Glycine Peptide Bond Formation Catalyzed by Faujasite

The catalysis of peptide bond formation between two glycine molecules on H‐FAU zeolite was computationally studied by the M08‐HX density functional. Two reaction pathways, the concerted and the stepwise mechanism, starting from three differently adsorbed reactants, amino‐bound, carboxyl‐bound, and hydroxyl‐bound, are studied. Adsorption energies, activation energies, and reaction energies, as well as the corresponding intrinsic rate constants were calculated. A comparison of the computed energetics of the various reaction paths for glycine indicates that the catalyzed reaction proceeds preferentially via the concerted reaction mechanism of the hydroxyl‐bound configuration. This involves an eight‐membered ring of the transition structure instead of the four‐membered ring of the others. The step from the amino‐bound configuration to glycylglycine is the rate‐determining step of the concerted mechanism. It has an estimated activation energy of 51.2 kcal mol^?1. Although the catalytic reaction can also occur via the stepwise reaction mechanism, this path is not favored.     

Structures and reaction mechanisms of propene oxide isomerization on H-ZSM-5: an ONIOM study

The isomerization mechanisms of propene oxide over H-ZSM-5 zeolite have been investigated via the utilization of 5T and 46T cluster models calculated by the B3LYP/6-31G(d,p) and the ONIOM(B3LYP/6-31G(d,p):UFF) methods, respectively. The reactions are considered to proceed through a stepwise mechanism: (1) the epoxide ring protonation, and concurrently the ring-opening, and (2) the 1,2-hydride shift forming the adsorbed carbonyl compound. Because of the asymmetric structure of propene oxide, two different C-O bonds (more or less substituted carbon atom sides) can be broken leading to two different types of products, propanal and propanone. The ring-opening step of these mechanisms is found to be the rate-determining step with an activation barrier of 38.5 kcal/mol for the propanal and of 42.4 kcal/mol for the propanone. Therefore, the propanal is predicted to be the main product for this reaction.     

Investigation of ethylene dimerization over faujasite zeolite by the ONIOM method.

Ethylene dimerization was investigated by using an 84T cluster of faujasite zeolite modeled by the ONIOM3(MP2/6-311++G(d,p):HF/6-31G(d):UFF) method. Concerted and stepwise mechanisms were evaluated. In the stepwise mechanism, the reaction proceeds by protonation of ethylene to form the surface ethoxide and then C--C bond formation between the ethoxide and the second ethylene molecule to give the butoxide product. The first step is rate-determining and has an activation barrier of 30.06 kcal mol(-1). The ethoxide intermediate is rather reactive and readily reacts with another ethylene molecule with a smaller activation energy of 28.87 kcal mol(-1). In the concerted mechanism, the reaction occurs in one step of simultaneous protonation and C--C bond formation. The activation barrier is calculated to be 38.08 kcal mol(-1). Therefore, the stepwise mechanism should dominate in ethylene dimerization.