APPLICATIONS OF ENDOR SPECTROSCOPY TO RADICAL CATIONS IN FREON MATRICES .10. STRUCTURES AND REARRANGEMENTS OF BICYCLO[1.1.0]BUTANE RADICAL CATIONS - AN ESR AND ENDOR STUDY

Radical cations were generated by gamma-irradiation in Freon matrices from bicyclo[1.1.0]butane (1), 1,3-dimethylbicyclo[1.1.0]butane (2), 1 -methylbicyclo[1.1.0]butane (3), tricyclo[3.1.0.0(2,6)]hexane (4), and tricyclo-[4.1.0.0(2,7)]heptane (5), as well as from some deuterio der ivatives of 1, 3, and 5. Under these conditions, the bicyclic radical cations 1.+, 2.+, and 3.+ were persistent and could be characterized b y their hyperfine data with the use of ESR and ENDOR spectroscopy. A d etailed analysis of these data indicates that a methyl group withdraws ca. 15% of the spin population from the substituted carbon atom 1 or 3 of bicyclobutane. In contrast to 1.+-3.+, the tricyclic radical cati ons 4.+ and 5.+ are not sufficiently long-lived for ESR and ENDOR stud ies, as they readily rearrange to the radical cations of cyclohexa-1,3 -diene (6) and cyclohepta-1,3-diene (8), respectively. The isomerizati on of 5.+ to 8.+ possibly proceeds via the radical cation of cis-bicyc lo[3.2.0]hept-6-ene (7) which undergoes ring opening to yield 8.+. Deu terium labelings of 5.+ and 8.+ point out that the initial step in thi s isomerization is the cleavage of a lateral bond in the bicyclobutane moiety, and the same statement should hold for the rearrangement of 4 .+ to 6.+. The present work emphasizes the importance of ENDOR spectro scopy for the full characterization of radical cations in Freon matric es. Not only can smaller coupling constants and differences in the lar ger ones be determined, but it is also possible to derive the absolute sign of these values from the anisotropic components of their ENDOR s ignals. Because of its lower resolving power, this information is not available with the use of ESR spectroscopy alone, in particular for ra dicals in rigid solutions.