Abstract
Previous theoretical work has shown that the lowest excited singlet state of pyrazine, the ..pi..* /sup 1/B/sub 3/u state, is best described in terms of interacting excitations localized on each nitrogen. The present work refines the localized excitation model and considers its implications for the geometry of the /sup 1/B/sub 3/u state. Hartree-Fock calculations show that the best single configuration description of the n..pi..* state has broken (/sup 1/B/sub 1/) symmetry with the excitation strongly localized at one end of the molcule. If the symmetry-restricted hf result is used for reference, this localization describes an important correlation effect. The excited-state geometry was probed using configuration interaction wave functions based on the symmetry-restricted orbitals, as well as properly symmetrized ''valance-bond'' wave functions based on the broken symmetry solutions. Both descriptions lead to a very flat potential for a b/sub 1u/ vibrational mode. This mode reduces the molecular geometry from D/sub 2h/ to C/sub 2v/. We present spectroscopic evidence of our own and of other workers which is consistent with such a flat potential.
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