Theoretical study of the interaction between HNZ (Z = O, S) and H(2)XNH(2) (X = B, Al). Conventional and dihydrogen bonds.

Theoretical calculations at the MP2/6-311++G(2d,2p) level are used to analyze the interaction between HNZ (Z = O, S) and H(2)XNH(2) (X = B, Al). In the most stable conformation, the complexes are cyclic, the molecules being held together by conventional NHZ hydrogen bonds and by XHHN dihydrogen bonds. Binding energies including ZPE- and BSSE-corrections lie in the range 6.2-6.9 kJ mol(-1) and there is little sensitivity to the nature of the X and Z atoms. In the XHHN dihydrogen bonds, the NH stretching vibrations are blue-shifted in the HNO complexes and red-shifted in the H(2)AlNH(2)-HNS complex. In the conventional NHZ hydrogen bonds, the NH stretching vibrations are red-shifted. The topological parameters at the bond critical point are in the usual range for hydrogen or dihydrogen bonds. A natural bond orbital analysis including the calculation of the atomic charges, hybridization, occupation of the antibonding orbitals and hyperconjugation energies shows that the shifts of the NH stretching vibrations in the conventional and dihydrogen bonds are mainly determined by the changes in occupation of the sigma*(NH) antibonding orbitals. The mechanism of intramolecular coupling is discussed and appears to be different for the HNO and HNS complexes. The analysis of all the theoretical data reveals that the NHZ bonds are stronger in the H(2)BNH(2) than in the H(2)AlNH(2) systems and that the XHHN dihydrogen bonds are stronger in the H(2)AlNH(2) than in the H(2)BNH(2) complexes.