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The Electronic Properties of Diradicals
Author(s) -
Salem Lionel,
Rowland Colin
Publication year - 1972
Publication title -
angewandte chemie international edition in english
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.831
H-Index - 550
eISSN - 1521-3773
pISSN - 0570-0833
DOI - 10.1002/anie.197200921
Subject(s) - intersystem crossing , diradical , singlet state , chemistry , triplet state , wave function , degenerate energy levels , singlet fission , chemical physics , molecular physics , physics , atomic physics , quantum mechanics , excited state
A review of the various possible definitions of diradicals leads the authors to describe these systems as having two odd electrons in degenerate or nearly‐degenerate molecular orbitals. A study of the wave‐function for the two odd electrons shows that its form depends entirely on whether the diradical is homo‐ or heterosymmetric. Energy schemes are given in these two cases, as well as in the intermediate “non‐symmetric” case. The extent of zwitterionic character in diradical states is also investigated. This is followed by a discussion of intersystem crossing between singlet and triplet diradical states via spin‐orbit coupling and other mechanisms. The electronic matrix elements for spin‐orbit coupling are calculated and evaluated numerically for various model cases. It is then possible to establish general rules for favorable (electronic) intersystem crossing. In 1,3 or 1,4 diradicals its efficiency is estimated to be comparable with that in aromatics. The role of the electron‐nuclear hyperfine interaction in mixing singlet and triplet states, particularly in CIDNP, is explained. Finally the question of whether diradicals actually occur as secondary minima on potential energy surfaces is examined. Recent quantum‐mechanical calculations, in contradiction to some thermochemical and kinetic evidence, lead to flat singlet surfaces without significant minima.