Methylenecyclopropenes radical cation

Data on the radical cations are much more definitive because the parent is well established as a fragment formed by the photoionization of benzene and its isomers, and its hetero derivatives such as pyridine and the three diazines. (This ion, its formation and thermochemistry, is reviewed in Ref. 76.) Unequivocally, the most stable C4H4 isomer is the radical cation of methylenecyclopropene. This last species is perhaps more explicitly and sensibly described as the -CH 2 derivative of cyclopropenium cation and so renamed (cyclopropenio)methyl radical (15). From these studies, the heat of formation of this... 

There is one substituted methylenecyclopropene for which the radical cation is well understood the 2,3-diphenyl, 4-hydroxy, 4-methoxy derivative (16). This ion is not formed by 1-electron oxidation or chemi-ionization from the methylenecyclopropene. Instead it arises from rearrangement of its tautomer, methyl 2,3-diphenylcyclopropene-l-carboxylate (17) in pro tic media . This represents another example of reversal of stability... 

The extensive mass spectral studies of the triafulvenes have been summarized The radical cation of 85 is observed in the mass spectrum of methylenecyclopropene and is the most stable C4H4 species . By analogy with cyclopropenones fragment ions derived from cycloreversion to alkynes are often observed. 

The spirit is to show some of the results, but also to guide users of the approach by pointing to the problems and limitations of the method. The review covers some of the newer applications in the spectroscopy of organic molecules acetone, methylenecyclopropene, biphenyl, bithiophene, the protein chromophores indole and imidazole, and a series of radical cations of conjugated polyenes and polyaromatic hydrocarbons. The applications in transition metal chemistry include carbonyl, nitrosyl, and cyanide complexes, some dihalogens, and the chromium dimer. 

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