Direct observation triplet carbenes

On the contrary. Chapman and McMahon found that H-transfer in o-tolylcar-bene can be directly observed at very low temperatures, through several different spectroscopic methods. Irradiation of Ar matrix isolated diazo compound 50 at 4.2 to 10 K gave triplet carbene 51, which could be characterized by EPR, IR, and UV/VIS spectroscopy. The various spectra of 51 slowly decayed at temperatures as low as 4.2 K (tin ca. 64 h), and o-xylylene (52) could be observed to grow correspondingly... 

The yield of trans product (18) is decreased by the presence of a radical scavenger such as 1,1-diphenylethylene and increased by dilution of the reactants with methylene chloride or butane, indicating this product to result from the triplet carbene. A heavy-atom effect on the carbene intermediate was observed by photolysis of a-methylmercuridiazoacetonitrile. With c/s-2-butene as the trapping agent either direct photolysis or triplet benzophenone-sensitized decomposition results in formation of cyclopropanes (19) and (20) in a 1 1 ratio ... 

In contrast to the behavior of BA, triplet sensitized formation of XA in the presence of alcohol gives exactly the same result as does the direct irradiation ether is formed in very high yield. Thus, even when 3XA is formed first, the chemical behavior observed is that of XA. This finding is consistent with a carbene having a singlet ground state a view at least supported by the absence of an epr spectrum for the triplet carbene as noted above. 

Although the equilibration of singlets and triplets has been many times inferred from classical studies of reaction products, and taken as a given by the community, this tour de force work for the first time allows the direct observation of the many interrelated reactive intermediates involved when a carbene precursor is irradiated. 

DIRECT OBSERVATION OF TRIPLET CARBENES 387 TABLE 9.3. The ZFS Parameters" for Some Typical Triplet Carbenes... 

The LFP techniques make it possible to observe directly the hydrogen atom abstraction process by triplet carbenes from appropriate H donors. The techniques also give useful kinetic information. 

The triplet dimer diradical DR2(Ti) finally will relax into thermal equilibrium (kT) with its singlet ground state DR2(So). As we have seen from the ESR spectra (see Fig, 10) the energy separation between the singlet and triplet diradical states is very low and thermally activated transitions occur even at low temperatures. Furthermore the ESR spectra have revealed an admixture of about 10% carbene character with the diradical intermediates. This carbene character may be important in determining the probability x of the side reactions (see Eq. (19)) for the DR -+ AC chain termination reaction. It surely is not, however, the only essential factor, otherwise there should be no difference in the optical and thermal termination reaction steps. Up to now a direct observation of the metastable triplet state Ti(M) has been possible only in two specific crystals where the polymerization reactions are very weak. 

Takeda and Wegner suggested a carbene-like structure for the active species of the topochemical reaction [38]. A strong EPR spectrum with a conspicuous fine structure is observed for the partially polymerized samples. This fine structure is due to the magnetic dipole interaction of two or more unpaired electron spins. It is strongly dependent on the direction of the external magnetic field. By the analysis of this anisotropy, various triplet carbenes and quintet dicarbenes are unambiguously identified. 

A considerable amount of information has been accumulated on the spectroscopic details of carbenes. Often, experiments are carried out in low-temperature matrices, as in the direct observation of di- -butylcarbene produced from the corresponding diazo compound by irradiation at 254 nm at 14 However, solution-phase studies are also important and a study of diadamantyldiazomethane has shown that the derived carbene behaves as a triplet state in solution. This behaviour is reported to be most unusual for a dialkyl carbene. ... 

Direct generation of triplet carbene from the diazo compound is usually by irradiation with a triplet photosensitizer. Although the distinctions between reactions of singlet carbenes and triplet carbenes can be clearly made , product studies are always subject to the limitation that the final product depends on the relative speeds of singlet-triplet interconversion and the rates of the reactions of these species to yield the observed products. 

The transient spectra of neither singlet nor triplet 1-naphthylcarbene have ever been observed upon LFP of 10. The kinetics of these invisible carbenes were obtained from the growth of their ylide reaction products and not by direct observation of either singlet or triplet 1 -naphthylcarbene. 

ESR parameters for triplet carbenes and nitrenes have been summarized, and it has been shown that phenylnitrene is produced predominantly (87-88%) in the singlet state by direct photolysis of phenyl azide in low-temperature matrices. The first spectroscopic observation of a singlet nitrene has been reported nanosecond-laser photolysis of 1-azidopyrene gives the So nitrene (A ax 450 nm) which has a lifetime of 22 nsec at room temperature (in benzene) and 34 nsec at 77 K in rigid solution. At room temperature it decays to the triplet ground state (Tj, A ax 415 nm) with a rate constant of about 4.4 x 10 sec T, is formed directly by biacetyl sensitized photolysis of the azide. The lifetime of the excited triplet (Tj) was about 7 nsec. Tj dimerizes to azopyrene. ... 

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