Vibrational spectrum modifications of the hydrogen fluoride due to complexation with aromatic azines. An ab initio study

MP2/6-31++G(d,p) ab initio calculations have been performed to estimate the effect of the H-bond on the vibrational properties of HF due to its complexation with aromatic azines (CnN6-nHn, with n = 5, 4 or 3). Our calculations have revealed that the greater is the number of nitrogen atoms in the aromatic ring, the smaller will be the HF stretching frequency downward shift and the increase of the H-F stretching intensity due to complexation. For example, the H-F frequency downward shift is 899 cm−1 in pyridine-HF, whereas its corresponding value is 597 cm−1 in 1,3,5-triazine-HF. Their infrared intensity ratios between the complexes and their corresponding monomers are 18.7 and 14.3 cm−1, respectively. We have found a relationship between the HF stretching frequency downward shift and the number and position of the nitrogen atoms in the aromatic ring. As expected, the HF stretching intensity is enormously increased upon complexation and its value can be predicted by using the ∂Pz/∂zH element of the hydrogen polar tensor, where z is the Cartesian axis along the H-F chemical bond.