Radical-pair model

All these observations necessitated theorists to discard the Overhauser explanation. The radical pair model was put forward (Gloss and Gloss,... 

G. R. Lawler, Chemically Induced Dynamic Polarization (CIDNP). II. The Radical-Pair Model," Accfs. Chem. Res. 5, 25 (1972). 

Equations (5) and (8) shows that the exponential kinetics of the square-root of tj can be obtained for either the random distribution model or the radical-pair model. The same kinetic curve will be obtained from the number density of C0 in the random distribution model and the characteristic distance r0 in the radical-pair model, if... 

The photolyses of 1,2-dipheny1-2-methyl-1-propa-none and its 2h and derivatives in micellar solution are now described and further demonstrate the enhanced cage and magnetic isotope effects of mlcelllzatlon. We report also the observation of CIDP during the photolyses of micellar solutions of several ketones, and demonstrate the validity of the radical pair model to these systems. Analyses of the CIDNP spectra in the presence and absence of aqueous free radical scavengers (e.g., Cu2+) allow us to differentiate between radical pairs which react exclusively within the micelle and those that are formed after diffusion into the bulk aqueous phase. In some cases this allows us to estimate a lifetime associated with the exit of free radicals from the micelles. 

The first discovery of chemically induced dynamic electron polarization (CIDEP) was made by Fessenden and Schuler in 1963 (58). These authors observed the abnormal spectra of the H atoms produced during the irradiation of liquid methane. The low-field line in the esr spectrum was inverted compared to the corresponding high-field line. The related chemically induced dynamic nuclear polarization effect (CIDNP) was reported independently four years later by Bargon et al. (22) and by Ward and Lawler (134). Because of the wider application of nmr in chemistry, the CIDNP effect immediately attracted considerable theoretical and experimental attention, and an elegant theory based on a radical-pair model (RPM) was advanced to explain the effect. The remarkable development of the radical-pair theory has obviously brought cross-fertilization to the then-lesser-known CIDEP phenomenon. 

As regards the proposed caged radical pair model, it is consistent with the results obtained upon photolysis of phenyl acetate in the vapor phase, which gives almost exclusively the escape product (phenol) [10, 11]. Conversely, in rigid matrixes (where the cage effect is enhanced), only radical recombination products are obtained [23]. 

The quantum yield of [Co(II)L(H20) ] (L = Me4[14]tetraeneN4) produced by the near-UV photolysis of [Co(III)L(H20)2] increases appreciably in the presence of alcohols/ The results are interpreted within the context of the radical-pair model with the formation of OH radicals, which can be intercepted by the alcohol component of the solvent cage. 

Surfactants have also been of interest for their ability to support reactions in normally inhospitable environments. Reactions such as hydrolysis, aminolysis, solvolysis, and, in inorganic chemistry, of aquation of complex ions, may be retarded, accelerated, or differently sensitive to catalysts relative to the behavior in ordinary solutions (see Refs. 205 and 206 for reviews). The acid-base chemistry in micellar solutions has been investigated by Drummond and co-workers [207]. A useful model has been the pseudophase model [206-209] in which reactants are either in solution or solubilized in micelles and partition between the two as though two distinct phases were involved. In inverse micelles in nonpolar media, water is concentrated in the micellar core and reactions in the micelle may be greatly accelerated [206, 210]. The confining environment of a solubilized reactant may lead to stereochemical consequences as in photodimerization reactions in micelles [211] or vesicles [212] or in the generation of radical pairs [213]. 

Norris J R, Morris A L, Thurnauer M C and Tang J 1990 A general model of electron spin polarization arising from the interactions within radical pairs J. Chem. Phys. 92 4239—49... 

Fig. 8.9 Possible mechanisms of the bioluminescence reaction of dinoflagellate luciferin, based on the results of the model study (Stojanovic and Kishi, 1994b Stojanovic, 1995). The luciferin might react with molecular oxygen to form the luciferin radical cation and superoxide radical anion (A), and the latter deproto-nates the radical cation at C.132 to form (B). The collapse of the radical pair might yield the excited state of the peroxide (C). Alternatively, luciferin might be directly oxygenated to give C, and C rearranges to give the excited state of the hydrate (D) by the CIEEL mechanism. Both C and D can be the light emitter.

The origin of CIDNP lies in the microscopic behaviour of radical pairs. Our discussion of this will follow fairly closely the model approach associated with the names of Gloss, Kaptein, OosterhofF, and Adrian, rather than the more formal kinetic treatments of Fischer (1970a) and Buchachenko et al. (1970b). 

C. The Dynamic Behaviour of Badical Pairs A simple model (Fig. 4) will assist in visualizing the time-dependent variation of the separation and hence the interaction of the components of a radical pair. 

A pictorial representation of the Tg-S mixing process follows from Fig. 6. Just as in normal n.m.r. or e.s.r. spectroscopy, precession can be represented by a vector model. When placed in an external magnetic field the two unpaired electrons of the radical pair 1 and 2 will precess... 

Hollander, 1972) using basically the Noyes diffusion model (Noyes, 1954, 1955, 1956, 1961), treats the radicals as if they go on a random walk . It must be remembered, however, that the two radicals must maintain the correlation of their electron spins (Jee f ) or tl distinction between T and S manifolds for a given radical pair would be lost. 

As it pertains to the solid state photodecarbonylation reaction, the model assumes that most aliphatic ketones have similar excitation energies, that reactions are more likely along the longer-lived triplet excited state, and that each reaction step must be thermoneutral or exothermic to be viable in the solid state. " Using acetone and its decarbonylation intermediates as a reference reaction (dashed lines in Fig. 7.24), we can analyze the energetic requirements to predict the effects of substituents on the stability of the radical intermediates. The a-cleavage reaction of triplet acetone generates an acetyl-methyl radical pair in a process that is 3.5 kcal/mol endothermic and the further loss of CO from acetyl radical is endothermic by 11.0... 

The cage effect was also analyzed for the model of diffusion of two particles (radical pair) in viscous continuum using the diffusion equation [106], Due to initiator decomposition, two radicals R formed are separated by the distance r( at / = 0. The acceptor of free radicals Q is introduced into the solvent it reacts with radicals with the rate constant k i. Two radicals recombine with the rate constant kc when they come into contact at a distance 2rR, where rR is the radius of the radical R Solvent is treated as continuum with viscosity 17. The distribution of radical pairs (n) as a function of the distance x between them obeys the equation of diffusion ... 

The substitution reaction of CP with methyl chloride, 2-chloroethyl radical, and allyl chloride has been treated by several different ab initio theoretical models. Depending on the method, the intrinsic barrier for the 5ivr2 process in allyl chloride is 7-11 kcalmoP higher than the barrier for the 5ivr2 reaction of methyl chloride. The reaction of CP with the 2-chloroethyl radical involves an intermediate complex, which is best described as an ethylene fragment flanked by a resonating chloride anion-chloride radical pair. There are many other points of interest. 

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