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

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.

Radiation Chemistry of Solid-State Carbohydrates Using EMR

“…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.…”

“…In search of (close to) primary radicals, Tarpan et al [18] and Vanhaelewyn et al [91] also investigated fructose single crystals after X-irradiation at 10 K, 80 K and annealing to 160 K. All proposed models for these radicals can be considered to be highly reliable if not certain. Both after irradiation at 10 K and 80 K a prominent pair (labeled T1/T1* and R1/R1* respectively) has been identified (again) as the C3(-H) species.…”

“…The LT study (10 K irradiation) by Tarpan et al [18] gave evidence for two hydroxyalkyl radicals: a C3(-H) (certain) and a C5(-H) species (plausible). Trapped electrons were not detected and no attempts were made to analyze weak features, probably due to alkoxy radicals.…”

“…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]. The radicals stabilized after irradiation at low temperatures (~ liquid He or N 2 ) are, however, more directly structurally related to those of the pristine molecules, and although such studies are experimentally far more challenging, the identification of the radical species is therefore in principle simpler [16][17][18][19][20][21]. Following the evolution of the EPR and ENDOR spectra towards the RT stable stage via thermal annealing is one approach to elucidate the structures of the stable species and to unveil their formation mechanisms.…”

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

“…1) and hence maybe contrary to expectations, β-D-fructose turns out to be a more difficult system with respect to identification of radiation-induced radicals. We recently successfully identified the dominant radical species after irradiation at 10 K in these crystals as resulting from net H-abstraction at two C atoms in the fructopyranose six-ring (C3 and C5) 5 . An earlier study after irradiation at 77 K had demonstrated the presence of similar C3…”