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

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 N(a)-H and N(b)-H coordinates are generated for CN(a)H:N(b)CH 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 CN(a)H:N(b)CH 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.