Singlet triplet surface crossings

Computational studies of singlet-triplet surface crossings for the ring opening of 4,6-dimethylidenebicyclo[3.1.0]hex-2-ene derivatives show that all the reaction paths exhibited characteristics suggestive of singlet-triplet intersections along their paths (Scheme 72) ° ... 

Reaction (69) takes place via the , singlet and triplet states of the ketone (Kellmann and Dubois, 1965). Using what are commonly termed Salem diagrams, Salem (1974) illustrated the manner in which the excited , state of the reactants correlates smoothly with the ground state products this is illustrated in Fig. 15. The surface crossing that is indicated is... 

The preceding discussion reveals a few of the complexities of the photochemistry that is typical of carbonyl and azo compounds. Results on the olefin-carbonyl Paterno-Buchi system,30 and on the photorearrangements of a, (i-enones,31 P, y-enones,32 azo compounds (diazomethane33 and cyclic diazoalkenes104), and acylcyclopropenes34 show similar features. In these examples, one encounters fourfold intersections as well as conical intersections and singlet-triplet crossings. Thus the potential surfaces are more complex than in hydrocarbon photochemistry. 

The reactions, 0(3P) + CS and 0(3P) + C Se, are very similar in their dynamics [450—452]. They are both direct reactions producing CO in high vibrational states with inverted population distributions ( 80 and 70%, respectively). The similarity of the dynamics is illustrated by the virtual identity of the CO vibrational population distributions for the two reactions when plotted against Fv [452]. It seems most likely in view of the direct nature of the reactions that they take place on a repulsive triplet surface without a crossing to the lower singlet surface corresponding to the stable species, OCS or OCSe. 

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