A THEORETICAL STUDY OF ELECTRONIC SYMMETRY BREAKING AND GEOMETRY DISTORTION IN THE $^{1}B_{3u}$ AND $^{1}B_{2g} n\pi^{*}$ STATE OF PYRAZINE
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Date
1980
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Ohio State University
Abstract
Extensive SCF and VB calculations of the energies and wave functions of the $^{1}B_{3u}$ and $^{1}B_{2g}, n\pi^{*}$ states of pyrazine have been carried out. The symmetry unrestrained SCF calculation of the $^{1}B_{3u}$ state is found to undergo symmetry breaking to $C_{2v}$ symmetry even though the nuclei are constrtrained to $D_{2h}$ geometry. The lower energy of the symmetry broken $^{1}B_{3u}$ state appears to arise from an improved correlation energy. This in turn occurs implicity in our calculation through configuration interaction between the $^{1}B_{3u}$ and $^{1}B_{2g}$ states which share a $common^{1}B_{1}$ symmetry in $C_{2v}$. To obtain a better description consistent with the enforced $D_{2h}$ geometry of the nuclei, a valence bond calculation was performed using the two equivalent symmetry broken SCF wave functions. Further improvement in the energy was obtained. Finally, both SCF and VB calculations were performed on $C_{2v}$ distorted nuclear geometries in these excited states. Depending upon the interatomic distances chosen for the $D_{2h}$ geometry either a shallow double minimum or a broad, flat potential is calculated for the $^{1}B_{3u}$ state when the nuclei are distorted along a $b_{1u}$ vibrational coordinate. The energy of the $^{1}B_{2g}$ state rises sharply for displacements along this same coordinate. Experimental evidence consistent with these observations will be cited.
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