The molecular force field of guanine and its deuterated species as determined from neutron inelastic scattering and resonance Raman measurements

Abstract

Neutron inelastic scattering (NIS) spectra from polycrystalline samples and ultraviolet resonance Raman scattering (RRS) spectra from aqueous solutions of guanine and CS-deuterated and (N9, NI, C2-amino)-deuterated guanine are reported. These measurements allowed theoretical simulations of the vibrational wavenumbers and intensities of the NIS and RRS bands to be performed. A valence force field enabled the normal mode wavenumbers, as well as the atomic displacements, to be calculated. The NIS intensities were simulated by considering multi-phonon interactions arising from the lattice mode couplings with the internal molecular 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 from the ground to the first electronic excited state. It is shown that NIS spectroscopy mainly provides information on the guanine out-of-plane modes of vibration, while RRS allows the in-plane stretching vibrational motions to be analyzed.