Theoretical study on HBO+ and HOB+ cations using multiconfiguration second-order perturbation theory.

The HBO(+) and HOB(+) cations have been reinvestigated using the CASSCF and CASPT2 methods in conjunction with the contracted atomic natural orbital (ANO) basis sets. The geometries of all stationary points in the potential energy surfaces were optimized at the CASSCF/ANO and CASPT2/ANO levels. The ground and the first excited states of HBO(+) are predicted to be X(2)Pi and A(2)Sigma(+) states, respectively. It was predicted that the ground state of HOB(+) is X(2)Sigma(+) state. The A(2)Pi state of HOB(+) has unique imaginary frequency. A bending local minimum M1 was found for the first time along the 1(2)A'' potential energy surface and the A(2)Pi state of HOB(+) should be the transition state of the isomerization reactions for M1<--> M1. The CASPT2/ANO potential energy curves (PECs) of isomerization reactions were calculated as functions of the HBO bond angle. Many of the CASSCF and CASPT2 calculated results were different from the previously published QCISD(T) results.