Planar and non-planar borepins

Abstract

Theoretical studies of borepin and its B-substituted derivatives show planar structures with weak out-of-plane bending modes, often leading to boat-shaped borepins. The lowest vibrational frequency of borepin is 167 cm^-1 at both HF/6-31G* and MP2/6-31G* levels, compared to 248 cm^-1 calculated for the isoelectronic tropylium ion. Chloro, methyl and amino substitution on boron leads to even lower frequencies. The MP2/6-31G* geometry of 1-chloroborepin is in good agreement with a recent X-ray structure. The low frequencies of out-of-plane bending of the ring atoms imply that suitable replacement of borepin hydrogens should lead to borepins having boat conformations. Several such cases are discussed.