A full understanding of an elementary reaction must include the functional dependence of the reaction rate (kinetics) on the driving force (thermodynamics), which should include all pertinent variables such as stereoelectronic factors and medium effects. Unimolecular dissociations of radical ions into radicals and ions (mesolytic scissions) ['] provide an opportunity to develop such a quantitative theory for a bond-breaking process. We present here the experimentally determined free-energy relationship for mesolytic cleavages of C-C bonds in 7c radical ions spanning a range of over 40 kcalmol-' for the driving force and over 17 powers of ten for the rate constants. We show that these fragmentations have low intrinsic barriers if the stereoelectronic factors are optimal, and that a parabolic, Marcus-type free-energy relation may be fitted to the data.The unpaired electron in the x radical ion resides in a x-type orbital localized on one bonding partner of the scissile bond. The fragmentation reaction is accompanied by the redistribution of electron density to the forming fragments according to one of the modes of electron apportionment"] [Equations (a)-(d)]. As illustrated by the curved arrows, the processes described by Equations (a) and (c) formally correspond to homolysis; the alternatives (b) and (d) are equivalent to heterolysis.We have prepared['. 3 * 4 1 a series of directly observable radical ions of substrates containing two benzylic units (1-5) that undergo unimolecular C-C bond scissions according to Equations (a)-(d). The radical ions were designed to provide optimal overlap of the x system bearing the unpaired electron with the scissile
C-CIn all cases p r~b e d ,~~.~] the C-C bond cleavage reactions were irreversible under the reaction conditions, as was found in experiments with stereochemically pure evythrojthreo or mesoldl radical ion precursors. The kinetic data (Table 1) photolysis, pulse radiolysis, time-resolved fluorescence spectroscopy, and cyclic voltammetry (CV). The primary fragments observed corresponded to the thermodynamically predicted apportionment of electrons.[39 41 In all cases, the isolated products were consistent with fragmentation of the central C-C bond. In our systems AGh has been approximated by experimentally accessible AGZ. The approximation is valid (k 3 kcalmol-') if the radical coupling has only a small activation barrier (that is, the reaction is diffusion-limited) and the thermolysis is carried out under conditions selected to minimize recombination within the solvent cage. The kinetic and thermodynamic data at 300 K (Table 1) were used to construct the free-energy relationships of Figure 1. The slope of the solid line corresponds to unity; that is, it depicts endergonic reactions with no kinetic "overhead". The overhead is defined here as the difference between AG: and AG,,, and represents the barrier to the reverse (exergonic) reaction (that is,
An den Radikal‐Ionen von Bibenzylverbindungen wie 1 wurden mesolytische Spaltungen von C‐C‐Bindungen in Lösung untersucht. eine parabolische Funktion vom Marcus‐Typ für die Gibbs‐Energige‐Beziehung konnte am besten an die experimentellen Daten angepaßt werden, die auch darauf hindeuten, daß die intrinsischen Aktivierungsenergien von Mesolysen klein sind (ca. 3 kcal mol−1). magnified image
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