The use of the electrotopological state as a basis for predicting hydrogen abstraction rate coefficients: a proof of principle for the reactions of alkanes and haloalkanes with OH

Structure-activity relationships (SARs) are essential components of detailed chemical models, where they are employed to provide kinetic information when high-quality experimental or theoretical data are unavailable. Notwithstanding, there are very few types of SARs that are routinely employed to estimate reaction kinetics. Accordingly, a new temperature-dependent and site-specific technique for rate coefficient estimation is presented, based on the electrotopological state (E-state), a fundamental property that can describe the substituent effect upon each hydrogen environment in a molecule. This accounts for the electronic character of individual atoms within molecules and their respective distances from one another. This method is applied to the hydrogen abstraction reactions of OH with alkanes and haloalkanes, where it was found to perform well compared with other approaches for molecules whose rate coefficients have been measured experimentally over a broad temperature range (similar to 200-1500 K). To extend this comparison, an efficient software tool for batch-estimated rate coefficients has been developed. By applying this software to fully enumerated lists of halocarbons containing from one to four carbon atoms, we were able to compare predictions of >100 000 species between techniques, and although experimental coverage is sparse, we could assess the degree of consensus between these estimates. Disagreement between methods was found to increase with carbon number, and differences of up to three orders of magnitude were observed in some cases. The reasons for these discrepancies and possible solutions are discussed. In a further demonstration of the utility of the E-state approach, we show that it can also be used to calculate bond-dissociation energy (BDE), which also compares favourably with a state-of-the-art literature method. The E-state approach not only provides accurate predictions of rate coefficients, but it does so with fewer fitting parameters and by being constrained by a fundamental molecular property. From this we conject that it is less prone to overfitting and more easily expanded to unfamiliar substituents than previous SAR approaches. The efficiency and robustness with which estimates of BDE and rate coefficients are made over a wide range of conditions will be of relevance to a variety of fields including atmospheric and combustion chemistry.