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 (KIEs) have been determined for the various routes by which • OH radicals react with ethanol (CH 3 CH 2 OH and CD 3 CD 2 OH) and methylamine (CH 3 NH 2 and CD 3 NH 2 ) in H 2 O solutions. The KIEs have been evaluated from overall rate constants and the yields in which the individual primary radicals were generated in the • 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) 6 3- for the reducing radicals (CH 3 C • HOH/CD 3 C • DOH and • CH 2 NH 2 / • CD 2 NH 2 ) and I - and hydroquinone for the oxidizing radicals (CH 3 CH 2 O • /CD 3 CD 2 O • and • NHCH 3 / • NHCD 3 ). The numerical results obtained also include, besides yields relative to total available • OH, absolute rate constants for most of these scavenging reactions. For the alcohol, the major process (almost 90%) is H/D abstraction from the C α bond with KIE = 1.96. For methylamine, abstraction from C α 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 ofβ-C-attached H/D in ethanol shows, also in qualitative agreement with expectation, a larger KIE = 3.4 than that for α-C H/D. An interestingly high KIE ≈ 50 was obtained for the 1,2 hydrogen shift that converts the CH 3 CH 2 O • and CD 3 CD 2 O • oxyl radicals into the corresponding α-hydroxyethyl radicals CH 3 C • HOH and CD 3 C • DOH. All of the results are discussed in light of existing literature data on relevant H/D isotope effects, the influence of solvent relative to gas phase, the selectivity of the • OH attack, and other mechanistic considerations. Specifically, the mechanism by which the • OH reacts with the amine seems likely to involve a transient, caged aminium/hydroxide ion pair.