An electronndash;nuclear double resonance study of the lowest triplet state of pyrimidine

摘要

Opticallyhyphen;detected electronndash;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 stateT0of pyrimidinehyphen;h4as a guest in a single crystal of benzenehyphen;d6. The electronhyphen;spinhyphen;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 intoT0, pyrimidine remains a planar molecule of approximatelyC2vsymmetry; the Cndash;Nndash;C angles, which are 115.5deg; in the ground state, and the angle between the directions of the nonbonding nitrogen orbitals both become 120deg;. Nearly 60percnt; of the total electronhyphen;spin density is located at the two nitrogen atoms, the two nonbonding orbitals each carrying a spin density of 0.21, and the pgr; orbitals a spin density of 0.08. We conclude that the lowest triplet state of pyrimidine is appropriately described in terms of annpgr;ast; excitation; we find no evidence for vibronic coupling of this state with higher lying3pgr;pgr;ast; states.