Methylene, from photolysis diazomethane

Flash photolysis studies<22) have indicated singlet methylene to be produced from the diazomethane-excited singlet upon loss of nitrogen followed by collisional deactivation to the triplet, the ground state multiplicity for this molecule. 

It will now be shown that the discrepancy can be resolved by considering the possibility of a more energetic methylene in the experiments of Bell and Kistiakowsky, who used diazomethane photolysis to generate methylene. Frey41 has shown that methylene from diazomethane may be more energetic than methylene from ketene by 10-15 kcal. 

Assuming that reaction (4) proceeds with unit collisional deactivation efficiency, then the value of is computed as 3 X 10 sec. . This value is close to the value obtained when diazomethane is used as the methylene precursor and implies that the total energy of the methylene produced by the photolysis of diazomethane and diazirine is the same. These results were rationalized on the basis that methylene from diazirine contained less translational energy and was more vibrationally excited than methylene from diazirine. 

There is much evidence that the photolysis of ketene, diazomethane, and diazirine produces methylene predominantly in the singlet state. It is, however, possible, by using mercury photosensitization or by photosensitization using benzophenone to produce triplet methylene. Since the triplet is the ground state for methylene, it is also possible to produce triplet methylene from singlet methylene by carrying out reactions in the presence of a large excess of inert gas. Recently, much evidence has accumulated to indicate that even in the normal photolysis of all three of the methylene precursors, some of the methylene is produced in the triplet state. ... 

The photolysis of diazirine °° at 3130 A yields ethylene and nitrogen methylene is probably formed in the primary process. The long wavelength absorption ( max = 3200 A) had been identified as the allowed (a, n ) transition and semi-empirical Huckel calculations indicate that the methylene produced must be (Ai) or (Bi). There is evidence that methylene produced from the photolysis of diazirine is more selective than methylene from diazomethane, owing to decreased excess translational energy. At low pressures 5-30 torr, diazomethane was identified as an intermediate by its absorption spectrum and the question arises, is methylene formed directly, or does it arise from decomposition of diazomethane The quantum yield of disappearance of diazirine is 2.0+0.5 and the quantum yield of diazomethane formation is about 0.2. The intermediate diazomethane is... 

On the basis of eqn. (36), the yields of triplet methylene from ketene are found to be roughly 15% at 2800 A, 15% at 3130 A, 30% at 3340 A and 40% at 3660 A . Equation (36) neglects the formation of singlet-type products from triplet methylene via radical recombination (35). If the reduction in yield of rra/ti-2-pentene and 2-methyl-2-butene by added oxygen is also considered to be the result of triplet CH2 reactions, a recalculation of triplet methylene fractions yields 20% triplet at 2800 A, 55% at 3340 A and 60% at 3660 A for ketene photolysis . Assuming that 10% added oxygen suppresses all triplet products and measuring product yields relative to an internal standard, Eder and Carr " arrived at 29 3% triplet CH2 at 3130 A, and 87 2% at 3660 A. For the diazomethane-troni-2-butene system, the fraction of triplet methylene was found to be 12% at 3550-4000 A . 

Early work on the gas-phase interaction of methylene with carbon-hydrogen bonds showed selective reactivity in the order, tertiary C-H > secondary C-H > primary C-H. Methylene from diazomethane was found to be less selective than methylene from ketone . The differences in the relative reactivity of methylene from the two sources have been attributed to excess translational energy of methylene from CH2N2 . Recently, however, it has been shown that the photolysis of ketene produces more triplet CH2 than the photolysis of diazomethane cf. preceeding section). Part of the reported selectivity must be attributed to triplet-methylene effects. 

Methylene (CHj) was generated from diazomethane in some of the earhest matrix studies of organic reactions, and its dimerization to ethene was observed, although its IR spectrum proved somewhat elusive.Various halomethylenes were also observed among the products from the vacuum-UV irradiation of halomethanes in matrices. Examples are CHF from photolysis of methyl fluoride and CClj from dichloromethane. ... 

Sensitization, which can populate the triplet manifold, was used in a number of instances. Sensitization with benzophenone was used in the photolysis of diazomethane to generate triplet methylene. The triplet methylene thus produced, however, failed to abstract much hydrogen from alkanes (cyclohexene), but... 

The difference in energy between a /l, and X B, has been estimated to be in the range from about 0.1 to about 1 eV. See a recent review by Chu and Dahler (211). Very recently a value of 0.27 eV was obtained as an upper limit of this energy difference from the occurrence of the process CH2CO > CH2(a1 /4,) + CO at 3370 A (258a). It has been known for many years that chemical reactivities of X3B, and a /t, states are very different. The Cl, state reacts 3 orders of magnitude faster than the iBl with H2 and CHa [Braun et al. (143)]. Both states are generated by the photolysis of ketene or diazomethane. Methylene is also a primary product of hydrocarbon photolysis in the vacuum ultraviolet [Ausloos and Lias (49)]. 

Vibrational excitation of triplet methylene appears to be possible. Photolysis of diazomethane in the presence of alkenes shows product distributions as a function of pressure that could indicate a change in the methylene singlet/triplet proportions in the mixture97. Internal conversion from singlet to triplet has been suggested115. These triplet methylene molecules should be excited. 

Two band systems of methylene have been observed by photolysis of diazomethane in presence of excess nitrogen (Herzberg, 1961 Herzberg and Shoosmith, 1959). The radical is short-lived (no absorption is detected 50 psec after the photolysing flash) and appears to be formed in an excited state from which it decomposes unless deactivated by collisions with the inert gas. 

Like diazomethane, on photolysis diazirine yields methylene and nitrogen as major primary products. In a preliminary communication it was reported that when photolyzed alone, ethylene and nitrogen were the major products, the ethylene resulting from an attack of methylene on the diazirine (trace amounts of HCN were also detected). Photolysis in the presence of a large excess of b ans-but-2-ene at a total pressure of 500 mm. yielded irans-l,2-dimethylcyclopropane, double bond was thus stereospecific, implying that the methylene was initially produced in the singlet state. 

Photolysis of diazirine in the presence of a large excess of propane yielded n- and isobutane and in the presence of n-butane yielded n- and isopentane. From the relative rates of attack on the primaiy and secondary carbon-hydrogen bonds in these compounds, it was concluded that methylene derived from diazirine showed approximately the same discrimination as methylene formed by the photolysis of ketene. The results obtained, using methylene derived from the photolysis of diazomethane, gave a product ratio closer to the simple statistical ratio of the number of carbon-hydrogen bonds without correction factors for the type involved and indicated almost no differentiation between the types. 

Recently, Moore and Pimentel have photolyzed diazirine isolated in a solid nitrogen matrix. They found that the photolysis did produce diazomethane. In order to decide whether the diazomethane arose by a photoisomerization reaction or by the reaction of methylene with the nitrogen of the matrix, experiments were carried out in a nitrogen matrix enriched in N. On the basis of these experiments they conclude that the photolysis of the diazirine produces methylene and nitrogen, and that the diazomethane arises from the reaction of the methylene with the matrix and not by photoisomerization. Photolysis of diazomethane does not... 

Photolysis of diazo compounds (305) generates nitrogen and the carbene 289. Photolysis of diazomethane (311, R = H), for example, generates methylene (H2C ),242 Benzophenone is often added as a sensitizer for the photolysis of diazomethane. Under these conditions, triplet methylene is formed via intersystem crossing (5i Ti)243 (sec. II.IO.B). Energy transfer from triplet benzophenone to triplet diazomethane followed, and triplet diazomethane decomposed to triplet methylene.244 a similar thermal reaction gives the carbene and the usual carbene reactions.245... 

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