Vibrational Predissociation of the HCl-(H2O)3 Tetramer

Mechanism of ionic dissociation of HCl in the smallest water clusters

The dissociation of strong acids into water is a fundamental process in chemistry and biology. Determining the minimum number of water molecules that can result in an ionic dissociation of hydrochloric acid (HCl → H+ + Cl-) remains a challenging subject. In this study, the reactions of H2O with HCl(H2O)n-1 (HCl-H2O cluster), i.e., HCl(H2O)n-1 + H2O (n = 3-7), were investigated by using the direct ab initio molecular dynamics (AIMD) method. Direct AIMD calculations were performed to set the collision energy of H2O to zero for all trajectories. For n = 3, no reaction occurred. In contrast, HCl dissociated to H+ + Cl- at n = 4, forming a contact ion pair (cIP) and solvent-separated ion pair (ssIP) as products. The reactions were expressed as HCl(H2O)3 + H2O → H3O+(H2O)2Cl- (ssIP), and HCl(H2O)3 + H2O → H3O+(Cl-)(H2O)2 (cIP). The ion pair (IP) products were dependent on the collision site of H2O relative to HCl(H2O)3. For n = 5-7, both IPs were formed through the reaction between H2O and HCl(H2O)n-1 (n = 5-7). The reaction between HCl and (H2O)4 (HCl + (H2O)4 → HCl(H2O)4) was non-reactive in IP formation. The reaction mechanism was discussed based on the theoretical results.

Ion Pairing in HCl-Water Clusters: From Electronic Structure Investigations to Multiconfigurational Force-Field Development

In the bulk, condensed-phase HCl exists as a dissociated Cl^- ion and a proton that is delocalized over solvating water molecules. However, in the gas phase, HCl is covalent, and even on the introduction of hydrating water molecules, the HCl covalent state dominates small clusters and is relevant at larger clusters including 21 water molecules. Electronic structure calculations (at the MP2 level) and ab initio metadynamics simulations (at the DFT level) have been carried out on HCl-(H₂O)_n clusters with n = 2-22 to investigate distinct solvation environments in clusters from covalent HCl structure, to contact ion pairs and solvent-separated ion pairs. The data were further used to train and validate a multiconfigurational force-field for HCl-water clusters that incorporates covalent HCl states into the MS-EVB3.2 formalism. Additionally, the many-body interaction of the Cl^- ion with water and the excess proton was modeled by the introduction of two geometric three-body terms that incorporates the dominant many-body interaction in an efficient noniterative manner.

Probing the Partial Activation of Water by Open-Shell Interactions, Cl(H2O)1-4

The partial chemical activation of water by reactive radicals was examined computationally for small clusters of chlorine and water, Cl^•(H₂O)_n=1-4. Using an automated isomer-search procedure, dozens of unique, stable structures were computed. Among the resulting structural classes were intact, hydrated-chlorine isomers, as well as hydrogen-abstracted (HCl)(OH)(H₂O)_n-1 configurations. The latter showed increased stability as the degree of hydration increased, until n = 4, where a new class of structures was discovered with a chloride ion bound to an oxidized water network. The electronic structure of these three structural classes was investigated, and spectral signatures of this hydration-based evolution were connected to these electronic properties. An ancillary outcome of this detailed computational analysis, including coupled-cluster benchmarks, was the calibration of cost-effective quantum chemistry methods for future studies of these radical-water complexes.

Vibrational predissociation of the phenol-water dimer: a view from the water

The vibrational predissociation (VP) dynamics of the phenol-water (PhOH-H₂O) dimer were studied by detecting H₂O fragments and using velocity map imaging (VMI) to infer the internal energy distributions of PhOH cofragments, pair-correlated with selected rotational levels of the H₂O fragments. Following infrared (IR) laser excitation of the hydrogen-bonded OH stretch fundamental of PhOH (Pathway 1) or the asymmetric OH stretch localized on H₂O (Pathway 2), dissociation to H₂O + PhOH was observed. H₂O fragments were monitored state-selectively by using 2+1 Resonance-Enhanced Multiphoton Ionization (REMPI) combined with time-of-flight mass spectrometry (TOF-MS). VMI of H₂O in selected rotational levels was used to derive center-of-mass (c.m.) translational energy (E_T) distributions. The pair-correlated internal energy distributions of the PhOH cofragments derived via Pathway 1 were well described by a statistical prior distribution. On the other hand, the corresponding distributions obtained via Pathway 2 show a propensity to populate higher-energy rovibrational levels of PhOH than expected from a statistical distribution and agree better with an energy-gap model. The REMPI spectra of the H₂O fragments from both pathways could be fit by Boltzmann plots truncated at the maximum allowed energy, with a higher temperature for Pathway 2 than that for Pathway 1. We conclude that the VP dynamics depends on the OH stretch level initially excited.

Predissociation dynamics of the HCl-(H2O)3 tetramer: An experimental and theoretical investigation

The cyclic HCl-(H₂O)₃ tetramer is the largest observed neutral HCl-(H₂O)_n cluster. The vibrational predissociation of HCl-(H₂O)₃ is investigated by theory, quasiclassical trajectory (QCT) calculations, and experiment, following the infrared excitation of the hydrogen-bonded OH-stretch fundamental. The energetically possible dissociation pathways are HCl + (H₂O)₃ (Pathway 1) and H₂O + HCl-(H₂O)₂ (Pathway 2). The HCl and H₂O monomer fragments are observed by 2 + 1 resonance enhanced multiphoton ionization combined with time-of-flight mass spectrometry, and their rotational energy distributions are inferred and compared to the theoretical results. Velocity map images of the monomer fragments in selected rotational levels are used for each pathway to obtain pair-correlated speed distributions. The fragment speed distributions obtained by experiment and QCT calculations are broad and structureless, encompassing the entire range of allowed speeds for each pathway. Bond dissociation energies, D₀, are estimated experimentally from the endpoints of the speed distributions: 2100 ± 300 cm^-1 and 2400 ± 100 cm^-1 for Pathway 1 and Pathway 2, respectively. These values are lower but in the same order as the corresponding calculated D₀: 2426 ± 23 cm^-1 and 2826 ± 19 cm^-1. The differences are attributed to contributions from vibrational hot bands of the clusters that appear in the high-speed tails of the experimental pair-correlated distributions. Satisfactory agreement between theory and experiment is achieved when comparing the monomer fragments' rotational energies, the shapes of the speed distributions, and the average fragment speeds and center-of-mass translational energies. Insights into the dissociation mechanism and lifetime are gained from QCT calculations performed on a previously reported many-body potential energy surface. It is concluded that the dissociation lifetime is on the order of 10 ps and that the final trimer products are in their lowest energy cyclic forms.

Acid Dissociation in HCl-Water Clusters is Temperature Dependent and Cannot be Detected Based on Dipole Moments

The dissociation of acids in aqueous environments at low temperatures in the presence of a limited amount of water is underlying a wealth of processes from atmospheric to interstellar science. For the paradigmatic case of HCl(H_{2}O)_{n} clusters, our extensive ab initio path integral simulations quantify in terms of free energy differences and barriers that n=4 water molecules are indeed required to dissociate HCl at low temperatures. Increasing the temperature, however, reverses the process and thus counteracts dissociation by fluctuation-driven recombination. The size of the electric dipole moment is shown to not correlate with the acid being in its dissociated or molecular state, thus rendering its measurement as a function of n unable to detect the dissociation transition.