Whither BCH2? An <i>ab initio</i> inquiry

Abstract

The boron methylene (BCH2) free radical has never been identified spectroscopically. We have undertaken a series of ab initio calculations to predict the molecular structures, vibrational frequencies, and energies of the ground and first three electronically excited states of BCH2 and its various isomers and isotopologues. In the ground state, we find that the global minimum is linear HBCH, with C2v BCH2 roughly 3935 cm−1 higher and the unstable CBH2 species is a C2v weakly bound structure at 16 384 cm−1, which readily isomerizes to the linear radical. HBCH and BCH2 are separated by a large isomerization barrier (12 825 cm−1), so it may be possible to prepare boron methylene in the gas phase and detect it with spectroscopic methods. The vibrational frequencies and rotational constants of four ground-state isotopologues of BCH2 have been calculated as an aid to future IR matrix isolation and gas-phase microwave studies. Similar calculations are reported for the ground-state linear HBCH species and its cis- and trans-bent excited states. The C̃2B2−X̃2A1 electronic transition in the 320–290 nm region is the only viable option for detecting BCH2 by gas-phase absorption or laser-induced fluorescence techniques. Franck–Condon simulations of the C̃−X̃ absorption and the allowed C̃−X̃ and C̃−B̃ emission transitions have been done for 11BCH2 and 11BCD2. In addition, the rotational structure expected for the 0-0 bands of both isotopologues under supersonic expansion conditions has been simulated. The ab initio data and predicted spectra should be invaluable for attempts to identify the boron methylene free radical in the gas phase.