Reactive Intersystem Crossing

By happenstance, Bi and B2 are not interchanged despite the different axis conventions of Figs. 4.3 and 9.1. 

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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. 

Their extensive kinetic study of the thermolysis of deuterated methylenepyra-zoline molecules at 170°C, and in particular the ratios of the differently labeled MCP molecules produced, led Crawford and Chang [40, 41] to reject reactive intersystem crossing of MP to TMM. They propose instead that, as in the case of 1-pyrazoline, initial rupture of one CN bond is followed by closure of the resulting diazenyl biradical to MCP with concomitant loss of N2. The rates of... 

Interconversion of the triplet components can be frozen out when the triplet is produced photochemically at very low temperatures in a matrix or host crystal. In these conditions, photolysis of diphenyldiazomethane produces the Ty(6i) component of diphenylcarbene selectively [8]. This finding is consistent with Fig. 9.1 if the carbene is produced in its closed shell singlet state and then crosses to the more stable triplet. It is also consistent with direct reactive intersystem crossing RISC from the photoexcited reactant molecule [9, footnote on p. 285]. 

The failure to observe photosubstitution in the presence of a sensitizer in which the latter is the principal absorber, the invariance of product quantum yield with wavelengths shorter than 350 nm (onset of n -> -n absorption), and the observation that chloride and bromide ions (known to catalyze S-+T intersystem crossing) strongly diminish the quantum yields of these reactions, strongly points to the lowest excited ir- n singlet state as the reactive species in these transformations. Excitation into the n->ir absorption band results in little product formation. A triplet state may, however, be involved in the photoamination of nitrobenzene.a41)... 

Despite the superficial similarities between phenylcarbene (la) and phenylni-trene (lb), there are significant differences between these reactive intermediates. Phenylcarbene undergoes intersystem crossing and intermolecular reactions... 

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