The low-lying electronic states and harmonic vibrational frequencies of HCB, HBC, HCB − and HBC −

Abstract

The low-lying singlet and triplet states of HCB and HBC, and doublet states of HCB − and HBC − are examined using ab initio molecular orbital theory at the HF/TZPP, FOCI/MCSCF level. For the neutral molecules, several new low-lying electronic states are found, and the linear triplet structures are found to have the lowest energy. For the anions, the 2 Σ + states are lower in energy than the 2 Π states. For HCB, the 3 Π state is 15.38 kcal mol −1 more stable than the 3 Σ − state, and for HBC the 3 Π is 2.51 kcal mol −1 below the 3 Σ − state at the first order configuration interaction (FOCI) level. The barriers to conversion of HBC( 3Π to HCB( 3Π) , HBC( 1Δ( 1 A 1)) to HCB( 1 A′C s) and HBC( 1 Δ( 1 A 2)) to HCB( 1A″ C s) are 23.21, 12.97 and 21.09 kcal mol −1, respectively. The activation energies to conversion of HBC − ( 2Σ +) to HCB − ( 2Σ +) and HCB − ( 2Π) to HBC − ( 2Π) are 18.13 and 25.88 kcal mol, respectively. The energies of fragmentation of HCB and HBC, and the harmonic vibrational frequencies of the neutral molecules and the anions are also reported to provide information for the possible experimental observation of these species.