Temperature-dependent low-frequency vibrational spectra of purine and adenine

Terahertz time-domain spectroscopy has been used to measure the vibrational spectra of polycrystalline purine and adenine over the temperature range 4–290 K. A number of well-resolved absorption peaks were observed in the frequency range 0.2–3.0 THz, which are interpreted as originating from intermolecular vibrational modes mediated by hydrogen bonds. We find that as the temperature is reduced, the observed absorption bands resolve into narrower peaks and some shift towards higher frequencies. We explain the temperature dependence of the spectra by the anharmonicity of the vibrational potentials and give an empirical expression to describe the frequency shift.