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

The effects of intermolecular hydrogen bonding on the vibrational frequenci es of uracil are discussed on the basis of ab initio calculations. DFT meth ods are applied to calculated vibrational frequencies of various uracil dim ers, namely six cyclic structures and four T-shaped arrangements, as well a s 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 indica te that cooperativity plays a fundamental role in a self-association proces s. Moreover the cooperative effects increase their contribution with the st rength of hydrogen bonding. The theoretical IR spectra of all the hydrogen bonded uracils considered here are compared with the FT-IR spectra of uraci l and thymine measured in an N-2 matrix at different concentrations.