Hyperconjugation effect in substituted methyl boranes: an orbital deletion procedure analysis

The hyperconjugation effect in the substituted methyl boranes, XCH(2)BH(2) (X = H, CH(3), NH(2), PH(2), OH, SH, F, Cl, Br), has been quantitatively evaluated by using the orbital deletion procedure (ODP), where the p(pi) orbital on boron is deactivated. Except for the case of X = NH(2), which forms a three-membered ring, the magnitude of the hyperconjugative stabilization in all other substituted methylborane ranges from 6.8 to 3.4 kcal/mol. Significant structural changes are observed, particularly the shortening of the central B-C bond distance and the reducing of the corresponding XCB and HCB bond angles. In general, the strength of the hyperconjugative interaction between the occupied sigma(C-X) bond and the vacant p(pi) orbital on boron is correlated to the electronegativity of X, and the competition between the donation ability of the sigma(C-X) and the sigma(C-H) bonds determines the preference of the staggered or eclipsed structure as the energy minimum state. When the donation abilities of the C-X and C-H bonds are comparable, other factors such as electron correlation and steric effect may play elaborate roles in the geometrical propensity of the most stable structures.