An ab initio study of the thermodynamical properties of competing reaction pathways in alkyl molecule oxidation
An ab initio study of the ethyl-, propyl-, butyl-, pentyl-, hexyl-, heptyl-, and octyl- peroxy radical molecules to compare the thermodynamical properties of two competing reactions: the 1,4 H-migration and the concerted elimination reactions at different abstraction sites. All molecules are studied using the CBS-QB3 and G4 composite methods which show good agreement with 6 different experimentally determined reaction enthalpies having rms deviations of 1.0 and 1.2 kcal mol-1 respectively. The entry pathway to the reaction, enthalpy of reaction, activation energy, and their Arrhenius pre-exponential factors are determined for each molecule and each transition along the carbon chains. The Skodje-Truhlar method is used to calculate the effect of tunneling on the reactions. With these values the rate of reaction was calculated for all molecule groups. There is a shift in dominance from the concerted elimination reaction pathway to the 1,4 H-migration as temperature approaches 450K due to the tunneling effect. The concerted elimination reaction is shown to be, overall, superior to its 1,4 H-migration counterpart. However, since tunneling affects hydrogen migration more profoundly, the advantage gained by the 1,4 H-migration is enough to gain dominance over the concerted elimination pathway at lower temperatures where tunneling is more important. It is also shown that the location of the reaction along the chain has a great effect on the rate, enthalpy, A-factor, and activation energy of the reactions. A hydrogen abstraction from a primary carbon is the most unfavorable reaction; it is proposed that this is due to the instability formed when creating a primary radical, which is known to be a high-energy molecule. Conversely, abstracting a hydrogen from a carbon atom that is more distant from the ends of the chain results in lower enthalpies of reaction and lower activation energies.
Francisco, Purdue University.
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