MOLECULAR VIBRATIONS OF SOLVATED URACIL - AB-INITIO REACTION FIELD CALCULATIONS AND EXPERIMENT

Harmonic vibrational frequencies and transition strengths in uracil ha ve been calculated in self-consistent reaction fields of low (epsilon = 1.53) and high (epsilon = 78.54) dielectric constant using ab initio Hartree-Fock and density functional theory methods at the 6-31+G() l evel of theory. Uniformly scaled frequencies calculated in low dielect ric medium agree well with infrared spectra of uracil in argon matrix, Delta nu(avg) = 2.2 cm(-1), although only partial agreement is obtain ed for individual matrix-induced frequency shifts and intensity change s. Reaction field calculations with a tighter spherical cavity or solu te cavity determined by the isodensity polarizable continuum method yi eld better match with experiment for certain vibrations, Iri a polar p rotic medium, the vibrational analysis is extended beyond neutral urac il to its (de)protonation derivatives selected by reaction field calcu lations. Unsealed vibrational frequencies, as well as infrared and Ram an intensities of the uracil-4-ol cation, neutral uracil, uracil N1-an ion, and uracil N1,3-dianion calculated in continuous high dielectric medium are found to agree fairly well with vibrational spectra of urac il in aqueous media recorded over a wide pH range. The deficiencies of the reaction field model, like hydrogen bonding and ion-solvent inter actions, are highlighted and their contributions quantitatively estima ted.