Radical Cations from Dicyclopropylidenemethane and Its Octamethyl Derivative

Abstract

The radical cations of dicyclopropylidenemethane (2) and its octamethyl derivative (2-Me 8 ) are prone to rearrangements into those of (2-methylallylidene)cyclopropane (2a) and its octamethyl derivative (2a-Me 8 ), respectively, by opening one three-membered ring. In contrast to the radical cations of bicyclopropylidene (1) and its octamethyl derivative (1-Me 8 ), 2 •+ and 2-Me 8 •+ are stable to opening of the second ring, because in this case the resulting species would be a non-Kekulé hydrocarbon with a quartet ground state. Similarly to 1, octamethyl substitution in 2 promotes the tendency to rearrangement. Thus, ESR and ENDOR studies indicate that the primary radical cation 2 •+ , which is formed upon γ-irradiation of 2 in a CFCl 3 matrix at 77 K, does not rearrange up to 150 K. On the other hand, when 2-Me 8 is treated in the same way, only the rearranged radical cation 2a-Me 8 •+ can be observed and characterized by its ESR and ENDOR spectra. Nevertheless, the existence of the two "missing" species, 2a •+ and 2-Me 8 •+ , is revealed by other methods. According to UV and IR studies, X irradiation of 2 in an Ar matrix leads directly to the ring-opened radical cation 2a •+ . Moreover, magnetic field effects on the decay of fluorescence, which appears upon recombination of the radical anion of p-terphenyl with a radical cation generated from 2-Me 8 in liquid octane, strongly suggest that 2-Me 8 •+ (and not 2a-Me 8 •+ ) is formed initially. From the temperature dependence of the decay, the activation energy of the ring-opening process 2-Me 8 •+ f 2a-Me 8 •+ is estimated. The radical cations 2a •+ and 2a-Me 8 •+ are formally distonic with the spin residing in the allylic moiety and the charge accommodated on the central carbon atom of the allene π-system. The intact cyclopropylidenemethylidene moiety assumes a "bisected" conformation, thus favoring an optimal interaction with the positively charged center on the π-system.

Structure and Stability of Pentafluoroaniline and 4-Aminononafluorobiphenyl Radical Anions: Optically Detected Electron Paramagnetic Resonance, Time-Resolved Fluorescence, Time-Resolved Magnetic Field Effect, and Quantum Chemical Study

“…Assuming that polyfluoroarene RAs possess the close rates of disappearing via the geminate recombination we can estimate the rate of 2 −• decay via the parallel chemical process using a fluorescence decay curve for some acceptor giving a stable RA. 34 Such approach is approximate, so the use of several reference RAs increases accuracy of estimate. We use as a reference the fluorescence decay curves, I 1 (t), obtained for alkane solutions of 1 and hexafluorobenzene.…”

Time-resolved magnetic field effect as a method of studying radical pairs with rapid evolution of spin state

“…The assumed decay should lead to a charged product and its recombination with a counterion does not induce fluorescence. The rate constant of this process was determined from the ratio of the kinetic curves to the corresponding curves for TFB solutions as described in [11]. The decay activation energy of ≈3.5 kcal/mol was determined from the temperature dependence of rate constant k(Т ) shown in Fig.…”

Electron spin resonance

“…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. 104 The radical cation of Dewar benzene and its ring-opening to the hexamethylbenzene radical cation was first observed using ESR by Rhodes. 105 In a more recent study, this species was generated and observed by optical spectroscopy in cryogenic matrices to which it reveals itself by a charge resonance band at 600 nm, in fact very similar in shape and position to that observed for the related radical cation of norbornadiene.…”