Classical and Non-Classical Decay Paths of Electronically Excited Conjugated Dienes
In 1969 Oosterhoff and van der Lugt1 investigated the butadiene → cyclobutene interconversion and pointed out that a reaction would take place only if there exist such states from which a driving force can derive ; this was an elegant way of showing on theoretical grounds that a molecule electronically excited in its Franck-Condon nuclear configuration will spontaneously distort itself (relax) if the new nuclear configuration corresponds to an energy minimum which correlates with the initial state along a coordinate devoid of significant barriers. If two electronic states do not correlate with each other (regardless ΔHO) or if they do through an exceedingly high barrier or in a continuous highly endothermic way, no adiabatic interconversion would occur (interconversion does however take place diabatically at avoided surface crossings). According to and within the restrictions of the above statements a molecule, whatever in the ground or in an excited state, is entitled to move on its hypersurface. These motions (changes of internuclear angles and distances) from energy well to energy well are determined by the shape of the potential surfaces; if one were to make any prediction on chemical (or physical) transformations one should obviously have some knowledge of the potential surfaces.
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