Radical Cation of a Trimethylenemethane with a Nondegenerate Ground State

摘要

Upon ionization by gamma-irradiation in frozen CFCl_3,or by X-irradiation in an Ar matrix,2,2,3,3-tetramethylmethylenecyclopropane(MCP-Me_4)readily undergoes ring opening to yield the radical cation of 1,1,2,2-tetramethyltrimethylenemethane(TMM-Me_4).The hyperfine-coupling constants for TMM-Me_4~(centre dot +)are(mT)-1.99(2H),+0.53(6H),and +0.19(6H),and the singly occupied orbital closely resembles one of the two degenerate nonbonding pi-MOs(NBMOs)of trimethylenemethane(TMM).Due to the expected effect of the methyl substituents,this "symmetric" NBMO,phi_(2+)(b_1),is energetically favored relative to its "antisymmetric" counterpart,phi_2-(a_2),so that the ground state assumes a structure with ~2B_1 symmetry in the C_(2v)point group.Calculations show that the ring opening in the primary radical cation MCP-Me_4~(centre dot +)to yield TMM-Me_4~(centre dot +)is spontaneous,whereas in the parent system(MCP(centre dot +)-> TMM~(centre dot +))a low barrier does exist.In contrast to the previously investigated case of the radical cation of tetramethyleneethane,the "electromer" of TMM-Me_4~(centre dot +),in which the unpaired electron occupies phi_(2-),cannot be attained photochemically.