Structure and energetics of the lowest 1A1 and 1B1 states of dichlorocarbene

A recent (1989) partial structural analysis of spectroscopic measurements by Choe, Tanner, and Harmony led to the conclusion that the first excited electronic state (1B1 state) of CCl2 has a longer bond distance than does the 1A1 ground state. This is consistent with typical Franck–Condon pictures of diatomic potential curves and with experimental results for the analogous CF2 system. However, the earlier (1985) theoretical study of Nguyen, Kerins, Hegarty, and Fitzpatrick came to the qualitatively opposite conclusion, predicting the 1B1 bond distance to be shorter by 0.044 Å, which agrees with the 1989 analysis of rotationally resolved spectra by Clouthier and Karolczak. In this research high level ab initio quantum mechanical methods are used to clarify this question. The most reliable values predicted here for the CCl2 bond distances are r0=1.713 Å (1A1) and r0=1.657 Å (1B1). Large basis sets and high levels of electron correlation are required for excellent agreement with experiment in both structures and energies. The differences between CF2 and CCl2 in the behavior of the C–X bond length upon excitation is discussed. It is also argued that most closed shell carbenes are not two configuration problems.