Analysis of the vibrational spectra of 1-methyluracil and its isotopic derivatives by ab initio calculations

Abstract

The vibrational spectrum of 1-methyluracil trapped in an argon matrix has been analysed based on ab initio Hartree—Fock SCF calculations with a split-valence 4–21 basis set. The directly computed theoretical harmonic force field was scaled with empirical scale factors which were transferred from uracil (except for the methyl vibrational modes) to provide an a priori prediction of fundamental frequencies and intensities. The average deviations between experiment and prediction were 9.8 cm −1 for the in-plane vibrations and 18.3 cm −1 for the ring out-of-plane modes. After a few corrections of assignment of the observed spectrum, a new set of scale factors was optimized to give the best force field available from combined consideration of the experimental and theoretical information. These scale factors reduced the average deviations to 6.7 cm −1 for the in-plane modes and to 11.7 cm −1 for the out-of-plane ones. The vibrational spectra of seven isotopic derivatives (-2 180, -4 180, -3d, -5d, -6d, -5, 6d 2 and -CD 3) of 1-methyluracil were predicted with the force field resulting from the optimized set of scale factors, and compared with the crystal-phase experimental data. A few misassignments in the experimental isotopic spectra have been corrected. Vibrational absorption intensities have been computed and compared with experiment and with an earlier calculation.