Can topological indices be used to predict gas-phase rate coefficients of importance to tropospheric chemistry? Free radical abstraction reactions of alkanes

摘要

As tropospheric chemical models become ever more complex, reliable rate coefficient data for gas-phase hydrogen abstraction reactions involving trace hydrocarbons becomes increasingly necessary. There are hundreds of non-methane hydrocarbons (NMHCs) of potential importance to tropospheric chemistry, many of which have not been subjected to experimental inquiry. This study—the first of its kind to investigate hydrocarbons ranging from C_2 to C_(10) using topological indices—aims to provide a reliable and accessible method of estimating rate coefficients for these species. Rate coefficients of free radical abstraction reactions of NMHCs were correlated with the Randic and Balaban topological indices for radical moieties OH, Cl and NO_3. These correlations were compared with those of an established frontier orbital approach based on calculated ionization potentials (IPs). The Randic index was found to correlate better than IP for each of the radicals studied, and correlated particularly effectively for the Cl radical. The Balaban index did not correlate for branched alkanes except for NO_3. Where only unbranched alkanes were considered, the Balaban index proved most reliable, demonstrating a clear linear relationship. Topological indices present an accurate and diverse method for estimating free radical abstraction rate coefficients that does not require the computing power, specialist software packages, or complex mathematics inherent in ab initio calculations. A further advantage of using topological indices is that they are calculated in an unambiguous manner. More reliable tools for estimating rate coefficients have direct implications for improving models, and may also provide a direction for future laboratory work, either by highlighting particularly reactive species or identifying potentially spurious rate coefficient data.