F. Callens

Ab initio EPR study of S and Se defects in alkali halides

F. Stevens, H. Vrielinck, F. Callens, E. Pauwels, V. Van Speybroeck, M. Waroquier
International Journal of Quantum Chemistry
102 (4), 409-414
2005
A1

Abstract 

Calculations using density functional theory are performed to study the electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) properties of S and Se impurities in alkali halide lattices. Cluster in vacuo models are used to describe the defect and the lattice surroundings. The trivacancy defect model proposed in the literature is able to reproduce both the experimental principal values and directions of the g tensor for S and Se defects doped in alkali halides. The alternative monovacancy model gives rise to important discrepancies with experiment and can be discarded. For the KCl lattice, the hyperfine tensors of the S and Semolecular ions also agree well with the available experimental data, giving further evidence to the trivacancy model. In addition, for NaCl:S and KCl:S computational results for the 23Na and 35Cl superhyperfine and quadrupole tensors are compared with experimental ENDOR parameters. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Study of radical defects in crystalline lattices from first-principles molecular dynamics simulations

V. Van Speybroeck, E. Pauwels, F. Stevens, F. Callens, M. Waroquier
International Journal of Quantum Chemistry
101 (6), 761-769
2005
A1

Abstract 

Theoretical calculations are presented to determine the structure of radical defects in crystalline lattices. The applications are concentrated on radical defects as they are induced by irradiation in organic crystals and paramagnetic molecular ions embedded in alkali halide lattices. Various approaches are possible to model the molecular environment: the single-molecule approach, the cluster approach, and periodic calculations. The latter are based on a Car-Parrinello formalism in which the molecular orbitals are expanded in a plane-wave basis set and in which the optimized structures at 0 K are obtained by a simulated annealing technique. The pros and cons of the various approaches are highlighted, and where possible comparison with experimental election paramagnetic resonance data are given. Due to the different natures of ionic and organic crystals, specific computational procedures are needed to get good correspondence with the experimental data. In various cases there is experimental evidence that some radical structures are submitted to noticeable changes with increasing temperature. These effects were theoretically reproduced by performing molecular dynamics calculations at elevated temperatures. (C) 2004 Wiley Periodicals, Inc. | Conference: 10th International Conference on the Applications of Density Functional Theory in Chemistry and Physics Location: Brussels, BELGIUM Date: SEP 05-12, 2003

Level of theory study of magnetic resonance parameters of chalcogen XY− (X, Y = O, S and Se) defects in alkali halides

F. Stevens, V. Van Speybroeck, E. Pauwels, H. Vrielinck, F. Callens, M. Waroquier
Physical Chemistry Chemical Physics (PCCP)
7 (2), 240-249
2005
A1

Abstract 

An extensive level of theory study is performed on diatomic chalcogen defects in alkali halide lattices by density functional theory methods. A variety of exchange correlation functionals and basis sets are used for the calculation of electron paramagnetic resonance (EPR) parameters of XY− (X, Y = O, S, Se) molecular ions doped in MZ (M = Na, K, Rb and Z = Cl, Br, I) lattices. Various factors contribute to the EPR values, such as geometrical effects, the choice of basis set and functional form. A sensitivity analysis is made by comparing experimental and theoretical magnetic resonance data. A flow scheme is proposed for obtaining the best agreement between experimental and calculated g-values for chalcogen defects in alkali halides.

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
A1

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.

DFT-EPR study of radiation-induced radicals in α-D-glucose

E. Pauwels, V. Van Speybroeck, F. Callens, M. Waroquier
International Journal of Quantum Chemistry
99 (2), 102-108
2004
A1

Abstract 

The structures of two radiation-induced radicals in solid-state α-d-glucose have been identified by means of single-molecule density function theory (DFT) calculations. Using the original crystalline structure as input, several radical models were created and their geometries optimized. Subsequently, electron paramagnetic resonance (EPR) parameters were calculated. During these calculations, the global orientation of the radical structure was kept fixed with respect to the crystal axes reference frame. This was essential to allow for an easy analysis of the hyperfine tensor principal directions, besides the isotropic and anisotropic coupling constants. By comparing these calculated EPR parameters with their experimentally determined counterparts, a plausible identification of two carbon-centered glucose radicals was possible. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

Article Experimental and Theoretical Electron Magnetic Resonance Study on Radiation-Induced Radicals in α-l-Sorbose Single Crystals

G. Vanhaelewyn, B. Jansen, E. Pauwels, E. Sagstuen, M. Waroquier, F. Callens
Journal of Physical Chemistry A
108 (16), 3308-3314
2004
A1

Abstract 

α-l-Sorbose single crystals were X-irradiated at 295 K (room temperature). A combined electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and ENDOR-induced EPR (EI-EPR) study at 120 K revealed a realm of radiation-induced free radicals in this sugar system. In the present work, a pair of closely related radicals is focused on, being dominant immediately after irradiation, but unstable with respect to long time storage or upon warming the samples. A density functional theory (DFT) study was carried out considering the complete hyperfine coupling tensors (principal axes and anisotropic and isotropic couplings) in comparison with the observed electron−proton interactions. This combined approach yielded very plausible models for both radicals, which are formed by a net hydrogen-abstraction from the C3 position of the six-membered sorbose ring. It appears that the difference between the two species is linked to the molecular disorder in the sorbose crystal structure. In addition, DFT calculations of the g tensors were performed for the plausible radical conformations.

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
A1

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.

Tentative Structures for the Radiation-Induced Radicals in Crystalline β-d-Fructose Using Density Functional Theory

E. Pauwels, P. Lahorte, G. Vanhaelewyn, F. Callens, F. De Proft, P. Geerlings, M. Waroquier
Journal of Physical Chemistry A
106 (51), 12370-12375
2002
A1

Abstract 

In this study, density functional theory calculations were used to identify the structure of the radiation-induced radicals in solid state β-d-fructose, using a single molecule approach. Four model radicals were proposed, and the electron paramagnetic resonance (EPR) parameters were calculated for the optimized geometries. These calculated parameters were subsequently compared with those of two radical species, observed in an experimental EPR and electron nuclear double resonance study on irradiated fructose (Vanhaelewyn, G.; Lahorte, P.; De Proft, F.; Mondelaers, W.; Geerlings, P.; Callens, F. Phys. Chem. Chem. Phys. 2001, 3, 1729). On the basis of this preliminary comparison, three model structures were rejected. By varying the main degree of freedom of the remaining model, a number of conformations were obtained that yielded isotropic and anisotropic hyperfine tensor components in close agreement with experimental results. To disentangle between these possible conformers, a detailed study was made of the hyperfine tensor eigenvectors. One conformation was found to be in close agreement with the experimental measurement of the hyperfine tensor of the two observed radical species. It was concluded that these experimental species are in fact manifestations of one and the same radical, with a structure conforming to our model but with slightly altered conformations.

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
A1

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.

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