Influence of the F-H⋯N hydrogen bond on the C-H isolated stretching in aromatic azines

MP2/6-31++G(d,p) ab initio molecular orbital calculations have been employed to investigate the effect of the F-H⋯N hydrogen bond between aromatic azines and hydrogen fluoride on the degree of aromaticity of these azines in considering the CH stretching region of its IR spectrum. Our results have revealed a lowering of the aromatic character of the azine ring. This corresponds to larger vibrational frequencies and smaller CH stretching intensities after complexation. An analysis based on a charge-charge flux-overlap model for infrared intensities indicates that this decrease on the intensity values is mainly due to the CH charge flux term. It becomes less negative upon the H-bond formation, whereas the hydrogen charge is only slightly increased.