THEORETICAL-STUDIES OF BORANAMINE AND ITS CONJUGATE BASE - COMPARISONOF THE B-N AND B-P PI-BOND ENERGIES

To answer questions that have arisen about the relative strengths of t he B-N and B-P pi bonds, we have investigated BH2NH2 and BH2NH- by the methods of ab initio molecular electronic structure theory, using bas is sets of high quality, and we have characterized them with respect t o-their molecular geometries, energies, dipole moments, normal vibrati onal modes, and harmonic vibrational frequencies. With the aid of the latter we have been able to resolve several disagreements in the liter ature on the interpretation of the BH2NH2 infrared absorption spectrum Aminoborane has two rotational transition states, one of C(s) symmetr y at 32.4 kcal/mol above the planar ground state and one of C2v symmet ry at 37.9 kcal/mol. The former has a BNH angle of 110.50 and an out-o f-plane angle at nitrogen of 55.4-degrees. Surprisingly, it is the d o rbitals of the nitrogen atom that give the C(s) transition state its l ower energy and its approximately tetrahedral geometry at nitrogen. Th e BH2NH- anion has planar geometry and a BN bond length of 1.366 angst rom. it has an inversion transition state at 20.8 kcal/mol above the g round state, but it has no rotational transition state. Finally, compa rison of BH2NH2 and BH2PH2 shows that the B-P pi bond is stronger than the B-N pi bond (40.5 versus 37.9 kcal/mol). Furthermore, it is shown that these bonds are overwhelmingly of ppi-ppi type.