Biradical magnetic field effect

Magnetic field effects on the reaction kinetics or yields of photochemical reactions in the condensed phase have been studied [20-23]. They have proved powerful for verifying the mechanism of photochemical reactions including triplet states. Previously, we obtained photogenerated triplet biradicals of donor-acceptor linked compounds, and found that the lifetimes of the biradicals were remarkably extended in the presence of magnetic fields up to 1T [24]. It has been reported that Cgo and its derivatives form optically transparent microscopic clusters in mixed solvents [25,26]. The clustering behavior of fullerene (C o) is mainly associated with the strong three-dimensional hydrophobic interactions between the C o units. Photoinduced... 

In some systems, triplet BET can occur, as deduced from time-resolved optical spectroscopy, magnetic field effects, CIDNP, or optoacoustic calorimetry. Triplet BET is governed by energetic factors, which determine rates, and by the relative topologies of the potential surfaces of parent molecule, radical ions, and of accessible triplet or biradical states. Divergent topologies for different states may cause rearrangements. 

Magnetic Field Effects on Chemical Reactions through Biradicals 8.1 Historical Introduction... 

Magnetic Field Effects on Thermal Reactions through Biradicals... 

The low-temperature formation of C +-P-C6o and its recombination to a triplet state allow some interesting and potentially useful magnetic field effects. In the presence of a small (20 mT) static magnetic field, the lifetime of the C +-P-C6o charge-separated state in 43 is increased by 50 % [155]. This is ascribed to the effect of the magnetic field on interconversion of the singlet and triplet biradicals. At zero field, the initially formed singlet biradical state is in equilibrium with the three triplet biradical sublevels, and all four states have comparable populations. Decay to the carotenoid triplet only occurs from the three triplet sublevels. In the presence... 

Magnetic field effects show a dependence of the modulated triplet yields for photogenerated biradicals of the type )n-2O" upon the methylene chain length SI. There is also an effect on the yield micellized triplet radical pairs2 2... 

Infra red spectra of triplet phenylnitrenes , zero-field splitting of the T, state in biradicals measured by magnetic field effects on fluorescence decay , and oiplet-triplet fluorescence and spin polarization of 1- and 2-naphthylphenylcarbenes ° are experimental studies reported on biradical species. 

Gloss, G. L., Forbes, M. D. E. and Piotrowiak, R, Spin and Reaction Dynamics in Flexible Polymethylene Biradicals As Studied by EPR, NMR and Optical Spectroscopy and Magnetic Field Effects. Measurements and Mechanisms of Scalar Electron Spin-Spin Coupling,/. Am. Chem. Soc., 114, 3285, 1992. 

Paramagnetic species possessing two unpaired electrons (biradicals, carbenes, and their analogues) have been intensively studied using CIDNP techniques in high and low magnetic fields. The involvement of biradicals in organic and biochemical reactions is postulated in a number of cases, but the physical methods (EPR, laser spectroscopy) have allowed the detection of their formation predominantly in the photolysis of cyclic ketones. These species have also served as models for detailed studies of CIDNP peculiarities for biradicals, as well as the distinction of these effects from the case of free radicals. 

Reference data on biradicals of cyclic ketones point to the appearance of CIDNP effects in a wide range of magnetic field strengths. The manifestations of various mechanisms of CIDNP formation depend on the structure of the biradical. 

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