Isomeric Equilibria, Nuclear Quantum Effects, and Vibrational Spectra of M+(H2O)n=1−3 Clusters, with M = Li, Na, K, Rb, and Cs, Through Many-Body Representations

Abstract

<div> <div> <div> <p>A quantitative characterization of the molecular mechanisms that regulate ion solvation is key to the microscopic understanding of fundamental processes taking place in aqueous environments, with major implications for different fields, from atmospheric chemistry to materials research and biochemistry. This study presents a systematic analysis of isomeric equilibria for small M<sup>+</sup>(H<sub>2</sub>O)<sub>n</sub> clusters, with M = Li, Na, K, Rb, and Cs, from 0 K to 200 K. To determine the relative stability of different isomers of each M<sup>+</sup>(H<sub>2</sub>O)<sub>n </sub>cluster as a function of temperature, replica exchange simulations are carried out at both classical and quantum levels with the recently developed many-body MB-nrg potential energy functions, which have been shown to exhibit chemical accuracy. Anharmonic vibrational spectra are then calculated within the local monomer approximation and found to be in agreement with the available experimental data, providing further support for the accuracy of the MB-nrg potential energy functions. The present analysis indicates that nuclear quantum effects become increasingly im- portant for larger M<sup>+</sup>(H<sub>2</sub>O)<sub>n </sub>clusters containing the heavier alkali metal ions, which is explained in terms of competing ion-water and water-water interactions along with the interplay between energetic and entropic effects. Directly connecting experimental measurements with molecular properties calculated at the quantum mechanical level, this study represents a further step toward the development of a consistent picture of ion hydration from the gas to the condensed phase. </p> </div> </div> </div>

Infrared Signatures of Isomer Selectivity and Symmetry Breaking in the Cs+(H2O)3 Complex Using Many-Body Potential Energy Functions

“…It should be noted that isomeric equilbria for Cs + (H 2 O) 3 were already reported in Ref. 69 at both classical (REMD) and quantum (REPIMD) levels using the (2B+NB)-MB-nrg PEF.…”

“…In the case of monoatomic ions, the charge of the ion dictates the bonding motifs of the surrounding water molecules, with the halide ions typically serving as HB-acceptors within the water HB network and the alkali-metal ions forming noncovalent, ion−dipole complexes. [69][70][71][72][73][74][75][76][77] Cesium occupies a special place in the periodic table since it is the most electropositive element and the most polarizable alkali metal. 78 When hydrated, cesium ions (Cs + ) form clusters that are predominantly bound by electrostatic interactions, which results in low-energy isomers that can interconvert, even at relatively low temperatures.…”

“…78 When hydrated, cesium ions (Cs + ) form clusters that are predominantly bound by electrostatic interactions, which results in low-energy isomers that can interconvert, even at relatively low temperatures. 26,52,69,74,[79][80][81] Both experimental measurements 36,37 and theoretical calculations 59,69 have demonstrated that the most stable structures of small Cs + (H 2 O) n clusters correspond to isomers in which the ion prefers to interact with a network of HB water molecules. A recent analysis of the hydration structure of Cs + in bulk water has shown that an accurate representation of individual low-order many-body interactions is necessary to quantitatively reproduce the experimental extended X-ray absorption fine structure (EXAFS) spectra.…”

“…This competition leads to a unique temperature dependence of the measured infrared spectrum of Cs + (H 2 O) 3 as multiple isomers are populated, even at cryogenic temperatures. 69 Although often observed via tagging experiments, recent advances in cryogenic infrared spectroscopy have enabled the trapping of individual isomers of ion−water clusters and measurements of the associated infrared spectra. 26,27,32,[34][35][36][37][38]40,43,[82][83][84][85][86][87][88][89][90] However, unambiguously dissecting and assigning isomer-specific spectroscopic signatures to de-fined vibrational modes remains challenging for both experiment and theory.…”

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.…”