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Hydrogen Atom Abstraction (hydrogen + atom_abstraction)
Selected AbstractsDimethyl Phosphite Mediated Hydrogen Atom Abstraction: A Tin-Free Procedure for the Preparation of Cyclopentane Derivatives.CHEMINFORM, Issue 51 2005Florent Beaufils No abstract is available for this article. [source] Gas-phase radical,radical recombination reactions of nitroxides with substituted phenyl radicalsINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 4 2004J. L. Heidbrink Fourier-transform ion cyclotron resonance mass spectrometry has been used to examine gas-phase reactions of four different nitroxide free radicals with eight positively charged pyridyl and phenyl radicals (some containing a Cl, F, or CF3 substituent). All the radicals reacted rapidly (near collision rate) with nitroxides by radical,radical recombination. However, some of the radicals were also able to abstract a hydrogen atom from the nitroxide. The results establish that the efficiency (kreaction/kcollision) of hydrogen atom abstraction varies with the electrophilicity of the radical, and hence is attributable to polar effects (a lowering of the transition-state energy by an increase in its polar character). The efficiency of the recombination reaction is not sensitive to substituents, presumably due to a very low reaction barrier. Even so, after radical,radical recombination has occurred, the nitroxide adduct was found to fragment in different ways depending on the structure of the radical. For example, a cationic fragment was eliminated from the adducts of the more electrophilic radicals via oxygen anion abstraction by the radical (i.e., the nitroxide adduct cleaves heterolytically), whereas adducts of the less electrophilic radicals predominantly fragmented via homolytic cleavage (oxygen atom abstraction). Therefore, differences in the product branching ratios were found to be attributable to polar factors. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 216,229 2004 [source] The OH, + CH3SH reaction: Support for an addition-elimination mechanism from ab initio calculationsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 6 2005Pedro L. Muiño Abstract Several intermediates for the CH3SH + OH, , CH3S, + H2O reaction were identified using MP2(full) 6-311+g(2df,p) ab initio calculations. An adduct, CH3S(H)OH,, I, with electronic energy 13.63 kJ mol,1 lower than the reactants, and a transition state, II,, located 5.14 kJ mol,1 above I, are identified as the entrance channel for an addition-elimination reaction mechanism. After adding zero-point and thermal energies, ,H(reactants , I) = ,4.85 kJ mol,1 and ,H(I , II,) = +0.10 kJ mol,1, which indicates that the potential energy surface is broad and flat near the transition state. The calculated imaginary vibrational frequency of the transition state, 62i cm,1, is also consistent with an addition,elimination mechanism. These calculations are consistent with experimental observations of the OH, + CH3SH reaction that favored an addition,elimination mechanism rather than direct hydrogen atom abstraction. An alternative reaction, CH3SH + OH, , CH3SOH + H,, with ,H = +56.94 kJ mol,1 was also studied, leading to a determination of ,H(CH3SOH) = ,149.8 kJ mol,1. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 612,618, 2005 [source] Reinvestigation of the Mechanism of the Free Radical Polymerization Photoinitiation Process by Camphorquinone,Coinitiator Systems: New ResultsMACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 17 2004Ilona Pyszka Abstract Summary: Comparative studies of photoinitiation processes using camphorquinone (CQ) and benzophenone (BP) as light absorbers were performed. The experimental results show that after the transformation of (phenylthio)acetic acid (PTAA) into its tetrabutylammonium salt (PTAA AS), a substantial decrease of the polymerization photoinitiation ability for the CQ,PTAA AS pair in comparison to the CQ,PTAA pair is observed. The mechanism of the photoinitiated polymerization for the tested photoredox pair was clarified based on laser flash photolysis experiments obtained using benzophenone as an electron acceptor and (phenylthio)acetic acid and its tetrabutylammonium salt as electron donors in solution in MeCN. It is documented and deduced that the photoreduction of benzophenone in the presence of (phenylthio)acetic acid and its tetrabutylammonium salt occurs by a photoinduced electron transfer process, while for CQ as initiator, the free radicals are formed by hydrogen atom abstraction by the triplet state of camphorquinone. Schematic of the transients formed after an electron-transfer process for benzophenone,PTAA and benzophenone,PTAA AS pairs. [source] Kinetic analysis of a reactive model enediyneJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 9 2004M. F. Semmelhack Abstract A model framework related to the calcheamicin family of enediyne toxins was evaluated in its reactivity toward cycloaromatization through the arene 1,4-diyl intermediates and hydrogen atom abstractions. The keto version 9 is relatively unreactive, with t1/2,=,1.5,h at 60°C. The product from hydride reduction of the ketone, alcohol 13b, is much more reactive, with t1/2=50,min at 0°C. In both cases, the rate of rearrangement is independent of the hydrogen atom donor, consistent with a rate-determining first step (cyclization). Copyright © 2004 John Wiley & Sons, Ltd. [source] | ||||||||||