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• W2017331852 endingPage "11997" @default.
• W2017331852 startingPage "11985" @default.
• W2017331852 abstract "Bimolecular rate constants have been measured for reactions that involve hydrogen atom transfer (HAT) from hydroxylamines to nitroxyl radicals, using the stable radicals TEMPO• (2,2,6,6-tetramethylpiperidine-1-oxyl radical), 4-oxo-TEMPO• (2,2,6,6-tetramethyl-4-oxo-piperidine-1-oxyl radical), di-tert-butylnitroxyl (tBu2NO•), and the hydroxylamines TEMPO-H, 4-oxo-TEMPO-H, 4-MeO-TEMPO-H (2,2,6,6-tetramethyl-N-hydroxy-4-methoxy-piperidine), and tBu2NOH. The reactions have been monitored by UV−vis stopped-flow methods, using the different optical spectra of the nitroxyl radicals. The HAT reactions all have |ΔG°| ≤ 1.4 kcal mol−1 and therefore are close to self-exchange reactions. The reaction of 4-oxo-TEMPO• + TEMPO-H → 4-oxo-TEMPO-H + TEMPO• occurs with k2H,MeCN = 10 ± 1 M−1 s−1 in MeCN at 298 K (K2H,MeCN = 4.5 ± 1.8). Surprisingly, the rate constant for the analogous deuterium atom transfer reaction is much slower: k2D,MeCN = 0.44 ± 0.05 M−1 s−1 with k2H,MeCN/k2D,MeCN = 23 ± 3 at 298 K. The same large kinetic isotope effect (KIE) is found in CH2Cl2, 23 ± 4, suggesting that the large KIE is not caused by solvent dynamics or hydrogen bonding to solvent. The related reaction of 4-oxo-TEMPO• with 4-MeO-TEMPO-H(D) also has a large KIE, k3H/k3D = 21 ± 3 in MeCN. For these three reactions, the EaD − EaH values, between 0.3 ± 0.6 and 1.3 ± 0.6 kcal mol−1, and the log(AH/AD) values, between 0.5 ± 0.7 and 1.1 ± 0.6, indicate that hydrogen tunneling plays an important role. The related reaction of tBu2NO• + TEMPO-H(D) in MeCN has a large KIE, 16 ± 3 in MeCN, and very unusual isotopic activation parameters, EaD − EaH = −2.6 ± 0.4 and log(AH/AD) = 3.1 ± 0.6. Computational studies, using POLYRATE, also indicate substantial tunneling in the (CH3)2NO• + (CH3)2NOH model reaction for the experimental self-exchange processes. Additional calculations on TEMPO(•/H), tBu2NO(•/H), and Ph2NO(•/H) self-exchange reactions reveal why the phenyl groups make the last of these reactions several orders of magnitude faster than the first two. By inference, the calculations also suggest why tunneling appears to be more important in the self-exchange reactions of dialkylhydroxylamines than of arylhydroxylamines." @default.
• W2017331852 created "2016-06-24" @default.
• W2017331852 creator A5007886088 @default.
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• W2017331852 creator A5068360992 @default.
• W2017331852 creator A5068671141 @default.
• W2017331852 creator A5077473574 @default.
• W2017331852 date "2009-07-20" @default.
• W2017331852 modified "2023-09-26" @default.
• W2017331852 title "Nitroxyl Radical Plus Hydroxylamine Pseudo Self-Exchange Reactions: Tunneling in Hydrogen Atom Transfer" @default.
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• W2017331852 doi "https://doi.org/10.1021/ja904400d" @default.
• W2017331852 hasPubMedCentralId "https://www.ncbi.nlm.nih.gov/pmc/articles/2775461" @default.
• W2017331852 hasPubMedId "https://pubmed.ncbi.nlm.nih.gov/19618933" @default.
• W2017331852 hasPublicationYear "2009" @default.

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