Structure–activity relationship (SAR) for the prediction of gas-phase ozonolysis rate coefficients: an extension towards heteroatomic unsaturated species

Abstract

Heteroatomic unsaturated volatile organic compounds (HUVOCs) are common trace components of the atmosphere, yet their diverse chemical behaviour presents difficulties for predicting their oxidation kinetics using structure–activity relationships (SARs). An existing SAR is adapted to help meet this challenge, enabling the prediction of ozonolysis rates with unprecedented accuracy. The new SAR index, x_H, correlates strongly with available literature measurements of ozonolysis rate coefficients (R² = 0.87), a database representing 110 species. It was found that capturing the inductive effect rather than the steric effect is of primary importance in predicting the reactivity of these species, which is to be anticipated since HUVOCs can possess a variety of functional groups with a range of electron-withdrawing and donating tendencies. New experimental measurements of ozonolysis rate coefficients were conducted for 1-penten-3-ol, 3-methyl; ethene, 1,1-dimethoxy; E-2-pentenoic acid; E-1,2-dichloroethene; Z-1,2-dichloroethene; trichloroethene; tetrachloroethene; 1-butene, 3-chloro and 2-chloropropene, and were determined to be 5.15 × 10⁻¹⁸, 4.82 × 10⁻¹⁶, 3.07 × 10⁻¹⁸, 8.05 × 10⁻²⁰, 4.88 × 10⁻²¹, 6.04 × 10⁻²², 1.56 × 10⁻²⁴, 2.26 × 10⁻¹⁸ and 1.13 × 10⁻¹⁹ cm³ molecule⁻¹ s⁻¹, respectively. The index of the inductive effect, i_H, is compared with other indices of the electron-withdrawing capacity of a substitution, notably the Taft σ* constants and the rate of reaction of a given species with the hydroxyl radical, both of which are expected to be unaffected by steric factors. i_H correlates strongly in both cases and suggests a universal response by olefinic species towards electrophilic addition.