Internal conversion in aromatic and N-heteroaromatic molecules

A previous semi-empirical approach to the calculation of the rate of internal conversion, regarded as a tunnelling process, is reformulated on a sounder theoretical basis. Following Robinson and Frosch, tunnelling rates are correlated with Franck-Condon factors for the associated transition. The total Franck-Condon factor, S2max, is a product of three terms, associated respectively with skeletal stretching, CH stretching, and skeletal angle bending vibrations. The value of S2max may be controlled by one term only of the product, or by two or more (mixed tunnelling). Tunnelling rates should be slowed down by deuteration only when CH vibrations participate significantly; specific predictions are made here. Skeletal-angle bending vibrations are of negligible importance in internal conversion in aromatic molecules, but they are very significant in transitions between states of π,π* and n,π* type in heteroaromatics. Correlation between S2max and estimated tunnelling rates is encouraging for four aromatic molecules and certain monocyclic azines; but the failure of pyridine and pyridazine to show luminescence is still unexplained. The case of pyrazine is discussed in detail.