Electron paramagnetic resonance and phosphorescence measurements of the triplet state of orotic acid and related pyrimidines

Abstract

With orotic acid an EPR triplet state signal (ΔM= ±2) was observed in a non-cracking rigid solvent at 77°K. This triplet state, lying at 470 mμ, has a mean half-life of 0.2 sec and is characterized by a large zero-field splitting. Excitation and phosphorescence spectra of related pyrimidines were measured, too. Besides uracil and cytosine, the excitation peak of these pyrimidines coincides with the large π—π* absorption peak. In the case of uracil and cytosine the excitation peak lies distinctly separated at the long wavelength side of the corresponding absorption peak. This probably indicates a n—π* excitation. The failure to record EPR triplet signals in these pyrimidines is attributed to large zero-field splitting.