M.A. Tarpan

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.

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.

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.

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
A1

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.

Identification of primary free radicals in trehalose dihydrate single crystals X-irradiated at 10 K

M.A. Tarpan, H. De Cooman, E. Sagstuen, M. Waroquier, F. Callens
Physical Chemistry Chemical Physics (PCCP)
13, 11294-11302
2011
A1

Abstract 

Primary free radical formation in trehalose dihydrate single crystals X-irradiated at 10 K was investigated at the same temperature using X-band Electron Paramagnetic Resonance (EPR), Electron Nuclear Double Resonance (ENDOR) and ENDOR-induced EPR (EIE) techniques. The ENDOR results allowed the unambiguous determination of six proton hyperfine coupling (HFC) tensors. Using the EIE technique, these HF interactions were assigned to three different radicals, labeled R1, R2 and R3. The anisotropy of the EPR and EIE spectra indicated that R1 and R2 are alkyl radicals (i.e. carbon-centered) and R3 is an alkoxy radical (i.e. oxygen-centered). The EPR data also revealed the presence of an additional alkoxy radical species, labeled R4. Molecular modeling using periodic Density Functional Theory (DFT) calculations for simulating experimental data suggests that R1 and R2 are the hydrogen-abstracted alkyl species centered at C5′ and C5, respectively, while the alkoxy radicals R3 and R4 have the unpaired electron localized mainly at O2 and O4′. Interestingly, the DFT study on R4 demonstrates that the trapping of a transferred proton can significantly influence the conformation of a deprotonated cation. Comparison of these results with those obtained from sucrose single crystals X-irradiated at 10 K indicates that the carbon situated next to the ring oxygen and connected to the CH2OH hydroxymethyl group is a better radical trapping site than other positions.

Open Access version available at UGent repository

Combined Electron Magnetic Resonance and Density Functional Theory Study of intermediate radicals in X-irradiated beta-D-Fructose single crystals

Poster

Conference / event / venue 

Euromar 2008, Magnetic Resonance Conference
St.Petersburg, Russia
zondag, 6 juli, 2008 t/m vrijdag, 11 juli, 2008

Combined Electron Magnetic Resonance and Density Functional Theory Study of Thermally Induced Free Radical Reactions in Fructose and Trehalose Single Crystals

Poster

Conference / event / venue 

European conference on the spectroscopy of biological molecules
Palermo, Italy
vrijdag, 28 augustus, 2009 t/m woensdag, 2 september, 2009

Electron Magnetic Resonance study of the Structure and Thermal Stability of Radiation-Induced Free Radicals in Trehalose Single Crystals

Poster

Conference / event / venue 

7th European Federation of EPR (EFEPR) Groups Meeting and Closing Meeting of COST P15
Antwerp, Belgium
zondag, 6 september, 2009 t/m vrijdag, 11 september, 2009

Pagina's

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