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 e xcitation, have been calculated using a combination of ab initio molec ular 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 no rmal mode displacements. It also requires the experimental measurement of the resonant electronic absorption band and the determination of t he absorption band's Kramers-Kronig transform. In this study, the grou nd and excited state properties (at the HF//6-31G and CIS//6-31G* lev els, respectively) of uracil, 1-methyluracil, 5-methyluracil (thymine) , and their 1,3-dideuterated derivatives were calculated. Good agreeme nt between the calculated and experimental spectra was obtained even t hough no adjustable parameters were used. The calculation of the UV re sonance Raman spectra of 1,3-dideuteriouracil using ground state prope rties 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* spec trum, showed better agreement with the experimental spectrum and there fore 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 frequen cies in the excited state was less than 5%. This implies that the Dusc hinsky effect in these molecules is not sufficiently large to prevent good agreement between the calculation and experiment using the transf orm theory in its simplest form.