Ab initio CASSCF and DFT investigations of (H₂O)₂⁺ and (H₂S)₂⁺: hemi-bonded vs proton-transferred structure

Abstract

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 (H₂O)₂⁺ and (H₂S)₂⁺ species. For (H₂O)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 (H₂S)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 (H₂O)₂⁺ as compared to (H₂S)₂⁺. An analysis of the nature of interactions involved in the (H₂O)₂⁺ and (H₂S)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 (H₂O)₂⁺ and (H₂S)₂⁺.