H. Vrielinck

ENDOR and HYSCORE analysis and DFT-assisted identification of the third major stable radical in sucrose single crystals X-irradiated at room temperature

H. De Cooman, E. Pauwels, H. Vrielinck, E. Sagstuen, S. Van Doorslaer, F. Callens, M. Waroquier
Physical Chemistry Chemical Physics (PCCP)
11 (7), 1105-1114
2009
A1

Abstract 

Recently, the chemical structure of two of the three major stable radicals (T2 and T3) produced in sucrose single crystals by X-irradiation at room temperature was identified by comparing Density Functional Theory (DFT) calculations of Electron Magnetic Resonance parameters with experimental results [H. De Cooman, E. Pauwels, H. Vrielinck, E. Sagstuen, F. Callens and M. Waroquier, J. Phys. Chem. B, 2008, 112, 7298–7307]. Ambiguities concerning an unusual proton hyperfine coupling (HFC) tensor prevented the identification of the third major stable radical (T1). In the present work, experimental results of continuous wave Electron Nuclear Double Resonance experiments on sucrose single crystals and Hyperfine Sublevel Correlation Spectroscopy experiments on sucrose powder are presented that lift these remaining ambiguities. Using the final set of experimental HFC tensors and employing advanced DFT calculations, the chemical structure of the T1 radical is established: an allylic-type radical with approximately half of the spin density localised on the C2′ carbon of the fructose unit, involving glycosidic bond cleavage at the fructose side and a concerted formation of a carbonyl group at the C1′ carbon. The electronic structure of the T1 radical is discussed in more detail by means of additional DFT calculations, yielding a better understanding of the peculiar properties of the unusual proton HFC tensor mentioned above.

Schonland ambiguity in the electron nuclear double resonance analysis of hyperfine interactions: Principles and practice

H. Vrielinck, H. De Cooman, M.A. Tarpan, E. Sagstuen, M. Waroquier, F. Callens
Journal of Magnetic Resonance
195 (2), 196-205
2008
A1

Abstract 

For the analysis of the angular dependence of electron paramagnetic resonance (EPR) spectra of low-symmetry centres with S = 1/2 in three independent planes, it is well-established—but often overlooked—that an ambiguity may arise in the best-fit tensor result. We investigate here whether a corresponding ambiguity also arises when determining the hyperfine coupling (HFC) tensor for nuclei with I = 1/2 from angular dependent electron nuclear double resonance (ENDOR) measurements. It is shown via a perturbation treatment that for each set of MS ENDOR branches two best-fit tensors can be derived, but in general only one unique solution simultaneously fits both. The ambiguity thus only arises when experimental data of only one MS multiplet are used in analysis or in certain limiting cases. It is important to realise that the ambiguity occurs in the ENDOR frequencies and therefore the other best-fit result for an ENDOR determined tensor depends on various details of the ENDOR experiment: the MS state of the fitted transitions, the microwave frequency (or static magnetic field) in the ENDOR measurements and the rotation planes in which data have been collected. The results are of particular importance in the identification of radicals based on comparison of theoretical predictions of HFCs with published literature data. A procedure for obtaining the other best-fit result for an ENDOR determined tensor is outlined.

Identification and Conformational Study of Stable Radiation-Induced Defects in Sucrose Single Crystals using Density Functional Theory Calculations of Electron Magnetic Resonance Parameters

H. De Cooman, E. Pauwels, H. Vrielinck, E. Sagstuen, F. Callens, M. Waroquier
Journal of Physical Chemistry A
112 (24), 7298-7307
2008
A1

Abstract 

One of the major stable radiation-induced radicals in sucrose single crystals (radical T2) has been identified by means of density functional theory (DFT) calculations of electron magnetic resonance parameters. The radical is formed by a net glycosidic bond cleavage, giving rise to a glucose-centered radical with the major part of the spin density residing at the C1 carbon atom. A concerted formation of a carbonyl group at the C2 carbon accounts for the relatively small spin density at C1 and the enhanced g factor anisotropy of the radical, both well-known properties of this radical from several previous experimental investigations. The experimentally determined and DFT calculated proton hyperfine coupling tensors agree very well on all accounts. The influence of the exact geometrical configuration of the radical and its environment on the tensors is explored in an attempt to explain the occurrence and characteristics of radical T3, another major species that is most likely another conformation of T2. No definitive conclusions with regard to the actual structure of T3 could be arrived at from this study. However, the results indicate that, most likely, T3 is identical in chemical structure to T2 and that changes in the orientation of neighboring hydroxy groups or changes in the configuration of the neighboring fructose ring can probably not account for the type and size of the discrepancies between T2 and T3.

Radiation-induced defects in sucrose single crystals, revisited: A combined electron magnetic resonance and density functional theory study

H. De Cooman, E. Pauwels, H. Vrielinck, A. Dimitrova, N.D. Yordanov, E. Sagstuen, M. Waroquier, F. Callens
Spectrochimica Acta Part A (Mol. & biomol.)
69 (5), 1372-1383
2008
A1

Abstract 

The results are presented of an electron magnetic resonance analysis at 110 K of radiation-induced defects in sucrose single crystals X-irradiated at room temperature, yielding a total of nine 1H hyperfine coupling tensors assigned to three different radical species. Comparisons are made with results previously reported in the literature. By means of electron paramagnetic resonance and electron nuclear double resonance temperature variation scans, most of the discrepancies between the present 110 K study and a previous 295 K study by Sagstuen and co-workers are shown to originate from the temperature dependence of proton relaxation times and hyperfine coupling constants. Finally, radical models previously suggested in the literature are convincingly refuted by means of quantum chemical density functional theory calculations.

Combined Electron Magnetic Resonance and Density Functional Theory Study of 10 K X-Irradiated β-d-Fructose Single Crystals

M.A. Tarpan, E. Sagstuen, E. Pauwels, H. Vrielinck, M. Waroquier, F. Callens
Journal of Physical Chemistry A
112 (17) , 3898-3905
2008
A1

Abstract 

Primary free radical formations in fructose single crystals X-irradiated at 10 K were investigated at the same temperature using X-band Electron Paramagnetic Resonance (EPR), Electron Nuclear Double Resonance (ENDOR) and ENDOR induced EPR (EIE) techniques. ENDOR angular variations in the three principal crystallographic planes and a fourth skewed plane allowed the unambiguous determination of five proton hyperfine coupling tensors. From the EIE studies, these hyperfine interactions were assigned to three different radicals, labeled T1, T1* and T2. For the T1 and T1* radicals, the close similarity in hyperfine coupling tensors suggests that they are due to the same type of radical stabilized in two slightly different geometrical conformations. Periodic density functional theory calculations were used to aid the identification of the structure of the radiation-induced radicals. For the T1/T1* radicals a C3 centered hydroxyalkyl radical model formed by a net H abstraction is proposed. The T2 radical is proposed to be a C5 centered hydroxyalkyl radical, formed by a net hydrogen abstraction. For both radicals, a very good agreement between calculated and experimental hyperfine coupling tensors was obtained.

EPR and ENDOR analysis of Fe3+ impurity centers in fluoroelpasolite lattices

F. Loncke, H. De Cooman, N.M. Khaidukov, H. Vrielinck, E. Goovaerts, P. Matthys, F. Callens
Physical Chemistry Chemical Physics (PCCP)
9 (39), 5320-5329
2007
A1

Abstract 

Fe3+ ions in hexagonal and cubic fluoroelpasolite crystals (AI2BIMIIIF6) have been investigated in a combined Electron Paramagnetic Resonance (EPR) and Electron Nuclear Double Resonance (ENDOR) study. A detailed analysis of the ENDOR spectra for the nearest 19F and 23Na shells in X (9.5 GHz) and Q band (34 GHz) allowed the complex EPR spectra to be disentangled and to determine the spin Hamiltonian parameters for the various S = 5/2 Fe3+ centres. W-band (95 GHz) EPR measurements as a function of temperature were performed to provide unambiguous evidence about the absolute signs of the Zero Field Splitting (ZFS) and SuperHyperFine (SHF) parameters for Fe3+ in Cs2NaAlF6 as already determined from the ENDOR work. It could be concluded that all principal 19F hyperfine values were positive, in agreement with earlier assignments in the literature for related systems. A comparative analysis of the 19F SHF data for Fe3+ at a perfectly octahedral site in the cubic crystal, and at two slightly trigonally distorted environments in the hexagonal crystals, indicates that the metal-to-ligand distance changes upon doping. The obtained set of parameters concerning one defect in various analogous environments can furthermore be used to test different methods of theoretical calculations for ZFS and SHF values.

X- (X = O, S) Ions in Alkali Halide Lattices through Density Functional Calculations. 1. Substitutional Defect Models

F. Stevens, H. Vrielinck, V. Van Speybroeck, E. Pauwels, F. Callens, M. Waroquier
Journal of Physical Chemistry B
110 (16), 8204–8212
2006
A1

Abstract 

Monoatomic X- (X = O, S) chalcogen centers in MZ (M = Na, K, Rb and Z = Cl, Br, I) alkali halide lattices are investigated within the framework of density functional theory with the principal aim to establish defect models. In electron paramagnetic resonance (EPR) experiments, X- defects with tetragonal, orthorhombic, and monoclinic g-tensor symmetry have been observed. In this paper, models in which X- replaces a single halide ion, with a next nearest neighbor and a nearest neighbor halide vacancy, are validated for the X- centers with tetragonal and orthorhombic symmetry, respectively. As such defect models are extended, the ability to reproduce experimental data is a stringent test for various computational approaches. Cluster in vacuo and embedded cluster schemes are used to calculate energy and EPR parameters for the two vacancy configurations. The final assignment of a defect structure is based on the qualitative and quantitative reproduction of experimental g and (super)hyperfine tensors.

X- (X = O, S, Se) Ions in Alkali Halide Lattices through Density Functional Calculations. 2. Interstitial Defect Models

V. Van Speybroeck, F. Stevens, E. Pauwels, H. Vrielinck, F. Callens, M. Waroquier
Journal of Physical Chemistry B
110 (16), 8213-8218
2006
A1

Abstract 

Density functional theory techniques are used to investigate the defect structure of X- (X = O, S, Se) ions in MZ (M = Na, K, Rb and Z = Cl, Br) alkali halides which exhibit monoclinic-I g-tensor symmetry, using cluster in vacuo, embedded cluster, and periodic embedding schemes. Although a perturbed interstitial defect model was suggested from electron paramagnetic resonance experiments (EPR), the nature of the perturbation is still unknown. An appropriate defect model is developed theoretically by comparing structural and energetical properties of various defect configurations. Further validation is achieved by cross referencing experimental and computed EPR data. On the basis of the computational results, the following defect model is proposed:  the X- ion is located interstitially with a charge compensating halide vacancy in its first coordination shell.

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

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.

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