AB-INITIO TRANSFORM CALCULATION OF RESONANCE RAMAN-SPECTRA OF URACIL, 1-METHYLURACIL, AND 5-METHYLURACIL

The UV resonance Raman spectra of several uracils, taken with 266 nm excitation, have been calculated using a combination of ab initio molecular orbital calculations and the transform theory of resonance Raman scattering. This formulation requires the calculation of the molecule's ground and excited state geometries, vibrational frequencies, and normal mode displacements. It also requires the experimental measurement of the resonant electronic absorption band and the determination of the absorption band's Kramers-Kronig transform. In this study, the ground and excited state properties (at the HF//6-31G* and CIS//6-31G* levels, respectively) of uracil, 1-methyluracil, 5-methyluracil (thymine), and their 1,3-dideuterated derivatives were calculated. Good agreement between the calculated and experimental spectra was obtained even though no adjustable parameters were used. The calculation of the UV resonance Raman spectra of 1,3-dideuteriouracil using ground state properties determined from a calculation at the MP2//6-31G* level was also performed and compared to the HF//6-31G* spectrum. The HF//6-31G* spectrum, showed better agreement with the experimental spectrum and therefore this basis set was used for the remainder of the study. From the calculation of the frequencies in the resonant excited state of uracil, it was also shown that the average change in the vibrational frequencies in the excited state was less than 5%. This implies that the Duschinsky effect in these molecules is not sufficiently large to prevent good agreement between the calculation and experiment using the transform theory in its simplest form.