F. Callens

Elucidating the promotional effect of a covalent triazine framework in aerobic oxidation

S. Abednatanzi, P. Gohari Derakhshandeh, P. Tack, F. Muniz-Miranda, Y-Y Liu, J. Everaert, M. Meledina, F. Vanden Bussche, L. Vincze, C. Stevens, V. Van Speybroeck, H. Vrielinck, F. Callens, K. Leus, P. Van der Voort
Applied Catalysis B: Environmental
269, 118769
2020
A1

Dominant stable radicals in irradiated sucrose: g tensors and contribution to the powder electron paramagnetic resonance spectrum

H. De Cooman, J. Keysabyl, J. Kusakovskij, A. Van Yperen-De Deyne, M. Waroquier, F. Callens, H. Vrielinck
Journal of Physical Chemistry B
117 (24), 7169–7178
2013
A1

Abstract 

Ionizing radiation induces a composite, multiline electron paramagnetic resonance (EPR) spectrum in sucrose, that is stable at room temperature and whose intensity is indicative of the radiation dose. Recently, the three radicals which dominate this spectrum were identified and their proton hyperfine tensors were accurately determined. Understanding the powder EPR spectrum of irradiated sucrose, however, also requires an accurate knowledge of the g tensors of these radicals. We extracted these tensors from angular dependent electron nuclear double resonance-induced EPR measurements at 110 K and 34 GHz. Powder spectrum simulations using this completed set of spin Hamiltonian parameters are in good agreement with experimentally recorded spectra in a wide temperature and frequency range. However, as-yet nonidentified radicals also contribute to the EPR spectra of irradiated sucrose in a non-negligible way.

Ti-functionalized NH2-MIL-47: an effective and stable epoxidation catalyst

K. Leus, G. Vanhaelewyn, T. Bogaerts, Y-Y Liu, F. Esquivel, F. Callens, G.B. Marin, V. Van Speybroeck, H. Vrielinck, P. Van der Voort
Catalysis Today
208, 97-105
2013
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Abstract 

In this paper, we describe the post-functionalization of a V-containing Metal-organic framework with TiO(acac)2 to create a bimetallic oxidation catalyst. The catalytic performance of this V/Ti-MOF was examined for the oxidation of cyclohexene using molecular oxygen as oxidant in combination with cyclohexanecarboxaldehyde as co-oxidant. A significantly higher cyclohexene conversion was observed for the bimetallic catalyst compared to the non-functionalized material. Moreover, the catalyst could be recycled at least 3 times without loss of activity and stability. No detectable leaching of V or Ti was noted. Electron paramagnetic resonance measurements were performed to monitor the fraction of V-ions in the catalyst in the +IV valence state. A reduction of this fraction by ∼17% after oxidation catalysis is observed, in agreement with the generally accepted mechanism for this type of reaction.

Room Temperature Radiation Products in Trehalose Single Crystals: EMR and DFT analysis

H. De Cooman, M. Tarpan, H. Vrielinck, M. Waroquier, F. Callens
Radiation Research
179 (3), 313-322
2013
A1

Abstract 

Radicals generated in trehalose single crystals by X radiation at room temperature were investigated by electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and ENDOR-induced EPR measurements, together with periodic density functional theory calculations. In the first days after irradiation, three radical species (I1, I2 and I3) were detected, two of which (I1 and I2) dominate the EPR spectrum and could be identified as H-abstracted species centered at C3′ (I1) and C2 (I2), the latter with additional formation of a carbonyl group at C3. Annealing the sample at 40°C for 3 days or storing it in ambient conditions for three months resulted in another, more stable EPR spectrum. Two major species could be characterized in this stage (S1 and S2), only one of which was tentatively identified as an H-abstracted, C2-centered species (S1). Our findings disagree with a previous EPR study [Gräslund and Löfroth (23)] on several accounts. This work stresses the need for caution when interpreting composite EPR spectra and thermally induced spectral changes of radiation-induced species, even in these relatively simple carbohydrates. It also provides further evidence that the pathways for radiation damage critically depend on the specific conformation of a molecule and its environment, but also that carbonyl group formation is a common process in the radiation chemistry of sugars and related compounds.

Radiation Products at 77 K in Trehalose Single Crystals: EMR and DFT Analysis

M. Tarpan, H. De Cooman, E. Hole, M. Waroquier, F. Callens
Journal of Physical Chemistry A
116 (13), 3377-3387
2012
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Abstract 

The radicals obtained in trehalose dihydrate single crystals after 77 K X-irradiation have been investigated at the same temperature using X-band electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and ENDOR-induced EPR (EIE) techniques. Five proton hyperfine coupling tensors were unambiguously determined from the ENDOR measurements and assigned to three carbon-centered radical species (T1, T1*, and T2) based on the EIE spectra. EPR angular variations revealed the presence of four additional alkoxy radical species (T3 to T6) and allowed determination of their g tensors. Using periodic density functional theory (DFT) calculations, T1/T1*, T2, and T3 were identified as H-loss species centered at C4, C1′, and O2′, respectively. The T4 radical is proposed to have the unpaired electron at O4, but considerable discrepancies between experimental and calculated HFC values indicate it is not simply the (net) H-loss species. No suitable models were found for T5 and T6. These exhibit a markedly larger g anisotropy than T3 and T4, which were not reproduced by any of our DFT calculations.

Early-Stage Evolution of the EPR Spectrum of Crystalline Sucrose at Room Temperature after High-Dose X Irradiation

H. Vrielinck, H. De Cooman, Y. Karakirova, N.D. Yordanov, F. Callens
Radiation Research
172 (2), 226-233
2009
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Abstract 

X irradiation of sucrose single crystals at room temperature leads to the production of stable radicals, which give rise to the dosimetric electron paramagnetic resonance (EPR) signal. In the first few hours after irradiation, however, the shape of the EPR spectrum changes drastically. Based on two-dimensional field-frequency electron nuclear double resonance (FF-ENDOR) measurements, we demonstrate that, after high-dose (5 kGy) and high-dose-rate irradiation, several species with limited stability at room temperature are produced next to the stable radicals. For two of these species, the main characteristics could be determined. Analysis of the time evolution of the FF-ENDOR and room-temperature EPR spectra in the first few hours after irradiation leads to the conclusion that these meta-stable radicals mainly recombine into diamagnetic species, while transformation into stable radicals is at most a marginal process.

Determination of the g Tensors for the Dominant Stable Radicals in X-Irradiated β-d-Fructose Single Crystals

M.A. Tarpan, H. Vrielinck, H. De Cooman, F. Callens
Journal of Physical Chemistry A
113 (28), 7994-8000
2009
A1

Abstract 

In spite of recent successful identifications of radicals produced after X-ray irradiation at 10 and 77 K in β-d-fructose, the structure of the two stable radicals dominating the electron paramagnetic resonance (EPR) spectrum after room temperature irradiation is still unclear. Based on the agreement between proton hyperfine (HF) tensors obtained in electron nuclear double resonance (ENDOR) experiments and the results of single molecule density functional calculations, a model for these radicals, involving OH abstraction at the C2 ring position, had previously been proposed, but this assignment could not be confirmed when the radical was embedded in a crystal environment. In this paper, we therefore provide additional experimental information for these radicals. First, their g tensors are determined from angular dependent ENDOR-induced EPR experiments. The relatively large anisotropy of these tensors is indicative of delocalization of the unpaired electron onto a neighboring oxygen atom. Second, EPR spectra of fructose powders, selectively enriched in 13C on various ring positions, are presented, demonstrating that the HF interaction with the carbon atom C3 is larger than with the C2. Combining the g tensor, proton and 13C HF data, we conclude that the structure of the stable radicals differs strongly from that of intact molecules and that further advanced quantum chemical modeling will be required to fully identify them.

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.

Radiation-Induced Radicals in Glucose-1-phosphate. II. DFT Analysis of Structures and Possible Formation Mechanisms

E. Pauwels, H. De Cooman, G. Vanhaelewyn, E. Sagstuen, F. Callens, M. Waroquier
Journal of Physical Chemistry B
112 (47), 15054-15063
2008
A1

Abstract 

Four radiation-induced carbon-centered radicals in dipotassium glucose-1-phosphate dihydrate single crystals are examined with DFT methods, consistently relying on a periodic computational scheme. Starting from a set of plausible radical models, EPR hyperfine coupling tensors are calculated for optimized structures and compared with data obtained from EPR/ENDOR measurements, which are described in part I of this work. In this way, an independent structural identification is made of all the radicals that were observed in the experiments (R1−R4) and tentative reaction schemes are proposed. Also, the first strong evidence for conformational freedom in sugar radicals is established: two species are found to have the same chemical composition but different conformations and consequently different hyperfine coupling tensors. Analysis of the calculated energies for all model compounds suggests that the radiation chemistry of sugars, in general, is kinetically and not necessarily thermodynamically controlled.

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