Transition state leading to β-O′ quinonemethide intermediate of p-coumaryl alcohol analyzed by semi-empirical molecular orbital calculation |
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Authors: | Mikiji Shigematsu Takayuki Kobayashi Hiroyasu Taguchi Mitsuhiko Tanahashi |
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Institution: | (1) Faculty of Applied Biological Sciences, Gifu University, Gifu 501-1193, Japan |
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Abstract: | The radical coupling reaction leading to the β-0′ quinonemethide intermediate of p-coumaryl alcohol was analyzed by semi-empirical molecular orbital calculation with MOPAC2002. By analyzing the radical monomer
in a one-electron oxidation, the spin density of the unpaired electron at the 4-oxygen was less than half of the values at
the C1, C3, C5, and Cβ positions. By analyzing the transition state during the radical coupling reaction, the activation enthalpy was evaluated
as 9.76 kcal/mol, which corresponds to the activation energies for the propagation of common vinyl polymers. From the analysis
of atomic interactions in the transition state, it was found that the activation enthalpy was largely composed of a high coulombic
repulsion between Cβ of the first monomer and the phenolic oxygen of the second monomer. After passing the transition state, the two radical monomers
formed a metastable quinone-methide intermediate. The optimum conformation of the quinonemethide intermediate was formed from
the meta-stable conformation through a second transition state with a small energy barrier.
Part of this article was presented at the 46th Lignin Symposium, held in Fukuoka, October 31-November 1, 2002 |
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Keywords: | Lignin Radical coupling reaction Transition state Molecular orbital calculation Activation enthalpy |
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