Identification of vibrational modes in thymine bases by broadband far-infrared absorption spectroscopy

Far-infrared and THz absorption spectra were obtained for thymine bases. Quantum chemical calculations were also conducted to simulate the far-infrared spectrum using density functional theory. Three models were used for the simulations. In one model, a thymine monomer was assumed. The second model assumed a thymine dimer, where two pyrimidine rings were aligned antiparallel. The last model assumed a thymine trimer, in which four hydrogen bonds were formed in three pyrimidine rings placed on the same plane. By comparing the measurements and simulations, almost all the absorption peaks appearing from 100 to 600 cm^-1 could be assigned to torsional or skeletal vibrations of a pyrimidine ring. On the other hand, absorption peaks appearing in the THz range from 16.7 to 167 cm^-1 were assigned to intermolecular vibrations.