Methylene intersystem crossing

Fluorinated hydrocarbons favoring intersystem crossing by dilution increase the proportion of triplet methylene up to about 56%... 

It is also possible to induce intersystem crossing from an initially formed singlet state to a lower energy triplet by forcing the singlet state to suffer collisions with an inert medium (no easy task for a species as reactive as methylene ). 

Intersystem crossing rate constants of ortho- and meta-substituted singlet phenylni-trenes are presented in Table 1Mono- and di-o-fluorine substituents have no influence on ISC rate constants.No effect with meta, meta-difluoro substitution is observed either. Pentafluoro substitution has no effect on fcisc in pentane although a modest acceleration is observed in the more polar solvent methylene chloride.i° - i... 

Thus we expect intersystem crossing of 1A1 methylene to 3B2 methylene. It is seen from eq. (8-5) that the transition rate is especially high when the overlap of the WlAi-v(0) and vP3Bi v(0) vibrational wave functions is a maximum this occurs when these vibrational states have vibrational kinetic... 

It is apparent that two collisional deactivation processes must be working in competition—one a very efficient removal of some intermediate which yields nonstereospecific products, and the other a relatively inefficient production of some other intermediate which yields nonstereospecific products. The very efficient process, which produces a sharp drop in the amount of tripletlike products at low pressures of inert gas, undoubtedly involves collisional stabilization of the hot cyclopropane formed by singlet addition, while the less efficient process would seem to involve intersystem crossing to triplet methylene, just as Anet and Frey proposed. 

The intersystem crossing from CH2( /4,) to (3B,) is induced by inert gases (143). A theory dealing with the collision-induced singlet to triplet transition of methylene is developed by Chu and Dahler (211). 

Ethoxyisoindolenone (50) sensitized cis-trans isomerization of c/s-piperylene has been used in a triplet counting experiment to measure the intersystem crossing efficiency. A linear triplet counting plot is obtained which gives an intersystem crossing efficiency of 0.96 0.01 and a triplet lifetime of 1.5 x 10 7 sec. for 50 in methylene chloride solvent at ambient temperature in the absence of olefin. 

The photolysis of diazomethane at 4360 and 3650 A was investigated by Nor-rish, who found (N2) 4 and methylene was postulated as the intermediate. Spin conservation rules require that methylene be formed in a singlet state, but rapid decay to the ground triplet occurs and much of the controversy in the literature centers on the spin state of methylene prior to chemical reaction with a double bond . The benzophenone or Hg( Pi) sensitized decompositions of diazomethane yields triplet methylene directly, presumably via electronically-excited (triplet) diazomethane . Recently it has been shown that triplet methylene can be formed in the photolysis of diazomethane at longer wavelengths by intersystem crossing , viz. 

Triplet sensitization did not lead to intramolecular adducts for higher alkanes but resulted in polymer formation (vide infra) a reaction scheme including excited complex formation in a primary step was proposed. The rate constant for excited complex formation, k, could be calculated as a function of the number of methylene units and diminished by lengthening of the chain (Table 8). The quantum yield of intersystem crossing was much lower (Table 8) than in the isolated chromophore, N-butylamalelmlde ( isc = 0.25), and substantially lower than the quantum yield of reaction, (Table 8). 

In hydrocarbon solvents, the reaction of methylene, generated photochemically from diazomethane, is faster than intersystem crossing to the triplet ground state, permitting the study of exclusively singlet carbene chemistry. "... 

The view that electronic states of different multiplicity need not be considered cannot easily be ruled out, since both deactivation of vibrationally excited carbenes and intersystem crossing between singlet and triplet states are brought about by collision with other molecules. The difficulty is not restricted to reactions in the gas phase in solution, collisional deactivation and collision-induced intersystem crossing can still be expected to compete with collisions leading to chemical reaction. However, the parallelism between the variation in stereospecificity in the gas-phase addition of methylene to the 2-butenes with the pressure of inert gas (Frey, 1959, I960 Anet et al., 1960 Bader and Generosa,... 

---