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. McGillen1, C. J. Percival1, G. Pieterse2, L. A. Watson3, and D. E. Shallcross3 1School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, The Sackville Building, Sackville Street, P.O. Box 88, Manchester M60 1QD, UK 2Department of Air Quality and Climate Change, Energy Research Centre for the Netherlands, P.O. Box 1, 1755 ZG Petten, The Netherlands 3Biogeochemistry Research Centre, School of Chemistry, The University of Bristol, Cantock's Close BS8 1TS, UK
Abstract. The reactivity of aromatic compounds is of great relevance to pure and
applied chemical disciplines, yet existing methods for estimating gas-phase
rate coefficients for their reactions with free radicals lack accuracy and
universality. Here a novel approach is taken, whereby strong relationships
between rate coefficients of aromatic hydrocarbons and a Randić-type
topological index are investigated, optimized and developed into a method
which requires no specialist software or computing power.
Measured gas-phase rate coefficients for the reaction of aromatic
hydrocarbons with OH radicals were correlated with a calculated Randić-type
index, and optimized by including a term for side chain length. Although
this method is exclusively for use with hydrocarbons, it is more diverse
than any single existing methodology since it incorporates alkenylbenzenes
into correlations, and can be extended towards other radical species such as
O(3P) (and tentatively NO3, H and Cl). A comparison (with species
common to both techniques) is made between the topological approach
advocated here and a popular approach based on electrophilic subsituent
constants, where it compares favourably.
A modelling study was carried out to assess the impact of using estimated
rate coefficients as opposed to measured data in an atmospheric model. The
difference in model output was negligible for a range of NOx
concentrations, which implies that this method has utility in complex
Strong relationships (e.g. for OH, R2=0.96) between seemingly diverse
compounds including benzene, multisubstituted benzenes with saturated,
unsaturated, aliphatic and cyclic substitutions and the nonbenzenoid
aromatic, azulene suggests that the Randić-type index presented here
represents a new and effective way of describing aromatic reactivity, based
on a quantitative structure-activity relationship (QSAR).
Citation: McGillen, M. R., Percival, C. J., Pieterse, G., Watson, L. A., and Shallcross, D. E.: 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, Atmos. Chem. Phys., 7, 3559-3569, doi:10.5194/acp-7-3559-2007, 2007.