FTIR and Raman spectra and fundamental frequencies of biomolecule: 5-Methyluracil (thymine)

Abstract

Raman (200–4000 cm −1) and IR (400–4000 cm −1) spectra of uracil and 5-methyluracil (thymine) have been recorded and analysed in the region 200–4000 cm −1 and vibrational assignment of 5-methyluracil (thymine) for all the 39 (26a′ + 13a′′) normal modes are proposed and distributed as 26a′ species of planar and 13a′′ species of non-planar vibrations. Consistent assignments have been made for the internal modes of the CH 3 group, especially for the anti-symmetric CH 3 stretching and bending modes. However, assuming 5-methyluracil to be non-planar molecule, splitting of all the frequencies could be very well explained. A possible explanation could be the planarity of the pyrimidine ring and non-planarity at carbon site of the methyl group which might cause splitting of the frequencies including three components. The non-equivalence of the three CH bonds of the CH 3 group suggests a difference in the strength of the other CH bond on the pyrimidine ring. These three CH 3 stretching frequencies are distinctly separated from the CH/NH ring stretching frequencies. All other bands have also been assigned different fundamentals/overtones/combinations.