H. Vrielinck

Density functional theory investigation of S2− in KCl: evidence for the existence of a di-vacancy site

F. Stevens, H. Vrielinck, F. Callens, M. Waroquier
Solid State Communications
132 (11), 787-790
2004
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Abstract 

Electron paramagnetic resonance experiments have shown that, depending on the doping procedure, two different S2− centers may coexist in KCl. These centers have the 2B2g and the 2B3gground state, respectively. As no experimental ligand hyperfine data are available, it could not be determined whether the S2− molecular ion replaces a single halide ion (mono-vacancy site) or two nearest neighbor halide ions (di-vacancy site). Also, other defect models could a priory be considered. In this work, cluster in vacuo density functional theory calculations of the g and 33S hyperfine tensors show that the S2− ion at a mono-vacancy site has the 2B2g ground state, whereas S2− in a di-vacancy exhibits a 2B3g ground state. For the latter center, the possibility of charge compensation by a cation vacancy is also considered. The calculations indicate that a possible vacancy is not in the direct vicinity (nearest or next-nearest neighbor) of the S2− ion.

Ab initio investigation of electron paramagnetic resonance parameters of S2-, SSe-, and Se2- radicals in alkali halides

F. Stevens, H. Vrielinck, F. Callens, E. Pauwels, M. Waroquier
Physical Review B
67 (10), 104429
2003
A1

Abstract 

Density functional theory (DFT) methods, as implemented in the Amsterdam Density Functional program, are used to calculate the electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) parameters of S2-, SSe-, and Se2- molecular ions doped into NaZ (Z=Cl,Br,I) and KI lattices. The calculations are performed on cluster in vacuo models, involving 88 atoms for the defect and its lattice surroundings, assuming that the molecular anions replace a single halide ion. In a previous study on the S2- ion, difficulties were encountered in calculating the superhyperfine and quadrupole principal values and axes of the neighbor cation nuclei. The observed discrepancies were partially attributed to the use of the frozen core approximation. In this work, the influence of this approximation on the calculated EPR and ENDOR parameters is evaluated. The DFT results for the S2-, SSe-, and Se2- molecular ions are in good agreement with the available experimental EPR data for all considered lattices, strongly supporting the monovacancy model for these diatomic defects.

Density-functional study of S2- defects in alkali halides

F. Stevens, H. Vrielinck, F. Callens, E. Pauwels, M. Waroquier
Physical Review B
66 (13), 134103
2002
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Abstract 

Density-functional methods, as implemented in the Amsterdam Density Functional program, are used to calculate the electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) parameters of the S2- defect in a halide monovacancy in various alkali halides (MZ:M=Na, K, Rb and Z=Cl, Br, I) lattices. The calculations were performed on cluster in vacuo models for the defect and its lattice surroundings, involving up to 88 atoms in order to limit boundary effects. For all MZ lattices, the calculated g and 33S hyperfine tensors of the S2- molecular ion are in very good agreement with the available EPR data, explicitly supporting the monovacancy model for the defect. In addition, computational results for the principal superhyperfine and quadrupole values and axes of the nearest shells of M+ and Z- ions are compared with experimental ENDOR data. The merits and shortcomings of the applied cluster in the vacuo method are critically evaluated.

Oxidation and Reduction Products of X Irradiation at 10 K in Sucrose Single Crystals: Radical Identification by EPR, ENDOR, and DFT

H. De Cooman, E. Pauwels, H. Vrielinck, E. Sagstuen, M. Waroquier, F. Callens
Journal of Physical Chemistry B
114 (1), 666–674
2010
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Abstract 

Electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and ENDOR-induced EPR (EIE) measurements on sucrose single crystals at 10 K after in situ X irradiation at this temperature reveal the presence of at least nine different radical species. Nine proton hyperfine coupling tensors were determined from ENDOR angular variations and assigned to six of these species (R1−R6) using EIE. Spectral simulations indicate that four of those (R1−R3 and R6) dominate the EPR absorption. Assisted by periodic density functional theory (DFT) calculations, R1 and R2 are identified as H-abstracted C1- and C5-centered radicals, R3 is tentatively assigned to an H-abstracted C6-centered radical, and R6 is identified as an alkoxy radical where the abstracted hydroxy proton has migrated to a neighboring OH group via intermolecular proton transfer. The latter radical had been characterized and identified in a previous study, but the present DFT calculations provide additional insight into its conformation and particular properties. This study provides the first step in unraveling the formation mechanism of the stable sucrose radicals detected after room-temperature irradiation and contributes to the understanding of the initial stages of radiation damage to solid-state carbohydrates.

Dosimetric characteristics of different types of saccharides: An EPR and UV spectrometric study

Y. Karakirova, N.D. Yordanov, H. De Cooman, H. Vrielinck, F. Callens
Radiation Physics and Chemistry
79 (5), 654-659
2010
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Abstract 

The time stability and dose response of the free radicals produced in various types of “less-studied” mono- and disaccharides by γ-radiation is studied by EPR (Electron Paramagnetic Resonance) and UV spectrometry. The time evolution of the shape of the EPR spectra of irradiated saccharides is investigated from 5 min to 5 months after irradiation. The intensity of the stable EPR signal is studied as a function of the absorbed γ-dose in the range 0.5–20 kGy. Aqueous solutions of irradiated solid saccharides exhibit a UV absorption maximum in the range 250–290 nm. A linear dependency is found between the magnitude of the UV absorption maximum and the absorbed γ-dose. The time stability of the UV absorption maximum is also studied for every saccharide. The results are compared with those obtained for irradiated sucrose.

Electron Magnetic Resonance and Density Functional Theory Study of Room Temperature X-Irradiated β-d-Fructose Single Crystals

M.A. Tarpan, E. Pauwels, H. Vrielinck, M. Waroquier, F. Callens
Journal of Physical Chemistry A
114 (47), 12417–12426
2010
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Abstract 

Stable free radical formation in fructose single crystals X-irradiated at room temperature was investigated using Q-band electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and ENDOR induced EPR (EIE) techniques. ENDOR angular variations in the three main crystallographic planes allowed an unambiguous determination of 12 proton HFC tensors. From the EIE studies, these hyperfine interactions were assigned to six different radical species, labeled F1−F6. Two of the radicals (F1 and F2) were studied previously by Vanhaelewyn et al. [Vanhaelewyn, G. C. A. M.; Pauwels, E.; Callens, F. J.; Waroquier, M.; Sagstuen, E.; Matthys, P. J. Phys. Chem. A 2006, 110, 2147.] and Tarpan et al. [Tarpan, M. A.; Vrielinck, H.; De Cooman, H.; Callens, F. J. J. Phys. Chem. A 2009, 113, 7994.]. The other four radicals are reported here for the first time and periodic density functional theory (DFT) calculations were used to aid their structural identification. For the radical F3 a C3 carbon centered radical with a carbonyl group at the C4 position is proposed. The close similarity in HFC tensors suggests that F4 and F5 originate from the same type of radical stabilized in two slightly different conformations. For these radicals a C2 carbon centered radical model with a carbonyl group situated at the C3 position is proposed. A rather exotic C2 centered radical model is proposed for F6.

Assessment of Periodic and Cluster-in-Vacuo Models for First Principles Calculation of EPR Parameters of Paramagnetic Defects in Crystals: Rh2+ Defects in NaCl as Case Study

N. Sakhabutdinova, A. Van Yperen-De Deyne, E. Pauwels, V. Van Speybroeck, H. Vrielinck, F. Callens, M. Waroquier
Journal of Physical Chemistry A
115(9), 1721-1733
2011
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Abstract 

In order to find a reliable and efficient calculation scheme for electron paramagnetic resonance (EPR) spectroscopic parameters for transition metal complexes in ionic solids from first principles, periodic and finite cluster-in-vacuo density functional theory (DFT) simulations are performed for g tensors, ligand hyperfine tensors (A), and quadrupole tensors (Q) for Rh2+-related centers in NaCl. EPR experiments on NaCl:Rh single crystals identified three Rh2+ monomer centers, only differing in the number of charge compensating vacancies in their local environment, and one dimer center. Periodic and cluster calculations, both based on periodically optimized structures, are able to reproduce experimentally observed trends in the ligand A and Q tensors and render very satisfactory numerical agreement with experiment. Taking also computation time into account as a criterion, a full periodic approach emerges as most appropriate for these parameters.The g tensor calculations, on the other hand, prove to be insufficiently accurate for model assessment. The calculations also reveal parameters of the complexes which are not directly accessible through experiments, in particular related to their geometry.

The coordinatively saturated vanadium MIL-47 as a low leaching heterogeneous catalyst in the oxidation of cyclohexene

K. Leus, M. Vandichel, Y-Y Liu, I. Muylaert, J. Musschoot, H. Vrielinck, F. Callens, G.B. Marin, C. Detavernier, Y.Z. Khimyak, M. Waroquier, V. Van Speybroeck, P. Van der Voort
Journal of Catalysis
285 (1) 196-207
2012
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Abstract 

A Metal Organic Framework, containing coordinatively saturated V+IV sites linked together by terephthalic linkers (V-MIL-47), is evaluated as a catalyst in the epoxidation of cyclohexene. Different solvents and conditions are tested and compared. If the oxidant TBHP is dissolved in water, a significant leaching of V-species into the solution is observed, and also radical pathways are prominently operative leading to the formation of an adduct between the peroxide and cyclohexene. If, however, the oxidant is dissolved in decane, leaching is negligible and the structural integrity of the V-MIL-47 is maintained during successive runs. The selectivity toward the epoxide is very high in these circumstances. Extensive computational modeling is performed to show that several reaction cycles are possible. EPR and NMR measurements confirm that at least two parallel catalytic cycles are co-existing: one with V+IV sites and one with pre-oxidized V+V sites, and this is in complete agreement with the theoretical predictions.

Open Access version available at UGent repository

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