Enthalpy and Entropy Barriers Explain the Effects of Topology on the Kinetics of Zeolite‐Catalyzed Reactions

The methylation of ethene, propene, and trans‐2‐butene on zeolites H‐ZSM‐58 (DDR), H‐ZSM‐22 (TON), and H‐ZSM‐5 (MFI) is studied to elucidate the particular influence of topology on the kinetics of zeolite‐catalyzed reactions. H‐ZSM‐58 and H‐ZSM‐22 are found to display overall lower methylation rates compared to H‐ZSM‐5 and also different trends in methylation rates with increasing alkene size. These variations may be rationalized based on a decomposition of the free‐energy barriers into enthalpic and entropic contributions, which reveals that the lower methylation rates on H‐ZSM‐58 and H‐ZSM‐22 have virtually opposite reasons. On H‐ZSM‐58, the lower methylation rates are caused by higher enthalpy barriers, owing to inefficient stabilization of the reaction intermediates in the large cage‐like pores. On the other hand, on H‐ZSM‐22, the methylation rates mostly suffer from higher entropy barriers, because excessive entropy losses are incurred inside the narrow‐channel structure. These results show that the kinetics of crucial elementary steps hinge on the balance between proper stabilization of the reaction intermediates inside the zeolite pores and the resulting entropy losses. These fundamental insights into their inner workings are indispensable for ultimately selecting or designing better zeolite catalysts.     

Theoretical study on the mechanism of the reaction acetone + OH: channels towards the formation of methanol

Two possible pathways for the acetone + OH reaction towards the formation of methanol have been examined theoretically. Our results show that both channels are characterized by a substantial activation barrier and reject the possibility of a significant CH₃CO + CH₃OH channel. This revised version was published online in August 2006 with corrections to the Cover Date.     

Local structure of highly dispersed lead species incorporated within zeolite: experimental and theoretical studies

XAFS (both XANES and FT-EXAFS) measurements revealed that the Pb²⁺ /ZSM-5 catalyst prepared from precursor H-ZSM-5 by a conventional ion-exchange method includes a highly dispersed 3-fold coordinated Pb²⁺ ion species within the zeolite framework. UV-irradiation of Pb²⁺ /ZSM-5 led to effective decomposition of NO and N₂O producing N₂. The photocatalytic decomposition of NO is found to be slightly preferable than that of N₂O. The isolated Pb²⁺ ions play a significant role in the decomposition of pollutant NO _x . Ab initio and DFT quantum chemical studies at the HF/Lanl2dz and B3PW91/Lanl2dz levels further shed light on local structures of the Pb²⁺ active site of lead-containing zeolites, as well as on their interactions with pollutant NO and N₂O molecules. In agreement with experiments, 3-fold coordination was found to be the most favorable state for the Pb²⁺ site within the zeolite framework.     

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.     

Theoretical insights into the mechanism for thiol/disulfide exchange

The mechanism for thiol/disulfide exchange has been studied with high-level theoretical calculations. Free energies, transition structures, charge densities, and solvent effects along the reaction pathway have been determined for the first time. Mechanistic results agree with experimental data, and support the idea that the thiolate is the reacting species and that the reaction indeed proceeds through an uncomplicated S(N)2 transition state. The transition structures have the charge density evenly concentrated in the attacking and leaving sulfur atoms. The charge densities allow us to rationalize the solvent effects. As transition structures have the charge density more widely distributed than reactants, hydrophobic environments catalyze the reaction. The effect can be so dramatic that disulfide exchange inside the active site of ribonucleotide reductase is estimated to be catalyzed 10(3) times faster than the reaction in water. It was also found that attack by thiol is much faster than previously assumed, if mediated through water chains. Although the present results, as well as experimental data, still suggest that thiolate is the main reaction species, water-mediated thiol attack is almost kinetically competitive, and can eventually become competitive under specific experimental conditions.     

Theoretical investigation of carbon-sulfur triple bonds

By means of first principle calculations we have investigated a set of molecules that are presumed to contain carbon-sulfur triple bonds, namely HCSOH, H(3)SCH, cis-FCSF, F(3)CCSF(3), and F(5)SCSF(3). For HCSOH, FCSF, and H(3)SCH we used the CCSD(T) methodology and the correlation-consistent basis sets. On the other hand, F(3)CCSF(3) and F(5)SCSF(3) were studied at the B3LYP, M06-2X, MP2, and G3 levels of theory. We found that none of these molecules display a carbon-sulfur adiabatic bond dissociation energy (ABDE) as strong as diatomic CS (170.5 kcal mol(-1)), or a diabatic bond dissociation energy (DBDE) larger than the one found in SCO (212.0 kcal mol(-1)), although the DBDE of FCSF comes quite close at 208.3 kcal mol(-1). The CS ABDEs of F(3)CCSF(3), F(5)SCSF(3), and H(3)SCH are comparable to that of a single C-S bond. In contrast with the experimental results, F(3)CCSF(3) and F(5)SCSF(3) are predicted to be linear with C(3v) and C(s) symmetry, respectively, at the B3LYP/6-311+G(3df,2p) level. MP2/6-311+G(2df,2p) calculations support the C(3v) symmetry for F(3)CCSF(3), despite F(5)SCSF(3) not having a perfect linear structure; the CSC angle is 174.6°, which is nearly 20° larger than the experimental value. The analysis of the carbene structures of HCSOH and H(3)SCH revealed that they are not significant, because the triplet state is dissociative in these cases. However, for F(3)CCSF(3) and F(5)SCSF(3) , the carbene triplet states lie 0.81 and 0.77 eV above the singlet state, respectively. In the same vein, our investigation supports the presence of a strong double bond for HCSOH. The conflicting evidence available for F(3)CCSF(3) and F(5)SCSF(3) makes it very difficult to determine the nature of the CS bonds. However, the bond dissociation energies and the singlet-triplet splittings clearly suggest that these compounds should be considered as masked sulfinylcarbenes. The analysis of the bond dissociation energies challenges the existence of a triple bond in these five molecules, but from a strictly thermodynamic standpoint, cis-FCSF is found to be the candidate most likely to exhibit triple-bond character.     

Theoretical investigations on the geometric and electronic structures of phenylene-acetylene macrocycles.

The geometric and electronic structures of a series of conjugated macrocycles (phenylene-acetylene macrocycles, PAMs) have been studied theoretically with ab initio and semiempirical molecular orbital methods. The ab initio calculations at the HF/6-31G* level demonstrate that the model molecules may have a planar conformation. Bigger macrocycles, for example, 7PAM, 8PAM, and 9PAM, result in several energy minima. The boatlike conformation is the most energetically favored form. Based on the conformational analysis, a novel method for analyzing the ring-strain energy was proposed and used. In view of their potential applications as electronic materials, the electronic structures of a series of PAMs are also investigated. The LUMO-HOMO gaps of the planar PAMs show an odd-even difference behavior. In addition, the HOMOs of the planar species 3PAM, 5PAM, 7PAM, and 9PAM are doubly degenerated.     

Theoretical Investigation of the OH.‐Initiated Oxidation of Benzaldehyde in the Troposphere

A mechanistic and kinetic study of the OH^.‐initiated oxidation of benzaldehyde is carried out using quantum chemical methods and classical transition state theory. We calculate the rate constant for this reaction within the temperature range of 200–350 K at atmospheric pressure. All possible hydrogen abstraction and OH^. addition channels are considered and branching ratios are obtained. Tunneling corrections are taken into account for abstraction channels, assuming unsymmetrical Eckart barriers. The aldehydic abstraction is by far the most important reaction channel within the entire range of temperatures studied, especially at room temperature and lower—the temperatures relevant to atmospheric chemistry. The relative importance of all the other possible channels increases slightly with temperature. Branching ratios show that addition at the ring and abstraction of an ortho hydrogen contribute about 1 % each at about 300 K, while the branching ratio for the main reaction decreases from 99 % at 200 K to 93 % at 350 K. The results are compared with available experimental measurements.     

Theoretical and photoluminescence studies on the d10-s2 AuI-TlI interaction in extended unsupported chains

The reactions of solutions of TlPF(6) and OPPh(3) in tetrahydrofuran or acetone with NBu(4)[AuR(2)] (R=C(6)Cl(5), C(6)F(5)) gave the new complexes [Au(C(6)Cl(5))(2)](2)[Tl(OPPh(3))][Tl(OPPh(3))(L)] (L=THF (1), acetone (2)) and the previously reported [Tl(OPPh(3))(2)][Au(C(6)F(5))(2)] (3). The crystal structures of complexes 1 and 2 display extended unsupported chains with short intermolecular interactions between alternating gold(I) and thallium(I) centres. Moreover, the Tl(I) centres show two different types of geometrical environments, such as pseudotetrahedral and distorted trigonal-bipyramidal, due to the presence of solvent molecules that act as ligands in the solid-state structure. Quasirelativistic and nonrelativistic ab initio calculations were performed to study the nature of the intermetallic Au(I)-Tl(I) interactions and are consistent with the presence of a high ionic contribution (80 %) and dispersion-type (van der Waals) interaction with a charge-transfer contribution (20 %) when relativistic effects are taken into account. All complexes are luminescent in the solid state at room temperature and at 77 K. Complexes 1 and 2 show site-selective excitation, probably due to the different environments around the Tl(I) centres. The DFT and time-dependent (TD)-DFT calculations are in agreement with the experimental excitation spectra for all complexes and confirm the site-selective excitation behaviour as a function of the Tl(I) geometrical environment.     

IRC analysis of methanol carbonylation reaction catalyzed by rhodium complex

In the reaction cycle for methanol carbonylation catalyzed by Rh complex, the structure geometries of the reactant, intermediates, transition states and product of each elemental reaction have been studied by using the energy gradient method at HF/LANL2DZ level, and the changes of their potential profiles have also been calculated. Through IRC analyses of the transition states for each elemental reaction, it is confirmed that the various structure geometries obtained are stationary points on the cycle reaction pathway of methanol carbonylation catalyzed by Rh complex, and the changes are given in energies and structure geometries of the reactant molecules along the reaction pathway of lowest energy. It has been proposed that the geometrical conversions of intermediates play an important role during the cycle reaction. Through analyses of structure geometries, it has been suggested that, in addition to cis- and trans-structure exchange linkage of catalysis reactive species, the two pathways, cis- and tra     

Ab initio study of the topology of the charge distribution of H3SiO(H)AlH3 conformers

The topologic properties of the electronic charge distribution of conformers of H₃SiO(H)AlH₃ molecule hydroxyl groups of zeolites are reported. The studied properties—total density, Laplacian density, and bond ellipticity—were evaluated at the position of the critical points of the O Si, O Al, and O H bonds, by using Hartree–Fock and second‐order Møller–Plesset levels of theory, and the STO/6‐31+G(d,p) standard basis set. For the H₃SiO(H)AlH₃ molecule, four conformers are identified. It is demonstrated that for these conformers, the total density and Laplacian density remain almost constant by effect of the rotations of the T H bonds, T=(Si, Al), around the corresponding O T bonds, respectively. However, these rotations induce sensible variations in the ellipticity at the position of the critical point of the O Al bonds, which are reflected in the OH bond distance, OH vibrational mode, and the stabilization energy of conformers. These results lead to a linear relationship between the magnitude of the bond ellipticity at the critical point of the O Al bonds and the frequency values of the OH bonds, with a correlation coefficient of r²=0.98. In addition, a good linear relationship between the ellipticity of the O Al bond and the pattern of the stabilization energy of conformers was also found. © 1999 John Wiley & Sons, Inc. Int J Quant Chem 76: 1–9, 2000     

Discrimination of Glycine Enantiomers by the Biologically Important Phosphocompounds:A Theoretical Prediction

With the aid of high-level B3LYP and MP2 calculations, three new neutral structures of glycine （iin, ivn and vn, see Fig. 2） were obtained and validated by frequency calculations. The structural and energetic analyses showed that iin, ivn and vn are enantiomers to the previous IIn, IVn and Vn （J. Am. Chem. Soc. 1992, 114, 9568.）, respectively. Owing to the presence of these novel conformers, a redistribution of the populations of glycine conformers is resulted in and causes the remarkable decrease of the most stabilized Ip （from 48% to 38%）. It indicated that the simple glycine molecule can show chirality under certain conditions. The interacting modes of glycine enantiomeric pairs （e.g., ivn and IVn） with PG showed large differences （Fig. 4）; in addition, their interaction energies corrected with basis set superposition errors （BSSE） were calculated to be --66.81 and -46.99 kJ tool^-1, respectively. Accordingly, the glycine enantiomers can be potentially applied to the chiral recognition in biological and pharmaceutical areas.     

Theoretical Study on the Complexes of Benzene with Isoelectronic Nitrogen‐Containing Heterocycles

The π–π interactions between benzene and the aromatic nitrogen heterocycles pyridine, pyrimidine, 1,3,5‐triazine, 1,2,3‐triazine, 1,2,4,5‐tetrazine, and 1,2,3,4,5‐pentazine are systematically investigated. The T‐shaped structures of all complexes studied exhibit a contraction of the C? H bond accompanied by a rather large blue shift (40–52 cm^?1) of its stretching frequency, and they are almost isoenergetic with the corresponding displaced‐parallel structures at reliable levels of theory. With increasing number of nitrogen atoms in the heterocycle, the geometries, frequencies, energies, percentage of s character at C, and the electron density in the C? H σ antibonding orbital of the complexes all increase or decrease systematically. Decomposition analysis of the total binding energy showed that for all the complexes, the dispersion energy is the dominant attractive contribution, and a rather large attraction originating from electrostatic contribution is compensated by its exchange counterpart.