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Abstract: 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 (C n N6-n H n , 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 899cm−1 in pyridine-HF, whereas its corresponding value is 597cm−1 in 1,3,5-triazine-HF. Their infrared intensity ratios between the complexes and their corresponding monomers are 18.7 and 14.3cm−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 ∂P z /∂z H element of the hydrogen polar tensor, where z is the Cartesian axis along the H–F chemical bond. [Copyright &y& Elsevier]

Abstract: Ab initio molecular orbital calculations have been performed at the MP2/6-31++G** theoretical level in order to obtain molecular properties of hydrogen-bonded complexes involving pyrazine and the HX linear acids with, X=F, NC, Cl, CN, and CCH. Both the 1:1 and 1:2 adducts have been investigated, i.e., Pyz⋯HX and XH⋯Pyz⋯HX. The H-bond strength for these complexes can be interpreted in terms of the H–X stretching frequency downward displacement. This latter shows a good linear correlation with the intermolecular charge transfer and follows the order: X=F>X=NC>X=Cl>X=CN>X=CCH. As expected, the H–X stretching intensity is much enhanced upon H-bond formation. Here these effects on the HX infrared spectra are much more pronounced than those previously found in complexes with unsaturated hydrocarbons as proton acceptors. On the other hand, no important change has been verified in pyrazine, except the inhibition of the lone-pair trans effect from the lone-pair of nitrogen to the σ* antibonding orbital of the CC chemical bond in pyrazine. The new vibrational modes arising from complexation show several interesting features, specially the bending modes of the proton-donor molecule and the intermolecular stretching mode. [Copyright &y& Elsevier]