EFFECT OF MOLECULAR ENVIRONMENT ON THE VIBRATIONAL DYNAMICS OF PYRIMIDINE-BASES AS ANALYZED BY NIS, OPTICAL SPECTROSCOPY AND QUANTUM-MECHANICAL FORCE-FIELDS

A complete set of vibrational spectra, obtained from several spectrosc opic techniques, i.e. neutron inelastic scattering (NIS), Raman scatte ring and infrared absorption (IR), has been used in order to assign th e vibrational modes of pyrimidine bases (uracil, thymine, cytosine) an d their N-deuterated species. The spectra of solid and aqueous samples allowed us to analyse the effects of hydrogen bonding in crystal and in solution. In a first step, to assign the observed vibrational modes , we have resorted to harmonic quantum mechanical force field, calcula ted at SCF + MP2 level using double-zeta 6-31G and D95V basis sets wit h non-standard exponents for d-orbital polarisation functions. In orde r to improve the agreement between the experimental results obtained i n condensed phases and the calculated ones based on isolated molecules , the molecular force field has been scaled. In a second step, to esti mate the effect of intermolecular interactions on the vibrational dyna mics of pyrimidine bases, we have undertaken additional calculations w ith the density functional theory (DFT) method using B3LYP functionals and polarised 6-31G basis sets. Two theoretical models have been cons idered: 1. a uracil embedded in a dielectric continuum (epsilon = 78), and 2. a uracil H-bonded to two water molecules (through N1 and N3 at oms). (C) 1997 Elsevier Science B.V.