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Homolysis

About: Homolysis is a(n) research topic. Over the lifetime, 4305 publication(s) have been published within this topic receiving 91400 citation(s).


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Journal ArticleDOI
Abstract: The best available values for homolytic bond dissociation energies (BDEs) of various classes of neutral compounds are considered in this review. (BDEs in ionic species is a legitimate subject that is touched on briefly and could easily be included in a longer review. The same can be said for heterolytic BDEs, which are not reviewed as such, although some of the ionic thermochemical data discussed yield values for these processes.) The major emphasis is on hydrocarbons and their nitrogen, oxygen, sulfur, halogen, and silicon-containing derivations, but limited data for inorganic molecules are included. The focus is particularly on prototypical radicals whose heats of formation, formerly thought to be well in hand, have recently been called into serious question. The intent is to include all the major types of sigma bonds, if not all specific cases where known or estimatable heats of formation allow bond dissociation energies to be generated. This review attempts to acknowledge all the standard techniques for measuring BDEs in polyatomic molecules and to offer critical analysis of selected portions of the literature. This leaves values that the authors recommend as the most likely to be correct at the time of this writing. 246 references, 9 tables.

1,824 citations

Journal ArticleDOI
TL;DR: In theory, activation could involve (1) heterolysis to OH- and NO2+ (delta rxn Gzero' = 13 kcal/mol at pH 7) or (2) homolysis
Abstract: Peroxynitrite [oxoperoxonitrate(1-), ONOO-] may be formed in vivo from superoxide and nitric oxide. The anion is stable, but the acid (pKa = 6.8) decays to nitrate with a rate of 1.3 s-1 at 25 degrees C. The experimental activation parameters of this process are delta H++ = +18 +/- 1 kcal/mol, delta S++ = +3 +/- 2 cal/(mol.K), and delta G++ = +17 +/- 1 kcal/mol. Peroxynitrite (or its protonated form) oxidizes some compounds such as thiols and thioethers in a biomolecular reaction. The reactions with glutathione and cysteine have activation enthalpies of 10.9 and 9.7 kcal/mol, respectively, which are lower than that of the isomerization reaction. Peroxynitrite reacts with other compounds such as dimethyl sulfoxide and deoxyribose in a unimolecular reaction for which the activation of peroxynitrite is rate-limiting. In theory, activation could involve (1) heterolysis to OH- and NO2+ (delta rxn Gzero' = 13 kcal/mol at pH 7) or (2) homolysis to .OH and .NO2 (delta rxn Gzero = 21 kcal/mol), and these processes also could be involved in the isomerization to nitrate. However, thermodynamic and kinetic considerations indicate that neither process is feasible, although binding to metal ions may reduce the large activation energy associated with heterolysis. An intermediate closely related to the transition state for isomerization of ONOOH to HONO2 may be the strongly oxidizing intermediate responsible for hydroxyl radical-like oxidations mediated by ONOOH. Thus, peroxynitrite reacts with different compounds by at least two distinct mechanisms, and the hydroxyl radical is not involved in either.

1,250 citations

Journal ArticleDOI
TL;DR: Although homolytic reactions of PUFA hydroperoxides have received the most attention, hydroper oxides are also susceptible to heterolytic transformations, such as nucleophilic displacement and acid-catalyzed rearrangement.
Abstract: Polyunsaturated fatty acids (PUFA) are readily susceptible to autoxidation. A chain oxidation of PUFA is initiated by hydrogen abstraction from allylic or biss-allylic positions leading to oxygenation and subsequent formation of peroxyl radicals. In media of low hydrogen-donating capacity the peroxyl radical is free to react further by competitive pathways resulting in cyclic peroxides, double bond isomerization and formation of dimers and oligomers. In the presence of good hydrogen donators, such as α-tocopherol or PUFA themselves, the peroxyl radical abstracts hydrogen to furnish PUFA hydroperoxides. Given the proper conditions or catalysts, the hydroperoxides are prone to further transformations by free radical routes. Homolytic cleavage of the hydroperoxy group can afford either a peroxyl radical or an alkoxyl radical. The products of peroxyl radicals are identical to those obtained during autoxidation of PUFA; that is, it makes no difference whether the peroxyl radical is generated in the process of autoxidation or from a performed hydroperoxide. Of particular interest is the intramolecular rearrangement of peroxyl radicals to furnish cyclic peroxides and prostaglandin-like bicyclo endoperoxides. Other principal peroxyl radical reactions are the β-scission of O2, intermolecular addition and self-combination. Alkoxyl radicals of PUFA, contraty to popular belief, do not significantly abstract hydrogens, but rather are channeled into epoxide formation through intramolecular rearrangement. Other significant reactions of PUFA alkoxyl radicals are β-scission of the fatty chain and possibly the formation of ether-linked dimers and oligomers. Although homolytic reactions of PUFA hydroperoxides have received the most attention, hydroperoxides are also susceptible to heterolytic transformations, such as nuleophilic displacement and acid-catalyzed rearrangement.

652 citations

Journal ArticleDOI
TL;DR: The photodegradation of pesticides is reviewed, with particular reference to the studies that describe the mechanisms of the processes involved, the nature of reactive intermediates and final products.
Abstract: The photodegradation of pesticides is reviewed, with particular reference to the studies that describe the mechanisms of the processes involved, the nature of reactive intermediates and final products. Potential use of photochemical processes in advanced oxidation methods for water treatment is also discussed. Processes considered include direct photolysis leading to homolysis or heterolysis of the pesticide, photosensitized photodegradation by singlet oxygen and a variety of metal complexes, photolysis in heterogeneous media and degradation by reaction with intermediates generated by photolytic or radiolytic means.

518 citations

Journal ArticleDOI
Abstract: The pyrolysis of TVSb has been investigated in a flow tube reactor using Dz and He carrier gases. For TVSb alone, the most likely pyrolysis reaction involves an Sbcentered reductive elimination pathway. A less likely possibility is pyrolysis via homolysis of the Sb-C bonds, yielding vinyl radicals. Unfortunately, examination of the organic byproducts in both He and D, yields insufficient information to form a definitive hypothesis. However, in He the pyrolysis rate for TVSb is more rapid than for TMSb. Since vinyl radicals form stronger bonds than methyl radicals, this datum contradicts the Sb-C bond homolysis mechanism. Again, the activation energy for pyrolysis is less than the expected Sb-vinyl bond strength. Finally, the addition of C7D, produces no CH,=CHD, indicative of the absence of vinyl radicals. To elucidate our understanding of GaSb growth by using TMGa and TVSb, the pyrolysis rates for this combination of reactants were also studied. CH, radicals from (CH3N), pyrolysis were found to enhance TVSb pyrolysis in He. TMGa also increases the TVSb pyrolysis rate, mainly due to the methyl radicals produced. A heterogeneous pyrolysis reaction appears a t high surface area. At V/III ratios normally used for OMVPE growth, carbonaceous deposits were formed. Thus, TVSb may be a useful precursor for OMVPE only a t V/III ratios less than unity.

473 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
20221
202174
202077
201991
201888
201781