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_2n–1COOCH₃) and Hydrogen Radical

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 HCOOCH ( 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.

Influence of Torsional Anharmonicity on the Reactions of Methyl Butanoate with Hydroperoxyl Radical

“…Good reproducibility of this study can be ensured by using the computational methods in which the grid size, convergence settings, and dispersion correction are default values recommended in Gaussian 09 . Such a computational approach has been adopted and validated against experiments in many previous studies of the authors − and others. , Transition states were identified through the intrinsic reaction coordinate (IRC) calculation by examining the connections of each saddle point to its local minima. The vibrational frequencies were not scaled by the common factor of 0.95–0.99 since it is believed to not cause significant influence on the present rate calculations .…”

Accurate Kinetics of Cyclization Reactions of the Large-Size Hydroperoxy Methyl-Ester Radicals Investigated by the Isodesmic Reaction Correction Method

“…In contrast, the ONIOM method operates as a multilayered QM/MM method, distinguishing itself from QM/MM by allowing the incorporation of various QM and QM methods and supporting more than two layers . Both the ONIOM method and its derivative, the CGA-ONIOM method, have demonstrated success in handling extensive systems, particularly in the context of methyl esters. − However, the accuracy of these methods predominantly hinges on the precision of the chemically active portion (CAP), which is limited to systems containing up to 10 heavy atoms.…”

Metric-Based Assessment Method for MS-T Formalism with Small Subsets of Torsional Conformers

“…In recent years, there are extensive theoretical studies on high-level single-point energy calculation of large molecules. Based on the well-known ONIOM (our own N -layered integrated molecular orbital and molecular mechanics), Zhang et al , proposed a two-layer ONIOM method, in which the QCISD­(T)/CBS and DFT/6-311++G­(d,p) methods are used for the high layer and the low layer, respectively. Li et al developed a generalized energy-based fragmentation (GEBF) approach, in which a large molecule is divided into small fragments and the total energy of the large molecule is obtained from synthesizing the small fragments.…”