Singlet-triplet intersection

Scheme 1 Electronic states involved in the absorbtion bands in the region of the first singlet—triplet intersection for octahedral, tetragonal and trigonal complexes of nickel(II).336 Solid arrows denote spin-allowed absorbtion transitions, dotted arrows connect pairs of interacting levels. (reprinted with permission from ref. 336 1998, American Chemical Society).

Fig. 12.7. Structures for the excited states Tj, T2, 5i, and S2 of styrene. Singlet-singlet conical intersections are labeled 1C (internal conversion). Singlet-triplet intersections are labeled ISC (intersystem crossing). Adapted from /. Am. Chem. Soc., 117, 6944 (1995), by permission of the American Chemical Society.

Loosely speaking, one might say that reactive intersystem crossing - depending as it does on the incursion of a weak magnetic interaction as the reactant approaches the singlet-triplet intersection - is an inherently improbable process. Spin-non-conservative reactions should therefore have low values of the pre-exponential factor, which would be expected to become more normal when heavy atoms are present in the reactant or the reaction medium. 

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) ° ... 

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. 

Location of Conical Intersections and Singlet-Triplet Crossing Points in Solution... 

Figure 6-2. Conical intersection and singlet-triplet crossing location scheme...

A simple example serves to illnstrate the similarities between a reaction mechanism with a conventional intermediate and a reaction mechanism with a conical intersection. Consider Scheme 9.2 for the photochemical di-tt-methane rearrangement. Chemical intnition snggests two possible key intermediate structures, II and III. Computations conhrm that, for the singlet photochemical di-Jt-methane rearrangement, structure III is a conical intersection that divides the excited-state branch of the reaction coordinate from the ground state branch. In contrast, structure II is a conventional biradical intermediate for the triplet reaction. 

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