Isotope effects in aqueous solvation of simple halides

Abstract

We present a path-integral-molecular-dynamics study of the thermodynamic stabilities of DOH⋯ X and HOD⋯ X (X = F, Cl, Br, I) coordination in aqueous solutions at ambient conditions. In agreement with experimental evidence, our results for the F case reveal a clear stabilization of the latter motif, whereas, in the rest of the halogen series, the former articulation prevails. The DOH⋯ X preference becomes more marked the larger the size of the ionic solute. A physical interpretation of these tendencies is provided in terms of an analysis of the global quantum kinetic energies of the light atoms and their geometrical decomposition. The stabilization of the alternative ionic coordination geometries is the result of a delicate balance arising from quantum spatial dispersions along parallel and perpendicular directions with respect to the relevant O-H⋯X axis, as the strength of the water-halide H-bond varies. This interpretation is corroborated by a complementary analysis performed on the different spectroscopic signals of the corresponding IR spectra.

Vapour Pressure Isotope Effect on Evaporation from Pure Organic Phases - a PIMD Approach

“…All things considered, it is important to mention that to the best of our knowledge, apart from water and halides 91 , there are no examples of isotope effects calculation for pure phase solvents using PIMD, or SCC-DFTB PIMD with the same purpose. Hence, this study serves as a robust test for the methods used, and provides evidence about the current limits of force fields potentials and SCC-DFTB in reproducing parameters rising from mainly non-covalent interactions of different kind: dispersion, hydrogen bonds, halogen bonds, etc.…”

Inclusion of nuclear quantum effects for simulations of nonlinear spectroscopy

“…65,66 Finally, the dynamics generated by the RPMD approximation preserve the initial Boltzmann distribution, which is an appealing feature for any semiclassical approximation to avoid leakage of energy between different modes. 67 The RPMD methodology has been successfully applied to a wide range of problems including rate theory, [68][69][70][71][72][73][74][75][76] simulations of linear spectroscopy in water clusters [77][78][79][80] and bulk water, 39,40,81,82 and in describing diffusion dynamics of liquid systems 21,[83][84][85] to name a few examples. In recent years, different efforts have been made to extend RPMD to describe nonadiabatic dynamics [86][87][88][89] and non-equilibrium conditions.…”

Quantum kinetic energy and isotope fractionation in aqueous ionic solutions

“…Path integral molecular dynamics (PIMD) simulations allow NQEs to be exactly included in the calculation of static equilibrium properties, such as QKEs and fractionation ratios, on a given potential energy surface. [35][36][37] Recent studies have combined path integral simulations and empirical fixed charge force fields to assess how NQEs affect the hydrogen bond and water exchange dynamics around monatomic alkali and halide ions, 38,39 the kinetic energy changes they engender in the water molecules around them, 39,40 and the effect of these changes on the fractionation ratios and infrared absorption spectra of the aqueous solutions. 40 In this work, we perform ab initio path integral molecular dynamics (AI-PIMD) simulations, which provide a quantum mechanical description of both the electrons and nuclei, of liquid water and aqueous solutions containing the monatomic Na + and Cl − ions and the polyatomic HPO 2− 4 ion.…”

“…[35][36][37] Recent studies have combined path integral simulations and empirical fixed charge force fields to assess how NQEs affect the hydrogen bond and water exchange dynamics around monatomic alkali and halide ions, 38,39 the kinetic energy changes they engender in the water molecules around them, 39,40 and the effect of these changes on the fractionation ratios and infrared absorption spectra of the aqueous solutions. 40 In this work, we perform ab initio path integral molecular dynamics (AI-PIMD) simulations, which provide a quantum mechanical description of both the electrons and nuclei, of liquid water and aqueous solutions containing the monatomic Na + and Cl − ions and the polyatomic HPO 2− 4 ion. Using these simulations, we demonstrate how the QKE of water molecules obtained from path integral simiulations can be decomposed in terms of their translational, rotational and vibrational degrees of freedom (DOFs).…”