Toward High-Level Theoretical Studies of Large Biodiesel Molecules: An ONIOM [QCISD(T)/CBS:DFT] Study of the Reactions between Unsaturated Methyl Esters (C nH2 n-1COOCH3) and Hydrogen Radical

Lidong Zhang; Qinghui Meng; Yicheng Chi; Peng Zhang

doi:10.1021/acs.jpca.8b02327

Toward High-Level Theoretical Studies of Large Biodiesel Molecules: An ONIOM [QCISD(T)/CBS:DFT] Study of the Reactions between Unsaturated Methyl Esters (C _nH_{2 n-1}COOCH₃) and Hydrogen Radical

J Phys Chem A. 2018 May 31;122(21):4882-4893. doi: 10.1021/acs.jpca.8b02327. Epub 2018 May 21.

Authors

Lidong Zhang¹, Qinghui Meng^{1

2}, Yicheng Chi², Peng Zhang²

Affiliations

¹ National Synchrotron Radiation Laboratory , University of Science and Technology of China , Hefei , China.
² Department of Mechanical Engineering , The Hong Kong Polytechnic University , Hung Hom , Hong Kong , China.

PMID: 29745664
DOI: 10.1021/acs.jpca.8b02327

Abstract

A two-layer ONIOM[QCISD(T)/CBS:DFT] method was proposed for the high-level single-point energy calculations of large biodiesel molecules and was validated for the hydrogen abstraction reactions of unsaturated methyl esters that are important components of real biodiesel. The reactions under investigation include all the reactions on the potential energy surface of C _nH_{2 n-1}COOCH₃ ( n = 2-5, 17) + H, including the hydrogen abstraction, the hydrogen addition, the isomerization (intramolecular hydrogen shift), and the β-scission reactions. By virtue of the introduced concept of chemically active center, a unified specification of chemically active portion for the ONIOM (ONIOM = our own n-layered integrated molecular orbital and molecular mechanics) method was proposed to account for the additional influence of C═C double bond. The predicted energy barriers and heats of reaction by using the ONIOM method are in very good agreement with those obtained by using the widely accepted high-level QCISD(T)/CBS theory, as verified by the computational deviations being less than 0.15 kcal/mol, for almost all the reaction pathways under investigation. The method provides a computationally accurate and affordable approach to combustion chemists for high-level theoretical chemical kinetics of large biodiesel molecules.