Sequence and Spacing of n→π Transitions in s-Triazine

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Abstract
Calculations are made by the MO method, including zeroth-order configuration interaction of the lowest singlet and triplet (n, π*) state energies in s-triazine in three approximations: (1) including only one- and two-center integrals evaluated theoretically; (2) including three-center integrals evaluated semiempirically; (3) different overlap approximation, integrals evaluated empirically. Under all three approximations, the four lowest singlet-singlet excitations comprising one allowed and three forbidden transitions are predicted to be grouped within 1 ev and the lowest energy excitation is predicted to be forbidden. The assignment of the lowest energy transition is dependent upon 3-center integral values and is tentatively 1E″←1A′1. The lowest n→π* singlet-triplet excitation is predicted to be orbitally forbidden (3E″←1A1′) with a split of ∼0.8 ev from the corresponding singlet, and approximately degenerate with the lowest energy (π→π*) multiplicity-forbidden transition (3A1′←1A1′). The dependence of the energy levels upon the n-orbital hybridization is investigated.