Vibrational spectra of 5,6-dihydrouracil. An experimental matrix isolation, solid state and theoretical study

Abstract

5,6-Dihydrouracil (DHU) is a rare pyrimidine base naturally occurring in transfer-RNAs. When formed (from cytosine) in DNA, it is mutagenic. This work presents the first theoretical and experimental investigations of the vibrational spectra of DHU. The molecular structure, vibrational frequencies, infrared intensities and Raman scattering activities have been calculated using ab initio MP2 and density functional (B3LYP and mPW1PW) methods. The optimized geometry of this molecule shows considerable puckering of the C5–C6–N1 part of the ring. The MP2-calculated barrier for the ring inversion is 1928 cm⁻¹. The infrared spectra of DHU have been measured in argon and nitrogen matrixes. The Raman and IR spectra of the solid compound have also been studied. The unequivocal assignment of the experimental spectra has been made on the basis of the calculated potential energy distribution. The vibrational spectra of 5,6-dihydrouracil are discussed in comparison with related compounds, uracil and glutarimide.