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Chain Mechanism (chain + mechanism)

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Chemistry100%

Selected Abstracts

A Radical-Anion Chain Mechanism Initiated by Dissociative Electron Transfer to a Bicyclic Endoperoxide: Insight into the Fragmentation Chemistry of Neutral Biradicals and Distonic Radical Anions

CHEMISTRY - A EUROPEAN JOURNAL, Issue 6 2008
David
Abstract The electron-transfer (ET) reduction of two diphenyl-substituted bicyclic endoperoxides was studied in N,N -dimethylformamide by heterogeneous electrochemical techniques. The study provides insight into the structural parameters that affect the reduction mechanism of the OO bond and dictate the reactivity of distonic radical anions, in addition to evaluating previously unknown thermochemical parameters. Notably, the standard reduction potentials and the bond dissociation energies (BDEs) were evaluated to be ,0.55±0.15,V and 20±3,kcal,mol,1, respectively, the last representing some of the lowest BDEs ever reported. The endoperoxides react by concerted dissociative electron transfer (DET) reduction of the OO bond yielding a distonic radical-anion intermediate. The reduction of 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]oct-5-ene (1) results in the quantitative formation of 1,4-diphenylcyclohex-2-ene- cis -1,4-diol by an overall two-electron mechanism. In contrast, ET to 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]octane (2) yields 1,4-diphenylcyclohexane- cis -1,4-diol as the major product; however, in competition with the second ET from the electrode, the distonic radical anion undergoes a ,-scission fragmentation yielding 1,4-diphenyl-1,4-butanedione radical anion and ethylene in a mechanism involving less than one electron. These observations are rationalized by an unprecedented catalytic radical-anion chain mechanism, the first ever reported for a bicyclic endoperoxide. The product ratios and the efficiency of the catalytic mechanism are dependent on the electrode potential and the concentration of weak non-nucleophilic acid. A thermochemical cycle for calculating the driving force for ,-scission fragmentation is presented, and provides insight into why the fragmentation chemistry of distonic radical anions is different from analogous neutral biradicals. [source]

A hybrid density functional theory study of the low-temperature dimethyl ether combustion pathways.

ISRAEL JOURNAL OF CHEMISTRY, Issue 2-3 2002
I: Chain-propagation
Dimethyl ether (DME) has been proposed to be a promising alternative to conventional diesel fuel because of its favorable compression ignition property (high cetane number) and its soot-free combustion. A radical chain mechanism for hydrocarbon autoignition has been proposed for DME at low temperatures. In this mechanism, the chain initiation step consists of DME undergoing hydrogen abstraction by a highly reactive species (typically ·OH). The CH3O·H2 created in the initiation step then combines with O2; the subsequent CH3OCH2OO· radical is involved in a Lindemann-type mechanism, which can lead to the production of formaldehyde (CH2 = O) and ·OH. This concludes the chain-propagating step: the one ·OH produced then sustains the chain-reaction by creating another CH3O·H2. A relatively stable intermediate (·CH2OCH2OOH), formed via isomerization of CH3OCH2OO· in the chain-propagation step, can combine with a second O2 to produce a radical (·OOCH2OCH2OOH) that can potentially decompose into two ·OH radical (and other products). This path leads to chain-branching and an exponential increase in the rate of DME oxidation. We have used spin-polarized density functional theory with the Becke-3-parameter Lee,Parr,Yang exchange-correlation functional to calculate the structures and energies of key reactants, intermediates, and products involved in (and competing with) the chain-propagating and chain-branching steps of low-temperature DME oxidation. In this article, Part I, we consider only the chain-propagation mechanism and its competing mechanisms for DME combustion. Here, we show that only certain conformers can undergo the isomerization to ·CH2OCH2OOH. A new transition state has been discovered for the disproportionation reaction ·CH2OCH2OOH , 2CH2O + ·OH in the chain-propagating step of DME autoignition that is much lower than previous barriers. The key to making this decomposition pathway facile is initial cleavage of the O,O rather than the C,O bond. This renders all transition states along the chain-propagation potential energy surface below the CH3O·H2 + O2 reactants. In contrast with the more well-studied CH3·H2 (ethyl radical) + O2 system, the H-transfer isomerization of CH3OCH2OO· to ·CH2OCH2OOH in low-temperature DME oxidation has a much lower activation energy. This is most likely due to the larger ring strain of the analogous transition state in ethane oxidation, which is a five-membered ring opposed to a six-membered ring in dimethyl ether oxidation. Thus low-temperature ethane oxidation is much less likely to form the ·ROOH (where R is a generic group) radicals necessary for chain-branching, which leads to autoignition. Three competing reactions are considered: CH3O·H2 , CH2O + ·CH3; ·CH2OCH2OOH , 1,3-dioxetane + ·OH; and ·CH2OCH2OOH , ethylene oxide + HOO·. The reaction barriers of all these competing paths are much higher in energy (7,10 kcal/mol) than the reactants CH3O·H2 + O2 and, therefore, are unlikely low-temperature paths. Interestingly, an analysis of the highest occupied molecular orbital along the CH3O·H2 decomposition path shows that electronically excited (1A2 or 3A2) CH2O can form; this can also be shown for ·CH2OCH2OOH, which forms two formaldehyde molecules. This may explain the luminosity of DME's low-temperature flames. [source]

A Radical-Anion Chain Mechanism Initiated by Dissociative Electron Transfer to a Bicyclic Endoperoxide: Insight into the Fragmentation Chemistry of Neutral Biradicals and Distonic Radical Anions

CHEMISTRY - A EUROPEAN JOURNAL, Issue 6 2008
David
Abstract The electron-transfer (ET) reduction of two diphenyl-substituted bicyclic endoperoxides was studied in N,N -dimethylformamide by heterogeneous electrochemical techniques. The study provides insight into the structural parameters that affect the reduction mechanism of the OO bond and dictate the reactivity of distonic radical anions, in addition to evaluating previously unknown thermochemical parameters. Notably, the standard reduction potentials and the bond dissociation energies (BDEs) were evaluated to be ,0.55±0.15,V and 20±3,kcal,mol,1, respectively, the last representing some of the lowest BDEs ever reported. The endoperoxides react by concerted dissociative electron transfer (DET) reduction of the OO bond yielding a distonic radical-anion intermediate. The reduction of 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]oct-5-ene (1) results in the quantitative formation of 1,4-diphenylcyclohex-2-ene- cis -1,4-diol by an overall two-electron mechanism. In contrast, ET to 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]octane (2) yields 1,4-diphenylcyclohexane- cis -1,4-diol as the major product; however, in competition with the second ET from the electrode, the distonic radical anion undergoes a ,-scission fragmentation yielding 1,4-diphenyl-1,4-butanedione radical anion and ethylene in a mechanism involving less than one electron. These observations are rationalized by an unprecedented catalytic radical-anion chain mechanism, the first ever reported for a bicyclic endoperoxide. The product ratios and the efficiency of the catalytic mechanism are dependent on the electrode potential and the concentration of weak non-nucleophilic acid. A thermochemical cycle for calculating the driving force for ,-scission fragmentation is presented, and provides insight into why the fragmentation chemistry of distonic radical anions is different from analogous neutral biradicals. [source]

Ring Opening of the Cyclobutane in a Thymine Dimer Radical Anion

CHEMISTRY - A EUROPEAN JOURNAL, Issue 32 2007
Chryssostomos Chatgilialoglu Dr.
Abstract The reactions of hydrated electrons (eaq,) with thymine dimer 2 and thymidine have been investigated by radiolytic methods coupled with product studies, and addressed computationally by means of BB1K-HMDFT calculations. Pulse radiolysis revealed that one-electron reduction of the thymine dimer 2 affords the radical anion of thymidine (5) with t1/2<35,ns. Indeed, the theoretical study suggests that radical anion 3, in which the spin density and charge distribution are located in both thymine rings, undergoes a fast partially ionic splitting of the cyclobutane with a half-life of a few ps. This model fits with the in vivo observation of thymine dimer repair in DNA by photolyase. ,-Radiolysis of thymine dimer 2 demonstrates that the one-electron reduction and the subsequent cleavage of the cyclobutane ring does not proceed by means of a radical chain mechanism, that is, in this model reaction the T,. is unable to transfer an electron to the thymine dimer 2. [source]