Semiempirical Calculation on the Electronic Structure of the Nitrogen-Containing Heterocyclic Molecules. II. Electronic Structure of Pyrazine with Particular Reference to Its n—π Transition

The semiempirical theory of n—π and π—π spectra of nitrogen analogs of aromatic hydrocarbons proposed in Part I is applied to calculate the lower excited levels of the pyrazine molecule. The doubly excited configurations as well as singly excited configurations are considered in CI calculation. It is shown that the usual assumption of sp² hybridization of the nonbonding orbital of the nitrogen atoms in the pyrazine molecule results in a considerably higher calculated value for the n—π transition energy as compared with experiment, even if the doubly excited configurations are considered. In order to bring the calculated values of the lowest ${}^1{A}_g{\to }^1{B}_{\text{1u}}{,}^1{B}_{\text{3g}}\mathrm{\hspace{0.17em}(n-\pi )}$ transition energies in agreement with the observed values, the s character of the nonbonding orbital of the nitrogen atom should be taken as 0.103. Discussions which support the lower s character of the nonbonding orbitals of the nitrogen atoms in this molecule are presented.