Predicting arene rate coe ffi cients with respect to hydroxyl and other free radicals in the gas-phase : a simple and e ff ective method using a single
Predicting arene rate coefficients with respect to hydroxyl and other free radicals in the gas-phase: a simple and effective method using a single topological descriptor M. R. McGillen, C. J. Percival, G. Pieterse, L. A. Watson, and D. E. Shallcross School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, The Sackville Building, Sackville Street, P.O. Box 88, Manchester M60 1QD, UK Department of Air Quality and Climate Change, Energy Research Centre for the Netherlands, P.O. Box 1, 1755 ZG Petten, The Netherlands Biogeochemistry Research Centre, School of Chemistry, The University of Bristol, Cantock’s Close BS8 1TS, UK Received: 13 November 2006 – Accepted: 2 February 2007 – Published: 26 February 2007 Correspondence to: C. J. Percival (c.percival@manchster.ac.uk)
Can topological indices be used to predict gas-phase rate coefficients of importance to tropospheric chemistry? Reactions of alkenes with OH, NO3 and O3.
Rate coefficients for the gas-phase reactions of unsaturated hydrocarbons with OH, NO(3) and O(3) are of vital importance to atmospheric modelling. Many of these rate coefficients are unknown--possibly resulting from the variety of these compounds and the inherent expense of deriving these data experimentally--and a reliable method for their prediction would therefore be of value to such models. This study presents a method for estimating rate coefficients for C(2)-C(10) olefins. Measured rate coefficients for the reaction of unsaturated hydrocarbons with OH and NO(3) radicals and O(3) are correlated with the Randić topological descriptor and an established correlation parameter, i.e. ionization potentials calculated using the frontier molecular orbital (FMO) approach. Although the ionization potential method produces better correlations in general, OH correlations of aliphatic species, subdivided into groups of varying total number of primary (1 degree), secondary (2 degrees) and tertiary (3 degrees) carbon atoms (Sigma x degrees (x)) produced several new trends with the Randić index that were not present in the ionization potential correlations. Strong correlation was observed with a Randić-type index optimized to include a term for side chain length (Sigma x degrees (x)). The presence of strong relationships, made predictable by molecular constitution, makes topological descriptors a useful and accessible tool for estimating rate coefficients for the reactions of OH with aliphatic species. This study indicates that alkene reactivity is affected strongly by constitution and that abstraction as well as addition may be important for some classes of compound.
Étude de la réactivité des radicaux NO3 avec le diméthyl-2,3 butène-2, le butadiène-1,3 et le diméthyl-2,3 butadiène-1,3
Les reactions du radical NO 3 avec des alcenes ont ete etudiees a 298K dans un reacteur a ecoulement couple a un spectrometre de masse. La reaction F + HNO 3 a ete utilisee comme source de NO 3 . La determination absolue des constantes de vitesse a donne les valeurs suivantes : (4,5 ± 0.4) × KF -11 , (9,7 ± 0.4) × 10 -14 , (1.96 ± 0,20) × 10 -12 (cm 3 molecule -1 s -1 ) respectivement pour le dimethyl-2.3 butene-2, le butadiene-1,3 et le dimethyl-2,3 butadiene-1,3. Ces donnees sont comparees a celles de la litterature. Elles montrent une tendance de reactivite qui est fonction de la structure des molecules. Par ailleurs, la detection des produits primaires des reactions etudiees permet d’interpreter partiellement les mecanismes d’oxydation atmospherique nocturne des alcenes consideres.