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The chemical compound hydantoin is commonly used in the synthesis of broad-spectrum medicaments with multiple pharmacological activities. There are several papers in literature describing hydantoin derivates, yet few are related to the isolated molecule. In this work, we have performed B3LYP and MP2 calculations with the 6-31++G(d,p) basis set in order to obtain the geometric, energetic, and electronic parameters of the isolated hydantoin and its dimers. We have performed a complete energetic analysis of these complexes considering corrections from the basis set superposition error ( BSSE) and zero-point energy (ZPE) approaches. On the basis of these results, we have proposed and tested an additive model which has successfully predicted the complexation energies of larges hydantoin complexes. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [ABSTRACT FROM AUTHOR]

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]

The present manuscript reports a systematic investigation of the basis set dependence of some properties of hydrogen-bonded (π type) complexes formed by propyne and a HX molecule, where X=F, Cl and CN. The calculations have been performed at Hartree–Fock, MP2 and B3LYP levels. Geometries, H-bond energies and vibrational have been considered. The more pronounced effects on the structural parameters of the isolated molecules, as a result of complexation, are verified on RC≡C and HX bond lengths. As compared to double-ζ (6-31G**), triple-ζ (6-311G**) basis set leads to an increase of R C≡C bond distance, at all three computational levels. In the case where diffuse functions are added to both hydrogen and ‘heavy’ atoms, the effect is more pronounced. The propyne–HX structural parameters are quite similar to the corresponding parameters of acetylene–HX complexes, at all levels. The largest difference is obtained for hydrogen bond distance, R H, with a smaller value for propyne–HX complex, indicating a stronger bond. Concerning the electronic properties, the results yield the following ordering for H-bond energies, ΔE: propyne⋯HF>propyne⋯HCl>propyne⋯HCN. It is also important to point out that the inclusion of BSSE and zero-point energies (ZPE) corrections cause significant changes on ΔE. The smaller effect of ZPE is obtained for propyne⋯HCN at HF/6-311++G** level, while the greatest difference is obtained at MP2/6-31G** level for propyne⋯HF system. Concerning the IR vibrational it was obtained that larger shift can be associated with stronger hydrogen bonds. The more pronounced effect on the normal modes of the isolated molecule after the complexation is obtained for H–X stretching frequency, which is shifted downward. [Copyright &y& Elsevier]