Thermal outer-sphere reactions

How important, though, is nuclear tunnelling for thermal outer-sphere reactions at ordinary temperature If we work in the Golden Rule formalism, an approximate answer was given some time ago. In harmonic approximation, one obtains from consideration of the Laplace transform of the transition probability (neglecting maximization of pre-exponential terms) the following expressions for free energy (AG ) and enthalpy (AH ) of... 

It was not very long ago that stereoselectivity in the thermal outer-sphere electron transfer reaction was reliably observed by Geselowitz and Taube (1980) [1]. Before their study, stereoselectivity had not been clearly observed even in the thermal outer-sphere electron transfer reaction. For instance, the stereoselectivity was reported in a thermal outer-sphere electron transfer reaction between [Co(phen)3]3 + and [Cr(phen)3]2+ [phen = 1,10-phenanthroline seeEq.(l)] [2], but the stereoselectivity was not observed by the different group [3], where A-and A-forms of [M(phen)3]3 + are shown in Scheme 1. 

As such, the thermal process in equation (60) proceeds via the same reactive intermediates (arising from an adiabatic electron transfer) as that observed in the photochemical processes in equations (57) and (58). The proposed electron-transfer activation for the thermal retropinacol reaction is further confirmed by the efficient cleavage of benzpinacol with tris-phenanthroline iron(III), which is a prototypical outer-sphere one-electron oxidant195 (equation 61). 

The theory of electron-transfer reactions presented in Chapter 6 was mainly based on classical statistical mechanics. While this treatment is reasonable for the reorganization of the outer sphere, the inner-sphere modes must strictly be treated by quantum mechanics. It is well known from infrared spectroscopy that molecular vibrational modes possess a discrete energy spectrum, and that at room temperature the spacing of these levels is usually larger than the thermal energy kT. Therefore we will reconsider electron-transfer reactions from a quantum-mechanical viewpoint that was first advanced by Levich and Dogonadze [1]. In this course we will rederive several of, the results of Chapter 6, show under which conditions they are valid, and obtain generalizations that account for the quantum nature of the inner-sphere modes. By necessity this chapter contains more mathematics than the others, but the calculations axe not particularly difficult. Readers who are not interested in the mathematical details can turn to the summary presented in Section 6. 

Romanian scientists compared one-electron transfer reactions from triphenylmethyl or 2-methyl benzoyl chloride to nitrobenzene in thermal (210°C) conditions and on ultrasonic stimulation at 50°C (lancu et al. 1992, Vinatoru et al. 1994, Chivu et al. 2006). In the first step, the chloride cation-radical and the nitrobenzene anion-radicals are formed. In the thermal and acoustic variants, the reactions lead to the same set of products with one important exception The thermal reaction results in the formation of HCl, whereas ultrasonic stimulation results in CI2 evolution. At present, it is difficult to elucidate the mechanisms behind these two reactions. As an important conclusion, the sonochemical process goes through the inner-sphere electron transfer. The outer-sphere electron transfer mechanism is operative in the thermally induced process. 

An ideal photosensitizer must satisfy several stringent requirements (Balzani et. al., 1986) 1) stability towards thermal and photochemical decomposition reactions 2) sufficiently intense absorption bands in a suitable spectral region 3) high efficiency of population of the reactive excited state 4) long lifetime in the reactive excited state 5) suitable ground state and excited state potentials 6) reversible redox behavior 7) good kinetic factors for outer sphere electron transfer reactions. 

For systems that are powerful excited-state reductants, photoreduction of alkyl halides is observed (6.16). This reaction was initially interpreted to be an outer-sphere electron transfer to form the radical anion, which rapidly decomposes to yield R- and X . Subsequent thermal reactions yield the observed products, an SrnI mechanism (Figure 3a). While such a mechanism, SrnI, appears plausible for a metal complex with E°(M2 /3M2 ) < -1.5 V (SSCE), it seems unlikely for complexes with E°(M2 /3M2 ) > -1.0 V (SSCE). Reduction potentials for alkyl halides of interest are generally more negative than -1.5 V (SSCE) (1/7). Alkyl halide photoreduction is observed for binudear d complexes whose excited-state reduction potentials are more positive than -1.0 V (SSCE) in CH3CN. 

Along with the orbital diagram for it electron transfer (Figure 2), equation 1 implies an outer-sphere redox (second-order) mechanism. The reduction potential (E°) is calculated from the recently tabulated free-radical reduction potentials for a variety of half-reactions (16) Table I presents reduction potentials of interest. Thus, the oxidation of I by 02 is thermodynamically unfavorable by an abiotic, thermal, one-electron transfer process. Chloride, bromide, and bisulfide one-electron oxidations with 02 are also thermodynamically unfavorable with E° values of -2.57, -2.08, and -1.24 V, respectively. 

It is also surprising that the reorganization energies obtained for the thermal backward ET of electrostatically associated complexes between metallocytochrome c and metallouropoii yrins are similar to those obtained for the systems with two hemes. These systems are very similar in terms of their reaction centers, but the environment of the heme poi iyrin is different from that of the uroporphyrin moiety. The latter is more exposed to bulk solvent and should have a higher outer-sphere contribution to the solvent reorganization. Actually, diporphyrin synthetic models react ca. 10 times faster in methanol than the cytochrome systems at similar although... 

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