Electron Spin Resonance of Irradiated Single Crystals of Orotic Acid

Abstract

Hydrogen abstraction from ${\rm N}_{(1)}$ occurs when single crystals of orotic acid are exposed to ionizing radiation at room temperature. The stable free radical has unpaired spin density on both ${\rm N}_{(1)}$ and ${\rm C}_{(5)}$ of the pyrimidine ring. Hyperfine coupling to the nitrogen nucleus shows axial symmetry, with $A_{\parallel}{}^{{\rm N}}=15.0$ gauss and $A_{\perp}{}^{{\rm N}}=0$. The hydrogen bonded to ${\rm C}_{(5)}$ has principal hyperfine values, in gauss, of $A_{{\rm x}}{}^{{\rm H}}=7.3$, $A_{{\rm y}}{}^{{\rm H}}=22.2$, and ${\rm A}_{{\rm z}}{}^{{\rm H}}=15.1$. From the hyperfine couplings, the spin densities calculated for ${\rm N}_{(1)}$ and ${\rm C}_{(5)}$, respectively, are 0.29 and 0.64. Principal values of the g-tensor are $g_{{\rm u}}=2.0059$, $g_{{\rm v}}=2.0052$, and $g_{{\rm w}}=2.0023$. The ESR analysis can be used to predict the molecular orientation in orotic acid crystals.