Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2009 Aug 26;131(33):11985-97.
doi: 10.1021/ja904400d.

Nitroxyl radical plus hydroxylamine pseudo self-exchange reactions: tunneling in hydrogen atom transfer

Adam Wu  1 , Elizabeth A Mader, Ayan Datta, David A Hrovat, Weston Thatcher Borden, James M Mayer
Affiliations

Nitroxyl radical plus hydroxylamine pseudo self-exchange reactions: tunneling in hydrogen atom transfer

Adam Wu et al. J Am Chem Soc. .

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 ((t)Bu(2)NO(*)), and the hydroxylamines TEMPO-H, 4-oxo-TEMPO-H, 4-MeO-TEMPO-H (2,2,6,6-tetramethyl-N-hydroxy-4-methoxy-piperidine), and (t)Bu(2)NOH. 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 |DeltaG (o)| < or = 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 k(2H,MeCN) = 10 +/- 1 M(-1) s(-1) in MeCN at 298 K (K(2H,MeCN) = 4.5 +/- 1.8). Surprisingly, the rate constant for the analogous deuterium atom transfer reaction is much slower: k(2D,MeCN) = 0.44 +/- 0.05 M(-1) s(-1) with k(2H,MeCN)/k(2D,MeCN) = 23 +/- 3 at 298 K. The same large kinetic isotope effect (KIE) is found in CH(2)Cl(2), 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, k(3H)/k(3D) = 21 +/- 3 in MeCN. For these three reactions, the E(aD) - E(aH) values, between 0.3 +/- 0.6 and 1.3 +/- 0.6 kcal mol(-1), and the log(A(H)/A(D)) values, between 0.5 +/- 0.7 and 1.1 +/- 0.6, indicate that hydrogen tunneling plays an important role. The related reaction of (t)Bu(2)NO(*) + TEMPO-H(D) in MeCN has a large KIE, 16 +/- 3 in MeCN, and very unusual isotopic activation parameters, E(aD) - E(aH) = -2.6 +/- 0.4 and log(A(H)/A(D)) = 3.1 +/- 0.6. Computational studies, using POLYRATE, also indicate substantial tunneling in the (CH(3))(2)NO(*) + (CH(3))(2)NOH model reaction for the experimental self-exchange processes. Additional calculations on TEMPO((*)/H), (t)Bu(2)NO((*)/H), and Ph(2)NO((*)/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.

PubMed Disclaimer

Figures

Figure 1
Figure 1
Van't Hoff plot for reactions 2−4 in CD3CN at 278−318 K.
Figure 2
Figure 2
(a) Overlay of UV-vis spectra for the reaction of 8.8 mM 4-oxo-TEMPO• with 88 mM TEMPO-H (eq 2) in MeCN over 5 s at 298 K. (b) Absorbance at 460 nm showing the raw data (○) and first order A → B fit using SPECFIT (—).
Figure 3
Figure 3
Plot of pseudo first order kobs versus [TEMPO-H(D)] for reaction 2 in MeCN (kH/kD = 23 ± 3) and in CH2Cl2 (kH/kD = 23 ± 4) at 298 K.
Figure 4
Figure 4
Eyring plots for (a) 4-oxo-TEMPO• + TEMPO-H (reaction 2) and (b) tBu2NO• + TEMPO-H (reaction 4), both in MeCN and CH2Cl2.
Figure 5
Figure 5
(a) Transition structure for hydrogen transfer between (CH3)2NOH and (CH3)2NO•. (b) H-bonded complex between (CH3)2NOH and (CH3)2NO•. Bond lengths are in Ångstroms.
Figure 6
Figure 6
Bond lengths (Å), atomic spin densities (in green), and activation energies (kcal mol−1) for the nitroxyl radicals, (a) tBu2NO•, (b) TEMPO•, and (c) Ph2NO•, and the transition structures and CVT activation energies for their hydrogen self-exchange reactions with the corresponding hydroxylamines.
Figure 7
Figure 7
SOMOs of the transition structures for hydrogen exchange reactions: (a) tBu2NO• + tBu2NOH, (b) TEMPO• + TEMPO-H , and (c) Ph2NO• + Ph2NOH.
See this image and copyright information in PMC

References

    1. Hynes JT, Klinman JP, Limbach H-H, Schowen RL, editors. Hydrogen-Transfer Reactions. Wiley-VCH; Weinheim: 2007.

    1. Hammes-Schiffer S. pp. 479–502.Fernandez-Ramos A, Ellingson BA, Garrett BC, Truhlar DG. In: Reviews in Computational Chemistry. Cundari TR, Lipkowitz KB, editors. Vol. 23. Wiley-VCH; Hoboken, NJ: 2007. pp. 125–232. In ref.

    1. Bell RP. The Tunnel Effect in Chemistry. Chapman and Hall; London: 1980. pp. 77–105.
    1. Carpenter BK. Determination of Organic Reaction Mechanisms. John Wiley & Sons; 1984. pp. 83–111.
    1. cf.
    2. Klinman JP. Phil. Trans. R. Soc. B. 2006;361:1323. - PMC - PubMed
    3. Sutcliffe MJ, Masgrau L, Roujeinikova A, Johannissen LO, Hothi P, Basran J, Ranaghan KE, Mulholland AJ, Leys D, Scrutton NS. Phil. Trans. R. Soc. B. 2006;361:1375. - PMC - PubMed
    4. Barroso M, Arnaut LG, Formosinho SJ. J. Phys. Org. Chem. 2009;22:254–263.
    5. Meyer TJ, Huynh MHV. Inorg. Chem. 2003;42:8140–8160. - PubMed
    6. Reinaudt OM, Theopold KH. J. Am. Chem. Soc. 1994;116:6979–6980.
    7. Mahapatra S, Halfen JA, Tolman WB. J. Am. Chem. Soc. 1996;118:11575–11586.
    8. Zheng H, Lipscomb JD. Biochemistry. 2006;45:1685–1692. - PubMed

Publication types