Energy transfer nonvertical

Hammond and SaltieP - proposed the concept of nonvertical energy transfer to account for the nonclassical behavior of ci.r-stilbene and cis- and rra/u-a-methylstilbene. 

At the present time it is not possible to decide whether one or more of these mechanisms are operative. 

Results of theoretical calculation at the B3LYP-D FT level using the 6-31 + G(d,p) basis set in the ground and triplet states of the (c-1, t-1) and of 2,3-stUbenes were 

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Nonvertical Energy Transfer. The concept of nonvertical energy transfer was introduced by Hammond and Saltiel to explain the observation that low energy sensitizers would photoisomerize stilbene, despite the fact that energy... 

The first report of triplet photochemistry of three-membered rings concerned the reversibility of the photosensitized isomerization of norbornadiene to quadricyclane.270 Since no spectroscopic triplet levels of cyclopropane are known, it was felt that this reaction represents an extreme sort of nonvertical energy transfer with actual bond breaking in the acceptor. 

Photostationary cis/trans ratios vary with sensitizer structure in a manner not yet perfectly understood, and both the time required for attainment of a photostationary state and actual flash spectroscopic quenching rates indicate that energy transfer to diphenylcyclopropane is quite inefficient. Such should be the case if nonvertical energy transfer with the production of a biradical is occurring. When care is taken so that only the sensitizer absorbs light, only cis-trans isomerization is observed. Direct excitation of the cyclopropane produces 1,3-diphenyl-propene and 1-phenylindane as well.298... 

The nonvertical energy transfer observed between low energy sensitizers and diphenylcyclopropane may thus be similar to that observed with the stilbenes, namely a transition to a triplet level not able to be observed spectroscopically. 

The exact mechanism of nonvertical energy transfer is still unclear. Originally Hammond thought of it as occurring directly, during a prolonged encounter between donor and acceptor.23 Now, however, isotope effects and other evidence favor the view that the nonspectroscopic, or phantom, triplet is formed... 

Hammond, Saltiel and co-workers subsequently observed that the rate constant of energy transfer becomes slower than that predicted by Equation 2.47 when the structure of the relaxed excited triplet state of the acceptor differs substantially from that in ground state, as is the case for stilbene.112 These findings initiated extensive investigations on the twisted triplet state geometry of stilbene and related compounds (Section 5.5). The expression nonvertical energy transfer (NVET) was coined to describe this situation. 

Balzani, V., Bolletta, F., Scandola, F., Vertica, Nonvertical Energy Transfer Processes. A General Classical Treatment, J. Am. Chem. Soc. 1980, 102, 2152 2163. 

When Ej is in excess of 60 kcal/mol, -/ran -butadiene Ej = 60 kcal/ mol), which strongly predominates in the thermal equilibrium, is excited and produces mainly divinylcyclobutanes. When the of the sensitizer is not high enough to excite 5-tra/t5-butadiene, energy transfer to 5-m-butadiene (Ef = 54 kcal/mol) occurs instead, yielding vinylcyclohexene. If Ej < 50 kcal/mol, nonvertical energy transfer to a twisted diene triplet is believed to occur (Liu etal., I%5). 

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