Piotr Pietrzyk

Application of the genetic algorithm joint with the Powell method to nonlinear least-squares fitting of powder EPR spectra

The application of the stochastic genetic algorithm (GA) in conjunction with the deterministic Powell search to analysis of the multicomponent powder EPR spectra based on computer simulation is described. This approach allows for automated extraction of the magnetic parameters and relative abundances of the component signals, from the nonlinear least-squares fitting of experimental spectra, with minimum outside intervention. The efficiency and robustness of GA alone and its hybrid variant with the Powell method was demonstrated using complex simulated and real EPR data sets. The unique capacity of the genetic algorithm for locating global minima, subsequently refined by the Powell method, allowed for successful fitting of the spectra. The influence of the population size, mutation, and crossover rates on the performance of GA was also investigated.

Resolving Conformation Dichotomy for Y‐ and T‐Shaped Three‐Coordinate NiI Carbonyl Complexes with Relativistic DFT Analysis of EPR Fingerprints

The chemistry of paramagnetic Ni ions has received considerable interest, particularly owing to the involvement of this species (whether in a ground state, transition state or as the intermediates) in many important heterogeneous and homogeneous catalytic systems including organometallic, enzymatic and industrial processes. For instance, nickelexchanged zeolites play an important role in studying the mechanistic aspects of selective catalytic reduction of NOx [2]

Heterogeneous Binding of Dioxygen: EPR and DFT Evidence for Side-On Nickel(II)–Superoxo Adduct with Unprecedented Magnetic Structure Hosted in MFI Zeolite

This article reports on the activation of dioxygen on nickel(I) dispersed inside the nanopores of the ZSM-5 zeolite, which can be regarded as a heterogeneous mimetic system (zeozyme) for Ni-bearing enzymes. The side-on η(2)-coordination of the resulting nickel-bound superoxo adduct was ascertained by detailed analysis of the EPR spectra of both (16)O(2) and (17)O(2) species supported by computer simulations of the spectra and relativistic DFT calculations of the EPR signatures. Molecular analysis of the g and A((17)O) tensors (g(xx) = 2.0635, g(yy) = 2.0884, g(zz) = 2.1675; |A(xx)| ≈ 1.0 mT, |A(yy)| = 5.67 mT, |A(zz)| ≈ 1.3 mT) and quantum chemical modeling revealed an unusual electronic and magnetic structure of the observed adduct (with g(zz)(g(max)) > g(yy)(g(mid)) > g(xx)(g(min)) and the largest O-17 hyperfine splitting along the g(mid) direction) in comparison to the known homogeneous and enzymatic nickel-superoxo systems. It is best described as a mixed metalloradical with two supporting oxygen donor ligands and even triangular spin-density redistribution within the η(2)-{NiO(2)}(11) magnetophore. The semioccupied molecular orbital (SOMO) is constituted by highly covalent δ overlap between the out-of-plane 2p(π(g)*) MO of dioxygen and the 3d(x(2)(-y(2))) MO of nickel. By means of the extended transition state-natural orbitals for the chemical valence approach (ETS-NOCV), three distinct orbital channels (associated with σ, π, and δ overlap) of congruent and incongruent charge and spin density flows within the η(2)-{NiO(2)}(11) unit, contributing jointly to activation of the attached dioxygen, were identified. Their individual energetic relevance was quantified, which allowed for explaining the oxygen binding mechanism with unprecedented accuracy. The nature and structure sensitivity of the g tensor was rationalized in terms of the contributions due to the magnetic field-induced couplings of the relevant molecular orbitals that control the g-tensor anisotropy. The calculated O-17 hyperfine coupling constants correspond well with the experimental parameters, supporting assignment of the adduct. To the best of our knowledge, the η(2)-{NiO(2)}(11) superoxo adducts have not been observed yet for digonal mononuclear nickel(I) centers supported by oxygen donor ligands.

DFT Analysis of g and 13C Hyperfine Coupling Tensors for Model NiI(CO)nLm (n = 1-4, L = H2O, OH-) Complexes Epitomizing Surface Nickel(I) Carbonyls

Relativistic calculations within the spin-orbit mean-field (SOMF) approximation, the zero-order regular approximation (ZORA), and the scalar relativistic method based on the Pauli Hamiltonian were performed for the prediction and interpretation of the electronic g tensor and (13)C hyperfine tensor for a set of model polycarbonyl nickel(I) complexes with aqua or hydroxy coligands. They exhibit extensive similarities with heterogeneous [Ni(I)(CO)(n)]-surface complexes produced upon adsorption of carbon monoxide on Ni(I) ions grafted on silica or inside the zeolite channels. Benchmark calculations showing the influence of the exchange-correlation functional on the g tensor were carried out for well-defined nickel(I) complexes of known structure. On this basis, the SOMF-B3LYP scheme was chosen for calculations of the g tensor, and the obtained results were in satisfactory agreement with literature EPR data found for the [Ni(I)(CO)(n)]/SiO(2) system. The calculated g and A((13)C) tensors allowed polycarbonyl complexes of various stereochemistries to be distinguished. The nature of the Deltag(ii) shifts was assessed in terms of the molecular orbital contributions due to the magnetic-field-induced couplings and their structure sensitivity. The noncoincidence of g and (13)C hyperfine principal axes and their orientation with respect to the molecular framework was also examined. The ability of DFT calculations to follow consistently variations of the EPR parameters induced by stereochemical changes around the Ni(I) center provides an invaluable reference for the interpretation of experimental results.

Diagnostic Features of EPR Spectra of Superoxide Intermediates on Catalytic Surfaces and Molecular Interpretation of Their g and A Tensors

The use of electron paramagnetic resonance spectroscopy to study the superoxide intermediates, generated by end-on and side-on adsorption of the naturally abundant and 17O-enriched dioxygen on catalytic surfaces is discussed. Basic mechanisms of O2− radical formation via a cationic redox mechanism, an anionic redox mechanism, and an electroprotic mechanism are illustrated with selected oxide-based systems of catalytic relevance. Representative experimental spectra of various complexities are analyzed and their diagnostic features have been identified and interpreted. The molecular nature of the g and A tensors of the electrostatic and covalent superoxide adducts is discussed in detail within the classic and density functional theory based approaches.

Co2+/Co0 redox couple revealed by EPR spectroscopy triggers preferential coordination of reactants during SCR of NOx with propene over cobalt-exchanged zeolites

Catalytic reduction of NOx with propene over Co2+ -exchanged beta and ZSM-5 zeolites occurs with formation of zero-valent cobalt; NOx preferentially adsorbed on Co2+ plays the role of a metal reducing agent while ligation of propene is favored for Co(0) centers.

Intimate Binding Mechanism and Structure of Trigonal Nickel(I) Monocarbonyl Adducts in ZSM-5 Zeolite—Spectroscopic Continuous Wave EPR, HYSCORE, and IR Studies Refined with DFT Quantification of Disentangled Electron and Spin Density Redistributions along σ and π Channels

Interaction of tetracoordinated nickel(I) centers generated inside the channels of ZSM-5 zeolite with carbon monoxide ((12,13)CO, pCO < 1 Torr) led to the formation of T-shaped, top-on monocarbonyl adducts with a unique trigonal nickel core, supported by two oxygen donor ligands. The mechanism of the formation of the {Ni(I)-CO}ZSM-5 species was accounted for by a quantitative molecular orbital correlation diagram of CO ligation. Detailed electronic and magnetic structure of this adduct was obtained from comprehensive DFT calculations, validated by quantitative reproduction of its continuous wave electron paramagnetic resonance (CW-EPR), hyperfine sublevel correlation (HYSCORE), and IR fingerprints, using relativistic Pauli and ZORA-SOMF/B3LYP methods. Molecular analysis of the stretching frequency, νCO = 2109 cm(-1), g and A((13)C) tensors (g(xx) = 2.018, g(yy) = 2.380, g(zz) = 2.436, A(xx) = +1.0 ± 0.3 MHz, A(yy) = -3.6 ± 0.9 MHz, A(zz) = -1.6 ± 0.3 MHz) and Q((27)Al) parameters (e(2)Qq/h = -13 MHz and η = 0.8) supported by quantum chemical modeling revealed that the Ni-CO bond results from the π overlap between the low-laying π(2p) CO states with the 3d(xz) and 3d(yz) orbitals, with a small σ contribution due to the overlap of σ(2p+2s) orbital and a protruding lobe of the in-plane 3d(xz) orbital. Two types of orbital channels (associated with the σ and π overlap) of the electron and spin density flows within the {Ni(I)-CO} unit were identified. A bathochromic shift of the νCO stretching vibration was accounted for by resolving quantitatively the separate contributions due to the σ donation and π back-donation, whereas the (13)C hyperfine coupling was rationalized by incongruent α and β spin flows via the σ and π channels. As a result the very nature of the carbon-metal bond in the Ni(I)-CO adduct and the molecular backbone of the corresponding spectroscopic parameters were revealed with unprecedented accuracy.

Chapter 2 DFT modeling and spectroscopic investigations into molecular aspects of DeNOx catalysis

Abstract Even though the nitric oxide is thermodynamically unstable toward decomposition (2NO → N 2 + O 2 , Δ r G ° = −41.3kcal/mol) and disproportionate (4NO → 2N0 2 + N 2 , Δ r G ° = −57.8kcal/mol; 4NO → O 2 + 2N 2 O, Δ r G ° = −32.7kcal/mol; 3NO Δ NO 2 + N 2 O, Δ r G ° = −24.6 kcal/mol), none of these reactions can occur to any appreciable extent without a catalyst, due to the quite large kinetic stability of NO [1]. However, despite of being thermodynamically favored, catalytic abatement of NO x still remains a challenging problem from both the fundamental and the practical viewpoints [2,3]. Indeed, a number of mechanistically important issues concerning the active site design, the nature of the key intermediates, as well as detailed molecular understanding of the reaction route have not been definitely resolved as yet.The wide scientific interest in the transition-metal ion (TMI)-exchanged zeolites stems from their unique nitrosyl chemistry and remarkable activity in direct decomposition of NO x , which is combined with the advantage of exhibiting a relatively simple structure. This makes such materials convenient model systems for basic mechanistic studies and quantum chemical modeling [4–10]. In this context, the interfacial chemistry of nitrogen oxides (NO, NO 2 , and N 2 0) with intrazeolite TMIs acting as the active sites of DeNO x reaction is a subject of the fundamental importance for elucidation of the reaction elementary processes such as coordination, charge and spin density redistribution, accompanying the N-N and O-O bond making. A noteworthy feature of those processes is a dramatic spin change on passing from reactants to products, which along with the orbital symmetry barrier creates principal molecular constraints for efficient decomposition of nitrogen oxides in a conceivably simple concerted way.The aim of this contribution is to provide a molecular insight through density functional theory modeling corroborated with spectroscopic investigations (both in static and flow regimes) into the binding and activation of nitrogen oxides on various TMIs of different electron configuration (d 5 –d 10 ) and spin multiplicity, which are heterogenized in siliceous and alumino-siliceous frameworks. In this chapter, we will illustrate a variety of mechanistic aspects of DeNO x process using selected examples coming from our laboratory and from literature, with emphasis on the CuZSM-5 system.

Speciation and structure of cobalt carbonyl and nitrosyl adducts in ZSM-5 zeolite investigated by EPR, IR and DFT techniques

Adsorption of CO and NO on CoZSM-5 leads to the formation of various carbonyl and nitrosyl adducts. Their speciation and structure was studied by joint use of EPR and IR spectroscopies corroborated by DFT calculation. Depending on the temperature and pressure the following species were observed: {CoCO} 7 , {Co(CO) 3 } 7 carbonyls and {CoNO} 8 {Co(NO) 2 } 9 , {CoNO} 7 nitrosyls. The bounding of CO and No leads to nucleophilic activation of the diatomic ligands with distinctly different electron density redistribution within the Co-CO and Co-NO moieties.

Molecular interpretation of EPR parameters - computational spectroscopy approaches

DFT and post HF computation machinery used for the calculation of EPR parameters have been surveyed. The role of electron correlation, relativity treatment, spin polarization and contamination, and basis set customisation has been outlined and illustrated using recent literature data. Various aspects of molecular interpretation of spin Hamiltonian parameters regarding the structural, dynamical and environmental effects have been covered and illustrated using selected examples. It is presumed that such non-technical precis of current theoretical framework of the computational EPR spectroscopy may be useful for the broader audience to follow the case studies, and may serve also as a practical guide of the present state of the art in this rapidly developing field.