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 extern al electric Fields and chemical substitution on the properties of complexes stabilized by N-H-N hydrogen bonds. Two-dimensional MP2/aug'-cc-pVTZ poten tial energy surfaces in the N-a-H and N-b-H coordinates are generated for C NaH:NbCH in the presence of external electric fields, and equilibrium dista nces and two-dimensional anharmonic dimer- and proton-stretching frequencie s are obtained. The N-N spin-spin coupling constant across the N-H-N hydrog en 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 do nors and seven nitrogen bases as proton acceptors. Equilibrium structures, binding energies, and harmonic frequencies and intensities for the proton-s tretching vibration are computed at MP2/6-31+G(d,p), Examination of both fi eld and substituent effects allows correlations to be established between s tructures, hydrogen bond type, and proton-stretching frequencies. In the ca se of CNaH:NbCH these correlations extend to the NMR properties of the hydr ogen bond. Both approaches suggest that proton-shared and ion-pair N-H-N hy drogen bonds are unlikely to form in neutral complexes.