Quantum chemical study of the thermal decomposition of o-quinone methide (6-methylene-2,4-cyclohexadien-1-one)

Document Type

Article

Publication Date

8-16-2007

Abstract

o-Quinone methide (o-QM), or 6-methylene-2,4-cyclohexadiene-1-one, has been identified as an important intermediate in lignin and alkyl benzene combustion, and the thermal decomposition of o-QM is therefore relevant to the combustion of transportation fuels (which contain toluene) and of biomass and low-rank coals (which contain lignin). We present a comprehensive reaction mechanism for the unimolecular conversion of o-QM to the reaction intermediates tropone and fulvene, calculated using theoretical quantum chemical techniques. Enthalpies of formation for all reactants, products, and intermediates are calculated using the CBS-QB3 theoretical method. Transition states are determined with the CBS-QB3 method, which we use to obtain rate constants as a function of temperature from transition-state theory, with Wigner tunneling corrections applied to hydrogen-shift reactions. Barrier heights are also calculated with the BB1K density functional theory (DFT) method for thermochemical kinetics. Reaction pathways are identified leading to tropone (which rapidly decomposes to benzene + CO) and to fulvene + CO, via initial hydrogen transfer to 2-hydroxyphenylcarbene and via ring opening to 1,3,5,6-heptatetraen-1-one, respectively. Quantum Rice-Ramsperger-Kassel (QRRK) theory analysis of the reaction kinetics indicates that the dominant reaction pathway is formation of tropone via 2-hydroxyphenylcarbene; the formation of fulvene + CO via initial ring opening constitutes a secondary pathway, which becomes more important with increasing temperature. Our calculations, using BB1K barrier heights, yield the rate equation k(T) [s-1] = 2.64 × 1014 exp(-35.9/T [K]) for o-QM decomposition, which is in relatively good agreement with the experimental rate equation. Calculations provide an apparent activation energy of 71.3 kcal mol-1, versus 67.2 kcal mol-1 from experiment. © 2007 American Chemical Society.

Identifier

34548153529 (Scopus)

Publication Title

Journal of Physical Chemistry A

External Full Text Location

https://doi.org/10.1021/jp073335c

ISSN

10895639

PubMed ID

17645323

First Page

7987

Last Page

7994

Issue

32

Volume

111

This document is currently not available here.

Share

COinS