M. Waroquier

An extensive level of theory study is performed on diatomic chalcogen defects in alkali halide lattices by density functional theory methods. A variety of exchange correlation functionals and basis sets are used for the calculation of electron paramagnetic resonance (EPR) parameters of XY− (X, Y = O, S, Se) molecular ions doped in MZ (M = Na, K, Rb and Z = Cl, Br, I) lattices. Various factors contribute to the EPR values, such as geometrical effects, the choice of basis set and functional form. A sensitivity analysis is made by comparing experimental and theoretical magnetic resonance data. A flow scheme is proposed for obtaining the best agreement between experimental and calculated g-values for chalcogen defects in alkali halides.

In this work, we present an extensive investigation of the radiation-induced +NH3−•CH−CO2- glycine radical, using ab initio density functional modeling. The geometry and electron paramagnetic resonance (EPR) characteristics of the radical have been calculated using several model space approaches, including a single molecule approach, cluster models, and periodic calculations. Consecutively, both the calculated structural and spectroscopic properties are compared with experimental values taken from the literature. This comparative study involves the reproduction of the hyperfine coupling constants and the principal directions of the hyperfine tensor. It is found that the accurate calculation of these two features represents a sensitive probe for the accuracy of the proposed methodology to describe the glycine radical. The best overall agreement with experimental EPR parameters is found for a cluster calculation, in which the molecular environment surrounding the radical was explicitly taken into account, not only for the geometry optimization but also for the calculation of the spectroscopic properties. In the case of the +NH3−•CH−CO2- glycine radical, apparently, the magnetic properties are indeed affected by the crystal environment.

The use of density functional methods for the elucidation of the structure of radiation-induced bio-radicals by comparison of computed and experimental EPR properties is discussed. Three case studies, radiation induced radicals of the amino acid alanine, steroid hormones and β-d-fructose, with increasing degree of uncertainty about the proposed radical structures, are investigated. Next to the analysis of the isotropic and anisotropic components of the hyperfine tensor, also the direction cosines of the principal axes of this tensor were investigated in greater detail in the case of the β-d-fructose radicals. Since all radicals considered in this contribution are formed in a solid matrix, also the question as to how to incorporate the effect of the molecular environment is addressed. It is concluded that the methodology outlined represents a powerful tool to aid experimentalists in the assignment of the contributions of various radicals contributing to the observed EPR spectra. Copyright © 2004 John Wiley & Sons, Ltd.

The formation of alkylammonium groups in amine-doped zeolites is studied using density functional theory on small clusters representing the chemically active site. The presence of both strong Lewis base and Brønsted acid sites leads to a significant lowering of reaction barriers as opposed to alkoxide formation in full-oxygen zeolites. Furthermore, amine-substituted zeolites suggest novel reaction pathways that are not solely centralized around the aluminum substitution but in which two tetrahedral sites are involved, maximizing use of the zeolitic acid site and its surroundings. An investigation of the proton mobility in these yet to be synthesized materials demonstrates the need for minimizing the amount of Al−NH−Si bridges, as to prevent protonation of the amine group.