Substituent effect on ammonia-borane donor-acceptor complexes:: a G2(MP2) molecular orbital study

H3BNHnMe3-n and Me3-nHnBNH3 (n = 0-3) donor-acceptor complexes have been studied with the G2(MP2) method. Predicted equilibrium structures and dissociation energies have been correlated to the degree of substitution on the donor and the acceptor separately. It is found that successive methyl substitutions on boron reduce the dissociation energies of the complexes, contrary to the successive substitutions on nitrogen. The NBO partitioning scheme suggests that there is no correlation between the charge transfer and the dissociation energies. These results are interpreted in terms of the changes in the HOMO-LUMO gap, the type of acceptor and donor involved, and distortion of the acceptor and donor moieties. (C) 1998 Elsevier Science B.V. All rights reserved.