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

Bimolecular rate constants have been measured for reactions that involve hydrogen atom transfer (HAT) from hydroxylamines to nitroxyl radicals, using the stable radicals TEMPO center dot (2,2,6,6-tetramethylpiperidine-1-oxyl radical), 4-oxo-TEMPO center dot (2,2,6,6-tetramethyl-4-oxo-piperidine-1-oxyl radical), di-tert-butylnitroxyl ((Bu2NO center dot)-Bu-t), and the hydroxylamines TEMPO-H, 4-oxo-TEMPO-H, 4-MeO-TEMPO-H (2,2,6,6-tetramethyl-N-hydroxy-4-methoxy-piperidine), and (Bu2NOH)-Bu-t. 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 vertical bar Delta G degrees vertical bar <= 1.4 kcal mol(-1) and therefore are close to self-exchange reactions. The reaction of 4-oxo-TEMPO center dot + TEMPO-H --> 4-oxo-TEMPO-H + TEMPO center dot occurs with k(2H,MeCN) = 10 +/- 1 M-1 s(-1) in MeCN at 298 K (K-2H,K-MeCN = 4.5 +/- 1.8). Surprisingly, the rate constant for the analogous deuterium atom transfer reaction is much slower: K-2H,K-MeCN = 0.44 +/- 0.05 M-1 s(-1) with K-2H,K-MeCN/k(2D,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 center dot 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 (Bu2NO center dot)-Bu-t + 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 (CH3)(2)NO center dot + (CH3)(2)NOH model reaction for the experimental self-exchange processes. Additional calculations on TEMPO((center dot)/H), (Bu2NO)-Bu-t((center dot)/H), and Ph2NO((center dot)/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.