The strengths of N-H---N and N-H---O hydrogen bonds in 15 nucleic acid base pairs have been investigated using different descriptors. Geometrical and energetic criteria, atoms in molecules topological parameters, natural bond orbital analysis, and spectroscopic measurements have been used to detect the H-bonds and evaluate their strengths in the intermolecular interactions between five nucleic acid bases. Different correlations have been obtained between many of these descriptors to provide a global view of H-bond interaction. We found good linear correlations for the dependence of some descriptors such as atomic interpenetration and hyperconjugation energy on density at bond critical point, while others like destabilization of H-atom energy, variation in N-H frequency, and NMR parameters correlate in a much worse fashion. The calculations suggest that almost all H-bonds in different base pairs belong to medium strength H-bonds. We found in thymine the H-bond interaction is more likely through the amide-type oxygen while the situation is reverse for uracil in which the urea-type oxygen is more accessible to form an H-bond. Cytosine and guanine can also form H-bonds via their amine-type or amide-type nitrogens. In cytosine, the amine-type nitrogen is involved in an N-H---O bond interaction, while, in guanine, the amide-type nitrogen has a greater contribution to H-bond interaction.

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In this paper, an alternative corresponding states correlation was applied to the perturbed hard-sphere equation of state (PHS EOS) to improve the predictive power of this EOS for modeling the volumetric properties of ionic liquids (ILs) and their mixtures. Two temperature-dependent parameters appearing in the EOS have been determined using two reliable scaling constants, the surface tension and the liquid density, both at room temperature. The predictive power of the proposed model has been assessed by comparing the results with 2366 experimental data points over a broad range of pressures and temperatures. The overall average absolute deviation (AAD) of the calculated densities from literature data was found to be 0.77%. Generally, this work shows that the PHS EOS based on the surface tension property of ILs outperforms the PHS EOS based on critical properties. Moreover, the improved PHS EOS has been applied to predict the volumetric properties of 16 binary mixtures involving ILs. The second partners of binary mixtures studied in this work are water, ethanol, methanol, acetone, acetonitrile, propylene carbonate, and 1-propanol. Furthermore, the binary mixtures of ILs have also been studied. From 1685 data points examined for the aforementioned binary mixtures, the AAD of the calculated densities and the excess volumes from those reported in the literature were found to be 0.38 and 0.56%, respectively. Finally, the nonadditivity behavior of the studied mixtures is also investigated. The sign of the nonadditivity parameter indicates a tendency toward attraction between the unlike molecules in the mixture. However, the value of this parameter is not large, which implies that the hard-sphere model is able to model the excess properties of the present mixtures.

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