Effects of hydrogen bonding with H2O on the resonance Raman spectra of uracil and thymine

Abstract

The effects of explicit hydrogen bonding with H2O on the resonance Raman spectra of uracil and thymine are examined computationally. The three bonding sites in uracil and thymine that form the lowest energy uracil-H2O and thymine-H2O complexes, as well as a more limited number of low-lying ones containing two waters, are considered. The ground state structures are optimized at the PBE0/aug-cc-pVTZ level of theory in H2O (C-PCM), and the gradients of the bright excited state (S1) are computed at the TD-CAM-B3LYP/aug-cc-pVTZ level of theory in H2O (C-PCM). As the resonance Raman spectrum is governed by the ground state normal modes and the excited state Cartesian gradient (within the Herzberg-Teller formalism), the differences between spectra of uracil- and thymine-(H2O)n, n=1 or 2, are compared in terms of these two factors. Explicit hydrogen bonding to water is found to cause changes in both relative peak positions and intensities for the resonance Raman spectra of uracil and thymine when compared to the isolated molecules. The effect of hydrogen bonding is primarily on the ground state normal mode character, especially for the high frequency modes (>1600cm−1), rather than on the excited state Cartesian gradients. Different hydrogen bonding sites are found to have different contributions to the resulting resonance Raman spectra, and inclusion of explicit hydrogen bonding on the carbonyl bond opposite to the ring nitrogen is necessary to obtain good agreement between the simulated and experimental resonance Raman spectra for uracil and thymine.