α- and π-bond strengths in main group 3-5 compounds

The sigma- and pi-bond strengths for the molecules BH(2)NH(2), BH(2)PH(2), AlH(2)NH(2), and AlH(2)PH(2) have been calculated by using ab initio molecular electronic structure theory at the CCSD(T)/CBS level. The adiabatic pi-bond energy is defined as the rotation barrier between the equilibrium ground-state configuration and the C(s)symmetry transition state for torsion about the A-X bond. We also report instrinsic pi-bond energies corresponding to the adiabatic rotation barrier corrected for the inversion barrier at N or P. The adiabatic sigma- bond energy is defined as the dissociation energy of AH(2)XH(2) to AH(2) + XH(2) in their ground states minus the adiabatic pi-bond energy. The adiabatic sigma-bond strengths for the molecules BH(2)NH(2), BH(2)PH(2), AlH(2)NH(2), and AlH2PH2 are 109.8, 98.8, 77.6, and 68.3 kcal/mol, respectively, and the corresponding adiabatic pi-bond strengths are 29.9, 10.5, 9.2, and 2.7 kcal/mol, respectively.