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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]

Factorial design and principal component models are used to determine how ab initio H-bond energies depend on characteristics of the molecular orbital wave functions of HCN–HX linear complexes and acetylene–HX T-shaped complexes, with X=F, NC, Cl, CN and CCH. The results obtained for the two sets of complexes show that factorial design and principal component analyses complement each other. The H-bond energies of the HCN–HX linear complexes are affected mostly by adding polarization functions and by MP2 treatment, which have opposite signs. When polarization functions are introduced in the basis set the calculated H-bond energies decrease by  ≈4 kJ·mol−1. In contrast, when the level of calculation is changed from Hartree–Fock to MP2, the energies are increased by nearly the same amount on average. The principal component analysis shows that for the HCN–HX linear complexes the ab initio results are grouped into four well-separated classes: (I) HF calculations with polarization functions (HF/6-nG** and HF/6-n++G**), with n=31 or 311; (II) MP2 calculations without polarization functions (MP2/6-nG and MP2/6-n++G); (III) MP2 calculations including polarization functions (MP2/6-nG** and MP2/6-n++G**) and (IV) HF results without polarization functions (HF/6-nG and HF/6-n++G). The ab initio results of the group (III) are those that show a better agreement with the available experimental values. The change from the Hartree–Fock to the MP2 level is the most significant effect for the acetylene–HX complexes, increasing the calculated H-bond energy values by about 4.5 kJ·mol−1 on average. Here the ab initio results can be essentially grouped into two classes: HF or MP2 calculations. The H-bond energies calculated from the factorial models for the two sets of complexes are found to deviate only 6.5% from the full ab initio values. [Copyright &y& Elsevier]