Author:
Byrne JP,McCoy EF,Ross IG
Abstract
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.
Cited by
173 articles.
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