Vibrational spectra of halide-water dimers: Insights on ion hydration from full-dimensional quantum calculations on many-body potential energy surfaces

Abstract

Full-dimensional vibrational spectra are calculated for both X(HO) and X(DO) dimers (X = F, Cl, Br, I) at the quantum-mechanical level. The calculations are carried out on two sets of recently developed potential energy functions (PEFs), namely, Thole-type model energy (TTM-nrg) and many-body energy (MB-nrg), using the symmetry-adapted Lanczos algorithm with a product basis set including all six vibrational coordinates. Although both TTM-nrg and MB-nrg PEFs are derived from coupled-cluster single double triple-F12 data obtained in the complete basis set limit, they differ in how many-body effects are represented at short range. Specifically, while both models describe long-range interactions through the combination of two-body dispersion and many-body classical electrostatics, the relatively simple Born-Mayer functions employed in the TTM-nrg PEFs to represent short-range interactions are replaced in the MB-nrg PEFs by permutationally invariant polynomials to achieve chemical accuracy. For all dimers, the MB-nrg vibrational spectra are in close agreement with the available experimental data, correctly reproducing anharmonic and nuclear quantum effects. In contrast, the vibrational frequencies calculated with the TTM-nrg PEFs exhibit significant deviations from the experimental values. The comparison between the TTM-nrg and MB-nrg results thus reinforces the notion that an accurate representation of both short-range interactions associated with electron density overlap and long-range many-body electrostatic interactions is necessary for a correct description of hydration phenomena at the molecular level.

Chemical accuracy in modeling halide ion hydration from many-body representations

“…These differences can be attributed to intrinsic deficiencies associated with the TTM-nrg PEFs which, as all common polarizable FFs, are unable to correctly describe charge transfer and penetration effects through purely classical expressions. It should be noted that fully full-dimensional vibrational spectra calculated for the four X -(H 2 O) dimers and the corresponding isotopologues, X -(D 2 O), at the quantum-mechanical level with both TTM-nrg and MBnrg PEFs have recently been reported [155]. In all cases, the MB-nrg vibrational spectra are in close agreement the the available experimental data, correctly reproducing frequency shifts and tunneling splittings, while significant deviations from the experimental values are found with the TTM-nrg PEFs, which again emphasize the difficulties of purely classical representations in correctly describing halide-water interactions.…”

“…As shown in Figure 3, however, the two water molecules are H-bonded to each other in the minimum energy structures of the X -(H 2 O) 2 complexes only in the presence of chloride, bromide, and iodide. This can be explained by considering the different size, charge density, and polarizability of each ion, which, directly affecting the relative strengths of halide-water and water-water H-bonds, progressively stabilize the cyclic configuration, going from Fto I - [120,155].…”

Chemical Accuracy in Modeling Halide Ion Hydration from Many-Body Representations

“…It should be noted that fully full-dimensional vibrational spectra calculated for the four X -(H 2 O) dimers and the corresponding isotopologues, X -(D 2 O), at the quantum-mechanical level with both TTM-nrg and MB-nrg PEFs have recently been reported[155]. In all cases, the MB-nrg vibrational spectra are in close agreement the the available experimental data, correctly reproducing frequency shifts and tunneling splittings, while significant deviations from the experimental values are found with the TTM-nrg PEFs, which again emphasize the difficulties of purely classical representations in correctly describing halide-water interactions.…”

The Effect of Ions on the Optical Absorption Spectra of Aqueously Solvated Chromophores

“…A variety of studies suggest that ions lead to the perturbation to the electronic structure of bulk water due to a combination of the electrostatic presence of the ion and the distortion of the hydrogen bond network in the first solvation shell of the ion. [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] Although the optical absorption of anionic chromophores is of interest in both biology and energy related applications, it is unclear how the non-absorbing ions in solution with the chromophore affect the optical absorption. Perturbations to the optical absorption may occur due to the continuum field caused by the background electrolyte or due to a local change to aqueous solvation about a chromophore caused by ion disruption of the hydrogen bond network of water.…”