Molecular Vibrations of Solvated Uracil. Ab Initio Reaction Field Calculations and Experiment

Abstract

Harmonic vibrational frequencies and transition strengths in uracil have been calculated in self-consistent reaction fields of low (ε = 1.53) and high (ε = 78.54) dielectric constant using ab initio Hartree−Fock and density functional theory methods at the 6-31+G* level of theory. Uniformly scaled frequencies calculated in low dielectric medium agree well with infrared spectra of uracil in argon matrix, Δν(avg) = 2.2 cm-1, although only partial agreement is obtained for individual matrix-induced frequency shifts and intensity changes. Reaction field calculations with a tighter spherical cavity or solute cavity determined by the isodensity polarizable continuum method yield better match with experiment for certain vibrations. In a polar protic medium, the vibrational analysis is extended beyond neutral uracil to its (de)protonation derivatives selected by reaction field calculations. Unscaled vibrational frequencies, as well as infrared and Raman intensities of the uracil-4-ol cation, neutral uracil, uracil N1-anion, and uracil N1,3-dianion calculated in continuous high dielectric medium are found to agree fairly well with vibrational spectra of uracil in aqueous media recorded over a wide pH range. The deficiencies of the reaction field model, like hydrogen bonding and ion−solvent interactions, are highlighted and their contributions quantitatively estimated.