Radical Cation of a Trimethylenemethane with a Nondegenerate Ground State

Abstract

Upon ionization by gamma-irradiation in frozen CFCl(3), 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 pi-MOs (NBMOs) of trimethylenemethane (TMM). Due to the expected effect of the methyl substituents, this "symmetric" NBMO, psi(2+) (b(1)), is energetically favored relative to its "antisymmetric" counterpart, psi(2-) (a(2)), so that the ground state assumes a structure with (2)B(1) symmetry in the C(2v) 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 psi(2-), cannot be attained photochemically.

Electron spin resonance

“…The hyperfine-coupling constants for the latter are (in Gauss) À19.9 (2H), +5.3 (6H), and +1.9 (6H), and the singly occupied orbital closely resembles one of the two degenerate nonbonding pi-MOs (NBMOs) of trimethylenemethane. 103 The radical cations of dicyclopropylidenemethane and its octamethyl derivative are prone to rearrange into the corresponding (2-methylallylidene)cyclopropane and its octamethyl derivative, by fracture of a single three-membered ring. Contrastingly, the radical cation of bicyclopropylidene (and its octamethyl derivative) is stable to opening of the second ring, since the resulting species would be a non-Kekule hydrocarbon with a quartet ground state.…”

Stable Hydrocarbon Diradical, An Analogue of Trimethylenemethane

“…Trimethylenemethane (TMM) diradical was first detected in an inert matrix at cryogenic temperatures by Dowd in 1966 with current relevance spanning a wide range of areas, such as theory of molecular structure and bonding, − gas-phase chemistry, , organometallic chemistry, organic synthesis, and organic magnetism. − The significant and diverse impact of TMM has resulted from the synthesis and studies of TMM derivatives, modified to improve stability (or persistence) and tailored to the desired properties. In this context, the minimally perturbed and stable derivatives of TMM are of general interest.…”

“…Trimethylenemethane (TMM) diradical was first detected in an inert matrix at cryogenic temperatures by Dowd in 1966. 1 Since then, both the parent TMM structure and its derivatives have been among the most studied reactive organic intermediates, 2 with current relevance spanning a wide range of areas, such as theory of molecular structure and bonding, [3][4][5][6] gas-phase chemistry, 7,8 organometallic chemistry, 9 organic synthesis, 10 and organic magnetism. [11][12][13][14][15][16] The significant and diverse impact of TMM has resulted from the synthesis and studies of TMM derivatives, modified to improve stability (or persistence) and tailored to the desired properties.…”