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Nitroxyl Radical Plus Hydroxylamine Pseudo Self-Exchange Reactions: Tunneling in Hydrogen Atom Transfer.

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Title: Nitroxyl Radical Plus Hydroxylamine Pseudo Self-Exchange Reactions: Tunneling in Hydrogen Atom Transfer.
Authors: Wu, Adam1, Mader, Elizabeth A.1, Datta, Ayan2,3, Hrovat, David A.2, Borden, Weston Thatcher2 borden@unt.edu, Mayer, James M.1 mayer@chem.washington.edu
Source: Journal of the American Chemical Society. 8/26/2009, Vol. 131 Issue 33, p11985-11997. 13p. 3 Diagrams, 3 Charts, 3 Graphs.
Subjects: Nitroxides, Hydroxylamine, Chemical reactions, Atomic hydrogen, Hydrogen bonding
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-tetramethylpi- peridine-1 -oxyl radical), 4-oxo-TEMPO (2,2,6,6-tetramethyl-4-oxo-piperidine-1-oxyl radical), di-tert-butylni- troxyl (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. [ABSTRACT FROM AUTHOR]
Copyright of Journal of the American Chemical Society is the property of American Chemical Society and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
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  Label: Title
  Group: Ti
  Data: Nitroxyl Radical Plus Hydroxylamine Pseudo Self-Exchange Reactions: Tunneling in Hydrogen Atom Transfer.
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  Data: <searchLink fieldCode="AR" term="%22Wu%2C+Adam%22">Wu, Adam</searchLink><relatesTo>1</relatesTo><br /><searchLink fieldCode="AR" term="%22Mader%2C+Elizabeth+A%2E%22">Mader, Elizabeth A.</searchLink><relatesTo>1</relatesTo><br /><searchLink fieldCode="AR" term="%22Datta%2C+Ayan%22">Datta, Ayan</searchLink><relatesTo>2,3</relatesTo><br /><searchLink fieldCode="AR" term="%22Hrovat%2C+David+A%2E%22">Hrovat, David A.</searchLink><relatesTo>2</relatesTo><br /><searchLink fieldCode="AR" term="%22Borden%2C+Weston+Thatcher%22">Borden, Weston Thatcher</searchLink><relatesTo>2</relatesTo><i> borden@unt.edu</i><br /><searchLink fieldCode="AR" term="%22Mayer%2C+James+M%2E%22">Mayer, James M.</searchLink><relatesTo>1</relatesTo><i> mayer@chem.washington.edu</i>
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  Data: <searchLink fieldCode="JN" term="%22Journal+of+the+American+Chemical+Society%22">Journal of the American Chemical Society</searchLink>. 8/26/2009, Vol. 131 Issue 33, p11985-11997. 13p. 3 Diagrams, 3 Charts, 3 Graphs.
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  Data: <searchLink fieldCode="DE" term="%22Nitroxides%22">Nitroxides</searchLink><br /><searchLink fieldCode="DE" term="%22Hydroxylamine%22">Hydroxylamine</searchLink><br /><searchLink fieldCode="DE" term="%22Chemical+reactions%22">Chemical reactions</searchLink><br /><searchLink fieldCode="DE" term="%22Atomic+hydrogen%22">Atomic hydrogen</searchLink><br /><searchLink fieldCode="DE" term="%22Hydrogen+bonding%22">Hydrogen bonding</searchLink>
– Name: Abstract
  Label: Abstract
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  Data: 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-tetramethylpi- peridine-1 -oxyl radical), 4-oxo-TEMPO (2,2,6,6-tetramethyl-4-oxo-piperidine-1-oxyl radical), di-tert-butylni- troxyl (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. [ABSTRACT FROM AUTHOR]
– Name: AbstractSuppliedCopyright
  Label:
  Group: Ab
  Data: <i>Copyright of Journal of the American Chemical Society is the property of American Chemical Society and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)
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        Value: 10.1021/ja904400d
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        Text: English
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        PageCount: 13
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      – SubjectFull: Nitroxides
        Type: general
      – SubjectFull: Hydroxylamine
        Type: general
      – SubjectFull: Chemical reactions
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      – SubjectFull: Atomic hydrogen
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      – SubjectFull: Hydrogen bonding
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      – TitleFull: Nitroxyl Radical Plus Hydroxylamine Pseudo Self-Exchange Reactions: Tunneling in Hydrogen Atom Transfer.
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              Text: 8/26/2009
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