A theoretical comparison of the lowest-lying singlet and triplet states of HCB and HBC

Abstract

The low-lying singlet and triplet states of HCB and HBC were examined using ab initio molecular orbital theory at the MP4/6–311G **//HF/6–31G * + ZPE level. For both of these molecules, linear triplet structures were found to have the lowest energy. The singlet triple-bond structures, HCB and HB −C +, are much less favorable. For HCB, the 3Π state is 12.7 kcal/mol more stable than the 3Σ − state, while for CBH the 3Σ − state is more stable than the 3Π by 9.4 kcal/mol. The energy difference between the lowest energy structures of HCB and HBC is only 3.9 kcal/mol, with the 3Π state of HCB being the most stable. The barrier to conversion of HBC into HCB is 23.7 kcal/mol, suggesting that both triplet species might exist independently. Harmonic vibrational frequencies also are reported to assist in the possible experimental observation of these species.