Structural aspects of donor-acceptor interactions

Quantum chemical calculations at the MP2(FC)/6-311 + G(2df,p) level have been performed for the donor-acceptor complexes BH3NH3, BCl3NH3, AlH3NH3, AlCl3NH3, and GaCl3NH3. Variations of the potential energy, total charge transfer, and geometrical parameters have been investigated along the d(AN) (A = B, Al, Ga) donor-acceptor distance from a compressed (1.5 Angstrom) arrangement up to dissociation. Among the well-known geometrical consequences of complex formation (lengthening of the A-L and N-H bonds, increase of the L-A-N and H-N-A bond angles), those in the AL(3) acceptors are well pronounced whereas those in NH3 are rather small. The geometrical effects show a gradual decrease with increasing dAN, but most of them can still be recognized at considerable elongation (by 2 Angstrom) from the equilibrium donor-acceptor distance. Among the investigated properties, the potential energy and the L-A-N angle are the most sensitive indicators of the interaction.