Determination of singlet and triplet methylene fractions

Although Skell s postulate cannot be justified theoretically, it has received ample experimental support. In many cases, the stereoselectivity of the addition decreases drastically with increasing amounts of an inert diluent . On the other hand, radical scavengers, such as oxygen, which are expected to trap either triplet methylene or the diradical intermediates of triplet methylene reactions, restore the stereospecificity of olefin addition - . The mercury-photosensitized decomposition of ketene and the benzophenone-sensitized 

For a further differentiation of singlet from triplet methylene it has been assumed that singlet methylene undergoes mainly insertion, reaction (34). and triplet methylene undergoes mainly abstraction, reaction (35) .  

The validity of this assumption has been questioned the total amount of insertion products R-CH was found to exceed the possible contribution from radical recombination, as estimated from the yields of R-R and CH ,-CH ). Ring and Rabinovitch have, therefore, suggested that methylene both in the singlet and triplet states inserts into carbon-hydrogen bonds. The calculations of Ring and Rabinovitch were based on the preposition that under their experimental conditions (800-fold excess of nitrogen, approximately 1.6 atm) methylene was entirely in its triplet state. It should be pointed out. however, that similar conditions did not afford the same relative amounts of stereo-isomeric cyclopropanes starting from either cis-or /rans-2-butene . 

The application of both criteria to gas-phase reactions is complicated further by the formation of vibrationally excited products. Both the insertion and addition reactions of methylene are exothermic by approximately 93 kcal. mole (based on recent estimates of AH (CH2) = 94 kcal.mole ). Vibrationally excited alkanes and alkenes may dissociate into free radicals, and excited cyclopropanes may undergo structural and geometrical isomerizations unless collisionally stabilized . The occurrence of hot molecule reactions excludes any reasonable estimation of singlet and triplet methylene fractions. The data presented in the following paragraphs have been taken from experiments at high-pressures , which are thought to ensure complete collisional deactivation of excited reaction products. 

For a methylene-olefin system, both the addition and abstraction reaction of triplet methylene should be taken into account. The photolysis of ketene and diazomethane in the presence of trani-2-butene has been studied in some detail. The fraction of triplet methylene formed can be determined by taking the ratio of the sum of triplet products to total products, where 

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