Raman and infrared spectra of thymine. A matrix isolation and DFT study

The Raman spectrum has been measured for thymine isolated in an Ar matrix using an FTRA instrument with infrared excitation at 1.064 mu m, thereby avoiding complications due to resonance Raman and fluorescence effects. This spectrum is used, together with the newly measured infrared spectrum, to establish the assignment of the vibrational spectra of isolated thymine. The assignment is assisted by DFT calculations made at the B3LYP/6-31G(d,p) level of theory, using GAUSSIAN 98W. The calculated spectra, including the Raman spectrum, are in surprisingly good agreement with the experimental spectra. The discrepancies that are observed for some parts of the spectrum result primarily from the failure of the harmonic approximation made in the calculation. The effect of the neglect of anharmonicity has been investigated by determining a set of effective force constants that reproduce the experimental frequencies and intensity patterns in the infrared and Raman spectra and by examining how the predicted spectra (intensities, frequencies, potential energy distributions (PEDs), infrared intensity distributions (IDs), and Raman intensity distributions (RIDs)) change. The final results provide an unambiguous assignment of the vibrational spectra for thymine isolated in an Ar matrix and illustrate that the concepts of PEDs, IDs, and RIDs are useful for the interpretation of the vibrational spectra of molecules the size of the pyrimidine bases. Comparison of the Raman spectrum of thymine isolated in the Ar matrix with that of a polycrystalline sample indicates that they are quite similar, in marked contrast to the differences in the corresponding infrared spectra.