Leveling effect, kinetic

Values of kcs and kcr for Sa-linked hairpins possessing G or Z single base donors and GG or GGG sequences, which are expected to be more readily oxidized than G, are summarized in Fig. 5. Values of kcs for Z, GG, and GGG donors are slightly faster than that for G. Similar results for GG and GGG have been reported by Davis et al. [33]. The small variation in kcs may reflect the moderately exergonic nature of these processes, which may place them near the maximum in the Marcus curve (Fig. 4), resulting in a kinetic leveling effect. In fact, Sistare et al. [34] report a value of kGGlkG=12, based on... 

The reaction of aryldiazoacetates with cyclohexene is a good example of the influence of steric effects on the chemistry of the donor/acceptor-substituted rhodium carbenoids. The Rh2(reaction with cyclohexene resulted in the formation of a mixture of the cyclopropane and the G-H insertion products. The enantios-electivity of the C-H insertion was high but the diastereoselectivity was very low (Equation (31)). 0 In contrast, the introduction of a silyl group on the cyclohexene, as in 15, totally blocked the cyclopropanation, and, furthermore, added sufficient size differentiation between the two substituents at the methylene site to make the reaction to form 16 proceed with high diastereoselectivity (Equation (32)).90 The allylic C-H insertion is applicable to a wide array of cyclic and acyclic substrates, and even systems capable of achieving high levels of kinetic resolution are known.90... 

The present state of the theories of atomic and molecular processes in condensed phases is characterized by great non-uniformity of its development. Matters are much problematic in the theory of the kinetics of processes at a molecular level. The kinetics of surface processes mainly employs models taking no account of the interaction of the adsorbed particles (the law of masses or surface action) [14-16]. This does not reflect the real properties of a gas solid interface. There is also a diversity of models when considering the interaction of the particles because various approximations are used (equilibrium is described with a view to the correlation effects, while kinetics ignores them). The problem of approximations is of a fundamental significance in the theory of condensed systems. Interaction between the particles causes all the particles to be bound to... 

The effective kinetic energy of the ions is modified by the fictitious electronic mass /i and this leads to uncertainties in the temperature of the system. This could be circumvented by defining the temperature as the configurational temperature [182] but this is too expensive in ab initio calculations and has not been tried so far. At a more empirical level, appropriate corrections to the atomic masses can be applied [183]. By satisfying the first condition given in the previous paragraph we can make the uncertainties for the temperature determination small. 

When very dilute solutions of benzophcnonc in ether (pseudo first-order concentrations) react with solutions of /-butyl- and isopropylniagnc-sium halide of increasing concentrations, a rate-levelling effect is observed at 0.1-0.2 M 26 The use of benzophenone in excess leads to extremely high reaction rates. These phettomena are of course remini.scent of the complex-type kinetics of acetone 42. As will be described below, /-butyl- and isopropylmagnesium halides react with benzophenone by an ET-radical mechanism (ET is electron transfer) and acetone by a concerted mechanism. It therefore seems strange to have complex effects for both. 

Bun H, Monjanel-Mouterde S, Noel F, Durand A, Cano J-P. Effects of age and antiepileptic drugs on plasma levels and kinetics of clobazam andN-desmethylclobazam. Pharmacol Toxicol 990) 61, 136-. ... 

The discussions so far have concentrated on equilibrium interactions between atoms, molecules, particles, and surfaces with little attention being paid to possible kinetic effects. In vacuum, one may assume that such effects will be negligible or absent until distances of separation approach molecular dimension. At that level effects may be seen that reflect any or all of the following processes ... 

The fact that the Marcus equation (see above, Eq. 3) predicts a positive increase in AG12 with an increasingly negative AG, beyond AG = Ai2, is well known, as is the fact that the quantitative effect predicted has never been observed. What is usually seen is a leveling in rate at the diffusion-controlled limit, as in a recent electrochemical study. Alternatively, products in long-lived electronically excited states may occur. The lifetimes are themselves sometimes evidence of slow electron transfer in the inverted region, since deactivation paths by electron transfer may be available thermodynamically, but not effective kinetically. In a recent study by Meyer and co-workers, deactivation rate constants fc r of chemiluminescent states of [OsLs] " (L = various bipyridyls and phenanthrolines) have been shown to follow the energy-gap law... 

Based on these assumptions, we reasoned that a more basic lithiating agent could effect kinetic deprotonation of 37 at the C-S position. This turned out to be the case as shown in Scheme 13. A practical lithiation level with a high regioselectivity (C-5,87% vs. C-6,5%) was attained when sec-BuLi was used in combination with iVA Af -tetramethylethylene-diamine (TMEDA).43 Since reaction of the C-5 lithiated species of 37 with electrophiles readily produces various types of 5-substituted products 39, this method provides a short and general route to 5-substituted uridines. - 5... 

Emitted with a high level of kinetic energy, the fission products may move a few micrometers before entering a UO2 oxide lattice site. This leads to two distinct effects the doping by the fission products and the irradiation damage. 

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