Self-exchange reactions of radical anions in n-hexane

Birch reduction of benzene in a low-temperature plasma

Selective and specific dihydrogenation of benzene and other arenes has been observed in a low-temperature helium plasma. A surface Birch reduction mechanism has been proposed in which benzene molecules adsorbed on the discharge surface capture low-energy surface-adsorbed electrons and subsequently undergo protonation (see picture). Gas-phase oxidation processes accompany the reduction reaction.

Radical Cations from Dicyclopropylidenemethane and Its Octamethyl Derivative

The radical cations of dicyclopropylidenemethane (2) and its octamethyl derivative (2-Me8) are prone to rearrangements into those of (2-methylallylidene)cyclopropane (2a) and its octamethyl derivative (2a-Me8), respectively, by opening one three-membered ring. In contrast to the radical cations of bicyclopropylidene (1) and its octamethyl derivative (1-Me8), 2*+ and 2-Me8*+ 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 gamma-irradiation of 2 in a CFCl3 matrix at 77 K, does not rearrange up to 150 K. On the other hand, when 2-Me8 is treated in the same way, only the rearranged radical cation 2a-Me8*+ can be observed and characterized by its ESR and ENDOR spectra. Nevertheless, the existence of the two "missing" species, 2a*+ and 2-Me8*+, 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-Me8 in liquid octane, strongly suggest that 2-Me8*+ (and not 2a-Me8*+) is formed initially. From the temperature dependence of the decay, the activation energy of the ring-opening process 2-Me8*+ --> 2a-Me8*+ is estimated. The radical cations 2a*+ and 2a-Me8*+ are formally distonic with the spin residing in the allylic moiety and the charge accommodated on the central carbon atom of the allene pi-system. The intact cyclopropylidenemethylidene moiety assumes a "bisected" conformation, thus favoring an optimal interaction with the positively charged center on the pi-system.

Electron Self-Exchange Kinetics Determined by MARY Spectroscopy: Theory and Experiment

The electron self-exchange between a neutral molecule and its charged radical, which is part of a spin-correlated radical ion pair, gives rise to line width effects in the fluorescence-detected MARY (magnetic field effect on reaction yield) spectrum similar to those observed in EPR spectroscopy. An increasing self-exchange rate (i.e., a higher concentration of the neutral molecule) leads to broadening and subsequent narrowing of the spectrum. Along with a series of MARY spectra recorded for several systems (the fluorophores pyrene, pyrene-d(10) and N-methylcarbazole in combination with 1,2- and 1,4-dicyanobenzene) in various solvents, a theoretical model is developed that describes the spin evolution and the diffusive recombination of the radical pair under the influence of the external magnetic field and electron self-exchange, thereby allowing the simulation of MARY spectra of the systems investigated experimentally. The spin evolution of the radicals in the pair is calculated separately using spin correlation tensors, thereby allowing rigorous quantum mechanical calculations for real spin systems. It is shown that the combination of these simulations with high resolution, low noise experimental spectra makes the MARY technique a novel, quantitative method for the determination of self-exchange rate constants. In comparison to a simple analytical formula which estimates the self-exchange rate constant from the slope of the linear part of a line width vs concentration plot, the simulation method yields more reliable and accurate results. The correctness of the results obtained by the MARY method is proved by a comparison with corresponding data from the well-established EPR line broadening technique. With its less stringent restrictions on radical lifetime and stability, the MARY technique provides an alternative to the classical EPR method, in particular for systems involving short-lived and unstable radicals.

Optically Detected ESR and Low Magnetic Field Signals from Spin Triads: 2-Imidazoline-1-Oxyl Derivatives in X-irradiated Alkane Liquids as a Method to Study Three-Spin Systems

This contribution reports the design and synthesis of a series of spin-labeled charge acceptors to produce three-spin systems of "radical ion/biradical ion" type in X-irradiated alkane liquids. This opens the way to study spin triads in experimental conditions, in which short-lived radical ion pairs are conventionally studied, thus offering optically detected techniques such as magneto-resonance OD ESR and level-crossing MARY spectroscopy. The structure of the synthesized 2-imidazoline-1-oxyl derivatives is A-Sp-R, where A is a positive or negative charge acceptor, R is a stable radical, and Sp is a hydrocarbon bridge. The set of 20+ compounds represent a convenient tool to construct experimental three-spin systems with various properties, e.g. with the "third" spin introduced into one or the other partner of the radical ion pair. The degree of exchange coupling between the two paramagnetic fragments in the biradical ion has been demonstrated to strongly depend on the type of the radical fragment R and the structure of the bridge Sp. As a result, a series of acceptors with systematically reduced exchange interaction has been synthesized, and optimal systems for the observation of low magnetic field effect have been found. In the most favorable case, an OD ESR signal from a spin triad living as short as ca. 100 ns has been registered as a single unresolved line. The exchange integral for this biradical anion (9) was estimated from OD ESR and ESR experiments to be ca. 10(3) G by the order of magnitude, which is much greater than the hyperfine couplings in the biradical ion but much smaller than the thermal energy kT.

Self-Promoted Electron Transfer from Cobalt(II) Porphyrin to p-Fluoranil To Produce a Dimer Radical Anion-Cobalt(III) Porphyrin Complex

Self-promoted electron transfer from a cobalt(II) porphyrin [Co(II)OEP] to p-fluoranil (F4Q) occurs, exhibiting a second-order dependence of the electron-transfer rate with respect to the F4Q concentration due to the formation of a strong complex between the dimer radical anion [(F4Q)2*-] and the resulting Co(III)OEP+.