Radical Cation of a Trimethylenemethane with a Nondegenerate Ground State

Upon ionization by γ-irradiation in frozen CFCl3, or by X-irradiation in an Ar matrix, 2,2,3,3-tetramethylmethylenecyclopropane (MCP-Me4) readily undergoes ring opening to yield the radical cation
of 1,1,2,2-tetramethyltrimethylenemethane (TMM-Me4). The hyperfine-coupling constants for TMM-Me4•+
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 π-MOs (NBMOs) of trimethylenemethane (TMM). Due to the expected
effect of the methyl substituents, this “symmetric“ NBMO, ψ2+ (b1), is energetically favored relative to its
“antisymmetric“ counterpart, ψ2- (a2), so that the ground state assumes a structure with 2B1 symmetry in
the C2v point group. Calculations show that the ring opening in the primary radical cation MCP-Me4•+ to
yield TMM-Me4•+ is spontaneous, whereas in the parent system (MCP•+ → TMM•+) a low barrier does
exist. In contrast to the previously investigated case of the radical cation of tetramethyleneethane, the
“electromer” of TMM-Me4•+, in which the unpaired electron occupies ψ2-, cannot be attained photochemically.