Vibrational studies of biomolecules-II. 2-Thiocytosine

Abstract

The Raman, i.r. and far-i.r. spectra of 2-thiocytosine are reported and vibrational assignments for all the 33 normal modes are proposed. In all, 12 pairs of frequencies have been observed, only six of which could be interpreted as Fermi doublets. However, assuming 2-thiocytosine to be a non-planar molecule, splitting of all the frequencies could be very well explained. A possible explanation could be the non-planarity of the pyrimidine ring and non-planarity at the nitrogen site of the amino group which might cause splitting of the frequencies into two components. Non-equivalence of the two NH bonds of the NH₂ group suggests a difference in the strengths of the two hydrogen bonds. Lowering of the C=S stretching frequency for 2-thiocytosine compared to 2-thiouracil by 19 cm^-1 gives an indication of the involvement of the sulphur atom of the former in the hydrogen bonding.