Pronounced non-Arrhenius behaviour of hydrogen-abstractions from toluene and derivatives by phthalimide-N-oxyl radicals: a theoretical study.
Abstraction of hydrogen atoms by pthalimide-N-oxyl radicals is an important step in the N-hydroxyphthalimide catalyzed autoxidation of hydrocarbons. In this contribution, the temperature dependency of this reaction is evaluated by a detailed transition state theory based kinetic analysis for the case of toluene. Tunneling was found to play a very important role, enhancing the rate constant by a factor of 20 at room temperature. As a result, tunneling, in combination with the existence of two distinct rotamers of the transition state, causes a pronounced temperature dependency of the pre-exponential frequency factor, and, as a consequence, marked curvature of the Arrhenius plot. This explains why earlier experimental studies over a limited temperature range around 300 K found formal Arrhenius activation energies and pre-factors that are 4 kcal mol(-1) and three orders of magnitude smaller than the actual energy barrier and the corresponding frequency factor, respectively. Also as a consequence of tunneling, substitution of a deuterium atom for a hydrogen atom causes a large decrease in the rate constant, in agreement with the measured kinetic isotope effects. The present theoretical analysis, complementary to the experimental rate coefficient data, allows for a reliable prediction of the rate coefficient at higher temperatures, relevant for actual autoxidation processes.
Hermans
Ive
I
Centre for Surface Chemistry and Catalysis, K.U. Leuven, Kasteelpark Arenberg 23, B-3001, Heverlee, Belgium. ive.hermans@chem.et
Jacobs
Pierre
P
Peeters
Jozef
J
D001547:Benzaldehydes / Q000737:chemistry; D019905:Benzyl Alcohol / Q000737:chemistry; D006859:Hydrogen / Q000737:chemistry*; D007554:Isotopes; D007700:Kinetics; D008956:Models, Chemical; D008958:Models, Molecular; D015394:Molecular Structure; D010797:Phthalimides / Q000737:chemistry*; D017382:Reactive Oxygen Species / Q000737:chemistry*; D014050:Toluene / Q000031:analogs & derivatives / Q000737:chemistry*
D001547:Benzaldehydes; D007554:Isotopes; D010797:Phthalimides; D017382:Reactive Oxygen Species; C037431:phthalimide; D014050:Toluene; D006859:Hydrogen; D019905:Benzyl Alcohol; C032175:benzaldehyde