Matrix isolation infrared spectroscopic and quantum chemical studies on the rotational isomers of orotic acid (6-carboxyuracil)

The infrared spectrum of orotic acid (6-carboxyuracil) isolated in a low-temperature argon matrix is presented, for the first time. This molecule is a key precursor in the biosynthesis of all pyrimidine nucleotides in living organisms. The comprehensive theoretical studies on the rotational isomerism of orotic acid have been performed by an ab initio MP2 and three density functional methods (B3LYP, M06 and M06-2X). All theoretical methods have predicted that four possible conformers of orotic acid may exist in the gas phase. The calculated barrier height for rotation of the COOH group around the CC bond (37kJmol−1, M06-2X) is much lower than the barriers for the OH rotation around the CO bond (47 and 51kJmol−1). The Gibbs free energies, relative stabilities and the mole fractions of isomers at different temperatures, in the gas phase, have been determined. The anharmonic vibrational frequencies, infrared intensities and potential energy distributions (PEDs) were computed for two isomers of the lowest energy (A and B) using the B3LYP method with the aug-cc-pVTZ basis set. The theoretical anharmonic IR spectra are in excellent agreement with the experiment. It is concluded that the most stable conformer (A) is the predominant form in a low-temperature argon matrix, while the mole fraction of the less stable B conformer can be assessed as ca. 15%. No spectral indications of the presence of other isomers (C and D) in the matrix were detected.