NEUTRON INELASTIC-SCATTERING, OPTICAL SPECTROSCOPIES, AND SCALED QUANTUM-MECHANICAL FORCE-FIELDS FOR ANALYZING THE VIBRATIONAL DYNAMICS OF PYRIMIDINE NUCLEIC-ACID BASES - THYMINE

As in the case of uracil, a complete set of vibrational spectra of thy mine and its N-deuterated species, obtained from several spectroscopic techniques, i.e., neutron inelastic scattering (NIS), Raman scatterin g, and infrared absorption (IR), has been used in order to assign the vibrational modes on the basis of an ab initio scaled quantum mechanic al (SQM) force field. NIS, Raman, and IR spectra of polycrystalline th ymine recorded at T = 15 K provide complementary data for analyzing di fferent groups of molecular vibrational modes. Solid-state spectra hav e been supplemented with FT Raman (lambda(exc) = 1.06 mu m) and IR spe ctra of aqueous solutions. The spectra from both phases allowed us to analyze the effects of the environment related to strong (crystal) or weak (solution) hydrogen bondings. The molecular fundamental wavenumbe rs calculated at the SCF + MP2 level, by using different types of mole cular orbitals, have first been compared with the experimental wavenum bers taken from the gas phase of thymine. Then the force field has bee n scaled in order to improve the agreement with experimental data from solid and aqueous phases. The scaling procedure is similar to that es tablished in the case of uracil. We have used the Pulay method in orde r to improve the wavenumbers and NIS intensities corresponding to the bond stretchings and angular bendings as well as those related to the methyl group vibrational motions. The majority of these scaling factor s lie close to unity (between 0.8 and 1.1), except for the methyl tors ion, for which a large value of 1.4 was needed. The other force consta nts related to the ring torsional and wagging motions have been scaled by the least-squares refinement of the off-diagonal force constant ex pressed in terms of internal coordinates.