Knowledge Bank

Extensive SCF and VB calculations of the energies and wave functions of the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B3u$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B2g,n\pi \ast$ states of pyrazine have been carried out. The symmetry unrestrained SCF calculation of the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B3u$ state is found to undergo symmetry breaking to $C2v$ symmetry even though the nuclei are constrtrained to $D2h$ geometry. The lower energy of the symmetry broken $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B3u$ state appears to arise from an improved correlation energy. This in turn occurs implicity in our calculation through configuration interaction between the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B3u$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B2g$ states which share a $common1B1$ symmetry in $C2v$. To obtain a better description consistent with the enforced $D2h$ 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 $C2v$ distorted nuclear geometries in these excited states. Depending upon the interatomic distances chosen for the $D2h$ geometry either a shallow double minimum or a broad, flat potential is calculated for the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B3u$ state when the nuclei are distorted along a $b1u$ vibrational coordinate. The energy of the $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B2g$ state rises sharply for displacements along this same coordinate. Experimental evidence consistent with these observations will be cited.