An ab initio molecular orbital study of the structures and energies of neutral and charged bimolecular complexes of NH3 with the hydrides AHn (A = N, O, F, P, S, and Cl)

Abstract

AbstractHartree‐Fock 6‐31G(d) structures for the neutral, positive ion, and negative ion bimolecular complexes of NH3 with the first‐ and second‐row hydrides AHn (AHn = NH3, OH2, FH, PH3, SH2, and ClH) have been determined. All of the stable neutral complexes except (NH3)2, the positive ion complexes with NH3 as the proton acceptor, and the negative ion complexes containing first‐row anions exhibit conventional hydrogen bonded structures with essentially linear hydrogen bonds and directed lone pairs of electrons. The positive ion complex NH4+ …︁ OH2 has the dipole moment vector of H2O instead of a lone pair directed along the intermolecular line, while the complexes of NH4+ with SH2, FH, and ClH have structures intermediate between the lone‐pair directed and dipole directed forms. The negative ion complexes containing second‐row anions have nonlinear hydrogen bonds. The addition of diffuse functions on nonhydrogen atoms to the valence double‐split plus polarization 6‐31G(d,p) basis set usually decreases the computed stabilization energies of these complexes. Splitting d polarization functions usually destabilizes these complexes, whereas splitting p polarization functions either has no effect or leads to stabilization. The overall effect of augmenting the 6‐31G(d,p) basis set with diffuse functions on nonhydrogen atoms and two sets of polarization functions is to lower computed stabilization energies. Electron correlation stabilizes all of these complexes. The second‐order Møller–Plesset correlation term is the largest term and always has a stabilizing effect, whereas the third and fourth‐order terms are smaller and often of opposite sign. The recommended level of theory for computing the stabilization energies of these complexes is MP2/6‐31+G(2d,2p), although MP2/6‐31+G(d,p) is appropriate for the negative ion complexes.