Christian Van Alsenoy

Ab initio study of the elastic properties of single-walled carbon nanotubes and graphene

Abstract The first all-electron ab initio study of Youngs modulus and Poisson ratio for a number of closed single-walled nanotubes is presented. At the Hartree–Fock 6-31G ∗ level, the results obtained compare well with experimental as well as previous theoretical studies, predicting a Youngs modulus higher than 1 TPa. The calculated Youngs modulus for a graphene layer is found to be smaller than for its (5,5)-nanotube counterpart.

Critical thoughts on computing atom condensed Fukui functions.

Different procedures to obtain atom condensed Fukui functions are described. It is shown how the resulting values may differ depending on the exact approach to atom condensed Fukui functions. The condensed Fukui function can be computed using either the fragment of molecular response approach or the response of molecular fragment approach. The two approaches are nonequivalent; only the latter approach corresponds in general with a population difference expression. The Mulliken approach does not depend on the approach taken but has some computational drawbacks. The different resulting expressions are tested for a wide set of molecules. In practice one must make seemingly arbitrary choices about how to compute condensed Fukui functions, which suggests questioning the role of these indicators in conceptual density-functional theory.

brabo: a program for ab initio studies on large molecular systems

Abstract A combination of the multiplicative integral approximation with the direct SCF approach is implemented in the program brabo . This approach is an efficient combination for ab initio calculations on large molecular systems. For example, it was recently used to calculate the HF/4-21G gradient optimised structures of several hexapeptides (containing 56 atoms) and the structure and harmonic force fields of molecules in crystal clusters (involving 1092 basis functions).

FT-IR, FT-Raman, SERS and computational study of 5-ethylsulphonyl-2-(o-chlorobenzyl)benzoxazole.

FT-IR, FT-Raman and surface-enhanced Raman scattering spectra of 5-ethylsulphonyl-2-(o-chlorobenzyl)benzoxazole were recorded and analyzed. The vibrational wavenumbers were examined theoretically using the Gaussian09 set of quantum chemistry codes, and the normal modes were assigned by potential energy distribution calculations. The presence of CH(2), SO(2) and CH(3) modes in the SERS spectrum indicates the nearness of the methyl group to the metal surface which affects the orientation and metal molecule interaction. The synthesis, NMR spectra and antibacterial properties are reported. The title compound shows more inhibitory effect against Pseudomonas aeruginosa than ampicillin and found to be more potent against Klebsiella pneumoniae and drug-resistant Bacillus subtilis than the other microorganisms. A computation of the first hyperpolarizability indicates that the compound may be a good candidate as a NLO material. The RMS errors of the observed Raman and IR bands are found to be 30.93, 29.77 for HF and 9.57, 6.75 for DFT methods, respectively.

Spectroscopic (FT-IR, FT-Raman), first order hyperpolarizability, NBO analysis, HOMO and LUMO analysis of 2,4-bis(2-methoxyphenyl)-1-phenylanthracene-9,10-dione by ab initio HF and density functional methods

Anthraquinone derivatives are most important class of a system that absorb in the visible region. In this work, the vibrational spectral analysis was carried out using FT-IR and FT-Raman spectroscopy for 2,4-bis(2-methoxyphenyl)-1-phenylanthracene-9,10-dione. Theoretical calculations were performed by ab initio HF and DFT methods using 6-31G(*) basis set. The complete vibrational assignments of wavenumbers were made on the basis of potential energy distribution. The HOMO and LUMO analysis is used to determine the charge transfer within the molecule. The stability of the molecule arising from hyper-conjugative interaction and charge delocalization has been analyzed using NBO analysis. The calculated geometrical parameters (DFT) are in agreement with that of similar derivatives. The calculated first hyperpolarizability of the title compound is 4.69×10(-30) esu, which is 36.08 times that of urea and the title compound and the series of compounds it represents are attractive candidates for further studies in non linear optical applications.

Hydrogen–hydrogen interaction in planar biphenyl: A theoretical study based on the interacting quantum atoms and Hirshfeld atomic energy partitioning methods

The nature of H‐H interaction between ortho‐hydrogen atoms in planar biphenyl is investigated by two different atomic energy partitioning methods, namely fractional occupation iterative Hirshfeld (FOHI) and interacting quantum atoms (IQA), and compared with the traditional virial‐based approach of quantum theory of atoms in molecules (QTAIM). In agreement with Baders hypothesis of HH bonding, partitioning the atomic energy into intra‐atomic and interatomic terms reveals that there is a net attractive interaction between the ortho‐hydrogens in the planar biphenyl. This falsifies the classical view of steric repulsion between the hydrogens. In addition, in contrast to the traditional QTAIM energy analysis, both FOHI and IQA show that the total atomic energy of the ortho‐hydrogens remains almost constant when they participate in the H‐H interaction. Although, the interatomic part of atomic energy of the hydrogens plays a stabilizing role during the formation of the HH bond, it is almost compensated by the destabilizing effects of the intra‐atomic parts and consequently, the total energy of the hydrogens remains constant. The trends in the changes of intra‐atomic and interatomic energy terms of ortho‐hydrogens during HH bond formation are very similar to those observed for the H2 molecule.

An investigation into intramolecular hydrogen bonding: impact of basis set and electron correlation on the ab initio conformational analysis of 1,2-ethanediol and 1,2,3-propanetriol

Abstract Electron correlation effects are especially important in systems with strong nonbonded interactions. Despite this, very few ab initio studies of polyfunctional alcohols have included correlation effects in their geometry optimizations. In order to better understand intramolecular hydrogen bonding and to develop more reliable energy and geometric parameters for future molecular modeling, we optimized the geometries of 10 conformers of 1,2-ethanediol (ethylene glycol) and 11 conformers of 1,2,3-propanetriol (glycerol) at the HF/4-21G, HF/6-311G∗∗ and MP2/6-311G∗∗ levels of theory. All three computational methods are able to predict differences between internal coordinates optimized in different regions of conformational space, to within typical experimental accuracies. However, the inclusion of electron correlation has a major impact on the absolute values of these internal coordinates and on the depths of the associated energy minima. Compared with our HF/6-311G∗∗ results, the MP2-optimized structures have longer CO bonds by up to 0.020 A, OH bonds that are up to 0.023 A longer, OCC angles that are often 1 ° smaller, HOC angles in excess of 4 ° smaller, and torsional angles that may deviate from ideal trans or gauche values by an additional 5 ° for heavy-atom torsions and 8 ° for HOCC torsions. The net effect of all these conformational rearrangements is to greatly enhance intramolecular hydrogen bonding. Nonbonded O … H distances invariably decrease, by up to 0.19 A, and the alignments of hydrogens with hypothetical lone-pair orbitals on oxygen acceptors improve. Electron correlation selectively stabilizes those conformers with more intramolecular hydrogen bonds, but decreases the energy differences among conformers with the same number of hydrogen bonds. The errors associated with single-point MP2 energy calculations at HF-optimized geometries appear to increase with the size of the system, as do differences between MP2- and HF-optimized OCCO torsional angles. Thus, molecular mechanics parameters derived from MP2/6-311G∗∗ optimizations of prototypical small molecules such as ethylene glycol and glycerol are expected to result in significantly different macromolecular energies and structures than those based on HF/6-311G∗∗ optimizations.

Accurate interaction energies at density functional theory level by means of an efficient dispersion correction.

This paper presents an approach for obtaining accurate interaction energies at the density functional theory level for systems where dispersion interactions are important. This approach combines Becke and Johnsons [J. Chem. Phys. 127, 154108 (2007)] method for the evaluation of dispersion energy corrections and a Hirshfeld method for partitioning of molecular polarizability tensors into atomic contributions. Due to the availability of atomic polarizability tensors, the method is extended to incorporate anisotropic contributions, which prove to be important for complexes of lower symmetry. The method is validated for a set of 18 complexes, for which interaction energies were obtained with the B3LYP, PBE, and TPSS functionals combined with the aug-cc-pVTZ basis set and compared with the values obtained at the CCSD(T) level extrapolated to a complete basis set limit. It is shown that very good quality interaction energies can be obtained by the proposed method for each of the examined functionals, the overall performance of the TPSS functional being the best, which with a slope of 1.00 in the linear regression equation and a constant term of only 0.1 kcal/mol allows to obtain accurate interaction energies without any need of a damping function for complexes close to their exact equilibrium geometry.

Vibrational spectroscopic (FT-IR, FT-Raman, 1H NMR and UV) investigations and computational study of 5-nitro-2-(4-nitrobenzyl) benzoxazole

The optimized molecular structure, vibrational frequencies, corresponding vibrational assignments of 5-nitro-2-(4-nitrobenzyl) benzoxazole have been investigated experimentally and theoretically using Gaussian09 software package. Potential energy distribution of the normal modes of vibrations was done using GAR2PED program. The energy and oscillator strength calculated by time dependent density functional theory almost compliments with experimental findings. Gauge-including atomic orbital (1)H NMR chemical shifts calculations were carried out by using B3LYP functional with 6-31G basis set. The HOMO and LUMO analysis is used to determine the charge transfer within the molecule. The stability of the molecule arising from hyper-conjugative interaction and charge delocalization have been analyzed using NBO analysis. MEP was performed by the DFT method and the predicted infrared intensities and Raman activities have also been reported. The calculated geometrical parameters are in agreement with that of similar derivatives.

Information theoretical study of chirality: enantiomers with one and two asymmetric centra.

In this work, the Kullback-Leibler information deficiency is probed as a chirality measure. It is argued that the information deficiency, calculated using the shape functions of the R and S enantiomers, considering one as reference for the other, gives an information theory based expression useful for quantifying chirality. The measure is evaluated for five chiral halomethanes possessing one asymmetric carbon atom with hydrogen, fluorine, chlorine, bromine, and iodine as substituents. To demonstrate the general applicability, a study of two halogen-substituted ethanes possessing two asymmetric carbon atoms has been included as well. The basic expression of the sum of the local information deficiency over all atoms can be decomposed into separate summations over coinciding and noncoinciding atoms, or into a global and a mixing entropy term, or into a local entropy contribution for each atom individually based on the Hirshfeld partitioning. Avnirs continuous chirality measure (CCM) has been computed and confronted with the information deficiency. Finally, the relationship between chirality and optical rotation is used to study the proposed measure. The results illustrate Mezeys holographic electron density theorem with an intuitively appealing division of the strength of propagation of the atomic chirality from an asymmetric carbon atom throughout the molecule. The local information deficiency of the carbon atom is proposed as a measure of chirality; more precisely, the difference in information between the R and the S enantiomer turns out to be a quantitative measure of the chirality of the system. It may be evaluated as the arithmetic mean of the different alignments, or considering only the alignment resulting in the highest similarity value, or using the QSSA alignment.