A theoretical comparison of the lowest-lying singlet and triplet states of H 2CBe and of HCBeH

Abstract

The low-lying singlet and triplet states of H 2CBe and HCBeH are examined using ab inito molecular orbital theory. In agreement with earlier results, the lowest-lying structure of H 2CBe has C 2v symmetry and is a triplet with one π electron ( 3 B 1). The results presented here suggest that the lowest-energy singlet structure is the ( 1B 1) open-shell singlet, also with C 2v symmetry, at least 2.5 kcal/mol higher in energy. The singlet C 2v structure with two π electrons ( 1A 1) is 15.9 kcal/mol higher than 3B 1. All of these structures are bound with respect to the ground state of methylene and the beryllium atom. In HCBeH, linear equilibrium geometries are found for the triplet ( 3Σ) and singlet ( 1Δ) states. The triplet is more stable than the singlet ( 1Δ) by 35.4 kcal/mol, and is only 2.9 kcal/mol higher in energy than triplet H 2 CBe. Since the transition structure connecting these two triplet molecules is found to be 50.2 kcal/mol higher in energy than H 2 CBe, both triplet equilibrium species might exist independently. The harmonic vibrational frequencies of all structures are also reported.