Accurate Borane Sequential Bond Dissociation Energies by High-Level ab Initio Computational Methods

Ab initio molecular orbital calculations at the G-2 and CBS-4 compound levels of theory were used to determine the sequential homolytic bond dissociation energies (BDE's) for a series of B-H, B-C, and B-F bonds in a variety of cyclic and acyclic boranes. The calculated average BDE's agreed very well with the limited experimental data available. However, the first sequential BDE's, which are the most relevant for understanding borane reactivity, were substantially higher than the average BDE's. In general, first BDE's were found to be larger for B-C and B-H bonds in organoboranes than for C-C and C-H bonds in hydrocarbons, even though average B-H and B-C BDE's are lower than average C-H and C-C BDE's. In all the boron substitution patterns examined, B-H and B-C bonds were found to be of almost identical strength, while B-F bonds were found to be much stronger. Moreover, the strengths of B-H and B-C bonds were found to be essentially independent of the electronegativity, pi-donating ability, and conjugative ability of the other substituents on boron. Thus, for instance, a phenyl group was found not to stabilize the odd electron of borane radicals and hence not to lead to reduced B-H or B-C bond strengths. However, B-H bonds of four-coordinate boron were slightly weaker than those of three-coordinate boron.