MOLECULAR-FORCE FIELDS OF URACIL AND THYMINE, THROUGH NEUTRON INELASTIC-SCATTERING EXPERIMENTS AND SCALED QUANTUM-MECHANICAL CALCULATIONS

Neutron inelastic scattering (NIS) data have been obtained from polycr ystalline samples of pyrimidine nucleic bases, i.e. uracil (U) and thy mine (T), These spectra and those previously recorded by optical techn iques (Raman scattering and infrared absorption) led us to carry out a b initio quantum mechanical calculations (SCF + MP2) in using split-va lence gaussian sets with different d-orbital polarization functions fo r carbon, nitrogen and oxygen. The F-x matrices (interatomic force con stant matrices on the basis of cartesian coordinates) derived from the se ab initio calculations have been then described in terms of interna l molecular coordinates, allowing vibrational normal modes to be assig ned in the framework of the so-called Wilson GF-matrix method. In addi tion, atomic displacements estimated from the normal mode calculations have been used to simulate the first-order NIS intensities by only co nsidering the fundamentals of the internal molecular vibrational motio ns, For both molecules (U and T), the internal vibrational modes are l ocated at higher wavenumbers than the lattice vibrational modes (exter nal vibrational motions). In order to scale the calculated vibrational modes the so-called Pulay method has been used. A good agreement betw een calculated and experimental data (vibrational wavenumbers as well as NIS intensities) has been obtained for most of the internal vibrati onal modes.