and CP2a) are found to lie 6.3 and 11.0 kcal mol^-1, respectively, below the reactants [OH + (H₂O)₃]. For one of the H-abstraction pathways the lowest classical barrier height is predicted to be much higher, 6.1 kcal mol^-1 (TS2a) above the reactants. For the other H-abstraction pathway the barrier height is even higher, 15.0 (TS3) kcal mol^-1. Vibrational frequencies and the zero-point vibrational energies connected to the PES are also reported. The energy barriers for the H-abstraction pathways are compared with those for the OH + (H₂O)₂ and OH + H₂O reactions, and the effects of the third water on the energetics are usually minor (0.2 kcal mol^-1).

Size-Specific Infrared Spectra of Benzene-(H2O)n Clusters (n = 1 through 7): Evidence for Noncyclic (H2O)n Structures

Resonant ion-dip infrared spectroscopy has been used to record size-specific infrared spectra of C(6)H(6)-(H(2)O)n clusters with n = 1 through 7 in the O-H stretch region. The O-H stretch spectra show a dramatic dependence on cluster size. For the n = 3 to 5 clusters, the transitions can be divided into three types-attributable to free, pi hydrogen-bonded, and single donor water-water O-H stretches-consistent with a C(6)H(6)-(H(2)O)n structure in which benzene is on the surface of a cyclic (H(2)O)n cluster. In n = 6 and 7 clusters, the spectra show distinct new transitions in the 3500 to 3600 wave number region. After comparison of these results with the predictions of ab initio calculations on (H(2)O)n clusters, these new transitions have been assigned to double donor O-H stretches associated with the formation of a more compact, noncyclic structure beginning with (H(2)O)(6). This is the same size cluster for which ab initio calculations predict that a changeover to noncyclic (H(2)O)n structures will occur.

Vibrational Spectra and Structure of CH3Cl:(H2O)2and CH3Cl:(D2O)2Complexes. IR Matrix Isolation and ab Initio Calculations

The infrared spectra of CH3Cl + H2O isolated in solid neon at low temperature have been investigated. High concentration studies of water (0.01%-4%) and subsequent annealing lead to the formation of the ternary CH3Cl:(H2O)2 complex. Detailed vibrational assignments were made on the observed spectra of water and deuterated water engaged in the complex. In parallel, structural, energetic, and vibrational properties of the complex have been studied at the second-order Møller-Plesset perturbation theory using several basis sets. Anaharmonic correction to the vibrational frequencies has been done with the standard second-order perturbation approach. It was shown that the ground state of the complex has a cyclic form for which the nonadditive three-body contribution was found to be around 10% of the interaction energy.

The effect of solvation on biomolecular conformation: 2-amino-1-phenylethanol

Small molecule neurotransmitters form one the most important classes of pharmaceutical molecules. While the behavior of these molecules in their neutral forms in the gas phase is well understood, their behavior in more biologically relevant scenarios (protonated and in aqueous solution) has received comparatively little attention. Here we address this problem by using molecular mechanics simulations to build up a detailed picture of the conformational behavior of 2-amino-1-phenylethanol, a noradrenaline analogue, in aqueous solution in both its neutral and protonated forms. For the sake of comparison, equivalent simulations are also performed on the gas-phase molecules and gas-phase hydrated clusters. These calculations reveal the important role that water has to play in determining the conformational preferences and dynamic behavior of the molecules. Water molecules are found to bridge between the various functional groups within the molecule, significantly affecting their relative stabilities in comparison to the gas-phase values. The reorganization of these solvation structures also provides a mechanism for conformational interconversion. The role of the solvent in mediating interactions between the various functional groups within the molecule suggests that in noradrenaline the catechol groups will be able to interact, albeit indirectly, with the other functional groups, thereby influencing the behavior of the molecule.

Water-carbon dioxide mixtures at high temperatures and pressures: local order in the water rich phase investigated by vibrational spectroscopy

Raman scattering combined with near- and mid-infrared absorption spectroscopies was used to investigate the evolution of the local order in the water rich phase of water-CO(2) mixtures under isobaric heating (T=40-360 degrees C, P=250 bars). The quantitative analysis of the spectra shows that tetramers and larger oligomers are the main constituents of water at moderate temperatures below 80 degrees C. As the temperature increases, the dimer and trimer concentrations considerably increase at the expense of larger oligomers. Finally, water dimers are predominant at the highest temperature investigated close to the temperature of total miscibility of the mixture (T=366 degrees C, P=250 bars). This result is consistent with our previous investigation [R. Oparin T. Tassaing, Y. Danten, and M. Besnard, J. Chem. Phys. 120, 10691 (2004)] on water dissolved in the CO(2) rich phase where we found that close to the temperature of total miscibility water also exists mainly under dimeric form. The current study combined with that mentioned above provides a model investigation of the evolution of the state of aggregation of water molecules in binary mixture involving a hydrophobic solvent in a wide range of temperature.

Dynamics of the Concerted Triple Proton Transfer in Cyclic Water Trimer Using the Multiconfiguration Molecular Mechanics Algorithm

Cyclic water clusters are important molecular species to understand the nature of hydrogen bonded networks. Theoretical studies for the dynamics of triple proton transfer in the cyclic water trimer were performed. The potential energy surface (PES) of triple proton transfer is generated by the multiconfiguration molecular mechanics (MCMM) algorithm. We have used the MP2/6-31G(d,p) level for high-level ab initio data (energies, gradients, and Hessians), which are used in the Shepard interpolation. Eight high-level reference points were added step by step, including two points for the critical configurations of the large curvature tunneling paths. The more high-level points are used, the better the potential energy surfaces become. The rate constant and kinetic isotope effect (KIE) for the triple proton transfer at 300 K, which have been calculated by the canonical variational transition-state theory with microcanonical optimized multidimensional semiclassical tunneling approximation, are 1.6 x 10(-3) s(-1) and 230, respectively. Tunneling is very important not only for the triple proton transfer but also for the triple deuterium transfer. The MCMM results show good agreement with those from the direct ab initio dynamics calculations.

Intermolecular Vibrations of the Water Trimer, a Matrix Isolation Study

Infrared spectra from 25 to 4000 cm(-1) have been recorded of water (H2O, D2O and H218O) matrix isolated in neon, argon, and krypton matrices. Intermolecular absorption bands of different isotopologues of the water trimer and tetramer have been assigned from concentration dependencies and diffusion behavior, using the well-known mid-infrared trimer and tetramer absorption bands as measures of the trimer and tetramer concentrations. The results are compared to ab initio calculations.

Infrared spectra of water clusters in krypton and xenon matrices

The infrared absorption spectra of the water molecules and small water clusters, (H(2)O)(n) with n = 2-6, trapped in solid argon, krypton, and xenon matrices have been investigated. The infrared bands of the water clusters with n = 5 and 6 in krypton and n = 3, 4, 5, and 6 in xenon matrices have been identified for the first time in the bonded OH stretching region. The frequency shifts in the bonded OH stretching band of the water dimer and trimer in xenon matrices show fairly large deviations to the red from the empirical correlation between the matrix shifts and the square root of the critical temperatures of the matrix material. The observed anomalous shifts suggest that the water dimer and trimer in solid xenon are trapped in multiple sites, and that the structures of the preferential trapping sites are different from those in argon and krypton matrices.

Infrared spectra of the water-nitrogen complexes (H2O)2–(N2)n(n=1–4) in argon matrices

The infrared spectra of the water-nitrogen complexes trapped in argon matrices have been studied with Fourier transform infrared absorption spectroscopy. The absorption lines of the H20-N2 1:1, 1:2, 1:n, and 2:1 complexes have been confirmed on the basis of the concentration effects. In addition, we have observed a few lines and propose the assignments for the 2:2, 2:3, and 2:4 complexes in the nu1 symmetric stretching and nu2 bending regions of the proton-acceptor molecule, and in the bonded OH stretching region of the proton-donor molecule. The redshifts in the bonded OH stretching mode and blueshifts in the OH bending mode suggest that the hydrogen bonds in the (H2O)2-(N2)n complexes with n = 1-4 are strengthened by the cooperative effects compared to the pure H2O dimer. Two absorption bands due to the 3:n complexes are also observed near the bonded OH stretching region of the H2O trimer.

Water Adsorption on Rh(111) at 20 K: From Monomer to Bulk Amorphous Ice

The adsorption of water (D(2)O) molecules on Rh(111) at 20 K was investigated using infrared reflection absorption spectroscopy (IRAS). At the initial stage of adsorption, water molecules exist as monomers on Rh(111). With increasing water coverage, monomers aggregate into dimers, larger clusters (n = 3-6), and two-dimensional (2D) islands. Further exposure of water molecules leads to the formation of three-dimensional (3D) water islands and finally to a bulk amorphous ice layer. Upon heating, the monomer and dimer species thermally migrate on the surface and aggregate to form larger clusters and 2D islands. Based on the temperature dependence of OD stretching peaks, we succeeded in distinguishing water molecules inside 2D islands from those at the edge of 2D islands. From the comparison with the previous vibrational spectra of water clusters on other metal surfaces, we conclude that the number of water molecules at the edge of 2D islands is comparable with that of water molecules inside 2D islands on the Rh(111) surface at 20 K. This indicates that the surface migration of water molecules on Rh(111) is hindered as compared with the cases on Pt(111) and Ni(111) and thus the size of 2D islands on Rh(111) is relatively small.

Structural evolution of aqueous NaCl solutions dissolved in supercritical carbon dioxide under isobaric heating by mid and near infrared spectroscopy

The local order in aqueous NaCl solutions diluted in supercritical carbon dioxide at constant pressure as a function of NaCl concentration and temperature has been investigated using near and mid infrared absorption spectroscopy. The near IR results have allowed us to estimate the water concentration in CO(2) rich phase, whereas the state of water aggregation in CO(2) phase was investigated using mid IR spectroscopy. The analysis of the band shape variations of the OD stretching mode of HOD led us to conclude that below 100 degrees C, water molecules dissolved in CO(2) exist only under their monomeric form, whatever the salt concentration is, whereas hydrogen-bonded species, namely, dimers start to appear at higher temperatures. Larger aggregates have a negligible concentration in the range of temperature-pressure investigated. Using near and mid infrared data, we have calculated the concentrations of water species in the CO(2) phase. Upon heating, it was found that the concentration of dimers considerably increases at the expense of the monomers and only dimers are detected in carbon dioxide at highest temperatures. Changing the salt concentration affects significantly the concentration of monomers and decreases strongly the dimers population as the solution becomes progressively saturated in salt. In the saturated solution, at 340 degrees C, the dimer concentration is at least two times smaller than in the binary water-CO(2) mixture. These findings are in qualitative agreement with existing thermodynamics data showing that addition of NaCl to the binary H(2)O-CO(2) system shifts the range of partial miscibility of water and CO(2) towards higher pressure and temperature.

The effect of cooperative hydrogen bonding on the OH stretching-band shift for water clusters studied by matrix-isolation infrared spectroscopy and density functional theory

Infrared spectra of the water clusters have been measured in the N2 + O2 matrix. The aggregation process of water in the matrix has been monitored by annealing the deposited samples up to 40 K and UV irradiation. The monomer, dimer, cyclic trimer and cyclic pentamer are found as water clusters in the matrix. For the hexamer, several structures such as chair, cage, prism, bag 1 and/or book 1 are likely to exist. By UV irradiation, the cyclic pentamer is predominantly formed from the monomer and dimer. On the other hand, by annealing the deposited sample, several hexamers are formed. The theoretical calculation for water clusters has revealed that the formation of one hydrogen bonding in a hydrogen-bonded chain cooperatively enhances or diminishes the strength of another hydrogen bond. Both proton donor (D) and acceptor (A) participating in a hydrogen-bonding pair DA are capable of forming hydrogen bonding with the other water molecules; D can additionally accept two protons and donate one proton, and A can additionally donate two protons and accept one proton. We have proposed the classification of hydrogen-bonding patterns considering the cooperativity, denoting as d'a'DAd''a'', where d and a are integers indicating the number of proton donors and acceptors to D (the single prime) and A (the double prime), respectively. Then, a magnitude given by MOH = -d' + a' + d'' - a'' has been introduced, which is very useful for connecting the hydrogen-bonding patterns to their OH wavenumbers. As a result, it is revealed that the OH stretching bands of water clusters are characterized by eight indicators (free and MOH = -2, -1, 0, 1, 2, 3 and 4). The classification proposed here is applicable to the OH band analysis for the hydrogen-bonded water and alcohols in a condensed phase.

A vibrational spectroscopic study of structure evolution of water dissolved in supercritical carbon dioxide under isobaric heating

A combination of Raman scattering spectroscopy and infrared absorption was applied to investigate the structural evolution of water dissolved in supercritical carbon dioxide under isobaric heating (T=40-340 degrees C, P=250 bar). Quantitative analysis of experimental spectra allowed us to determine that at relatively moderate temperatures water dissolved in CO(2)-rich phase exists only under monomeric form (solitary water surrounding by CO(2) molecules), but hydrogen-bonded species, namely, dimers, begin to appear upon heating. At the same time, the ratio of dimers to monomers concentration increases with further temperature increase and at temperatures close to the temperature of total miscibility of the mixture (T=366 degrees C, P=250 bar), water dimers only are present in the CO(2)-rich phase.