A theoretical investigation into thiophenic derivative cracking mechanism over acidic and cation-exchanged beta zeolites

Bimolecular and monomolecular cracking mechanisms of alkanes simultaneously occur and have a competitive relationship, which strongly influences the product distribution. In this work, the density functional theory (DFT) calculation is first carried out to elucidate two cracking mechanisms in HZSM-5 and HY zeolites. It is found that the overall apparent reaction barrier for the monomolecular cracking reaction at 750 K in the HZSM-5 zeolite is 5.30 kcal/mol, much lower than that (23.12 kcal/mol) for bimolecular cracking reaction, indicating that the monomolecular mechanism is predominant in the HZSM-5 zeolite. In contrast, the bimolecular mechanism is predominant in the HY zeolite because of a lower apparent reaction barrier energy barrier (6.95 kcal/mol) for bimolecular cracking reaction than that (24.34 kcal/mol) for the monomolecular cracking reaction. Moreover, the intrinsic reason for the different mechanisms is further elucidated. The confinement effect can effectively decrease the energy barrier whe...

Apparent inhibition of thermal decomposition of hydrogencarbonate ion by poly(acrylic acid). The effect of molar mass and end-group functionality

While the 1,5-sigmatropic hydrogen shift in cyclopentadiene is generally thought to be a unimolecular pericyclic reaction, Yamabe proposed a more complex bimolecular mechanism proceeding through the exo dimer of cyclopentadiene. DFT computations by Yamabe were claimed to show that the bimolecular mechanism was kinetically more favorable than the unimolecular mechanism. Reinvestigation of the unimolecular concerted mechanism and Yamabe's bimolecular mechanism with ωB97X-D and DLPNO-CCSD(T) calculations demonstrates a 25 kcal/mol preference for the unimolecular mechanism relative to the bimolecular mechanism. While Yamabe's calculations were performed with the less accurate B3LYP functional, the incorrect conclusion was the result of a different error discovered here. We have also computed corrections for tunneling that result in computed activation barriers within 1.5 kcal/mol of the experimental values.

The rearrangement of fulvene to benzene is believed to play an important role in the formation of soot during hydrocarbon combustion. Previous work has identified two possible mechanisms for the rearrangement─a unimolecular path and a hydrogen-atom-assisted, bimolecular path. Computational results to date have suggested that the unimolecular mechanism faces a barrier of about 74 kcal/mol, which makes it unable to compete with the bimolecular mechanism under typical combustion conditions. This computed barrier is about 10 kcal/mol higher than the experimental value, which is an unusually large discrepancy for modern electronic structure theory. In the present work, we have reinvestigated the unimolecular mechanism computationally, and we have found a second transition state that is approximately 10 kcal/mol lower in energy than the previously identified one and, therefore, in excellent agreement with the experimental value. The existence of two transition states for the same rearrangement arises because there is a conical intersection between the two lowest singlet states which occurs in the vicinity of the reaction coordinates. The two possible paths around the cone on the lower adiabatic surface give rise to the two distinct saddle points. The lower barrier for the unimolecular mechanism now makes it competitive with the bimolecular one, according to our calculations. In support of this conclusion, we have reanalyzed some previous experimental results on anisole pyrolysis, which leads to benzene as a significant product and have shown that the unimolecular and bimolecular mechanisms for fulvene → benzene must be occurring competitively in that system. Finally, we have identified that similar conical intersections arise during the isomerizations of benzofulvene and isobenzofulvene to naphthalene.

A two-layer ONIOM study on the hydrodesulfurization mechanism of thiophene in H-FAU and M-FAU (M = Li(+), Na(+), and K(+)) has been carried out. The calculated results reveal that in H-FAU, for a unimolecular mechanism, the rate-determining step is hydrogenation of alkoxide intermediate. The assistance of H2O and H2S molecules does not reduce the difficulty of the C-S bond cracking step more effectively. A bimolecular hydrodesulfurization mechanism is more favorable due to the lower activation barriers. The rate-determining step is the formation of 2-methylthiophene, not the C-S bond cracking of thiophene. Moreover, the ring opening of thiophene is much easier to occur than the desulfurization step. A careful analysis of energetics indicates that H2S, propene, and methyl thiophene are the major products for the hydrodesulfurization process of thiophene over H-FAU zeolite, in good agreement with experimental findings. In M-FAU zeolites, both unimolecular and bimolecular cracking processes are difficult to occur because of the high energy barriers. Compared to the case on H-FAU, the metal cations on M-FAU increase the difficulty of occurrence of bimolecular polymerization and subsequent C-S bond cracking steps. Graphical abstract Hydrodesulfurization process of thiophene can take place in H-FAU zeolite. Two different mechanisms, unimolecular and bimolecular ones, have been proposed and evaluated in detail. The bimolecular mechanism is more favorable due to lower activation barrier as described in the picture above. Our calculated data indicate that H2S, propene, and methylthiophene are the major products, in good agreement with experimental observations. The effect of metal cations on the reaction mechanism is also investigated in this work.

Scorpion primers can be used to detect PCR products in homogeneous solution. Their structure promotes a unimolecular probing mechanism. We compare their performance with that of the same probe sequence forced to act in a bimolecular manner. The data suggest that Scorpions indeed probe by a unimolecular mechanism which is faster and more efficient than the bimolecular mechanism. This mechanism is not dependent on enzymatic cleavage of the probe. A direct comparison between Scorpions, TaqMan and Molecular Beacons on a Roche LightCycler indicates that Scorpions perform better, particularly under fast cycling conditions. Development of a cystic fibrosis mutation detection assay shows that Scorpion primers are selective enough to detect single base mutations and give good sensitivity in all cases. Simultaneous detection of both normal and mutant alleles in a single reaction is possible by combining two Scorpions in a multiplex reaction. Such favourable properties of Scorpion primers should make the technology ideal in numerous applications.

Catalytic decomposition of formic acid on oxide catalysts: III. IOM model approach to bimolecular mechanism

Dehydration reactions are important in the petroleum and petrochemical industries, especially for the feedstock production. In this work, the catalytic activity of zeolites with different acidities for the dehydration of ethanol to ethylene and diethylether is investigated by density functional calculations on cluster models of three isomorphous B, Al, and Ga substituted H-ZSM-5 zeolites. Both unimolecular and bimolecular mechanisms are investigated. Detailed reaction profiles for the dehydration reaction, assuming either a stepwise or a concerted mechanism, were calculated by using the ONIOM(MP2:M06-2X) + SCREEP method. The adsorption energies of ethanol are -21.6, -28.1, and -27.7 kcal mol-1 on H-[B]-ZSM-5, H-[Al]-ZSM-5, and H-[Ga]-ZSM-5 zeolites, respectively. The activation energies for the rate-determining step of the unimolecular concerted mechanism for the ethylene formation are 48.5, 42.6, and 43.6 kcal mol-1 on H-[B]-ZSM-5, H-[Al]-ZSM-5, and H-[Ga]-ZSM-5 zeolites, respectively. The activation energies for the ethoxy formation as the rate-determining step for the bimolecular formation of diethylether are 42.3, 40.0, and 41.1 kcal mol-1 on H-[B]-ZSM-5, H-[Al]-ZSM-5, and H-[Ga]-ZSM-5 zeolites, respectively. The results indicate that the catalytic activities for the dehydration of ethanol decrease in the order H-[Al]-ZSM-5 ~ H-[Ga]-ZSM-5 > H-[B]-ZSM-5. Besides the acid strength, the zeolite framework affects the reaction by stabilizing the reaction intermediates, leading to more stable adsorption complexes and lower activation barriers.

Affinity alkylation of human placental 3β -hydroxy-5-ENE-steroid dehydrogenase and steroid 5→4-ene-isomerase by 2α-bromoacetoxyprogesterone: Evidence for separate dehydrogenase and isomerase sites on one protein

REACTIONS of organic halides in solution which involve substitution by a nucleophilic reagent at a saturated carbon atom have been extensively studied by Hughes, Ingold and co-workers1. These authors conclude that reactions of this type may occur by two possible mechanisms, a unimolecular mechanism or a bimolecular mechanism . The increase in the unimolecular reaction-rate of the halide R – X as R varies along the series methyl, ethyl, sec.-propyl, tert.butyl, is attributed by these authors to the increase in electron accession to the reaction centre2. The decrease in the bimolecular reaction-rate along this series3 is also attributed to the increase in electron accession to the reaction centre ; this electron accession is assumed to inhibit the approach of the nucleophilic reagent.

: Isotope effects have been determined for (1) the decarbonylation of mesitaldehyde vs. deuteromesitaldehyde (MesCDO) in 60 to 100% sulfuric acid, and (2) the decarbonylation of mesitaldehyde in 60 to 100% sulfuric vs. deuterosulfuric acid. The observed isotope effects are not explained by a termolecular mechanism. With certain reasonable assumptions, a bimolecular mechanism of successive proton transfer steps, equations 2 and 3, is consistent with the isotope effects. All previous observations are explainable on the basis of the bimolecular mechanism, including (1) the shapes of the plots of k(obsd) vs. % sulfuric acid for 2,4,6-trimethyl-, triethyl- and triisopropylbenzaldehyde, and (2) the fact that for the mesitaldehyde decarbonylation, the change in k(obsd) with sulfuric acid percentage is in apparent agreement with the unimolecular mechanism, 4, over part of the acid range. (Author)

Interactions in Monoalkylbenzenes Disproportionation among Zeolite Characteristics and Reaction Mechanisms

Catalytic arene alkylation over H-Beta zeolite: Influence of zeolite shape selectivity and reactant nucleophilicity

Mechanisms of Heptane Isomerization on Bifunctional Pd/H-Beta Zeolites

1,2-trans-Glycosides hydrolyze through different mechanisms at different pH values, but systematic studies are lacking. Here, we report the pH-rate constant profile for the hydrolysis of 4-nitrophenyl β-D-glucoside. An inverse kinetic isotope effect of k(H3O+)/k(D3O+) = 0.65 in the acidic region indicates that the mechanism requires the formation of the conjugate acid of the substrate for the reaction to proceed, with the heterolytic cleavage of the glycosidic C-O bond. Reactions in the pH-independent region exhibit general catalysis with a single proton in flight, a normal solvent isotope effect of kH/kD = 1.5, and when extrapolated to zero buffer concentration show a small solvent isotope effect of k(H2O)/k(D2O) = 1.1, consistent with water attack through a dissociative mechanism. In the basic region, solvolysis in 18O-labeled water and H2O/MeOH mixtures allowed the detection of bimolecular hydrolysis and neighboring group participation, with a minor contribution of nucleophilic aromatic substitution. Under mildly basic conditions, a bimolecular concerted mechanism is implicated through an inverse solvent isotope effect of k(HO-)/k(DO-) = 0.5 and a strongly negative entropy of activation (ΔS‡ = -13.6 cal mol-1 K-1). Finally, at high pH, an inverse solvent isotope effect of k(HO-)/k(DO-) = 0.5 indicates that the formation of 1,2-anhydrosugar is the rate-determining step.