A comparative ab initio study of hydrogen abstraction from n-propyl benzene

Aromatics form an integral part of typical aviation fuels with n-propyl benzene selected as a representative molecule for inclusion in several EU and US surrogate blends used for design calculations. Despite the practical relevance, kinetic and thermodynamic data obtained using comparatively accurate ab initio methods have to date not been compared with currently used reaction class based estimates. The use of ab initio methods for comparatively complex molecules also necessitates an assessment of the relative benefits of higher levels of theory as it is typically desirable to balance the accuracy of the treatment of individual reactions with the need to consider more complete reaction sequences. The current study examines six hydrogen extractions, via the hydrogen or methyl radicals from the n-propyl side chain. Potential energy surfaces were determined using 10 different approaches, including state-of-the-art DFT (M06, M06-2X and M08-SO) and contemporary composite methods (G4, G4MP2, CBS-QB3 and CBS-4M). Results are presented relative to data obtained using the CCSD(T)/jun-cc-pVTZ//M06-2X/6-311++G(3df,3pd) coupled cluster based method. Rate parameters were determined using transition state theory combined with (i) small curvature tunnelling and energetics at the M06-2X/6-31G(2df,p) level and (ii) Eckart tunnelling corrections and energetics at the CCSD(T)/jun-cc-pVTZ level. Results were found to agree comparatively well with modest differences in rates for several reactions. However, it is also shown that substantial deviations can arise with respect to reaction class based estimation techniques.