Reactions of a hydrogen atom with haloacetates in aqueous solutions: Computational evidence for proton-coupled electron transfer and competing mechanisms

A computational study of the mechanisms and kinetics of the aqueous reactions of a hydrogen atom with haloacetates is presented. Several mechanisms in the close competition are observed, such as proton-coupled electron transfer (PCET), hydrogen atom transfer (HAT), and halogen abstraction (XA). Computations predict that dechlorination takes place via PCET mechanisms and not via XA, as stated earlier, while XA is the fastest mechanism forIAc - . The reaction rate constants are reasonably well predicted within the theoretically most reliable canonical variational transition state theory with small curvature tunneling corrections and compared with the experimental ones. To reproduce the experimental rate constants of the debromination process it is necessary to include the PCET and XA cumulative values. Small curvature tunneling corrections to the rate constants are the highest for HAT and PCET mechanisms, up to 70 times larger than the Wigner, while variational effects for XA mechanisms are very small.

Kinetics of chain reaction driven by proton-coupled electron transfer: α-hydroxyethyl radical and bromoacetate in buffered aqueous solutions

We measured and computed the rate constants of the reaction between the α-hydroxyethyl radical (˙CH(CH3)OH) and bromoacetate (BrCH2CO2-) in the non-buffered (NB), as well as in the bicarbonate (HCO3-) and hydrogen phosphate (HPO42-) buffered aqueous solutions in the presence of ethanol. These complex multistep reactions are initiated by the proton-coupled electron transfer (PCET) which reduces BrCH2CO2- and incites its debromination. The PCET is followed by the step in which the resulting carboxymethyl radical propagates a radical chain reaction thus recovering ˙CH(CH3)OH and enhancing the debromination yields. It is found that the rate constants for the initial PCET step (k1) are raised by ca. an order of magnitude in the presence of the buffers (k1(NB) = 1.4 × 105 dm3 mol-1 s-1; k1(HCO3-) = 1.4 × 106 dm3 mol-1 s-1; k1(HPO42-) = 1.1 × 106 dm3 mol-1 s-1). To rationalize this, we used density functional theory at the M06-2X-D3/6-311+G(2d,p) level in conjunction with the polarizable continuum model (PCM) for an implicit description of the aqueous environment. To acceptably reproduce the measured rate constants, the minimal solute, consisting of ˙CH(CH3)OH, BrCH2CO2- and the buffer anion, has to be expanded by at least 2-3 explicit molecules of the water solvent. The used kinetic model consisting of a set of coupled differential equations indicates the sigmoid dependence of yields vs. k1 thereby confirming the autocatalytic trait of these reactions. The computations unravel the profound influence of the presence of buffers on these reaction systems. On the one hand, the buffer anions promote the PCET by accelerating the proton transfer; on the other hand, they slow down the propagation step by forming the strong hydrogen bonds with the carboxymethyl radical. The two opposing effects cancel out and cause the Br- yields to remain approximately comparable in the non-buffered and buffered media.

Comparing the performances of various density functionals for modelling the mechanisms and kinetics of bimolecular free radical reactions in aqueous solution

We systematically tested the performances of 18 density functionals for the mechanisms and kinetics of reactions of the α-hydroxyisopropyl radical with 9 organic substrates.

Proton-coupled electron transfer is the dominant mechanism of reduction of haloacetates by the α-hydroxyethyl radical in aqueous media

Reactions of α-hydroxyethyl radical with four haloacetates in non-buffered and buffered aqueous solutions preferably follow the proton-coupled electron transfer pathway.