Ab Initio CASSCF and DFT Investigations of (H_2O)_2~+ and (H_2S)_2~+: Hemi-Bonded vs Proton-Transferred Structure

摘要

High level ab initio calculations using a complete active space self-consistent field (CASSCF) and multiconfigurational quasi-degenerate perturbation theory (MCQDPT2) methods as well as density functional theory (DFT)-based calculations with different exchange-correlation energy density functionals have been performed for predicting the relative stability of the proton-transferred vs hemi-bonded isomers of (H_2O)_2~+ and (H_2S)_2~+ species. For (H_2O)_2~+, DFT calculation using conventional exchange-correlation functionals predicts the hemi-bonded structure to be the ground state while use of full or half Hartree-Fock exchange and local correlation predicts a higher stability of the proton-transferred structure in agreement with ab initio results. For the (H_2S)_2~+ system, all of the methods lead to the prediction of lower energy for the hemi-bonded isomer. No regular trend of the exchange-correlation energy component with the total energy difference is however observed. Dynamical electron correlation effect incorporated through MCQDPT2 is found to be much stronger in (H_2O)_2~+ as compared to (H_2S)_2~+. An analysis of the nature of interactions involved in the (H_2O)_2~+ and (H_2S)_2~+ systems within the framework of Bader's topological theory of atoms in molecules is also presented through the plots of the Laplacian ▽~2ρ of the electron density ρ(r) and also other related quantities at the bond critical points with the objective of rationalizing the relative stability of the two isomers in both (H_2O)_2~+ and (H_2S)_2~+.