Kinetics of Hydrogen Abstraction Reactions of Methyl Palmitate and Octadecane by Hydrogen Atoms.

Hydrogen abstractions play a crucial role in the consumption of fuel molecules during fuel pyrolysis and combustion processes. In this study, a generalized energy-based fragmentation approach was used to obtain CCSD(T)-F12a/cc-pVTZ energy barriers of hydrogen abstraction reactions by hydrogen atoms from methyl palmitate (C15H31COOCH3), a key component of biodiesel. The accuracy of M06-2X/6-311++G(d,p) for obtaining the energy barriers was evaluated against the CCSD(T) results. Based on the quantum chemical results, the high-pressure-limit rate constants for C15H31COOCH3 + H were calculated and compared with those of octadecane ( n-C18H38) reacting with H. The treatment of hindered internal rotations for such long-chain molecules was discussed and the rate rules for different abstraction sites were summarized. The results show that in the C15H31COOCH3 + H system, the α hydrogen abstraction no longer plays a dominant role as in small methyl esters, and the hydrogen atoms of CH2 groups far away from the ester group are more easily abstracted than those near the ester group.

An adaptive distance-based group contribution method for thermodynamic property prediction.

In the search for an accurate yet inexpensive method to predict thermodynamic properties of large hydrocarbon molecules, we have developed an automatic and adaptive distance-based group contribution (DBGC) method. The method characterizes the group interaction within a molecule with an exponential decay function of the group-to-group distance, defined as the number of bonds between the groups. A database containing the molecular bonding information and the standard enthalpy of formation (Hf,298K) for alkanes, alkenes, and their radicals at the M06-2X/def2-TZVP//B3LYP/6-31G(d) level of theory was constructed. Multiple linear regression (MLR) and artificial neural network (ANN) fitting were used to obtain the contributions from individual groups and group interactions for further predictions. Compared with the conventional group additivity (GA) method, the DBGC method predicts Hf,298K for alkanes more accurately using the same training sets. Particularly for some highly branched large hydrocarbons, the discrepancy with the literature data is smaller for the DBGC method than the conventional GA method. When extended to other molecular classes, including alkenes and radicals, the overall accuracy level of this new method is still satisfactory.

Theoretical Analysis of the Effect of C═C Double Bonds on the Low-Temperature Reactivity of Alkenylperoxy Radicals.

Biodiesel contains a large proportion of unsaturated fatty acid methyl esters. Its combustion characteristics, especially its ignition behavior at low temperatures, have been greatly affected by these C═C double bonds. In this work, we performed a theoretical analysis of the effect of C═C double bonds on the low-temperature reactivity of alkenylperoxy radicals, the key intermediates from the low-temperature combustion of biodiesel. To understand how double bonds affect the fate of peroxy radicals, we selected three representative peroxy radicals from heptane, heptene, and heptadiene having zero, one, and two double C═C bonds, respectively, for study. The potential energy surfaces were explored at the CBS-QB3 level, and the reaction rate constants were computed using canonical/variational transition state theories. We have found that the double bond is responsible for the very different bond dissociation energies of the various types of C-H bonds, which in turn affect significantly the reaction kinetics of alkenylperoxy radicals.