Steric and hyperconjugative contributions to rotation barriers in diboron analogs B2L4 (L=F, OH, NH2, CH3, and H)

Abstract

The rotational barriers of B(2)L(4) (where L = F, OH, NH(2), CH(3), and H) were investigated with respect to the hyperconjugation and steric effects at the levels of RHF, MP2, and B3LYP using a basis set of 6-311G(d,p). At all levels of theory, the LBBL dihedral angle of the minimum energy conformation was found to increase with decreasing electronegativity of the atom bonded to boron. The L substituents that possess no lone pairs have greater steric exchange energy, destabilizing the planar conformation relative to the perpendicular. However, the planar conformation of compounds with L substituents having lone pairs on the heavy atom is counterstabilized by conjugative delocalization into the "empty" p-orbitals associated with boron. For B(2)(NH(2))(4), neither factor dominates and the optimal geometry is skewed at a dihedral angle of 37.62 degrees (MP2).