AN ELECTRON NUCLEAR DOUBLE-RESONANCE STUDY OF THE LOWEST TRIPLET-STATE OF PYRAZINE

The hyperfine and quadrupole tensors of the two nitrogen atoms and the hyperfine tensors of the four hydrogen atoms of pyrazine in the lowest triplet state T0 are obtained from optically-detected electron-nuclear double resonance (ENDOR) experiments on pyrazine-h4 in a single crystal of benzene-d6 at 1.2 K. Analysis of these tensors shows that pyrazine is in good approximation a planar molecule of D2h, symmetry in the lowest triplet state. The in-plane structure changes significantly upon excitation into T0. An increase of the C-N-C angles is observed and a reduction of the C-N bond lengths. From the hyperfine tensors the distribution of the electron-spin density is derived. The nitrogen nonbonding orbitals together carry 36% of the total spin density and it is shown that the lowest triplet state of pyrazine is appropriately described in terms of a 3n-pi* excitation. The results of the optically-detected ENDOR experiments give no indication of vibronic coupling of the lowest 3n-pi* state with nearby 3-pi-pi* states.