H-atom shift isomerization and ring closure are two important reaction categories in the oxidation and pyrolysis of hydrocarbon fuels. In this work, alkyl radical H-atom shift and alkenyl radical cyclization reactions are systematically investigated using electronic structure calculations at the CBS-QB3 level of theory, and high-pressure rate estimation rules are proposed based on the reactivity-structure relationships. The rules for the H-atom shift reactions of alkyl radicals cover the 1,2- up to 1,7-H shifts. The rules for the cycloaddition reactions of alkenyl radicals are for both the endo- and exo-cycloaddition and include the formation of 3- to 7-member ring products. The activation energies for these reactions are discussed in terms of a combination of the barrier for the corresponding bimolecular reaction plus a component to account for ring strain. The pre-exponential factors are rationalized in terms of entropy losses associated with the reduction in free rotors in the transition state. The impact of resonance stabilization is examined for both types of reactions. Such structures can be formed, for example, by the addition of vinyllic and allylic radicals to 1,3-butadiene. This adduct can then undergo H-shifts and/or cyclizations that may be important pathways to formation of aromatics. The results are in good agreement with available experiment measurements and other theoretical studies.

Impact of resonance stabilization on H-atom shift isomerization and ring closure reactions

249th National Meeting (2015)

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