Hydrogen structures atomic transfer kinetics

Figure 15.5 Transition-state structure for rate-determining hydrogen atom transfer in the methane metathesis reaction of methyllutetiocene. Note that the kinetics for this narcissistic reaction may be followed by using a label either in the reacting methane or in the methyl group of the starting organometallic...

Paramagnetic centers containing a sulfur atom in different oxidation states, (=Si-0)3Si-0-S = O, (=Si-0)3Si-0-S 02, (=Si-0)3Si-0-S02-0, and (=Si-0)3Si-0-S02-0-0, were obtained in Ref. [118]. Their radio-spectroscopic parameters were determined, and the mechanism of free radical oxidation of S02 molecules in this system was established. The mechanism of the initial steps of free radical polymerization and copolymerization of hydrogen- and fluorine-substituted unsaturated hydrocarbons was studied in Ref. [117]. The pathways were found and the kinetic parameters were determined for reactions of intramolecular H(D) atom transfer between r (CH3, CD3, CH2-CH3) and r (CH2-CH2, CD2-CD2), in the structure of (=Si-0)2Si(r)(rI) [120]. 

Quantitative kinetic studies of absolute rate constants for hydrogen atom transfer from substituted phenols to polystyrene peroxyl radicals by Howard and Ingold in the 1960s provided the first reliable data on substiment effects on antioxidant activities of phenols. Later, a very detailed report appeared providing data on substituent and structural effects on various classes of monohydroxy phenols. In addition, detailed reviews were given of substituent etfects ". These reports provide the basis for understanding how substituent and strucmral effects control the antioxidant activities of phenols and will be summarized in part below. 

Figure 8.8 Optimized structure of the kinetic product of proton transfer from CpWH(CO)3 to ( PCP)NiH. Selected bond lengths are reported (A). Hydrogen atoms of the PCP ligand are omitted for clarity.

The stoichiometry (equation 2) requires eight electrons transferred from vanadium(II) ions to dinitrogen atoms (six electrons) and hydrogen atoms (two electrons) and this kinetic equation (3) was interpreted as evidence for a polynuclear structure of the transition state during the reduction of dinitrogen. An alternative mechanism was suggested.145... 

Adam W, Arnold MA, Nau WM, Pischel U, Saha-Moller CR (2001b) Structure-dependent reactivity of oxyfunctionalized acetophenones in the photooxidation of DNA base oxidation and strand breaks through photolytical radical formation (spin trapping, EPR spectroscopy, transient kinetics) versus photosensitization (electron transfer, hydrogen-atom-abstraction). Nucleic Acids... 

The kinetic deuterium/hydrogen isotope effects for cyclohexane and adaman-tane are approximately 5 [30] (in contrast with, for example, radical bromination of cyclohexane with Br which gives a KIE value of approximately 2.4 [37] or 4 [38]) which shows that the transferred hydrogen atom lies about half-way between the carbon centers in the transition structure [30]. 

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