Ab initio prediction of the geometry and IR frequencies of the mono- and dihydrated complexes of the oxo-amino-tautomers of guanine

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The results of ab initio post-Hartree–Fock study of the molecular structures, relative stabilities, and Hartree–Fock level calculations of IR frequencies and intensities for mono- and dihydrated oxo-amino-tautomers of guanine are reported. The geometries of the local minima were optimized without symmetry restrictions at the second-order Møller–Plesset perturbation theory. The standard 6-31 G(d) basis set was used. Local minima were verified by calculations of the matrix of energy second derivatives [Hessian at the HF/6-31 G(d) level]. The single-point calculations were also performed at the MP4(SDQ)/6-31G(d)//MP2/6–31G(d) and MP2/6-31++G(d, p)//MP2/6-31G(d) levels of theory. The total energies were corrected for the zero-point energy contributions scaled by a factor of 0.9. The structural parameters of mono- and dihydrated complexes were analyzed and related to the characteristics of hydrogen bonds. We found that the interaction of guanine tautomers with two water molecules changes the order of the gas-phase stability: 7GUA≈9GUA into the order which corresponds to the stability of the guanine tautomers in the polar solvent: 9GUA≫7GUA. The predicted IR HF/6-31G(d) level frequencies and intensities are presented. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 759–765, 1997