EPR spectra for a range of pyrimidine and s-triazine radical cations

Abstract

Pyrimidine and s-triazine radical cations have been prepared by exposing dilute solutions in freons to ⁶⁰Co γ-rays at 77 K, and studied by EPR spectroscopy. Spectral interpretation is difficult because of the unknown angle (θ) subtended between the principle directions of the ¹⁴N hyperfine axes. Various deuteriated derivatives have been used, together with a range of computer-simulated spectra, which show how the spectra for a two-equivalent-nitrogen system change with θ. From these, the best spectra have been selected for comparison with experiment. The results are discussed in terms of two alternative interpretations offered previously. It is concluded that neither is correct, because of the neglect of weak outer features which are only clearly defined for the 2,4,6-trimethyl s-triazine derivative. The new results show that the SOMO is largely concentrated on two nitrogen ‘lone-pair ’in-plane orbitals with a nodal-surface passing through positions 2 and 5. The estimated p : s ratio on each nitrogen is ca. 4.8.