H-atom transfer reaction

Pulsed source techniques have been used to study thermal energy ion-molecule reactions. For most of the proton and H atom transfer reactions studied k thermal) /k 10.5 volts /cm.) is approximately unity in apparent agreement with predictions from the simple ion-induced dipole model. However, the rate constants calculated on this basis are considerably higher than the experimental rate constants indicating reaction channels other than the atom transfer process. Thus, in some cases at least, the relationship of k thermal) to k 10.5 volts/cm.) may be determined by the variation of the relative importance of the atom transfer process with ion energy rather than by the interaction potential between the ion and the neutral. For most of the condensation ion-molecule reactions studied k thermal) is considerably greater than k 10.5 volts/cm.). 

Table III shows that in the gas phase at a pressure of 40 torr the relative rates of the H2 transfer reactions from the cyclopentane ion to the various additives differ drastically from those derived from liquid phase radiolysis experiments. This indicates that the changes in density may profoundly affect the relative rates of the two competitive reactions, Reactions 22 and 28. Experimental results, which will be described in a later publication, indicate that in the liquid phase an increased importance of the H2 transfer reaction to some of the additives occurs at the expense of the H atom transfer reaction, Reaction 23.

The Hj ion, recently detected in the interstellar medium via infrared transitions,25 can subsequently react with a variety of neutral atoms present in the gas. The reaction with oxygen leads to a chain of reactions that rapidly produce the hy-dronium ion H30+ via well-studied H atom-transfer reactions ... 

Hydrogen atom transfer implies the transfer of hydrogen atoms from the chain carrier, which is the stereo-determining step in enantioselective hydrogen atom transfer reactions. These reactions are often employed as a functional group interconversion step in the synthesis of many natural products wherein an alkyl iodide or alkyl bromide is converted into an alkane, which, in simple terms, is defined as reduction [ 19,20 ]. Most of these reactions can be classified as diastereoselective in that the selectivity arises from the substrate. Enantioselective H-atom transfer reactions can be performed in two distinct ways (1) by H-atom transfer from an achiral reductant to a radical complexed to a chiral source or alternatively (2) by H-atom transfer from a chiral reductant to a radical. 

Scheme 11 Enantioselective H-atom transfer reaction with hydrogen bonding catalyst... 

For the primary and secondary a-alkoxy radicals 24 and 29, the rate constants for reaction with Bu3SnH are about an order of magnitude smaller than those for reactions of the tin hydride with alkyl radicals, whereas for the secondary a-ester radical 30 and a-amide radicals 28 and 31, the tin hydride reaction rate constants are similar to those of alkyl radicals. Because the reductions in C-H BDE due to alkoxy, ester, and amide groups are comparable, the exothermicities of the H-atom transfer reactions will be similar for these types of radicals and cannot be the major factor resulting in the difference in rates. Alternatively, some polarization in the transition states for the H-atom transfer reactions would explain the kinetic results. The electron-rich tin hydride reacts more rapidly with the electron-deficient a-ester and a-amide radicals than with the electron-rich a-alkoxy radicals. 

Another strategy for modifying potentials is to alter the composition of the bridge between the metal centers. This can be done by protonation, which produces (2) and (3) in nonaqueous media. Larsen et al. [82] measured the energetics of electron-, proton-, and H-atom-transfer reactions for [Mn202(phen)4]"" complexes in CH3CN and found that protonation greatly increases the ease of reduction. For example the for... 

Lynch and Truhlar (2003a) and Zhao et al. (2004) 6-31+G(d,p) basis set the Reaction column refers to the atomization enthalpies for six molecules chosen to be representative of a larger set in a fashion analogous to the H-atom transfer reactions, namely, SiO, S2, silane, propyne, glyoxal, and cyclobutane. 

Intramolecular H-abstraction (radical translocation) has attracted a lot of attention this year. 1,2-1,5 H-atom transfer reactions have been studied theoretically using UHF-AM1 methods. The predicted activation energies were compared to experimentally measured data.91 Ah initio studies into the 1,2-1,6 translocation of the 2-methylhexyl radical predicted that 1,5-H-transfer would be die fastest isomerization process.92 The effects of various groups (dioxolane, acetoxy, TBS ether) on the relative ability of 1,5-1,7 radical translocation have been examined.93... 

H-atom-transfer reactions. Hypericin exhibits a rise in its fluorescence signal, whereas the methylated derivatives do not. 

Free-Radical H-Atom Transfer. In competition with molecular H-atom transfer reactions, radical-induced transfer may occur,... 

Of particular interest (although not a true collision-induced process) has been the effect of added NO on the yield of CO. The H-atom transfer reaction... 

The solvent influence on rates of bimolecular H-atom-transfer reactions R + H-X R-H + X has been theoretically studied [580], Rates for the model... 

The absolute rate constants for these reversible H-atom transfer reactions [reaction (17)] in three model peptides (N-Ac-Cys-Glyg, N-Ac-Cys-Glyj-Asp-Glyj and N-Ac-Cys-Alaj-Asp-Alaj) were measured by means of pulse radiolysis. ... 

JK mol is effectively unchanged for all ring transition states involved in intramolecular H atom transfer reactions, so that A = 10 s These data give A (l,4s) = 10 exp(—8540/T) s and A (l,7p) = 10 exp(—12150/T) s for the two transfers involving 2-hexylperoxy radicals. Later it will be shown that these calculated parameters are seriously in error. 

The current status of the models of fluctuational and deformational preparation of the chemical reaction barrier is discussed in the Section 3. Section 4 is dedicated to the quantitative description of H-atom transfer reactions. Section 5 describes heavy-particle transfer models for solids, conceptually linked with developing notions about the mechanism of low-temperature solid-state chemical reactions. Section 6 is dedicated to the macrokinetic peculiarities of solid-state reactions in the region of the rate constant low-temperature plateau, in particular to the emergence of non-thermal critical effects determined by the development of energetic chains. 

The IE growth with a temperature rise from 4.2 to 50 K was observed by Iwasaki and co-workers in the reactions of H- and D-atom abstraction from saturated hydrocarbon molecules in crystalline xenon matrices [95]. The temperature rise causes a growing selectivity of various CH-bond breaks under H-atom abstraction. The ratio of the accumulation rates of radicals formed under a break of primary, secondary, and tertiary CH bonds in isobutane is 1 1 1 at 4.2 K and 1 6 14 at 30 K [96-98], These data also indicate the existence of the pretunneling molecular shifts of the reagents within the nearest coordination spheres. It is assumed in ref. 82 that these shifts are quasi-reversible, and the kinetic pattern of H-atom transfer reactions has the form... 

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