A Topological Study of the Transition States of the Hydrogen Exchange and Dehydrogenation Reactions of Methane on a Zeolite Cluster

Topological Analysis of the Electronic Charge Density in the Ethene Protonation Reaction Catalyzed by Acidic Zeolite

In the present work, the distribution of the electronic charge density in the ethene protonation reaction by a zeolite acid site is studied within the framework of the density functional theory and the atoms in molecules (AIM) theory. The key electronic effects such as topological distribution of the charge density involved in the reaction are presented and discussed. The results are obtained at B3LYP/6-31G(**) level theory. Attention is focused on topological parameters such as electron density, its Laplacian, kinetic energy density, potential energy density, and electronic energy density at the bond critical points (BCP) in all bonds involved in the interaction zone, in the reactants, pi-complex, transition state, and alkoxy product. In addition, the topological atomic properties are determined on the selected atoms in the course of the reaction (average electron population, N(Omega), atomic net charge, q(Omega), atomic energy, E(Omega), atomic volume, v(Omega), and first moment of the atomic charge distribution, M(Omega)) and their changes are analyzed exhaustively. The topological study clearly shows that the ethene interaction with the acid site of the zeolite cluster, T5-OH, in the ethene adsorbed, is dominated by a strong O-H...pi interaction with some degree of covalence. AIM analysis based on DFT calculation for the transition state (TS) shows that the hydrogen atom from the acid site in the zeolitic fragment is connected to the carbon atom by a covalent bond with some contribution of electrostatic interaction and to the oxygen atom by closed shell interaction with some contribution of covalent character. The C-O bond formed in the alkoxy product can be defined as a weaker shared interaction. Our results show that in the transition state, the dominant interactions are partially electrostatic and partially covalent in nature, in which the covalent contribution increases as the concentration and accumulation of the charge density along the bond path between the nuclei linked increases.

Reactivity of Alkanes on Zeolites: A Computational Study of Propane Conversion Reactions

In this work, quantum chemical methods were used to study propane conversion reactions on zeolites; these reactions included protolytic cracking, primary hydrogen exchange, secondary hydrogen exchange, and dehydrogenation reactions. The reactants, products, and transition-state structures were optimized at the B3LYP/6-31G level and the energies were calculated with CBS-QB3, a complete basis set composite energy method. The computed activation barriers were 62.1 and 62.6 kcal/mol for protolytic cracking through two different transition states, 30.4 kcal/mol for primary hydrogen exchange, 29.8 kcal/mol for secondary hydrogen exchange, and 76.7 kcal/mol for dehydrogenation reactions. The effects of basis set for the geometry optimization and zeolite acidity on the reaction barriers were also investigated. Adding extra polarization and diffuse functions for the geometry optimization did not affect the activation barriers obtained with the composite energy method. The largest difference in calculated activation barriers is within 1 kcal/mol. Reaction activation barriers do change as zeolite acidity changes, however. Linear relationships were found between activation barriers and zeolite deprotonation energies. Analytical expressions for each reaction were proposed so that accurate activation barriers can be obtained when using different zeolites as catalysts, as long as the deprotonation energies are first acquired.

C-H···H-C interactions in organoammonium tetraphenylborates: another look at dihydrogen bonds

The crystal structures of the tetraphenylborates of the dabcoH+, guanidinium (MeCN solvate), and biguanidinium cations are shown to contain a variety of C-H···H-C dihydrogen (DB) bonds of nominally zero polarity, as well as a variety of N-H···N, C-H···N, N-H···Ph, and C-H···Ph hydrogen (HB) bonds. These intermolecular bonds have been characterized topologically after multipole refinement of the structures. The coexistence of the DBs and HBs in each of the structures makes it possible to establish their relative strength hierarchy. It also illustrates the importance of the DBs in satisfying the tendency of these structures to maximize the total intermolecular bonding engagement. To compare the above DBs with other DBs, the results of an extensive set of MP2/6-31G(d,p) calculations (supplied by I. Alkorta) were analyzed for reference correlations between the bond-critical parameters. Thus, for an X-H···H-Y bond, the difference Δε(H)m between the Mulliken charges on the H atoms in the uncomplexed X-H and H-Y components correlates quite well with the X-H···H-Y parameters and can be used for predicting the topological strength of an X-H···H-Y bond. The use of the difference Δε(H)c in the bond does not appear to change the correlation significantly; closer correlations are observed when the amount of charge transferred on formation of the H···H bond is used instead of Δε(H)m or Δε(H)c. Bonding interactions are obtained even between like or symmetry-related H atoms as a consequence of induced-dipole interactions, which accounts for the existence of the above intermolecular C-H···H-C bonds with d(H···H) = 2.18–2.57 Å, electron density at the bond-critical point of ~0.05–0.08 e/Å3, and a rough estimate of the H···H binding energy of ~1-5 kcal/mol. Examination of the bond-critical parameters of X-H···H-Y bonds also suggests a criterion of stability of these bonds with respect to the transition from non-shared (closed-shell) X-H···H-Y interaction to covalent (shared-shell) X···H-H···Y interaction. This transition appears to be discontinuous.Key words: bond-critical parameters, bond topology, dihydrogen bonds, hydrogen bonds, organoammonium tetraphenylborates.