Can Proton-Shared or Ion-Pair N−H−N Hydrogen Bonds Be Produced in Uncharged Complexes? A Systematic ab Initio Study of the Structures and Selected NMR and IR Properties of Complexes with N−H−N Hydrogen Bonds

Abstract

Ab initio calculations are carried out to investigate the effects of external electric fields and chemical substitution on the properties of complexes stabilized by N−H−N hydrogen bonds. Two-dimensional MP2/aug‘-cc-pVTZ potential energy surfaces in the Na−H and Nb−H coordinates are generated for CNaH:NbCH in the presence of external electric fields, and equilibrium distances and two-dimensional anharmonic dimer- and proton- stretching frequencies are obtained. The N−N spin−spin coupling constant across the N−H−N hydrogen bond, and the chemical shift of the hydrogen-bonded proton are computed for the equilibrium structure at each field strength. Substitutent effects on the properties of complexes stabilized by N−H−N hydrogen bonds are also examined in complexes with pyrrole and di-substituted pyrroles as proton donors and seven nitrogen bases as proton acceptors. Equilibrium structures, binding energies, and harmonic frequencies and intensities for the proton-stretching vibration are computed at MP2/6-31+G(d,p). Examination of both field and substituent effects allows correlations to be established between structures, hydrogen bond type, and proton-stretching frequencies. In the case of CNaH:NbCH these correlations extend to the NMR properties of the hydrogen bond. Both approaches suggest that proton-shared and ion-pair N−H−N hydrogen bonds are unlikely to form in neutral complexes.