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

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.

Radiation Chemistry of Solid-State Carbohydrates Using EMR

“…From a statistical MLCFA study on EPR powder spectra, it became clear that at least four stable radicals contributed to the spectra. This finding was confirmed later by ENDOR and EIE investigations [14,81]. Two dominant radicals, labeled F1/F2, were found to exhibit quite comparable HF interactions and, based on single molecule DFT calculations, a C2-centered model was proposed for both of them [89,122] (an alternative labeling of the fructose atoms was used in that work).…”

“…In these first attempts to properly take into account the full lattice environment of the radical, SH parameter calculations were still performed on clusters cut out of the periodically optimized structure, because g and HF tensors reflect local properties, which are known not to be well reproduced by plane wave basis sets [8,9,14,[17][18][19]55]. A further major improvement in the calculations came from the implementation of HF [98] and g tensor calculations [99,100] in periodic codes (CP2K) using Gaussian and augmented plane wave basis sets.…”

“…Two dominant radicals, labeled F1/F2, were found to exhibit quite comparable HF interactions and, based on single molecule DFT calculations, a C2-centered model was proposed for both of them [89,122] (an alternative labeling of the fructose atoms was used in that work). Although Tarpan et al in later work also determined the g tensors of F1/F2 and applied selective 13 C enrichment in fructose powders [13,14,81], the identity of these main stable radicals remains uncertain. Using the most advanced DFT methods presently available, a C3(-H) model, featuring a neighboring carbonyl group and possibly also ring opening, seems the most plausible.…”

“…Using the most advanced DFT methods presently available, a C3(-H) model, featuring a neighboring carbonyl group and possibly also ring opening, seems the most plausible. Still, overall good agreement between experimental and computational data has not been reached [14,81].Also radicals F3 and F6 were proposed to be C3-centered. F6 is H-abstracted with a carbonyl group at C2, whereas a C3(-OH) model seems plausible for radical F3 [14,81].…”

“…This resulted primarily from a detailed comparison of proton hyperfine (HF) interactions, determined from single crystal electron nuclear double resonance (ENDOR) experiments, with the results of highlevel density functional theory (DFT) calculations. The situation for other sugars and related carbohydrates is similar: although stable radical production has been reported for several decades, reliable identifications of their structures are in gen-eral much more recent [10][11][12] or has to date not yet been achieved [13,14]. A first important reason for this is that stable radical structures in organic solids very often differ substantially from the pristine molecule (crystal) structures and are the result of a complex reaction chain after the initial ionization or electron capture process [15].…”