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Reactions of 2-Propanol Radical with Halogenated Organics in Aqueous Solution: Theoretical Evidence for Proton-Coupled Electron Transfer and Competing Mechanisms

The reactions of α-hydroxyalkyl radicals in aqueous medium are of interest because they exhibit a rich variety of fundamentally important competing mechanisms, such as proton- coupled electron transfer (PCET), hydrogen atom transfer, free radical substitutions, abstractions and additions, etc. We present a theoretical study of the mechanism and kinetics of the aqueous reactions of α-hydroxyisopropyl (2-propanol) radical with four halogenated organic substrates: iodoacetate (IAc), iodoacetamide (IAm), 5-bromouracil (5-BrU), and carbon tetrachloride (CCl4). The reactions are studied using density functional theory (DFT) (M06-2X), and the solvent is modeled as a polarizable continuum, either without the explicit solvent molecules or with one added water molecule. For an additional refinement, the double hybrid DFT B2PLYP energies were calculated at the M06-2X stationary points. Within this framework, for each substrate, we determine the most favorable radical-induced decomposition pathway among the several found and compare the thermochemical predictions against the experimental kinetics. The following dominant decomposition mechanisms are inferred: PCET for IAc, PCET-H2O and the I-atom abstraction for IAm, the ortho-addition to the double bond for 5-BrU, and the Cl-atom abstraction for CCl4. These pathways are invariably characterized by the negative apparent activation energies. Whereas for 5-BrU and CCl4 the transition state theory rate constants are in good agreement with the experiment, the rate constants for IAc and IAm— the two substrates reacting preferably via the PCET—are difficult to predict correctly. Consequently, the corresponding reaction barriers necessitate lowering by 1–3 kcal mol–1 to bring them in accord with experiment. The B2PLYP method provides a worthwhile improvement over the M06-2X energetics although the largest errors remain for the two PCET processes.

Free radicals ; Aqueous Solution ; Haloorganics ; Proton-Coupled Electron Transfer ; Mechanism ; Kinetics ; Density Functional Theory ; Transition State Theory

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