On the origin of internal rotation in ammonia borane

The internal rotation in ammonia borane (AB) was studied on the basis of natural orbitals for chemical valence (NOCV) and eigenvectors for Pauli repulsion (NOPR). We found that the total hyperconjugation stabilization (ca. 5 kcal mol(-1)), based on the charge transfer from the occupied σ (B-H) orbitals into the empty σ*(N-H), slightly favors the staggered conformation over the eclipsed one; however, the barrier to internal rotation in ammonia borane can be understood predominantly in a 'classical' way, as originating from the steric (Pauli) repulsion contributions (of the kinetic origin) that act solely between N-H and B-H bonds. Repulsion between the lone pair of ammonia and the adjacent B-H bonds was found to be dominant in absolute terms; however, it does not determine the rotational barrier. Similar conclusions on the role of CH↔HC repulsion appeared to be valid for isoelectronic ethane.