An electron-nuclear double resonance study of the lowest triplet state of pyrimidine

Optically-detected electron–nuclear double resonance was employed to measure the entire hyperfine and quadrupole tensors of the two nitrogen nuclei and the hyperfine tensors of the four hydrogen nuclei in the lowest triplet state T0 of pyrimidine-h4 as a guest in a single crystal of benzene-d6. The electron-spin-density distribution and the molecular geometry in the lowest triplet state have been determined as well as the orientation of the molecule in the benzene crystal. It is found that upon excitation into T0, pyrimidine remains a planar molecule of approximately C2v symmetry; the C–N–C angles, which are 115.5° in the ground state, and the angle between the directions of the nonbonding nitrogen orbitals both become >120°. Nearly 60% of the total electron-spin density is located at the two nitrogen atoms, the two nonbonding orbitals each carrying a spin density of 0.21, and the π orbitals a spin density of 0.08. We conclude that the lowest triplet state of pyrimidine is appropriately described in terms of an nπ* excitation; we find no evidence for vibronic coupling of this state with higher lying 3ππ* states.