New Evidence for Hydroxyalkyl Radicals and Light- and Thermally Induced Trapped Electron Reactions in Rhamnose.

Abstract

Radical formation and trapping of radicals in X-irradiated crystals of rhamnose at 6 K were investigated using electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR) and ENDOR-induced EPR (EIE) techniques, complemented with periodic density functional theory (DFT) calculations. The two major radical species at 6 K were the O4-centered alkoxy radical and the intermolecularly trapped electron (IMTE), previously also detected by other authors. The current experimental results provided hyperfine coupling constants for these two species in good agreement with the previous data, thus providing a consistency check that improves their credibility. In addition to the O4-centered alkoxy radical and the IMTE, the C3-centered and C5-centered hydroxyalkyl radicals are the most prominent primary species at 6 K. The C3-centered radical appears in two slightly different conformations at 6 K, designated C and D. The C5-centered radical exhibits a coupling to a methyl group with tunneling rotation at 6 K, and analysis of one of the rotational substates (A) of the spin system yielded an understanding of the structure of this radical. Visible light bleaching of the IMTE at 6 K led to the C3-centered radical C, and thermal annealing above 6 K resulted in a conversion of the C to the D conformation. In addition, thermal annealing releases the IMTE, apparently resulting in the formation of the C2-centered radical. It is possible that the thermal decay of the IMTE also contributes to a small part of the C3-centered radical (D) population at 85 K. There are several other products trapped in rhamnose crystals directly after irradiation at 6 K, among which are resonance lines due to the C2 H-abstraction product. However, these other products are minority species and were not fully characterized in the current work.