E. Pauwels

How should we calculate multi-dimensional potential energy surfaces for an accurate reproduction of partition functions?

P. Vansteenkiste, V. Van Speybroeck, E. Pauwels, M. Waroquier
Chemical Physics
314 (1-3), 109-117
2005
A1

Abstract 

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.

Ab initio EPR study of S and Se defects in alkali halides

F. Stevens, H. Vrielinck, F. Callens, E. Pauwels, V. Van Speybroeck, M. Waroquier
International Journal of Quantum Chemistry
102 (4), 409-414
2005
A1

Abstract 

Calculations using density functional theory are performed to study the electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) properties of S and Se impurities in alkali halide lattices. Cluster in vacuo models are used to describe the defect and the lattice surroundings. The trivacancy defect model proposed in the literature is able to reproduce both the experimental principal values and directions of the g tensor for S and Se defects doped in alkali halides. The alternative monovacancy model gives rise to important discrepancies with experiment and can be discarded. For the KCl lattice, the hyperfine tensors of the S and Semolecular ions also agree well with the available experimental data, giving further evidence to the trivacancy model. In addition, for NaCl:S and KCl:S computational results for the 23Na and 35Cl superhyperfine and quadrupole tensors are compared with experimental ENDOR parameters. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Study of radical defects in crystalline lattices from first-principles molecular dynamics simulations

V. Van Speybroeck, E. Pauwels, F. Stevens, F. Callens, M. Waroquier
International Journal of Quantum Chemistry
101 (6), 761-769
2005
A1

Abstract 

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

Level of theory study of magnetic resonance parameters of chalcogen XY− (X, Y = O, S and Se) defects in alkali halides

F. Stevens, V. Van Speybroeck, E. Pauwels, H. Vrielinck, F. Callens, M. Waroquier
Physical Chemistry Chemical Physics (PCCP)
7 (2), 240-249
2005
A1

Abstract 

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.

Evaluation of Different Model Space Approaches Based on DFT to Examine the EPR Parameters of a Radiation-Induced Radical in Solid-State α-Glycine

E. Pauwels, V. Van Speybroeck, M. Waroquier
Journal of Physical Chemistry A
108 (51), 11321-11332
2004
A1

Abstract 

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.

Density functional theory as a tool for the structure determination of radiation-induced bioradicals

F. De Proft, E. Pauwels, P. Lahorte, V. Van Speybroeck, M. Waroquier, P. Geerlings
Magnetic Resonance in Chemistry
42 (Sp. Iss. S1), S3-S19
2004
A1

Abstract 

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.

Open Access version available at UGent repository

DFT-EPR study of radiation-induced radicals in α-D-glucose

E. Pauwels, V. Van Speybroeck, F. Callens, M. Waroquier
International Journal of Quantum Chemistry
99 (2), 102-108
2004
A1

Abstract 

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

Article Experimental and Theoretical Electron Magnetic Resonance Study on Radiation-Induced Radicals in α-l-Sorbose Single Crystals

G. Vanhaelewyn, B. Jansen, E. Pauwels, E. Sagstuen, M. Waroquier, F. Callens
Journal of Physical Chemistry A
108 (16), 3308-3314
2004
A1

Abstract 

α-l-Sorbose single crystals were X-irradiated at 295 K (room temperature). A combined electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and ENDOR-induced EPR (EI-EPR) study at 120 K revealed a realm of radiation-induced free radicals in this sugar system. In the present work, a pair of closely related radicals is focused on, being dominant immediately after irradiation, but unstable with respect to long time storage or upon warming the samples. A density functional theory (DFT) study was carried out considering the complete hyperfine coupling tensors (principal axes and anisotropic and isotropic couplings) in comparison with the observed electron−proton interactions. This combined approach yielded very plausible models for both radicals, which are formed by a net hydrogen-abstraction from the C3 position of the six-membered sorbose ring. It appears that the difference between the two species is linked to the molecular disorder in the sorbose crystal structure. In addition, DFT calculations of the g tensors were performed for the plausible radical conformations.

Ab initio investigation of electron paramagnetic resonance parameters of S2-, SSe-, and Se2- radicals in alkali halides

F. Stevens, H. Vrielinck, F. Callens, E. Pauwels, M. Waroquier
Physical Review B
67 (10), 104429
2003
A1

Abstract 

Density functional theory (DFT) methods, as implemented in the Amsterdam Density Functional program, are used to calculate the electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) parameters of S2-, SSe-, and Se2- molecular ions doped into NaZ (Z=Cl,Br,I) and KI lattices. The calculations are performed on cluster in vacuo models, involving 88 atoms for the defect and its lattice surroundings, assuming that the molecular anions replace a single halide ion. In a previous study on the S2- ion, difficulties were encountered in calculating the superhyperfine and quadrupole principal values and axes of the neighbor cation nuclei. The observed discrepancies were partially attributed to the use of the frozen core approximation. In this work, the influence of this approximation on the calculated EPR and ENDOR parameters is evaluated. The DFT results for the S2-, SSe-, and Se2- molecular ions are in good agreement with the available experimental EPR data for all considered lattices, strongly supporting the monovacancy model for these diatomic defects.

Application of molecular cluster models to study the amino acid L-α-alanine and its derived radicals in the crystalline state

E. Pauwels, V. Van Speybroeck, M. Waroquier
International Journal of Quantum Chemistry
91(3), 511-516
2003
A1

Abstract 

In this study, molecular cluster methods are used to simulate the crystalline environment of l-α-alanine. Two clusters are constructed based on neutron diffraction data, containing 7 and 14 molecules respectively. The geometry of a central alanine molecule is optimised at several levels of theory, while the surrounding molecules in the cluster are kept fixed in space. By evaluating the difference between the optimised and the experimental geometries, an assessment is made of the different levels of theory used. These vary from semi-empirical to full DFT-B3LYP treatments of the cluster. Special attention is paid to hybrid methods, involving the ONIOM scheme. These cluster methods are then used to study a radiation-induced radical of alanine, since they allow unbiased geometry optimisation of the radical under study. The influence of the theoretical treatment is considered on several calculated hyperfine coupling constants. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

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