Vibrations of Guanine-Cytosine pair in chloroform: an anharmonic computational study
We compute at the anharmonic level the vibrational spectra of the Watson-Crick dimer formed by guanosine (G) and cytidine (C) in chloroform, together with those of G, C and the most populated GG dimer. The spectra for deuterated and partially deuterated GC are also computed. We use DFT calculations, with B3LYP and CAM-B3LYP as reference functionals. Solvent effects are included via the Polarizable Continuum Model (PCM), and by performing tests on models including up two chloroform molecules. Both B3LYP and CAM-B3LYP calculations reproduce the shape of the experimental spectra well in the fingerprint (1500-1700~cm-1) and in the N-H stretching region (2800-3600 cm-1), with B3LYP providing better quantitative agreement to experiments. According to our calculations, the N-H amido streching mode of G falls at ~2900 cm-1, while the N-H amino of G and C fall at ~3100 cm-1 when hydrogen-bonded, or ~3500 cm-1 when free. Overtone and combination bands strongly contribute to the absorption band at ~3300 cm-1. Inclusion of bulk solvent effects significantly increases the accuracy of the computed spectra, while solute-solvent interactions have a smaller, though still noticeable, effect. The presence of the sugar ring modulates the relative intensity of the different peaks in the fingerprint region, without affecting the N-H stretching modes. Based on a critical comparison between the computed spectra and the available experimental results, many key aspects of the anharmonic treatment of strongly vibrationally coupled supermolecular systems and the related methodological issues are discussed.