Electron spin resonance studies of heterocyclopropane radical cations generated radiolytically in freon matrices

Several heterocyclopropane radical cations radiolytically generated in Freon matrices were studied by electron spin resonance spectroscopy. The results show: (a) The radical cations of cyclopropane and its derivatives are ring-closed with ²A₁ structures in CFCL₃ between 80 and 160 K and in CF₂CLCFCL₂ at low temperatures. On annealing in CF₂CLCFCL₂, however, these ring-closed cations all undergo ring opening irreversibly to form localized structures. The ring-opened cyclopropane radical cation further rearranges to the propene cation by U.V. irradiation, while the ring-opened trimethylcyclopropane cation further rearranges to the 4-methyl-2-pentene cation either by U.V. irradiation or by annealing. The possible mechanisms of ring-opening were discussed, (b) Both the ring-opened delocalized planar structure (²A₂) and the localized structure of the oxirane radical cation were observed in SF₆ at low temperatures from studies of oxirane, 1,2-D₂-oxirane, and the 1,2-¹³C oxirane radical cations. On annealing to above the phase transition at 95 K, the delocalized form is changed to the localized form irreversibly, (c) The aziridine radical cation in CFCL₃ possesses the ring-opened planar structure with spin mostly on two carbons, (d) The monomer radical cation of thiirane in CFCL₃ is the ring-closed ²B₁(π) state with most of the spin on the sulfur atom. The dimer radical cation of thiirane with a σ²σ^1* bond is produced in CF₂CLCFCL₂ and CF₃CCL₃ and on annealing it dissociates with the extrusion of an ethylene molecule, (e) The cyclopropylamine radical cation readily ring opens to form •CH₂CH₂CH=NH₂⁺, which does not rearrange to the nitrogen-centered radical cation CH₃CH₂CH=NH^+•; in contrast, the corresponding ring-opened neutral radical is CH₃CH₂CH=N• rather than •CH₂CH₂CH=NH. Cyclobutylamine behaves similarly, the results again showing a remarkable reactivity difference between the radical cation and the neutral radical. In addition, the present studies report that the radical cation center of 1,1-dimethylallene radical cation is localized on the dimethyl-substituted side of the molecule. Ion-molecule reactions were studied in the CF₂CLCFCL₂ matrix for radical cations of spiro[2.5]octane, cyclopentene oxide, exo-epoxynorbornane, tetramethylethylene, and dimethyl sulfide. The results show the possible importance of hydrogen-abstraction processes in some of the ion-molecule reactions.