Calculation of n→II Transition Energies in N-Heterocyclic Molecules by a One-Electron Approximation

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Abstract
The one‐electron LCAO‐MO formalism is applied to n→π* transition energies in N‐heterocyclic molecules. |βnπspec| is shown to be ∼1 ev smaller than |1βππspec| for (n, π*) states possessing no nodes through nitrogen atoms. Reasonable values are 1βnπspec≈—2.3 ev, 3βnspec≈—2.0 ev. Calculations of transition energies, both singlet‐singlet and singlet‐triplet, are made for 16 molecules, yielding semiquantitative agreement with experiment. Prediction of the assignment of the lowest excited singlet and triplet levels is made for a number of cases. The singlet‐triplet split in the lowest (n, π*) configuration is predicted to undergo a strong orientation dependence in the 6‐membered azines. Substitution effects are considered and orientation effects on the n→π* blue shift upon conjugative substitution are treated. (n, π*) states with nodes through nitrogen atoms are discussed qualitatively.
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