E. Pauwels

The potential energy of n-hexane is studied since it constitutes a typical example of a single chain molecule in which various internal rotations are present and a large number of conformations are existing, which cannot be reached by using one-dimensional rotational energy profiles. For an accurate reproduction of the global partition function and all derived thermodynamic properties an adequate description of all possible conformers is necessary. The full three-dimensional potential energy surface of the internal rotations in n-hexane (3D-PES) is calculated at an ab initio level and compared with one-dimensional schemes to reproduce the energy. Due to the higher dimensionality of the relevant potential energy surface, the computational cost is very high. A new approximate scheme based on two-dimensional cuts is proposed that gives good accuracy for the relative conformational energies and kinetic energies at a reasonable computational cost. This scheme is of general use for any long chain molecule.

Theoretical calculations are presented to determine the structure of radical defects in crystalline lattices. The applications are concentrated on radical defects as they are induced by irradiation in organic crystals and paramagnetic molecular ions embedded in alkali halide lattices. Various approaches are possible to model the molecular environment: the single-molecule approach, the cluster approach, and periodic calculations. The latter are based on a Car-Parrinello formalism in which the molecular orbitals are expanded in a plane-wave basis set and in which the optimized structures at 0 K are obtained by a simulated annealing technique. The pros and cons of the various approaches are highlighted, and where possible comparison with experimental election paramagnetic resonance data are given. Due to the different natures of ionic and organic crystals, specific computational procedures are needed to get good correspondence with the experimental data. In various cases there is experimental evidence that some radical structures are submitted to noticeable changes with increasing temperature. These effects were theoretically reproduced by performing molecular dynamics calculations at elevated temperatures. (C) 2004 Wiley Periodicals, Inc. | Conference: 10th International Conference on the Applications of Density Functional Theory in Chemistry and Physics Location: Brussels, BELGIUM Date: SEP 05-12, 2003

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 structures of two radiation-induced radicals in solid-state α-d-glucose have been identified by means of single-molecule density function theory (DFT) calculations. Using the original crystalline structure as input, several radical models were created and their geometries optimized. Subsequently, electron paramagnetic resonance (EPR) parameters were calculated. During these calculations, the global orientation of the radical structure was kept fixed with respect to the crystal axes reference frame. This was essential to allow for an easy analysis of the hyperfine tensor principal directions, besides the isotropic and anisotropic coupling constants. By comparing these calculated EPR parameters with their experimentally determined counterparts, a plausible identification of two carbon-centered glucose radicals was possible. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004