Electronic and geometrical structure of H2BOBH2. An ab initio study

The structure of the diboroxane H2BOBH2 has been studied by means of ab initio calculations at the MP4/6–311G∗∗//MP2/6–311G∗∗ level. Two minima have been characterized on the potential energy surface (PES): one is of D2d symmetry, analogous to the isoelectronic allene molecule, and the other is of C2v symmetry, lying 0.6 kcal/mol lower in energy than the D2d one. The D2h structure lies 11.3 kcal mol−1 above the absolute minimum and is a peak on the PES (two imaginary frequencies). The preference for a bent structure of C2v symmetry is rationalized through an orbital analysis. A reaction path between the two minima has been computed and may be described as a conrotatory motion of the two BH2 planes, coupled with the BOB angle opening. The associated activation energy is 1.2 kcal mol−1. These results are discussed in light of the experimental data on substituted R2BOBR2 diboroxanes.