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.