DFT calculations and vibrational (FT-IR, FT-Raman) spectra on an uracilyl-pyridinium salt

In the present article we have studied the structural and vibrational properties of the N-protonated derivative of a cross-conjugated heterocyclic mesomeric betaine formed by an uracilyl and a dimethylamino substituted pyridinium group. Fourier transform infrared and Raman spectra were recorded from solid samples. Quantum chemistry density functional theory calculations were performed at the B3PW91/6-31G** level. Geometrical parameters, charge distribution and natural bond orders were calculated for the ground-state minimum energy structure. The optimized geometry was found twisted with respect to the two aromatic planes. The pyridinium ring presents a quinoid structure, while the uracilyl ring only presents a localized double-bond, which is far from the carbonyl groups. The molecule is strongly polar along the longest molecular axis, being largely originated by the charge separation between the heterocycles. A force field and normal coordinate calculation was performed in order to correctly assign the measured infrared and Raman bands. Assignments suggest the existence of centro-symmetrical structures in solid state bonded by the C=O and N-H groups of the uracilyl ring. The calculated force constants in vibrational internal coordinates are in agreement with the structural parameters. (c) 2006 Elsevier B.V. All rights reserved.