THE MOLECULAR-FORCE FIELD OF GUANINE AND ITS DEUTERATED SPECIES AS DETERMINED FROM NEUTRON INELASTIC-SCATTERING AND RESONANCE RAMAN MEASUREMENTS

Neutron inelastic scattering (NIS) spectra from polycrystalline sample s and ultraviolet resonance Raman scattering (RRS) spectra from aqueou s solutions of guanine and C8-deuterated and (N9, N1, C2-amino)-deuter ated guanine are reported. These measurements allowed theoretical simu lations of the vibrational wavenumbers and intensities of the NIS and RRS bands to be performed. A valence force field enabled the normal mo de wavenumbers, as well as the atomic displacements, to be calculated. The NIS intensities were simulated by considering multi-phonon intera ctions arising from the lattice mode couplings with the internal molec ular vibrational modes. The RRS intensities were simulated within the framework of the so-called ''small shift approximation'', by using the molecular bond-order changes induced by the electronic transition fro m the ground to the first electronic excited state. It is shown that N IS spectroscopy mainly provides information on the guanine out-of-plan e modes of vibration, while RRS allows the in-plane stretching vibrati onal motions to be analyzed.