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/TZP, FOCI/MCSCF level. For the neutral molecules, several new low-lying electroinc states are found, and the linear triplet structures are found to have the lowest energy. For the anions, the ~2∑~+ states are lower in energy than the ~2Π states. For HCB, the ~3Π state is 15.38 kcal·mol~(-1) more stable than the ~3∑~- state, and for HBC the ~3Π is 2.51 kcal·mol~(-1) below the ~3∑~- state at the first order configuration interaction (FOCI) level. The barriers of the conversion of HBC(~3Π) to HCB(~3Π), HBC(~1Δ(~1A_1)) to HCB(~1A′C_s) and HBC(~1Δ(~1A_2)) to HCB(~1A″C_s) are 23.21, 12.97 and 21.09 kcal·mol~(-1), respectively. The activation energies of the conversion of HBC~-(~2∑~+) to HCB(~2∑~+) and HCB~-(~2Π) to HBC~-(~2Π) are 18.13 and 25.88 kcal·mol~(-1), respectively. A symmetry breaking phenomenon is observed in several states. 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.

Key words: HCB, Spectra excited states, FOCI/MCSCF