A quantum chemical investigation is presented for the determination of accurate kinetic and thermodynamic parameters for hydrocarbon radical reactions. First, standard enthalpies of formation are calculated at different levels of theory for a training set of 58 hydrocarbon molecules, ranging from C 1 to C 10 , for which experimental data are available. It is found that the CBS-QB3 method succeeds in predicting standard enthalpies of formation with a mean absolute deviation of 2.5 kJ/mol, after a systematic correction of -1.29 kJ/mol per carbon atom and -0.28 kJ/mol per hydrogen atom. Even after a systematic correction, B3LYP density functional theory calculations are not able to reach this accuracy, with mean absolute deviations of 9.2 (B3LYP/6-31G(d)) and 12.9 kJ/mol (B3LYP/6-311G(d,p)), and with increasing deviations for larger hydrocarbons. Second, highlevel transition state geometries are determined for 9 carbon-centered radical additions and 6 hydrogen additions to alkenes and alkynes and 10 hydrogen abstraction reactions using the IRCMax(CBS-QB3//B3LYP/6-311G-(d,p)) method. For carbon-centered radical addition reactions, B3LYP/6-311G(d,p) slightly overestimates the length of the forming C-C bond as compared to the IRCMax data. A correlation to improve the agreement is proposed. For hydrogen addition reactions, MPW1K density functional theory (MPW1K/6-31G(d)) is able to locate transition states. However, the lengths of the forming C-H bonds are systematically longer than reference IRCMax data. Here, too, a correlation is proposed to improve the agreement. Transition state geometries for hydrogen abstraction reactions obtained with B3LYP/6-311G(d,p) show good agreement with the IRCMax reference data. Third, the improved transition state geometries are used to calculate activation energies at the CBS-QB3 level. Comparison between both CBS-QB3 and B3LYP density functional theory predictions shows deviations up to 25 kJ/mol. Although main trends are captured by B3LYP DFT, secondary trends due to radical nucleophilic effects are not reproduced accurately.

The influence of dimethyl disulfide (DMDS), which is widely used as an additive in ethylene plants, on coke formation during the steam cracking of hydrocarbons was investigated in a continuous-flow stirred-tank reactor (CSTR) setup with n-hexane as the feed and in a pilot-plant setup with ethane as the feed. Both of the reactors were made of Incoloy 800HT. Experiments were carried out at conditions relevant to industrial steam crackers. DMDS was applied by presulfidation, continuous addition, and presulfidation followed by continuous addition. Application of DMDS suppresses CO production. The influence of DMDS on coke formation was found to depend on the application method and the amount of DMDS used. SEM examination of the coke samples obtained from the steam cracking of n-hexane indicated that application of DMDS leads to a significant change in the coke morphology. EDX analysis indicated that application of DMDS causes a significant change in the metal content and distribution in both the alloy surface and the coke layers. The mechanism of the influence of DMDS on the coke formation is discussed.

Thermochemical and kinetic data were calculated at four cost-effective levels of theory for a set consisting of five hydrogen abstraction reactions between hydrocarbons for which experimental data are available. The selection of a reliable, yet cost-effective method to study this type of reactions for a broad range of applications was done on the basis of comparison with experimental data or with results obtained from computationally demanding high level of theory calculations. For this benchmark study two composite methods (CBS-QB3 and G3B3) and two density functional theory (DFT) methods, MPW1PW91/6-311G(2d,d,p) and BMK/6-311G(2d,d,p), were selected. All four methods succeeded well in describing the thermochemical properties of the five studied hydrogen abstraction reactions. High-level Weizmann-1 (W1) calculations indicated that CBS-QB3 succeeds in predicting the most accurate reaction barrier for the hydrogen abstraction of methane by methyl but tends to underestimate the reaction barriers for reactions where spin contamination is observed in the transition state. Experimental rate coefficients were most accurately predicted with CBS-QB3. Therefore, CBS-QB3 was selected to investigate the influence of both the 1D hindered internal rotor treatment about the forming bond (1D-HR) and tunneling on the rate coefficients for a set of 21 hydrogen abstraction reactions. Three zero curvature tunneling (ZCT) methods were evaluated (Wigner, Skodje & Truhlar, Eckart). As the computationally more demanding centrifugal dominant small curvature semiclassical (CD-SCS) tunneling method did not yield significantly better agreement with experiment compared to the ZCT methods, CD-SCS tunneling contributions were only assessed for the hydrogen abstractions by methyl from methane and ethane. The best agreement with experimental rate coefficients was found when Eckart tunneling and 1D-HR corrections were applied. A mean deviation of a factor 6 on the rate coefficients is found for the complete set of 21 reactions at temperatures ranging from 298 to 1000 K. Tunneling corrections play a critical role in obtaining accurate rate coefficients, especially at lower temperatures, whereas the hindered rotor treatment only improves the agreement with experiment in the high-temperature range.

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