Theoretical study of the HNS+/HSN+ radical cations

Abstract

The three dimensional CASSCF-MRCI potential energy functions have been calculated for the X2A′ and A2A″ electronic states of HNS+ and for the X2A′ state of the HSN+ isomer. In HNS+, the two lowest states form a linear-bent Renner–Teller pair, the MRCI barrier to linearity for the X-state has been calculated to be 1876cm−1. Both isomers possess a conical intersection resulting from the crossing between the 2Σ+ and 2Π states along the colinear H(2S)+NS+(X1Σ+) dissociation path. The minima of both isomers are found to be separated by a large barrier of 18,042cm−1 (CASSCF), the HNS+ isomer is calculated to be more stable than HSN+ by 12,202cm−1 (34.9kcal/mol) (RCCSD(T)). The proton affinity of the NS radical (NS (X2Π)+H+) has been calculated to be 7.59eV (RCCSD(T)), the dissociation energy of HNS+→H(2S)+NS+(X1Σ+) to be 2.86eV (RCCSD(T)). Due to several low-lying electronic states of NS+, the protonation and the exothermic charge transfer reactions H++NS→H+NS+ are found to proceed via very complex reaction paths including vibronic and spin–orbit couplings. The results of Renner–Teller variational calculations of the rovibronic levels for the electronic ground state of HNS+ are presented. The onset of this coupling is calculated already in the first bending overtone of the X-state. Also the vibrational levels obtained variationally for the X2A′ state of the HSN+ isomer are given.