Q-Band EPR and ENDOR of Low Temperature X-Irradiated β-d-Fructose Single Crystals

Abstract

Beta-D-fructose single crystals were in situ X-irradiated at 80 K and measured using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and ENDOR-induced EPR (EIE) techniques at Q-band (34 GHz) microwave frequencies. The measurements revealed the presence of at least four carbon-centered radicals stable at 80 K. By means of ENDOR angular variations in the three principal crystallographic planes, six proton hyperfine coupling tensors could be determined and were assigned to four different radicals by the aid of EIE. Two of the radicals exhibit only beta-proton hyperfine couplings and reveal almost identical EIE spectra. For the other two radicals, the major hyperfine splitting originates from a single alpha-proton hyperfine coupling and their EIE spectra were also quite similar. The similarity of the EIE spectra and hyperfine tensors led to the assumption that there are only two essentially different radical structures. The radical exhibiting only beta-proton hyperfine couplings was assigned to a C3 centered radical arising from H3 abstraction and the other radical suggested to be an open-ring species with a disrupted C2-C3 bond and a double C2-O2 bond. A possible formation mechanism for the latter open-ring radical is presented. By means of cluster density functional theory (DFT) calculations, the structures of the two radicals were determined and a fairly good agreement between the calculated and experimental hyperfine tensors was found.

Radiation Chemistry of Solid-State Carbohydrates Using EMR

“…An obvious improvement consisted in embedding of the molecular radical in neighboring intact molecules for the geometry optimization, while the HF tensor calculations still were performed on single molecules [91]. This allowed for more realistic atomic relaxations around the radical center.…”

“…Trapped electrons were not detected and no attempts were made to analyze weak features, probably due to alkoxy radicals. After irradiation at 80 K, C3(-H) species was still observed [91], but C5(-H) was not. At this temperature, however, another radical was seen, identified (certain) as a ring-opened species with the spin localized at C3 (R2/R3).…”

“…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.…”

“…It is clear that the convincing identification of these radicals puts severe constraints on the precise identity of the F1/F2 C3-centered radicals (also assumed to have a broken ring) mentioned above. The C3-centered F3 and F6 radicals were proposed to have an intact ring [81,91]. Finally the last fructose radical with a probable identification (Q1), stable up to at least 160 K, is a C1(-H) radical species [81].…”

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

“…centered, H-abstracted radicals, in addition to ring-opened species 4 . For the dominant RT stable radicals doubts have arisen concerning the model assignment, based on single molecule DFT calculations, involving an OH-abstraction at the C2 position 2,3 .…”