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 semiempi rical PM3 harmonic force fields were determined by fitting to the Rama n and IR spectra of polycrystalline uracil and thymine. Both in-plane and out-of-plane vibrational modes have been interpreted. The transfer ability of the scale factors between uracil and thymine and the perfor mance of different computational methods were discussed. The Fermi res onance of the overtones of the out-of-plane deformation vibrations of oxygens with their stretching modes have been proposed as an explanati on for the band splitting observed in the 1600-1800 cm-1 region of ura cil.