Degenerate electron exchange

Time-resolved CIDNP experiments have been used to measure the rates of the degenerate electron exchange between the radical cation of 1,6-diphenylhexa-... 

Methionine adds another degree of complexity because the radical cation, which is formed by the same pathway as in the cysteine case, namely, by exclusive electron transfer from sulphur, can stabilize by the formation of a cyclic structure with a two-centre three-electron bond between sulphur and nitrogen. CIDNP has provided unequivocal evidence for this species and allowed to probe its spin distribution, through the polarization pattern.Only an unprotonated amino group can function as a donor and effect the cyclic stabilization, so there is a pronounced dependence of the polarization pattern on the pPI. ° This pH-dependence was explained by a reversible pair substitution of the open-chain and cyclic radical cations, but later reinterpreted as arising from a fast (on the CIDNP timescale) equilibrium between the different forms in conjunction with degenerate electron exchange. ... 

The observation that the motion of counter-ions can determine the rate of intramolecular ET in weakly polar media is not without a precedent. It has been elegantly demonstrated by the electron paramagnetic resonance (EPR) work of Mazur et al., in which the rate of degenerate electron exchange in radical anions of diketones was studied (17). 

The broadening of the MARY lines was also analyzed to study the degenerate electron exchange in the irradiated alkane solutions involving cw-decalin radical cation [21] and hexafluorobenzene radical anion [20, 21]. It was found that for cis-decalin radical cation, the rate of charge transfer was determined by the number of diffusion collisions, while for hexafluorobenzene radical anion, it was lower and reached the diffusion-controlled limit only at high viscosity. 

Our model is thus of a metal, with a small number of carriers in the conduction band and 3d- or 4f-moments antiferromagnetically coupled to each other by direct exchange. The carriers may be either inserted by doping or by overlap from the lower Hubbard band (cf. Chapter 4, Section 3). The most striking prediction of the model, however, is that the degenerate electron gas should have a much enhanced Pauli magnetism. Suppose that EF is the Fermi... 

Bis(trifluoromethyl) A3-iodane 6a undergoes degenerate ligand exchange with added alkoxide PhC(CF3)2OK more rapidly (second-order rate constant = 49 M 1s 1 at 56 °C) than that of dimethyl A3-iodane 6b (second-order rate constant =61 M 1s 1 at 93 °C), in which an associative mechanism involving the formation of [12-1-4] species was proposed [16]. The CF3 substituents, which lower the electron density on iodine(III) relative to the CH3 substituents, make the iodine of 6a more susceptible to attack by alkoxide ion. Dynamic 19F NMR of A3-iodane 7 showed an intramolecular ligand exchange via intermediacy of bicyclic tetracoordinated iodate with a AG of ca. 12 kcal/mol at - 80 °C [17]. 

Although the low field effects on chemical reactions through radical pairs had been explained by the LCM, Timmel et al. [14] proposed that the so-called low field effects arose also fi om coherent superpositions of degenerate electron-nuclear spin states in a radical pair in zero field. They made some model calculations for their mechanism. At first, let us consider the case of a radical pair with a single spin-1/2 nucleus, e.g., a proton. When the exchange term is not included (J= 0 J), its spin Hamiltonian (H) can be expressed from Eq. (3-3) as... 

The CD of the CT transitions has attracted little direct interest in the literature. Mason (10) has postulated that it arises from an exciton mechanism in which the CT transition is broken up into three degenerate oscillators or chromophores with different origins. Such a procedure is unjustified quantum mechanically, as each chromophore must have negligible electron exchange with any other chromophore considering that a common metal d state is involved in the definition of each such chromophore or oscillator, such a model is, despite its pictorial appeal, theoretically inconsistent. Mason supports his model by noting that the CT CD exhibits the exciton structure characteristic of... 

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