Diffusion controlled rate, triplet carbenes

Absolute rate constants for the representative triplet carbene-Oa reaction in solution at room temperature are listed in Table 9.12. The second-order rate constants are close to 10 M s, near the diffusion-controlled rate. Thus, the reaction with O2 is taken as evidence for the presence of the triplet state of the carbene. 

Assuming that singlet nitrene reacts with alkanes at near diffusion controlled rates allowed deduction of a rate constant of singlet-to-triplet nitrene intersystem crossing (ISC) of 2-8 X 10 s . This ISC rate is slower than in carbenes, but significantly faster than with arylnitrenes, which are discussed in a subsequent section. 

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