Development of a polarizable and flexible model of the hydrated ion potential to study the intriguing case of Sc(III) hydration

Abstract

The hydration number of \(\hbox {Sc}^{3+}\) aquaion is still an ongoing debate from both experimental and theoretical perspectives, in fact values between 6 and 9 have been proposed. This theoretical study presents the development of \(\hbox {Sc}^{3+}\)–\(\hbox {H}_{2}\hbox {O}\) intermolecular potentials based on ab initio potential energy surfaces of two scandium hydrates: \([\hbox {Sc}(\hbox {H}_{2}\hbox {O})_6]^{3+}\) and \([\hbox {Sc}(\hbox {H}_{2}\hbox {O})_7]^{3+}\). A flexible and polarizable cation and water model has been employed based on the mobile charge density harmonic oscillator (MCDHO) potential. Two classical molecular dynamics simulations of \(\hbox {Sc}^{3+}\) in water were carried out with these two new potentials. Data analysis shows very similar results from both simulations: The hydration number obtained is 6 and the average Sc–O distance for the first hydration shell is \(2.15\pm 0.01\) A. Estimated hydration enthalpy is very close to the experimental one. The simulated Sc K-edge EXAFS spectrum obtained from the structural information provided by the MD simulation agrees fairly well with the experimental spectrum, as well as the main bands of the vibrational power spectra with the IR and Raman spectra.