F. Callens

One of the major stable radiation-induced radicals in sucrose single crystals (radical T2) has been identified by means of density functional theory (DFT) calculations of electron magnetic resonance parameters. The radical is formed by a net glycosidic bond cleavage, giving rise to a glucose-centered radical with the major part of the spin density residing at the C1 carbon atom. A concerted formation of a carbonyl group at the C2 carbon accounts for the relatively small spin density at C1 and the enhanced g factor anisotropy of the radical, both well-known properties of this radical from several previous experimental investigations. The experimentally determined and DFT calculated proton hyperfine coupling tensors agree very well on all accounts. The influence of the exact geometrical configuration of the radical and its environment on the tensors is explored in an attempt to explain the occurrence and characteristics of radical T3, another major species that is most likely another conformation of T2. No definitive conclusions with regard to the actual structure of T3 could be arrived at from this study. However, the results indicate that, most likely, T3 is identical in chemical structure to T2 and that changes in the orientation of neighboring hydroxy groups or changes in the configuration of the neighboring fructose ring can probably not account for the type and size of the discrepancies between T2 and T3.

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.

Monoatomic X- (X = O, S) chalcogen centers in MZ (M = Na, K, Rb and Z = Cl, Br, I) alkali halide lattices are investigated within the framework of density functional theory with the principal aim to establish defect models. In electron paramagnetic resonance (EPR) experiments, X- defects with tetragonal, orthorhombic, and monoclinic g-tensor symmetry have been observed. In this paper, models in which X- replaces a single halide ion, with a next nearest neighbor and a nearest neighbor halide vacancy, are validated for the X- centers with tetragonal and orthorhombic symmetry, respectively. As such defect models are extended, the ability to reproduce experimental data is a stringent test for various computational approaches. Cluster in vacuo and embedded cluster schemes are used to calculate energy and EPR parameters for the two vacancy configurations. The final assignment of a defect structure is based on the qualitative and quantitative reproduction of experimental g and (super)hyperfine tensors.

Density functional theory techniques are used to investigate the defect structure of X- (X = O, S, Se) ions in MZ (M = Na, K, Rb and Z = Cl, Br) alkali halides which exhibit monoclinic-I g-tensor symmetry, using cluster in vacuo, embedded cluster, and periodic embedding schemes. Although a perturbed interstitial defect model was suggested from electron paramagnetic resonance experiments (EPR), the nature of the perturbation is still unknown. An appropriate defect model is developed theoretically by comparing structural and energetical properties of various defect configurations. Further validation is achieved by cross referencing experimental and computed EPR data. On the basis of the computational results, the following defect model is proposed:  the X- ion is located interstitially with a charge compensating halide vacancy in its first coordination shell.