Quantum kinetic energy and isotope fractionation in aqueous ionic solutions

Abstract

At room temperature, the quantum contribution to the kinetic energy of a water molecule exceeds the classical contribution by an order of magnitude. The quantum kinetic energy (QKE) of a water molecule is modulated by its local chemical environment and leads to uneven partitioning of isotopes between different phases in thermal equilibrium, which would not occur if the nuclei behaved classically. In this work, we use ab initio path integral simulations to show that QKEs of the water molecules and the equilibrium isotope fractionation ratios of the oxygen and hydrogen isotopes are sensitive probes of the hydrogen bonding structures in aqueous ionic solutions. In particular, we demonstrate how the QKE of water molecules in path integral simulations can be decomposed into translational, rotational and vibrational degrees of freedom, and use them to determine the impact of solvation on different molecular motions. By analyzing the QKEs and isotope fractionation ratios, we show how the addition of the Na + , Cl − and HPO 2− 4 ions perturbs the competition between quantum effects in liquid water and impacts their local solvation structures.

Comparison of Implicit and Explicit Solvent Approaches in Ab Initio Evaluation of Thermochemistry in Solution: Application in Studying Boron Isotope Fractionation in Water

“…In addition to NMA which has been the earliest theoretical approach for the evaluation of equilibrium constant of isotope exchange reactions, another rigorous approach which has been widely used for this purpose is the thermodynamic integration of kinetic energies associated with isotope effects via Path Integral Molecular Dynamics (PIMD). ,,− Based on this approach, the equilibrium constant of an isotope exchange reaction is calculated as K eq = prefix− 3 2 ln ( m 2 m 1 ) + 1 k normalB T true∫ m 1 m 2 false⟨ K normalE ( m ) false⟩ m normald m …”

Supplementary material to "Fragmentation inside PTR-based mass spectrometers limits the detection of ROOR and ROOH peroxides"

“…Recent work has shown that simulations of protein ions in the gas phase are more or less accurate using parameter sets designed for solvated biomolecules, and rather large reductions in the partial charges are needed to cause appreciable effects (Lee et al, 2019). As far as the nitrogen model is concerned, it has been used previously for simulations of gas phase nitrogen and has been shown to perform well (Lee and Kim, 2014;Wang et al, 2020).…”

Theoretical evaluation of equilibrium constant for boron isotopes exchange between boric acid and borate in pure and saline water

“…In addition to NMA which has been the earliest theoretical approach for evaluation of equilibrium constant of isotope exchange reactions, another rigorous approach which has been widely used for this purpose is the thermodynamic integration of kinetic energies associated with isotope effects via Path Integral Molecular Dynamics (PIMD) [17,29,[40][41][42][43]. Based on this approach, the equilibrium constant of an isotope exchange reaction is calculated as:…”