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, π-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.