Intersystem crossing phenylnitrene

The dimerization of phenylnitrene to form azobenzene is a characteristic reaction of the triplet state of an arylnitrene. Thus, some intermediate formed along the reaction coordinate must undergo intersystem crossing to the triplet manifold. 

Although AEst of phenylnitrene is small in comparison to that of imidogen, it is still very large compared to phenylcarbene. Thus, while singlet and triplet phenylcarbene interconvert rapidly, singlet to triplet intersystem crossing of phen-... 

Figure 11.5. The magnitude of ko s decreases with decreasing temperature until 170 K, whereupon it reaches a value of 3.2 x 10 s. Below this temperature, koBs remains constant. " The breakpoint in the Arrhenius plot is 180-200 K, which is in exactly the same temperature range in which the solution phase chemistry changes from the trapping of ketenimine 30 with diethylamine to the dimerization of 33t. Thus, the low-temperature data of Figure 11.5 were associated with k]sc, the rate constant for intersystem crossing of singlet to triplet phenylnitrene, and the high temperature data with k., the rate constant for rearrangement of 33t.

TABLE 11.3. Intersystem Crossing Rate Constants of Ortho and Meta Substituted Phenylnitrenes"... 

In the liquid phase, singlet phenylnitrene is rapidly relaxed by collision with solvent and cannot surmount the barrier to form cyanocyclopentadiene at ambient temperature. Under these conditions PN isomerizes over a small barrier to form cyclic ketenimine K. Later, computational work of Karney and Borden would show this to be a two-step process involving benzazirine BZ, the species trapped by ethanethiol (Scheme 2). In the liquid phase, PN prefers rearrangement to intersystem crossing (ISC) to the lower-energy triplet state at ambient temperature. Intersystem crossing is not an activated process and its rate is not expected to vary with temperature. The rate of... 

Marcinek, A. Platz, M. S. Deduction of the activation parameters for ring expansion and intersystem crossing in fluorinated singlet phenylnitrenes, J. Phys. Chem. 1993, 97, 12674-12677. 

That no intermolecular aromatic substitution was observed has been attributed to rapid intersystem crossing of the thermally generated singlet nitrene to the triplet. Even when phenylnitrene was... 

It was impossible to study the effect of strong Ji-donor substituents on the rate of cyclization, because with para-methoxy and dimethylamino substituents the phenylnitrene underwent intersystem crossing to the triplet faster than cyclization at all temperatures. ... 

---