Ewa Broclawik

Electronic Structure of Selected {FeNO}7 Complexes in Heme and Non-Heme Architectures: A Density Functional and Multireference ab Initio Study

The multiconfigurational CASSCF/CASPT2 approach, along with various functionals of density functional theory, is applied to selected iron(II)-nitrosyl ({FeNO}(7)) complexes, both with heme and nonheme groups. The energetics of the lowest doublet and quartet spin states at the correlated ab initio (CASPT2) level is presented for the first time. Comparison of the CASSCF and (unrestricted) DFT spin densities indicates that the nonhybrid functionals yield the spin densities most closely to the ab initio ones. The analysis of the multiconfigurational CASSCF wave function in terms of the localized active orbitals allows one to resolve the nature of Fe-NO bonding as a mixture of Fe(II)-NO(0) and Fe(III)-NO(-) resonance structures (in comparable contributions) for both spin states and various ligands.

Time-dependent DFT study on electronic states of vanadium and molybdenum oxide molecules

Spin-unrestricted time-dependent density functional theory (TDDFT) calculations for excited states of VO and MoO molecules have been undertaken to validate its applicability to highly open-shell systems. Equilibrium geometries, vibrational frequencies and excitation energies are compared with experimental data and ΔSCF DFT calculations where available. Overall good performance of TDDFT for intricate spectroscopic properties of transition metal (TM) oxides is found. Examples where discrepancies between experiment and theory could be expected are spotted and discussed.

Density functional theory calculations of the reaction pathway for methane activation on a gallium site in metal exchanged ZSM‐5

Density functional theory is used to describe the reaction profile for methane dissociation on Ga‐exchanged ZSM‐5. Stable structures on the reaction pathway are characterized as weakly adsorbed methane molecule and the C–H dissociation product. The transition state is also explicitly defined and optimized. The nonlocal density functional approximation is invoked to calculate the energy parameters of the reaction. The activation barrier is estimated at about 120 kJ/mol, in excellent agreement with other similar reactions. From vibrational analysis the reaction coordinate is deduced and transformation of a methane molecule on adsorption is discussed.Density functional theory is used to describe the reaction profile for methane dissociation on Ga‐exchanged ZSM‐5. Stable structures on the reaction pathway are characterized as weakly adsorbed methane molecule and the C–H dissociation product. The transition state is also explicitly defined and optimized. The nonlocal density functional approximation is invoked to calculate the energy parameters of the reaction. The activation barrier is estimated at about 120 kJ/mol, in excellent agreement with other similar reactions. From vibrational analysis the reaction coordinate is deduced and transformation of a methane molecule on adsorption is discussed.

The interaction of CO, N2 and NO with Cu cations in ZSM-5: quantum chemical description and IR study

In this paper we study the properties of Cu ions and their interactions with diatomic molecules in Cu-exchanged ZSM-5. We present DFT quantum chemical calculations for models composed of the Cu site and a diatomic molecule accompanied by IR investigations for various forms of CuZSM-5. Two series of calculations with density functional theory have been undertaken in order to investigate the influence of zeolitic framework on properties of exchanged cations: (i) for small models built of free mono- and divalent copper cation interacting with CO, N2 and NO and (ii) 5- or 6-member ring models of ZSM-5 hosting the cation and a diatomic molecule. Comparison of calculated and experimental IR X–Y frequencies supports our model and brings some insight into the activation mechanism.

ON THE ELECTRONIC STRUCTURE OF THE PALLADIUM MONOXIDE AND THE METHANE ADSORPTION : DENSITY FUNCTIONAL CALCULATIONS

Electronic structure of the palladium monoxide and its interaction with a methane molecule has been investigated by means of density functional theory. The two triplets, 3Π and 3Σ−, lie very close in energy, with the indication at the 3Π ground state of the oxide. A methane molecule interacts with the open shell PdO and forms two stable adsorption complexes: in collinear on palladium and bridging conformations. The scission of the C–H bond in adsorbed methane requires moderate activation energy of 24.5 kcal/mol and the dissociation product is very stable, however, the singlet–triplet crossing occurs at the transition state.

DFT and Ab Initio Study of Iron-Oxo Porphyrins: May They Have a Low-Lying Iron(V)-Oxo Electromer?

The energetics of various electromeric states for two heme complexes with an iron-oxo (FeO(3+)) group, FeO(P)(+) and FeO(P)Cl (P = porphin), have been investigated, employing DFT and correlated ab initio methods (CASPT2, RASPT2). Our interest focused in particular on tri- and pentaradicaloid iron(IV)-oxo porphyrin radical states as well as iron(V)-oxo states. Surprisingly, the iron(V)-oxo ground state is predicted for both models in vacuo. However, the presence of a polarizable medium, such as a solvent or a protein environment, favors the iron(IV)-oxo porphyrin radical cation, which is predicted to be the actual ground state of FeO(P)Cl under such conditions. Nonetheless, the iron(V)-oxo electromer is still expected to lie only a few kcal/mol above the ground state-a conclusion coming from both CASPT2 and RASPT2 calculations with a very large active space and further supported by a calibration with respect to coupled cluster CCSD(T) calculations for a simplified small model. The DFT results turn out to be strongly functional-dependent and thereby inconclusive. The widely used B3LYP functional-although correctly predicting the iron(IV)-oxo porphyrin radical ground state for FeO(P)Cl-seems to place the iron(V)-oxo states much too high in energy, as compared to the present CASPT2, RASPT2, and CCSD(T) results.

T–O–T skeletal vibration in CuZSM-5 zeolite: IR study and quantum chemical modeling

The location of Cu cations in CuZSM-5, properties of cationic sites and their interaction with guest molecules have been studied by quantum chemical (DFT) modeling and IR spectroscopy based on the frequency shift of antisymmetric T–O–T vibration of oxygen rings. The shift has been found sensitive both to the framework interaction with cations and to the interaction with adsorbed molecules. It has been measured and estimated theoretically from parameters characterising framework distorsion by Cu+ and Cu2+, with MgZSM-5 and NaZSM-5 used as ‘‘reference samples’’. It was found that the ordering of the cation perturbing effect was: Na+<Cu+<Mg2+<Cu2+. NO interaction with Cu cations was much stronger than that of CO and N2 . Divalent copper showed polarized 2-electron covalent bonding with NO strengthening its bond while moderate bonding ability of monovalent copper led to NO bond activation, in accordance with high catalytic activity of Cu+ZSM-5.

DENSITY FUNCTIONAL STUDY ON THE ACTIVATION OF METHANE OVER PD2, PDO, AND PD2O CLUSTERS

Simple functional models for elementary steps in the total oxidation of methane over supported palladium catalysts were investigated using density functional theory. Three simple cluster models were proposed, namely, the palladium dimer and PdO diatomic and linear Pd2O, to probe the mechanism of the methane activation on metallic and oxidized palladium phases, respectively. The strongest adsorption was found on Pd2, where also the C(SINGLE BOND)H bond became easily activated; however, no stable product of the C(SINGLE BOND)H bond scission was indicated. Similar hydrogen activation took place on Pd2O and, in addition, adsorbed methyl and OH species formed the most stable system after crossing a moderate energy barrier. The same product was previously found stable also in the case of PdO dimer but the activation barrier was high. On the Pd2O cluster, the process of energy barrier crossing was accomplished in two steps: easy formation of a free hydrogen moiety and actual oxidation, which made the overall process less demanding energetically.

The role of tungsten in formation of active sites for no SCR on the V-W-O catalyst surface — quantum chemical modeling (DFT)

This study concerns quantum chemical modeling of water behavior on basal surface of WO3‐V2O5 solid solution crystallites with V2O5 structure. It was undertaken in order to validate the hypothesis that the presence of W atoms in vanadia-like surface species of V-W-O catalysts facilitates low temperature water dissociation leading to formation of OH groups being active sites in selective NOx reduction by ammonia. Quantum chemical calculations were done with the use of modern electronic structure methodology based on the density functional theory (DFT). The calculations were performed for small clusters representing two adjacent metal sites in pentacoordinated oxygen environment, analogous to bipiramidal clusters introduced in description of the basal face of vanadium pentoxide. The results indicate that adsorption of water strongly depends on the presence of tungsten atom at the surface. Dissociative water adsorption leading to Bronsted acid centers creation is promoted. The proton acidity of the centers decreases with the increase of tungsten concentration in V 2O5 matrix leading to the increase of V 4C W 6C /V 5C W 6C ratio.

Evolution of the surface species of the V2O5–WO3 catalysts

Abstract Vanadia-related species formed as a result of vanadium segregation at the surface of V–W oxide bronze crystallites were investigated. The structures of these species and their transformations induced by oxygen removal and oxygen adsorption were monitored using photoelectron spectroscopy and the FT Raman technique. Assignments of the MeO vibrational bands, based on the results of DFT calculations for model clusters, have been proposed. Two kinds of surface species are dominant depending on the tungsten content: V 4+ –O–W 6+ at low tungsten content and V 5+ –O–W 5+ at higher tungsten concentration.