Configuration Interaction Studies of Ground and Excited States of Polyatomic Molecules II. The Electronic States and Spectrum of Pyrazine

Ab initio SCF and CI treatments of the ground state and

n → π*

and

π → π*

excited states of pyrazine, C₄H₄N₂, are reported for the molecule in its ground state equilibrium geometry. In the case of

n → π*

transitions, the two symmetry combinations of nitrogen lone pair orbitals

n_{1}

and

n_{2}

are found to correspond to the description

n_{+} {≈ n}_{1} {+ n}_{2} + λσ

and

n_{-} {≈ n}_{1} {− n}_{2}

in which the

n_{+}

orbital has the higher orbital energy and is significantly more delocalized than

n_{-}

by interaction with the pyrazine ring sigma system. This result leads to a significant splitting at the CI level of description of 1.4 eV between the

B_{3u}

and

B_{2g}

excited states which are derived from the orbital promotions

n_{+} → π*

and

n_{-} → π*

, respectively. Calculated transition energies to the lowest

n → π*

excited states

^{3} B_{3u}

and

^{1} B_{3u}

of 3.56 and 4.22 eV, respectively, are in good agreement with experimental values. A low‐lying

^{3} B_{1u} (π → π*)

state for which there exists indirect experimental evidence is found to occur between the

^{3} B_{3u}

and

^{1} B_{3u}

states. Also, an allowed transition to the

^{1} B_{2u} (π → π*)

state is calculated at 5.29 eV, slightly above the observed range of absorption. Numerous other excited states are calculated, but of these only the

^{1} B_{1u} (π → π*)

and

^{1} B_{2u} (π → π*)

states can be identified with observed transitions and in these cases the calculated transition energies are too high by approximately 2.5 eV.

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