Hybridization of Nitrogen Determines Hydrogen Bond Acceptor Strength: Gas Phase Comparison of Redshifts and Equilibrium Constants.

Gas-phase Fourier transform infrared spectroscopy and quantum chemical calculations are combined to illustrate the effect of hybridization on the hydrogen-bond acceptor strength of nitrogen by a comparison of nine bimolecular complexes. We present gas-phase results for the complexes of methanol, ethanol, and 2,2,2-trifluoroethanol with acetonitrile (sp-hybridized N) and find that the structure of these complexes is nearly linear and dominated by the OH center dot center dot center dot NI hydrogen bond with no experimental indication of an OH-pi bonded structure. We compare experimental redshifts and equilibrium constants, obtained by combining experiments and theory, for these complexes to the corresponding complexes with pyridine (sp(2)-hybridized N) and trimethylamine (sp(3)-hybridized N). The comparison clearly illustrates that increasing the s-character of the nitrogen lone pair decreases the hydrogen-bond acceptor strength (sp(3) > sp(2) > sp). The observed trend correlates with the basicity of the acceptors and can be explained by the partial charge on the accepting nitrogen atom and the degree of localization of the nitrogen lone pair.