Theoretical studies of simple organoboron compounds: Structures and stabilities of BC2H4 isomers

An ab initio study of the BC2H4 has been carried out. Several isomers have been studied: boron-ethylene complexes resulting from perpendicular and sigma interactions, boron-methylcarbene type structures, open and cyclic insertion products with a B-H bond, and isomers with two B-H bonds. Geometries at the second-order Moller-Plesset level have been obtained for ten species, and their characteristics have been tested through computation of vibrational frequencies. Special attention has been paid to their molecular structure, trying to rationalize the bonding in these species, particularly in the case of cyclic isomers. Electronic energies have been computed at the fourth-order Moller-Plesset level. Our theoretical calculations predict that the global minimum is the H2BCCH2 isomer (B-2(2) electronic state), with the HBH plane perpendicular to the CCH2 moiety, and that it lies about 66 kcal mol(-1) (276 kJ mol(-1)) below groundstate boron+ethylene. Nevertheless there are other isomers which are also quite stable and might be accessible to experimental detection, such as HBCHCH2 and H2BCHCH (both with (2)A' electronic states) which lie 8 (33.5) and 13 kcal mol(-1) (54.4 kJ mol(-1)), respectively, above the global minimum, as well as two cyclic structures, the insertion product HBC2H3((2)A '') and the boron-ethylene pi-complex BC2H4 ((2)A(1)), which are relatively stable since they lie 12 (50) and 22 kcal mol(-1) (92 kJ mol(-1)), respectively, higher than H2BCCH2.