BCH2 exists

The elusive boron carbene (BCH2) free radical has been observed spectroscopically for the first time. The reactive species was generated in a free expansion discharge jet with a precursor mixture of trimethylboron (TMB) in high pressure argon. A 11BCH2 laser-induced fluorescence spectrum consisting of only two bands (000 and 201) was found in the 318-305 nm region, in accord with our prior ab initio predictions. Experiments utilizing TMB-d9 and isotopically enriched 10B TMB precursors confirmed the existence of the boron carbene species, despite the fact that it is predicted to be some 4000 cm-1 less stable than the HBCH global minimum. High resolution studies of the 0-0 bands of 11BCH2, 11BCD2, 10BCH2, and 10BCD2 have yielded the rotational constants and molecular structures of boron carbene in the excited C̃2B2 and ground X̃2A1 states. In sharp contrast to the other group 13 carbenes (MCH2; M = Al, Ga, In), which have 2B1 ground states and metal-carbon single bonds, X̃2A1 BCH2 has a boron-carbon double bond with a length of 1.391(1) Å, a consequence of the better orbital match of the boron 2s and 2p orbitals with those of methylene.