A DFT study on recombination of alkyl radicals to C2-C17 normal alkanes & branched C8 alkanes and corresponding C-C bond pyrolysis reaction

Abstract

Alkane C–C dissociation is one of the key elementary reactions in combustion and pyrolysis. Corresponding rate constants are important in kinetic models. However, reliable rate constants for long and branched alkanes are still limited, and accurate rate constant calculation for large alkanes is time-consuming. In this work we have systematically investigated the high-pressure-limit rate constants of alkyl recombination to normal CnH2n+2 (n=2–17) and some branched alkanes with UM06-2X/cc-pvdz//VRC-TST method.Corresponding dissociation rate constants have been converted using calculated free energies at G4MP2 level. This combined method is found to reproduce dissociation rate constants well (error < 101) for small alkanes at 700–2000 K. Our results suggested that it’s a good approximation to use short alkane (C3 for example) dissociation rate constants for longer alkanes above 800 K. Dissociation reactions leading to CH3 radical are much harder than those leading to other alkyls.It's a good approximation to use rate constant of dissociation leading to C2H5 radical for other alkyl dissociations other than CH3. Both modified and a four-parameter Arrhenius equations give good fitting to calculated rate constants. Our results give further insight into C–C bond dissociation of long and branched alkanes, and provide alternative kinetic data for relevant combustion/pyrolysis kinetic models.