Ammonia and phosphine complexes with proton donors. Hydrogen bonding from the backside of the N(P) lone pair

Abstract

Quantum chemical calculations at the MP2/aug-cc-pVTZ level of theory were carried out to study the molecular Y…HX (Y = N, P; X = O, Cl, F) complexes formed by ammonia and phosphine with H2O, HCl and HF molecules with the orientation of the XH group to the backside of the N(P) lone pair (referred to as anti-complexes). Various methods used for analysis identify the Y…HX intermolecular interaction in the anti-complexes as hydrogen bonding. The NBO analysis shows that the stabilization of the ammonia and phosphine complexes is mainly determined by the nY → σ∗XH interaction between the acceptor lone pair and anti-σ bond XH of the proton donor. The values of topological parameters at the bond critical point for the H…Y contact are typical for neutral H-bonded molecular complexes; while the bridging hydrogen and acceptor atoms demonstrate mutual penetration, and all criteria concerning the integrated properties of the H atom are satisfied. The binding energy in complexes with the anti-orientation of monomers calculated with the BSSE correction was found to vary in the range of 1–3 kcal/mol. The anti-complexes, like typical H-bonded syn-complexes, show lengthening of the covalent XH bond, red-shift in the XH stretching frequency in IR spectra, and downfield chemical shift of the bridging hydrogen atom. The inversion of the NH3(PH3) group leads to transition of the molecular system between the anti- and syn-configurations corresponding to minima on the potential energy surface.