On the hydrogen bonding in uracil: its effect on the vibrational spectrum

The effects of intermolecular hydrogen bonding on the vibrational frequencies of uracil are discussed on the basis of ab initio calculations. DFT methods are applied to calculated vibrational frequencies of various uracil dimers, namely six cyclic structures and four T-shaped arrangements, as well as eight uracil–water complexes. Frequency shifts experienced by some normal modes in the dimerisation are clearly correlated with acidity or basicity of the interaction sites. Interaction energies and frequency changes indicate that cooperativity plays a fundamental role in a self-association process. Moreover the cooperative effects increase their contribution with the strength of hydrogen bonding. The theoretical IR spectra of all the hydrogen bonded uracils considered here are compared with the FT-IR spectra of uracil and thymine measured in an N2 matrix at different concentrations.