Reaction kinetics of hydrogen addition reactions to methyl butenoate.

To investigate the kinetics of hydrogen addition reactions of unsaturated methyl esters, we selected two representative molecules that are isomers with C[double bond, length as m-dash]C double bonds at different locations, i.e. methyl 2-butenoate and methyl 3-butenoate for study. An appropriate quantum chemical method was determined to compute the potential energy surfaces. The high-pressure limit rate constants were computed by applying multi-structural canonical variational transition state theory including tunneling by the multi-dimensional small-curvature tunneling approximation. The master equation analysis was followed to study the pressure-dependence of the rate constants of H addition and the subsequent dissociation reactions. The results show that it is easier for the H atom to add to the C[double bond, length as m-dash]C than to the C[double bond, length as m-dash]O bond because of the lower barrier heights, and the hydrogen addition reactions are faster for both methyl 2-butenoate and methyl 3-butenoate, except that the hydrogen abstraction is dominant at above 1700 K for methyl 2-butenoate. Using our computed rate constants, the prediction for methyl propanoate mole fraction agreed better with experimental data of methyl 2-butenoate combustion.

On the Prediction of Standard Enthalpy of Formation of C2-C4 Oxygenated Species.

In this study, to determine an efficient and accurate method for predicting standard enthalpy of formation (ΔfHo) of oxygenated species, we calculated ΔfHo for several typical C2-C4 oxygenated species using atomization and isodesmic reactions in combination with various quantum chemical methods, including six density functional theory methods, three compound methods, and CCSD(T)/CBS. Compared with experimental values, at the same quantum chemical level, ΔfHo values predicted by using isodesmic reactions are more accurate than those using atomization reactions. Comparing various quantum chemical methods when isodesmic reactions are used, the performance of G4 is the best with a mean unsigned deviation (MUE) of 0.3 kcal/mol and a standard deviation (SD) of 0.3 kcal/mol, while M06-2X can predict ΔfHo efficiently and accurately with an MUE of 0.6 kcal/mol and SD of 0.5 kcal/mol. Using the best methods we have found, we calculated the enthalpies of formation and other thermodynamic properties for dimethyl carbonate (DMC) and its associated species and then applied them in a DMC combustion model for predicting ignition delay times. Better agreement with the experiments is achieved when the newly computed thermodynamic properties are adopted.

Effect of hindered internal rotation treatments on predicting the thermodynamic properties of alkanes.

When considering hindered internal rotation, we usually have several options, including (i) single structure harmonic oscillator (SS-HO) approximation that considers the lowest-energy conformer only and approximates all molecular vibrations as harmonic oscillations, (ii) one-dimensional (1-D) internal rotation treatment that replaces the corresponding vibrational mode with one-dimensional torsion, and (iii) the multistructural method with torsional anharmonicity (MS-T) that considers the multiple-structure and torsional anharmonicity. These methods differ greatly in computational cost and accuracy. To evaluate the effect of different treatments on predicting thermodynamic properties, we calculated enthalpy, entropy, and heat capacity for a series of normal and branched alkanes using six different methods, including the SS-HO treatment, three 1-D methods, the MS-T method, and the group additivity (GA) method. The comparison of the computational results with experimental data shows that GA and two 1-D methods proposed in this study are more suitable for reliable and rapid predictions of thermodynamic properties for large hydrocarbons with many carbon-carbon single bonds.