engleski

Kinetic isotope effect for hydrogen abstraction by OH radicals from normal and carbon-deuterated ethyl alcohol and methylamine in aqueous solutions

Selective H/D kinetic isotope effects (KIE) have been determined for the various routes by which &middot ; ; ; ; ; ; ; OH radicals react with ethanol (CH3CH2OH and CD3CD2OH) and methylamine (CH3NH2 and CD3NH2) in H2O solutions. The KIEs have been evaluated from overall rate constants and the yields in which the individual primary radicals were generated in the &middot ; ; ; ; ; ; ; OH reaction with these compounds. The analytical method applied for the yield determinations was redox titration with suitable scavengers, namely, methyl viologen and Fe(CN)63- for the reducing radicals (CH3C&middot ; ; ; ; ; ; ; HOH/CD3C&middot ; ; ; ; ; ; ; DOH and &middot ; ; ; ; ; ; ; CH2NH2/&middot ; ; ; ; ; ; ; CD2NH2), and I- and hydroquinone for the oxidizing radicals (CH3CH2O&middot ; ; ; ; ; ; ; /CD3CD2O&middot ; ; ; ; ; ; ; and &middot ; ; ; ; ; ; ; NHCH3/&middot ; ; ; ; ; ; ; NHCD3). The numerical results obtained include, besides yields relative to total available &middot ; ; ; ; ; ; ; OH, also absolute rate constants for most of these scavenging reactions. For the alcohol the major process (almost 90%) is H/D- abstraction from the Ca-bond with a KIE = 1.96. For methylamine abstraction from Ca-H/D occurs with only 37% (H) and 26% (D), but at a similar KIE = 1.86. The remainder, denoting the major process in this case, accounts for the formation of aminyl radicals. The secondary KIEs for N-H cleavage, and O-H cleavage in the case of the alcohol, are close to unity, reflecting the expected negligible influence of C-H/D substitution in the attached alkyl groups. Abstraction of b-C-attached H/D in ethanol shows, also in qualitative agreement with expectation, a larger KIE = 3.4 than that for a-C H/D. An interestingly high KIE ≈ 50 was obtained for the 1, 2-hydrogen shift which converts the CH3CH2O&middot ; ; ; ; ; ; ; and CD3CD2O&middot ; ; ; ; ; ; ; oxyl radicals into the corresponding a-hydroxyethyl radicals CH3C&middot ; ; ; ; ; ; ; HOH and CD3C&middot ; ; ; ; ; ; ; DOH. All the results are discussed in the light of existing literature data on relevant H/D isotope effects, influence of solvent relative to gas phase, selectivity of the &middot ; ; ; ; ; ; ; OH attack, and other mechanistic considerations. Specifically, the mechanism by which the &middot ; ; ; ; ; ; ; OH reacts with the amine seems likely to involve a transient, caged aminium/hydroxide ion pair.

isotope effect; kinetic; radicals

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