Quantum-Chemical Investigations

METHANOL CATALYTIC OXIDATION

I.V. Malakhova, A.A. Shubin

Kinet. Catal., 50(4) (2009) pp. 583-586.

The quantum-chemical calculations of the hydroxymethyl radical •CH₂OH were performed for the first time and a theoretical EPR spectrum of this radical was constructed. The formation of the hydroxymethyl radical in the reaction of methanol oxidation is thermodynamically favourable. The shape and parameters of the constructed spectrum differed from those for radicals experimentally detected in the catalytic oxidation of methanol using the matrix isolation method. However, they are consistent with the spectrum ascribed to the EPR spectrum of •CH₂OH observed in the direct photolysis of methanol. This result allows one to refine the identification of the nature of radicals formed in the catalytic reaction of methanol oxidation.

NITROSONIUM NITRITE ISOMER OF N₂O₃: QUANTUM-CHEMICAL DATA

I.I. Zakharov*,**, O.I. Zakharova**

(*The Boreskov Institute of Catalysis, Novosibirsk , Russia;
**Severodonetsk Technological Institute, V. Dal West Ukrainian National University, Severodonetsk, Ukraine)

J. Struct. Chem., 50(2) (2009) pp. 212-218.

The geometrical, electronic, and thermodynamic parameters of three known isomers of dinitrogen trioxide N₂O₃ were calculated by the density functional theory DFT/B3LYP method using the 6-311++G(3df) basis. The structure of the new isomer, NONO₂, was calculated. From the calculation of vibrational frequencies it follows that the structure of NONO₂ has a local potential energy minimum and corresponds to the stationary state of the N₂O₃ isomer. The molecular structure of NONO₂ is characterized by a substantial negative charge on the NO₂ fragment and positive charge on the NO fragment. The electronic structure of the NO⁺NO₂^- isomer can be characterized as nitrosonium nitrite, which can be oxidized to nitrite and participate in nitrosylation in accordance with the biogenic characteristics of the NOx intermediate, assumed to be formed in biological systems during the oxidation of NO.

EFFECT OF MASS TRANSFER ON THE REACTION RATE IN A MONOLITHIC CATALYST WITH POROUS WALLS

O.P. Klenov, S.A. Pokrovskaya, N.A. Chumakova, S.N. Pavlova, V.A. Sadykov, A.S. Noskov

Catal. Today, 144(3-4) (2009) pp. 258-264.

The influence of reacting gas flow on the heterogeneous reaction in catalyst volume of honeycomb porous monolith with triangular channels was studied. The spatial distributions of the reacting gas flow, the rates of local mass transfer between channel walls and gas flow, as well as the interaction of transfer processes and a catalytic reaction were determined on the basis of the computational fluid dynamics. The reactions of deep oxidation and steam reforming of methane were considered as model reactions.
It was shown that there is no stabilization of the reacting flow over the whole channel length under studied conditions. The most intensive changing of the gas streams appears near the channel inlet, which causes the highest local rates of interphase exchange processes, the rate difference is up to two orders of magnitude. A higher reaction rate exists in the initial part due to penetration of feed components into the catalyst volume through the frontal surface. This leads to increasing the effectiveness factor whose value is above unity near the channel inlet. The reaction rate limitation by the transport of reagents inside porous wall was observed along the monolith length.

OXIDATION OF FORMALDEHYDE TO FORMIC ACID OVER V₂O₅/TiO₂ CATALYSTS: A DFT ANALYSIS OF THE MOLECULAR REACTION MECHANISMS

V.I. Avdeev, V.N. Parmon

J. Phys. Chem.,
113(7) (2009) pp. 2873-2880.

Oxidation of formaldehyde to formic acid on the surface of titania-supported vanadia is studied with density functional theory (DFT) and a dioxo-vanadyl (O=V-O-V=O) catalyst model. Two probable mechanisms (a redox Mars-van Krevelen mechanism and an associative mechanism) are considered and transition states and intermediates along these reaction pathways are investigated. The key intermediate of the redox mechanism is a dioxymethylene complex. Its successive transformation leads to a stable surface formate complex. The subsequent transformation of formate to formic acid can occur only in the presence of dioxygen. The key intermediate of the associative mechanism is a peroxo-oxo-methylene complex. Its successive transformation leads to the adsorbed formic acid. On the basis of the calculated activation barriers, the associative mechanism appears to be more probable.

ELECTRONIC STRUCTURE AND STABILITY OF PEROXIDE DIVANADATE SPECIES V(O−O) ON THE TiO₂(001)

V.I. Avdeev, V.M. Tapilin

J. Phys. Chem. C, 113(33) (2009) pp. 14941-14945.

Electronic structure and stability of the VOx species forming on the anatase-TiO₂(001)-(4×1) reconstructed surface at redox cycle V⁵⁺ −O−V⁵⁺ ↔ V⁴⁺ −O−V⁴ have been investigated on the basis of periodic density functional theory calculations. It is shown that oxidizing of the key intermediate of Mars−van Krevelen mechanism [O−V⁴⁺−O−V⁴⁺] by the gas phase dioxygen leads to the substantial structure relaxation of the surface VOx species within a monolayer. As a result, the binding energy of the vanadyl bond V═O decreased and the stable peroxide complex V(O−O) can be formed on the fully oxidized VO_x/TiO₂ surfaces. This complex manifests itself as a peak near Fermi level (−0.23 eV) in the total density of states (DOS) and O-projected density of states (PDOS).

QUANTUM CHEMICAL CALCULATION OF THE CATALYTIC REACTION OF ETHANE DEHYDROGENATION ON GALLIUM OXIDE- HYDROXIDE BINUCLEAR CLUSTERS IN OXIDIZED GaO/ZSM-5 ZEOLITE

I.V. Kuz’min*, G.M. Zhidomirov, V.N. Solkan*, V.B. Kazanskii*
(*Zelinsky Institute of Organic Chemistry, Moscow, Russia)

Kinet. Catal., 50(5) (2009) pp. 752-759.

The catalytic activity of oxidized GaO/HZSM-5 in the reaction of alkane dehydrogenation can be due to hydrogenated gallium oxide clusters stabilized in the cationic positions of the zeolite. The binuclear gallium oxide clusters [Ga₂O₂]²⁺ in oxidized gallium- substituted high-silica zeolite HZSM-5, which are isomeric to two gallyl ions [GaO]⁺ stabilized on two spatially separated lattice aluminum ions, were considered using the DFT method within the framework of a cluster approach. It was found that, even in the case of a relatively large distance between these aluminum ions, gallium oxide particles in oxidized GaO/HZSM-5 can occur as charged planar [Ga₂O₂]²⁺ four-membered rings. These cluster particles exhibited a high affinity to hydrogen, and they were readily hydrogenated with the retention of their structural integrity. It was demonstrated that this partially hydrogenated cluster could be responsible for the catalytic process of ethane dehydrogenation. In the first step, ethane dissociatively added to the [Ga₂O₂H₂]²⁺ cluster. Then, the ethylene molecule was eliminated from the resulting intermediate to leave the [Ga₂O₂H₄] ²⁺ cluster. The cycle was closed by the elimination of a hydrogen molecule with the formation of the initial structure of [Ga₂O₂H₂] ²⁺.

ON THE STABILITY OF COPPER(II) ORGANIC COMPOUNDS WITH THE σ BOND Cu-C: A QUANTUM-CHEMICAL STUDY

E.N. Golubeva*, G.M. Zhidomirov, A.I. Kokorin**
(*Moscow Lomonosov State University, Moscow, Russia; **Semenov Institute of Chemical Physics, Moscow, Russia)

Doklady Chem., 426(2) (2009) pp. 143-145.

In this paper, the results of quantum chemical calculations of the structure and properties of copper(II) organic compounds are reported with the aim to demonstrate the possibility of their existence by the example of Cl_nCu(II)R_1–n, where R is C and C₃, and determine their geometric parameters and the Cu–C bond energy. In addition, it was important to calculate the energies of possible mono- and bimolecular reactions involving Cl_nCu(II)R_1–n and justify the most probable mechanism of Cl_nCu(II)R_1–n transformations.

DFT ANALYSIS OF PROPANE CYCLIZATION OVER BINUCLEAR Ga-CLUSTERS IN MORDENITE

A.V. Larin*, G.M. Zhidomirov, D.N. Trubnikov*, D.P. Vercauteren**
(*Moscow Lomonosov State University, Moscow, Russia; **University of Namur, Namur, Belgium)

J. Mol. Catal. A: Chem., 305(1-2) (2009) pp. 90-94.

Molecular and dissociative adsorption of propane on binuclear (Ga₂O₂)²⁺ cluster located at the cation positions of Ga-exchanged mordenite zeolite are modelled using DFT calculations via the isolated cluster approach. Relative energies of the dissociative intermediates show the most stable bidentate complexes, i.e., via both primary C atoms of propane to both Ga atoms. This structure really is the suitable precursor of cyclo-propane formation. The hypothesis of cyclo-propane intermediate was proposed by Derouane et al. to explain the ¹³C/¹²C exchange migration within propane molecules in Ga-exchanged zeolites.

ION-EXCHANGED BINUCLEAR CLUSTERS AS ACTIVE SITES OF SELECTIVE OXIDATION OVER ZEOLITES

G.M. Zhidomirov, A.V. Larin*, D.N. Trubnikov*, D.P. Vercauteren**
(*Moscow Lomonosov State University, Moscow, Russia; **University of Namur, Namur, Belgium)

J. Phys. Chem., C, 113(19) (2009) pp. 8258-8265.

A new catalytic oxidation cycle over binuclear Zn- and Ca-cation clusters in zeolites is proposed. Intermediate active clusters appear due to trapping of dioxygen. CO oxidation is considered as a model reaction over a cluster located in an 8-membered (8R) ring. Geometries of active clusters involved in the catalytic cycle vary depending on the nature and size of the cation. Reagents, transition states, and products have been optimized at the isolated cluster level considering DFT functionals (B3LYP, B3P86, and B3PW91) with the 6-31G* or 6-311++G** basis sets. Moderate activation energies of 23.0 and 35.7 kcal/mol at the B3LYP/6-31G* level are obtained for CO oxidation over Zn- and Ca-clusters, respectively.

GENERATION OF O― RADICAL ANIONS ON MgO SURFACE: LONG-DISTANCE CHARGE SEPARATION OR HOMOLYTIC DISSOCIATION OF CHEMISORBED WATER?

S.E. Malykhin, A.M. Volodin, A.F. Bedilo, G.M. Zhidomirov

J. Phys. Chem., C, 113(24) (2009) pp. 10350-10353.

O^― radical anions were observed over a partially hydroxylated MgO surface after illumination by monochromatic light with λ = 280 and 303 nm. As all comer oxygen atoms are covered with adsorbed hydroxyl groups after activation at 450°C, this process is initiated by selective excitation of 3-coordinated complexes [Mg²⁺ ―O^―]_3C containing chemisorbed water. A new mechanism for generation of the O^― radical anions is suggested. It is based on homolytic dissociation of chemisorbed water followed by migration of a hydrogen atom, probably, to a different nanoparticle rather than on long-distance separation of charges. The surface structure remaining after its detachment consists of an •OH radical stabilized near the corner oxygen atom. The imminent charge transfer generates a hole at the corner oxygen atom stabilized by a hydroxyl group [OH―···Mg²⁺ ―O^―]_3C. DFT simulation of this structure showed that it reproduces the main characteristics of the radical anions O―_3C. The overall structure is electrically neutral. The charge of the hole is compensated by the nearby hydroxyl group. So, no long-distance charge separation is required.

INTERACTION OF A SINGLE WATER MOLECULE WITH A SINGLE GRAPHITE LAYER: AN INTEGRATED ONIOM STUDY

N.U. Zhanpeisov, G.M. Zhidomirov, H. Fukumura* (*Tohoku University, Japan)

J. Phys. Chem., C, 113(15) (2009) pp. 6118-6123.

Integrated ONIOM calculations using the two- and/or three-layer model chemistries were applied to mimic the interaction of water with the basal graphitic layer of HOPG material. The high-level layer in the ONIOM approach is treated at the electron correlated second-order Moller-Plesset perturbation (MP2/6-31G*) level. On the basis of the results obtained, the formation of structural defect sites, molecular and dissociative adsorption complexes of water on a regular and on structural defect sites, the discrepancy in the nature of the surface O-H and C-H groups appeared on SWNT and HOPG materials has been briefly discussed. It was shown that the dissociation of water is highly endothermic on regular sites of HOPG while the energy needed for dissociation decreases substantially if it proceeds on structural defect sites. This finding points out indirectly that the dissociation probability of water increases on SWNT materials as compared to HOPG because of the presence of a larger number of structural defect sites

MP2 STUDY ON DECOMPOSITION OF NITROUS OXIDE ON THE Ga-ZSM-5

V.N. Solkan*, G.M. Zhidomirov
(*Zelinsky Institute of Organic Chemistry, Moscow, Russia)

Int. J. Quantum Chem., 108(14) (2008) pp. 2732-2743.

The reaction mechanism for nitrous oxide decomposition has been studied on gallium site in Ga-ZSM-5 using the MP2/6-31+G(d) method. The active centers were taken to be mononuclear [Ga]⁺, [Ga═O]⁺, and [GaO₂]⁺ and the surrounding portion of the zeolite was represented by a 3T cluster, namely [AlSi₂O₄H₈]^-. The first elementary step of N₂O decomposition involves the formation of [GaO]⁺ and the release of N₂. The metal-oxo species produced in this step then reacts with N₂O again, to release N₂ and form [GaO₂]⁺. The calculated activation energies at MP2 level for N₂O dissociation on Ga-ZSM-5 and GaO-ZSM-5 are 15.7 and 26.5 kcal/mol at 298 K, respectively. The third elementary step of N₂O decomposition on GaO₂-ZSM-5 involves the formation of [GaO₃]⁺ and the release of N₂. The calculated activation energy at MP2 level for N₂O dissociation on GaO₂-ZSM-5 is 43.7 kcal/mol. Four- order perturbation theory (MP4//MP2) predicts that the activation barriers for nitrous oxide dissociation at 298 K on Ga-ZSM-5, GaO-ZSM-5, and GaO₂-ZSM-5 are 13.9, 13.0, and 34.4 kcal/mol, respectively. The calculated energy for desorption of singlet O₂ from the 3T^-[Ga(O)₃]⁺ cluster at MP2 level is 33.4 kcal/mol. When one takes into account the entropy gained on desorption of singlet O₂, the contribution of entropy to the free energy of desorption is TΔS = 12.3 kcal/mol at 298 K. The calculated energy of the singlet oxygen desorption from 3T^-[OGaO₂]⁺ cluster ΔH (298 K) = +43.57 kcal/mol and ΔG(298 K) = 30.13 kcal/mol is significantly higher than the barriers of the singlet molecular oxygen desorption from 3T^-[Ga(O)₃]⁺ cluster.

SURFACE SPECIES OF TITANIUM(IV) AND TITANIUM(III) IN MgCl₂-SUPPORTED ZIEGLER-NATTA CATALYSTS. A PERIODIC DENSITY FUNCTIONAL THEORY STUDY

D.V. Stukalov, I.L. Zilberberg, V.A. Zakharov

Macromol., 42(21) (2009) pp. 8165-8171.

A systematic consideration of different Ti(IV) and Ti(III) species on the (104) and (110) MgCl₂ surfaces has been implemented within DFT using cyclic boundary conditions. Some new mononuclear and dinuclear surface complexes of Ti(IV) and Ti(III) were obtained due to implication of zip coordination mode. A possible spin state of dinuclear Ti(III) species was thoroughly studied: antiferromagnetic (ESR silent) state proved to be the most preferable in a number of cases. The zip antiferromagnetic Ti₂Cl₆ complexes residing on the dominant (104) MgCl₂ surface make it possible to rationalize the fact that the most part of Ti(III) incorporated in activated MgCl₂ is ESR silent. Besides, these species produce aspecific active sites, thus explaining that aspecific centers significantly prevail over stereospecific one according to kinetic data on the simplest TiCl₄/MgCl₂ + AlR₃ system.

THEORETICAL STUDY OF CARBON SPECIES ON Pd(111): COMPETITION BETWEEN MIGRATION OF C ATOMS TO THE SUBSURFACE INTERLAYER AND FORMATION OF C_N CLUSTERS ON THE SURFACE

S.M. Kozlov, I.V. Yudanov, H.A. Aleksandrov*,**, N. Rösch*
(*Technische Universität München, Garching, Germany;
**University of Sofia, Sofia, Bulgaria)

Phys. Chem. Chem. Phys., 1(46) (2009) pp. 10955-10963.

Subsurface carbon species of Pd catalysts recently attracted considerable attention because they affect the selectivity of hydrogenation reactions. The migration of C atoms from the Pd(111) surface to interstitial subsurface sites was calculated to be energetically favorable. Yet, thermodynamically more stable is a graphene-like phase on the Pd surface. Applying a density functional method on periodic models, the formation of C_n (n = 2–4) clusters on Pd(111) was explored. At low coverage, carbon monomers on the surface and at octahedral subsurface sites were calculated to be more stable than dimer species, C₂, on the surface. However, at a C coverage of about half a monolayer, the formation of C₂ and C₃ species, precursors of a graphene phase, becomes competitive with migration of C monomers to octahedral subsurface sites. While discussing these findings, the authors also addressed the problem of C₁ formation on Pd catalysts from simple organics.

MECHANISM OF SELECTIVE HYDROGENATION OF α,β-UNSATURATED ALDEHYDES ON SILVER CATALYSTS: A DENSITY FUNCTIONAL STUDY

K.H. Lim*,**, A.B. Mohammad*, I.V. Yudanov, K.M. Neyman***, M. Bron****, P. Claus****, N. Rösch*
(*Technische Universität München, Garching, Germany; **Nanyang Technological University, Singapore; ***ICREA, Barcelona, Spain;
****Technische Universität Darmstadt, Darmstadt, Germany)

J. Phys. Chem. C,
113(30) (2009) pp 13231-13240.

Supported silver catalysts exhibit a remarkably high selectivity in the industrially important hydrogenation of α,β unsaturated aldehydes to unsaturated alcohols. Density functional calculations have been carried out to clarify factors that affect the catalytic function of silver in hydrogenating unsaturated aldehydes. The activity and the selectivity of model silver catalysts for acrolein, the simplest, yet most difficult unsaturated aldehyde to be selectively hydrogenated was examined. The focus has been made on describing bulky catalyst particles, represented by sites on extended silver surfaces, on the regular clean Ag(110) surface and the surface O_sub/Ag(111) with subsurface oxygen centers. On Ag(110) the results imply propanal, the undesired saturated aldehyde, to be the main product. In contrast, the calculations suggest a very high selectivity of O_sub/Ag(111) for the corresponding unsaturated alcohol, allyl alcohol, although the activity of this system is lower than that of clean silver. At variance with Pt(111), where the selectivity to allyl alcohol is strongly reduced by the hindered desorption of the latter, allyl alcohol and propanal products are predicted to desorb easily from both Ag(110) and O_sub/Ag(111) at common reaction temperatures. Inherent limitations for an accurate description of the chemical regioselectivity by contemporary computational methods have been also analyzed.

KOOPMANS’ THEOREM IN THE RESTRICTED OPEN-SHELL HARTREE−FOCK METHOD: I. A VARIATIONAL APPROACH

B.N. Plakhutin, E.R. Davidson*
(*University of Washington, Seattle, Washington)

J. Phys. Chem. A, 113(45) (2009) pp 12386-12395.

A general formulation of Koopmans’ theorem is derived for high-spin half-filled open shells in the restricted open-shell Hartree−Fock (ROHF) method based on a variational treatment of both the initial (nonionized) open-shell system under study, e.g., X, and the corresponding high-spin ions X_k⁺, X_m⁺, and X_v⁻ having a hole or an extra electron in the closed, open, and virtual shell, respectively. The ions are treated within a FCI-RAS (full CI in the restricted active space) method with a use of arbitrary ROHF orbitals optimal for the initial system. It was shown that the desired canonical ROHF orbitals and orbital energies satisfying Koopmans’ theorem, first defined within the canonical ROHF treatment [Plakhutin; et al. J. Chem. Phys. 2006, 125, 204110], generally appear as the natural CI orbitals and the eigenvalues of CI matrices for the respective ions X^±. A comparison is performed between the results derived with the present CI approach and the canonical ROHF method for the specific case where the canonical orbital energies satisfying Koopmans’ theorem do not satisfy the Aufbau principle.

PAIRED ORBITALS FOR DIFFERENT SPINS EQUATIONS

I.L. Zilberberg, S.F. Ruzankin

J. Comput. Chem., 31(1) (2009) pp. 84-89.

Eigenvalue-type equations for Löwdin-Amos-Hall spin-paired (corresponding) orbitals are developed to provide an alternative to the standard spin-polarized Hartree-Fock or Kohn-Sham equations in dealing with broken-symmetry (BS) states. To derive paired orbitals for different spins (PODS) equations there has been applied Adams-Gilbert “localizing” operator approach. The PODS equations contain different operators for different spins the eigenvectors of which are paired orbitals associated with the same eigenvalue for each pair. Preliminary applications to simple systems show viability of this approach. Although the spectrum of possible applications of the PODS equations seems to be quite wide, they would be especially useful for obtaining and analyzing the S_z = 0 BS solutions for the systems with antiferromagnetic structure.

PROTON ENVIRONMENT OF REDUCED RIESKE IRON−SULFUR CLUSTER PROBED BY TWO-DIMENSIONAL ESEEM SPECTROSCOPY

D.R.J. Kolling*, R.I. Samoilova**, A.A. Shubin, A.R. Crofts**, S.A. Dikanov*
(*University of Illinois, Urbana, Illinois; **Institute of Chemical Kinetics and Combustion, Novosibirsk, Russia)

J. Phys. Chem. A, 113(4) (2009) pp 653-667.

The proton environment of the reduced [2Fe-2S] cluster in the water-soluble head domain of the Rieske iron−sulfur protein (ISF) from the cytochrome bc₁ complex of Rhodobacter sphaeroides has been studied by orientation-selected X-band 2D ESEEM. The 2D spectra show multiple cross-peaks from protons, with considerable overlap. Samples in which ¹H₂O water was replaced by ²H₂O were used to determine which of the observed peaks belong to exchangeable protons, likely involved in hydrogen bonds in the neighborhood of the cluster. By correlating the cross- peaks from 2D spectra recorded at different parts of the EPR spectrum, lines from nine distinct proton signals were identified. Assignment of the proton interaction with electrons of Fe(III) and Fe(II) ions, using the high-resolution structure of ISF from Rb. sphaeroides. Analysis of experimental and calculated tensors has led to conclude that even 2D spectra do not completely resolve all contributions from nearby protons. Particularly, the seven resolved signals from nonexchangeable protons could be produced by at least 13 protons. The contributions from exchangeable protons were resolved by difference spectra (¹H₂O minus ²H₂O), and assigned to two groups of protons with distinct anisotropic hyperfine values. The largest measured coupling exceeded any calculated value. This discrepancy could result from limitations of the point dipole approximation in dealing with the distribution of spin density over the sulfur atoms of the cluster and the cysteine ligands, or from differences between the structure in solution and the crystallographic structure. The approach demonstrated here provides a paradigm for a wide range of studies in which hydrogen-bonding interactions with metallic centers has a crucial role in understanding the function

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