A Theoretical Study on the Kinetics of Disproportionation versus Association Reaction of CH3 + C2H5

The merit of pressure dependent kinetic modelling in steam cracking

Renewable cracking feedstocks from plastic waste and the need for novel reactor designs related to electrification of steam crackers drives the development of accurate and fundamental kinetic models for this process, despite its large scale implementation for more than half a century. Pressure dependent kinetics have mostly been omitted in fundamental steam cracking models, while they are crucial in combustion models. Therefore, we have assessed the importance of pressure dependent kinetics for steam cracking via in-depth modelling and experimental studies. In particular we have studied the influence of considering fall-off on the product yields for ethane and propane steam cracking. A high-pressure limit fundamental kinetic model is generated, based on quantum chemical data and group additive values, and supplemented with literature values for pressure dependent kinetic parameters for β-scission reactions and homolytic bond scissions of C₂ and C₃ species. Model simulations with high-pressure limit rate coefficients and pressure dependent kinetics are compared to new experimental measurements. Steam cracking experiments for pure ethane and propane feeds are performed on a tubular bench-scale reactor at 0.17 MPa and temperatures ranging from 1058 to 1178 K. All important product species are identified using a comprehensive GC × GC-FID/q-MS. For homolytic bond scissions, the inclusion of pressure dependent kinetics has a significant effect on the conversion profile for ethane steam cracking. On the other hand, pressure dependence of C₂ β-scissions significantly influences conversion and product species profiles for both ethane and propane steam cracking. The C₃ β-scissions pressure dependence has a negligible effect in ethane steam cracking, while for propane steam cracking the effect is non-negligible on the product species profiles.

Reaction Profiles and Kinetics for Radical-Radical Hydrogen Abstraction via Multireference Coupled Cluster Theory

Radical-radical abstractions in hydrocarbon oxidation chemistry are disproportionation reactions that are generally exothermic with little or no barrier yet are underappreciated and poorly studied. Such challenging multireference electronic structure problems are tackled here using the recently developed state-specific multireference coupled cluster methods Mk-MRCCSD and Mk-MRCCSD(T), as well as the companion perturbation theory Mk-MRPT2 and the established MRCISD, MRCISD+Q, and CASPT2 approaches. Reaction paths are investigated for five prototypes involving radical-radical hydrogen abstraction: H + BeH → H₂+ Be, H + NH₂ → H₂ + NH, CH₃ + C₂H₅ → CH₄ + C₂H₄, H + C₂H₅ → H₂ + C₂H₄, and H + HCO → H₂ + CO. Full configuration interaction (FCI) benchmark computations for the H + BeH, H + NH₂, and H + HCO reactions prove that Mk-MRCCSD(T) provides superior accuracy for the interaction energies in the entrance channel, with mean absolute errors less than 0.3 kcal mol^-1 and percentage deviations less than 10% over the fragment separations of relevance to kinetics. To facilitate combustion studies, energetics for the CH₃ + C₂H₅, H + C₂H₅, and H + HCO reactions were computed at each level of theory with correlation-consistent basis sets (cc-pVXZ, X = T, Q, 5) and extrapolated to the complete basis set (CBS) limit. These CBS energies were coupled with CASPT2 projected vibrational frequencies along a minimum energy path to obtain rate constants for these three reactions. The rigorous Mk-MRCCSD(T)/CBS results demonstrate unequivocally that these three reactions proceed with no barrier in the entrance channel, contrary to some earlier predictions. Mk-MRCCSD(T) also reveals that the economical CASPT2 method performs well for large interfragment separations but may deteriorate substantially at shorter distances.

Potential Energy Surface for Large Barrierless Reaction Systems: Application to the Kinetic Calculations of the Dissociation of Alkanes and the Reverse Recombination Reactions

The isodesmic reaction method is applied to calculate the potential energy surface (PES) along the reaction coordinates and the rate constants of the barrierless reactions for unimolecular dissociation reactions of alkanes to form two alkyl radicals and their reverse recombination reactions. The reaction class is divided into 10 subclasses depending upon the type of carbon atoms in the reaction centers. A correction scheme based on isodesmic reaction theory is proposed to correct the PESs at UB3LYP/6-31+G(d,p) level. To validate the accuracy of this scheme, a comparison of the PESs at B3LYP level and the corrected PESs with the PESs at CASPT2/aug-cc-pVTZ level is performed for 13 representative reactions, and it is found that the deviations of the PESs at B3LYP level are up to 35.18 kcal/mol and are reduced to within 2 kcal/mol after correction, indicating that the PESs for barrierless reactions in a subclass can be calculated meaningfully accurately at a low level of ab initio method using our correction scheme. High-pressure limit rate constants and pressure dependent rate constants of these reactions are calculated based on their corrected PESs and the results show the pressure dependence of the rate constants cannot be ignored, especially at high temperatures. Furthermore, the impact of molecular size on the pressure-dependent rate constants of decomposition reactions of alkanes and their reverse reactions has been studied. The present work provides an effective method to generate meaningfully accurate PESs for large molecular system.

On the origin of regio- and stereoselectivity in singlet oxygen addition to enecarbamates

The reactions of excited state singlet molecular oxygen ((1)Δ(g),(1)O(2)) continue to witness interesting new developments. In the most recent manifestation, (1)O(2) is tamed to react with enecarbamates in a stereoselective manner, which is remarkable, in view of its inherently high reactivity (Acc. Chem. Res. 2008, 41, 387). Herein, we employed the CAS-MP2(8,7)/6-31G* as well as the CAS-MP2(10,8)/6-31G* computations to unravel the origin of (i) diastereoselectivities in dioxetane or hydroperoxide formation and (ii) regioselectivity leading to a [2 + 2] cycloadduct or an ene product when (1)O(2) reacts with an oxazolidinone tethered 2-phenyl-1-propenyl system. The computed Gibbs free energy profiles for E- and Z-isomers when (1)O(2) approaches through the hindered and nonhindered diastereotopic faces (by virtue of chiral oxazolidinone) of the enecarbamates exhibit distinct differences. In the case of E-isomer, the relative energies of the transition structures responsible for hydroperoxide (ene product) are lower than that for dioxetane formation. On the other hand, the ene pathway is predicted to involve higher barriers as compared to the corresponding dioxetane pathway for Z-isomer. The energy difference between the rate-determining diastereomeric transition structures involved in the most favored ene reaction for E-enecarbamate suggests high diastereoselectivity. In contrast, the corresponding energy difference for Z-enecarbamate in the ene pathway is found to be diminishingly close, implying low diastereoselectivity. However, the dioxetane formation from Z-enecarbamate is predicted to exhibit high diastereoselectivity. The application of activation strain model as well as the differences in stereoelectronic effects in the stereocontrolling transition structures is found to be effective toward rationalizing the origin of selectivities reported herein. These predictions are found to be in excellent agreement with the experimental observations.

Predictive theory for the combination kinetics of two alkyl radicals

An ab initio transition state theory based procedure for accurately predicting the combination kinetics of two alkyl radicals is described. This procedure employs direct evaluations of the orientation dependent interaction energies at the CASPT2/cc-pvdz level within variable reaction coordinate transition state theory (VRC-TST). One-dimensional corrections to these energies are obtained from CAS+1+2/aug-cc-pvtz calculations for CH3 + CH3 along its combination reaction path. Direct CAS+1+2/aug-cc-pvtz calculations demonstrate that, at least for the purpose of predicting the kinetics, the corrected CASPT2/cc-pvdz potential energy surface is an accurate approximation to the CAS+1+2/aug-cc-pvtz surface. Furthermore, direct trajectory simulations, performed at the B3LYP/6-31G* level, indicate that there is little local recrossing of the optimal VRC transition state dividing surface. The corrected CASPT2/cc-pvdz potential is employed in obtaining direct VRC-TST kinetic predictions for the self and cross combinations of methyl, ethyl, iso-propyl, and tert-butyl radicals. Comparisons with experiment suggest that the present dynamically corrected VRC-TST approach provides quantitatively accurate predictions for the capture rate. Each additional methyl substituent adjacent to a radical site is found to reduce the rate coefficient by about a factor of two. In each instance, the rate coefficients are predicted to decrease quite substantially with increasing temperature, with the more sterically hindered reactants having a more rapid decrease. The simple geometric mean rule, relating the capture rate for the cross reaction to those for the self-reactions, is in remarkably good agreement with the more detailed predictions. With suitable generalizations the present approach should be applicable to a wide array of radical-radical combination reactions.

Rate Constants and Kinetic Isotope Effects for Unimolecular 1,2-HX or DX (X = F or Cl) Elimination from Chemically Activated CF3CFClCH3-d0, -d1, -d2, and -d3

Chemically activated CF(3)CFClCH(3), CF(3)CFClCD(3), CF(3)CFClCH(2)D, and CF(3)CFClCHD(2) molecules with 94 kcal mol(-1) of internal energy were formed by the combination of CF(3)CFCl radicals with CH(3), CD(3), CH(2)D, and CHD(2) radicals, which were generated from UV photolysis of CF(3)CFClI and CH(3)I, CD(3)I, CH(2)DI, or CHD(2)I. The total (HF + HCl) elimination rate constants for CF(3)CFClCH(3) and CF(3)CFClCD(3) were 5.3 x 10(6) and 1.7 x 10(6) s(-1) with product branching ratios of 8.7 +/- 0.6 in favor of HCl (or DCl). The intermolecular kinetic isotope effects were 3.22 and 3.18 for the HCl and HF channels, respectively. The product branching ratios were 10.3 +/- 1.9 and 11.8 +/- 1.8 (10.8 +/- 3.8 and 11.6 +/- 1.7) for HCl/HF and DCl/DF, respectively, from CF(3)CFClCH(2)D (CF(3)CFClCHD(2)). The intramolecular kinetic-isotope effects (without correction for reaction path degeneracy) for HCl/DCl and HF/DF elimination from CF(3)CFClCH(2)D (CF(3)CFClCHD(2)) were 2.78 +/- 0.16 and 2.98 +/- 0.12 (0.82 +/- 0.04 and 0.91 +/- 0.03), respectively. Density function theory at the B3PW91/6-311+G(2d,p) and B3PW91/6-31G(d',p') levels was investigated, and the latter was chosen to calculate frequencies and moments of inertia for the molecules and transition states. Rate constants, branching ratios and kinetic-isotope effects then were calculated using RRKM theory with torsional motions treated as hindered internal rotations. Threshold energies for HF and HCl elimination from CF(3)CFClCH(3) were assigned as 61.3 +/- 1.5 and 58.5 +/- 1.5 kcal mol(-1), respectively. The threshold energy for Cl-F interchange was estimated as 67 kcal mol(-1). The difference between the transition states for HCl and HF elimination is discussed.

Ab initio studies of alkyl radical reactions: Combination and disproportionation reactions of CH3 with C2H5, and the decomposition of chemically activated C3H8

This paper reports the first quantitative ab initio prediction of the disproportionation/combination ratio of alkyl+alkyl reactions using CH3+C2H5 as an example. The reaction has been investigated by the modified Gaussian-2 method with variational transition state or Rice-Ramsperger-Kassel-Marcus calculations for several channels producing (1) CH4+CH2CH2, (2) C3H8, (3) CH4CH3CH, (4) H2+CH3CHCH2, (5) H2+CH3CCH3, and (6) C2H6+CH2 by H-abstraction and association/decomposition mechanisms through singlet and triplet potential energy paths. Significantly, the disproportionation reaction (1) producing CH4+C2H4 was found to occur primarily by the lowest energy path via a loose hydrogen-bonding singlet molecular complex, H3CHC2H4, with a 3.5 kcal/mol binding energy and a small decomposition barrier (1.9 kcal/mol), instead of a direct H-abstraction process. Bimolecular reaction rate constants for the formation of the above products have been calculated in the temperature range 300-3000 K. At 1 atm, formation of C3H8 is dominant below 1200 K. Over 1200 K, the disproportionation reaction becomes competitive. The sum of products (3)-(6) accounts for less than 0.3% below 1500 K and it reaches around 1%-4% above 2000 K. The predicted rate constant for the disproportionation reaction with multiple reflections above the complex well, k1=5.04 x T(0.41) exp(429/T) at 200-600 K and k1=1.96 x 10(-20) T(2.45) exp(1470/T) cm3 molecule(-1) s(-1) at 600-3000 K, agrees closely with experimental values. Similarly, the predicted high-pressure rate constants for the combination reaction forming C3H8 and its reverse dissociation reaction in the temperature range 300-3000 K, k2(infinity)=2.41 x 10(-10) T(-0.34) exp(259/T) cm3 molecule(-1) s(-1) and k(-2)(infinity)=8.89 x 10(22) T(-1.67)exp(-46 037/T) s(-1), respectively, are also in good agreement with available experimental data.