SCALED QUANTUM-MECHANICAL FORCE-FIELDS AND VIBRATIONAL-SPECTRA OF SOLID-STATE NUCLEIC-ACID CONSTITUENTS .5. THYMINE AND URACIL

The scale factors of the ab initio SCF STO-3G and MINI-1, and semiempirical PM3 harmonic force fields were determined by fitting to the Raman and IR spectra of polycrystalline uracil and thymine. Both in-plane and out-of-plane vibrational modes have been interpreted. The transferability of the scale factors between uracil and thymine and the performance of different computational methods were discussed. The Fermi resonance of the overtones of the out-of-plane deformation vibrations of oxygens with their stretching modes have been proposed as an explanation for the band splitting observed in the 1600-1800 cm-1 region of uracil.