Study of rhamnose radicals in the solid state adopting a density functional theory cluster approach

A theoretical study is performed on the radiation-induced radicals in crystalline alpha-l-rhamnose, using density functional theory (DFT) calculations. Irrespective of earlier structural assignments, a host of possible radical models is examined in search for a structure that accurately reproduces experimental electron paramagnetic resonance (EPR) properties. A cluster approach is followed, incorporating all hydrogen bond interactions between radical and crystalline environment. Hyperfine coupling tensors as well as g tensors are determined and a comparison is made with available experimental data. Three carbon-centered hydroxyalkyl radicals are validated, in accordance with experimental suggestions for their structure. The occurrence of a carbon-centered oxygen anion radical for one of the radical species is rejected on theoretical grounds, and instead an altered hydroxyalkyl structure is suggested. Our cluster calculations are able to determine g and hyperfine tensors for the oxygen-centered alkoxy radical in rhamnose, in accordance with one of the two measurements for this species. For all radical models, quantitative agreement with experimental hyperfine tensors is obtained by performing full cluster DFT calculations. The inclusion of the molecular environment for the determination of this EPR property proved to be essential.