HCl hydrates as model systems for protonated water

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.

Decomposition of the Experimental Raman and Infrared Spectra of Acidic Water into Proton, Special Pair, and Counterion Contributions

Textbooks describe excess protons in liquid water as hydronium (H₃O⁺) ions, although their true structure remains lively debated. To address this question, we have combined Raman and infrared (IR) multivariate curve resolution spectroscopy with ab initio molecular dynamics and anharmonic vibrational spectroscopic calculations. Our results are used to resolve, for the first time, the vibrational spectra of hydrated protons and counterions and reveal that there is little ion-pairing below 2 M. Moreover, we find that isolated excess protons are strongly IR active and nearly Raman inactive (with vibrational frequencies of ∼1500 ± 500 cm^-1), while flanking water OH vibrations are both IR and Raman active (with higher frequencies of ∼2500 ± 500 cm^-1). The emerging picture is consistent with Georg Zundel's seminal work, as well as recent ultrafast dynamics studies, leading to the conclusion that protons in liquid water are primarily hydrated by two flanking water molecules, with a broad range of proton hydrogen bond lengths and asymmetries.

Insights into acid dissociation of HCl and HBr with internal electric fields

A critical electric field exerted by the solvent on the ionizable group leads to acid dissociation.

Molecular Modes and Dynamics of HCl and DCl Guests of Gas Clathrate Hydrates

Recent years have yielded advances in the placement of unusual molecules as guests within clathrate hydrates (CHs) without severe distortion of the classic lattice structures. Reports describing systems for which observable but limited distortion does occur are available for methanol, ammonia, acetone, and small ether molecules. In these particular examples, the large-cage molecules often participate as non-classical guests H-bonded to the cage walls. Here, we expand the list of such components to include HCl/DCl and HBr as small-cage guests. Based on FTIR spectra of nanocrystalline CHs from two distinct preparative methods combined with critical insights derived from on-the-fly molecular dynamics and ab initio computational data, a coherent argument emerges that these strong acids serve as a source of molecular small-cage guests, ions, and orientational defects. Depending on the HCl/DCl content the ions, defects and molecular guests determine the CH structures, some of which form in sub-seconds via an all-vapor preparative method.

Electric dipole moments of nanosolvated acid molecules in water clusters

The electric dipole moments of (H2O)nDCl (n=3-9) clusters have been measured by the beam-deflection method. Reflecting the (dynamical) charge distribution within the system, the dipole moment contributes information about the microscopic structure of nanoscale solvation. The addition of a DCl molecule to a water cluster results in a strongly enhanced susceptibility. There is evidence for a noticeable rise in the dipole moment occurring at n≈5-6. This size is consistent with predictions for the onset of ionic dissociation. Additionally, a molecular-dynamics model suggests that even with a nominally bound impurity an enhanced dipole moment can arise due to the thermal and zero-point motion of the proton and the water molecules. The experimental measurements and the calculations draw attention to the importance of fluctuations in defining the polarity of water-based nanoclusters and generally to the essential role played by motional effects in determining the response of fluxional nanoscale systems under realistic conditions.

Aqueous solutions: state of the art in ab initio molecular dynamics

The simulation of liquids by ab initio molecular dynamics (AIMD) has been a subject of intense activity over the last two decades. The significant increase in computational resources as well as the development of new and efficient algorithms has elevated this method to the status of a standard quantum mechanical tool that is used by both experimentalists and theoreticians. As AIMD computes the electronic structure from first principles, it is free of ad hoc parametrizations and has thus been applied to a large variety of physical and chemical problems. In particular, AIMD has provided microscopic insight into the structural and dynamical properties of aqueous solutions which are often challenging to probe experimentally. In this review, after a brief theoretical description of the Born-Oppenheimer and Car-Parrinello molecular dynamics formalisms, we show how AIMD has enhanced our understanding of the properties of liquid water and its constituent ions: the proton and the hydroxide ion. Thereafter, a broad overview of the application of AIMD to other aqueous systems, such as solvated organic molecules and inorganic ions, is presented. We also briefly describe the latest theoretical developments made in AIMD, such as methods for enhanced sampling and the inclusion of nuclear quantum effects.

Solvation structures of protons and hydroxide ions in water

X-ray Raman spectroscopy (XRS) combined with small-angle x-ray scattering (SAXS) were used to study aqueous solutions of HCl and NaOH. Hydrated structures of H(+) and OH(-) are not simple mirror images of each other. While both ions have been shown to strengthen local hydrogen bonds in the hydration shell as indicated by XRS, SAXS suggests that H(+) and OH(-) have qualitatively different long-range effects. The SAXS structure factor of HCl (aq) closely resembles that of pure water, while NaOH (aq) behaves similar to NaF (aq). We propose that protons only locally enhance hydrogen bonds while hydroxide ions induce tetrahedrality in the overall hydrogen bond network of water.

Understanding the solubility of triamino-trinitrobenzene in hydrous tetramethylammonium fluoride: a first principles molecular dynamics simulation study

With the aim to understand the relatively high solubility of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), an important energetic material with a high degree of inter- and intra-molecular hydrogen bonding, in fluoride anion containing ionic liquids (ILs), first principles molecular dynamics simulations in the isobaric-isothermal ensemble were carried out for a system using hydrous tetramethylammonium fluoride as the prototypical solvent. Simulations initiated from both molecular TATB and its Meisenheimer complex (i.e., a σ-complex of the fluoride and the electrophilic ring of TATB) yield a Zundel-type complex where a proton is shared between an amino group and an F(-) ion, whereas the Meisenheimer complex is found to be only transiently stable. An analysis of the electronic structure probing the Wannier function centers supports the finding of a proton-sharing complex with a three-center four-electron like bond. The Zundel-type complex also yields an electronic absorption spectrum consistent with the experimentally observed color change. This study provides evidence that the remarkable solubility of otherwise hard-to-dissolve molecular crystals in ILs can be aided by chemical modification of the solute.

Apparent stoichiometry of water in proton hydration and proton dehydration reactions in CH3CN/H2O solutions

Gradual solvation of protons by water is observed in liquids by mixing strong mineral acids with various amounts of water in acetonitrile solutions, a process which promotes rapid dissociation of the acids in these solutions. The stoichiometry of the reaction XH(+) + n(H(2)O) = X + (H(2)O)(n)H(+) was studied for strong mineral acids (negatively charged X, X = ClO(4)¯, Cl¯, Br¯, I¯, CF(3)SO(3)¯) and for strong cationic acids (uncharged X, X = R*NH(2), H(2)O). We have found by direct quantitative analysis preference of n = 2 over n = 1 for both groups of proton transfer reactions at relatively low water concentrations in acetonitrile. At high water concentrations, we have found that larger water solvates must also be involved in the solvation of the proton while the spectral features already observed for n = 2, H(+)(H(2)O)(2), remain almost unchanged at large n values up to at least 10 M of water.

Direct observation of atomic hydrogen generated from the water framework of clathrate hydrates

We demonstrate that the N(2)-induced ionic hydrate system can be a solution to produce the hydrogen radical from water without direct energy sources such as H(2) and CH(4).

The structure of the hydrogen ion (H(aq)+) in water

The hydrogen ion in water, H(aq)(+), is a unique H(13)O(6)(+) entity that defines the boundary of positive-charge delocalization. Its central unit is neither a C(3v) H(3)O(+) Eigen-type ion nor a typical H(5)O(2)(+) Zundel-type ion. IR spectroscopy indicates that the H(13)O(6)(+) ion has an unexpectedly long central O...O separation (>>2.43 A), showing that in comparison with the gas and solid phases, the environment of liquid water is uniquely proficient in delocalizing positive charge. These results will change the description of H(aq)(+) in textbooks of chemistry, and a more extensive delocalization of positive charge may need to be incorporated into descriptions of mechanisms of aqueous proton transport.

Theoretical study on hydrogen-bond effects in IR spectra of high- and low-temperature phases of nitric acid dihydrate

The low- and high-temperature phases (alpha and beta, respectively) of solid nitric acid dihydrate (NAD) are studied in depth by DFT methods. Each phase contains two types of complex structures (H(3)O(+)) x (H(2)O), designated A and B, with different hydrogen-bonding (HB) characteristics. The theoretical study reveals that type A complexes are weakly bound and could be described as (H(3)O)(+) and H(2)O aggregates, with decoupled vibrational modes, whereas in type B structures the proton is situated close to the centre of the O...O bond and induces strong vibrational coupling. The proton-transfer mode is predicted at quite different wavenumbers in each complex, which provides an important differentiating spectral feature, together with splitting of some bands in beta-NAD. Theoretical spectra are estimated by using two GGA parameterizations, namely, PBE and BLYP. The potential-energy surface for each type of HB in NAD is also studied, as is the spectral influence of displacement of the shared H atom along the O-O bond. The results are compared to literature infrared spectra recorded by different techniques, namely, transmission and reflection-absorption, with both normal and tilted incident radiation. This work provides a thorough assignment of the observed spectra, and predictions for some spectra not yet available. The usefulness of high-level theoretical calculations as performed herein to discriminate between two phases of a solid crystal is thus evidenced.

Clathrate hydrates with hydrogen-bonding guests

Clathrate hydrates (CHs) are inclusion compounds in which "tetrahedrally" bonded H(2)O forms a crystalline host lattice composed of a periodic array of cages. The structure is stabilized by guest particles which occupy the cages and interact with cage walls via van der Waals interactions. A host of atoms or small molecules can act as guests; here the focus is on guests that are capable of strong to intermediate H-bonding to water (small ethers, H(2)S, etc.) but nevertheless "choose" this hydrate crystal form in which H-bonding is absent from the equilibrium crystal structure. These CHs can form by exposure of ice to guest molecules at temperatures as low as 100-150 K, at the (low) guest saturation pressure. This is in contrast to the "normal" CHs whose formation typically requires temperatures well above 200 K and at least moderate pressures. The experimental part of this study addresses formation kinetics of CHs with H-bonding guests, as well as transformation kinetics between different CH forms, studied by CH infrared spectroscopy. The accompanying computational study suggests that the unique properties of this family of CHs are due to exceptional richness of the host lattice in point defects, caused by defect stabilization by H-bonding of water to the guests.

Role of charge transfer in the structure and dynamics of the hydrated proton

Although it has long been recognized that multiple water molecules strongly associate with an extra proton in bulk water, some models and conceptual frameworks continue to utilize the classical hydronium ion (H(3)O(+)) as a fundamental building block. In this work, the nature of the hydronium ion in aqueous systems is examined using an ab initio energy decomposition analysis (EDA) that evaluates both the magnitude of and energetic stabilization due to charge transfer among H(3)O(+) and the surrounding water molecules. The EDA is performed on structures extracted from dynamical bulk-phase simulations and used to determine how frequently the pure hydronium ion, where the excess charge is primarily localized on H(3)O(+), occurs under dynamic conditions. The answer is essentially never. The energetic stabilization of H(3)O(+) due to charge delocalization to neighboring water molecules is found to be much larger (16-49 kcal/mol) than for other ions (even Li(+)) and to constitute a substantial portion (20-52%) of the complex's binding energy. The charge defect is also shown to have intrinsic dynamical asymmetry and to display dynamical signatures that can be related to features appearing in IR spectra.