Use of quantum-chemical descriptors to analyse reaction rate constants between organic chemicals and superoxide/hydroperoxyl (O₂^•−/HO₂^•)

Abstract

The reaction between superoxide (O 2 2) and organic chemicals is of interest in many scientific disciplines including biology and synthetic chemistry, as well as for the evaluation of chemical fate in the environment. Due to limited data and lack of congeneric modelling, the involvement of superoxide in many complex processes cannot be adequately evaluated. In this study, we developed new quantitative structure-property relationship (QSPR) models for the prediction of the aqueous-phase rate constant for the reaction between superoxide and a wide variety of organic chemicals reacting via one-electron oxidation, reduction and hydrogen-transfer. It is shown that the relative importance of these pathways is related to frontier molecular orbital (FMO) interaction and to pH. The class-specific QSPRs developed have good statistics (0.84 R 2 0.92). For non-congeneric chemicals it is demonstrated that the reactivity toward superoxide can be described by applying explicit descriptions for competition kinetics and speciation. Therefore, the relationships developed in this study are useful as a starting point to evaluate more complex molecules having, for example, multiple reactive functional groups, labile H bonds, or delocalised cationic charges. However, additional kinetic data and more rigorous computation are needed to evaluate such molecules.

Thermochemical unification of molecular descriptors to predict radical hydrogen abstraction with low computational cost

“…Electron transfer versus hydrogen abstraction. For electron transfer by OH (eqn (2) and (3)), the s LFER (Hammett substituent constant) value, AE2SD, is 2.2 AE 0.2 p(k r )/eV, similar to that of HOO /O 2 À (s LFER = 2.1-2.5 p(k r )/eV) including ET 21.…”

“…Comparison of D(DG r ) with experimental data allowed to approximate s LFER for various radicals (e.g. O 2 À /HOO and aryl radicals 20,21 ). Curiously, for those radicals, s LFER was identical (i.e.…”

“…within 2 standard deviations). Given these findings, 18,[20][21][22] we hypothesized s LFER to be a constant factor for a broader set of radicals.…”

“…The dashed line denotes interaction between frontier orbitals allowing oxidative outersphere electron transfer. Data for O 2 À (both abstraction and addition reactions) and HOO s indicated by squared symbols were taken from ref 21…”

A universal free energy relationship for both hard and soft radical addition in water

“…Shapes of potential curves ( r ) affect importance of relative to (endo/exothermicity, Figure 1), via values (Equation ). [ 19,38,39 ] From literature, [ 19,40,41 ] we obtained a constant value of 10 2.3 M −1 s −1 eV −1 for . Via a radical channel/pathway, electrophilic aromatic substitution for various reagents adheres to CS: Transition energies (−Δh ν /2.3RT) correlate with log( k R1 / k R2 ) with a slope of ~1; thus, = 10 2.3 M −1 s −1 eV −1 .…”