Eckhard Koglin

Conformational dependence of Raman frequencies and intensities in alkanes and polyethylene

Abstract Raman vibrational frequencies and intensities of several octane conformers, taken as models for corresponding conformational sequences in polyethylene, were calculated using quantum mechanical ab initio methods. Wherever direct comparison with experimental data for polyethylene was possible, agreement was very satisfactory. The present data suggest that ab initio calculated Raman data on relatively short oligomers may provide valuable information regarding the interpretation of polymer Raman spectra, in particular concerning issues where interpretation based on experimental verification is not possible.

The vibrational spectra of metabolites of methabenzthiazuron: 2-amino-benzothiazole and 2-(methylamino)benzothiazole

Abstract The interaction of metabolites such as 2-amino-benzothiazole (ABT) and 2-(methylamino)benzothiazole (MABT), originating from pesticides, is of significant importance with respect to the development of our ecosystem. The study of such complex systems requires the use of multiple analytical techniques. In order to enable such an approach, the relevant part of the vibrational spectra of ABT and MABT are interpreted using quantum mechanically calculated vibrational frequencies and intensities. The B3LYP and BLYP DFT methods were employed, using 6-31G(d, p) and aug-cc-pVDZ basis sets. Using the shifts induced by 13 C substitution of one of the carbon atoms in ABT and MABT, the major spectral lines in the fingerprint spectral region were assigned.

On the problem of theoretical evaluation of the rotational barrier in aromatics with adjacent conjugated group: benzaldehyde and N-methylbenzamide

Abstract The rotational barrier in benzaldehyde was evaluated using DFT methods and CCSD. Compared to experimental values, the calculated values are typically a factor 2 too high. Evidence is provided this also applies to N -methylbenzamide. The results once more suggest that molecules comprising a benzene ring with a π-conjugated substituent form a special problem with respect to calculation of the rotational barrier around the C(sp2)–C(aryl) bond. It must be concluded that current DFT functionals cannot properly handle such barriers. Conversely, successfully describing these rotational barriers may be an appropriate and stringent test for new functionals.

An ab-initio study of the relative stability of the ggg and the gtg conformer in hexane

Abstract Earlier ab-initio work suggested, on the basis of MP2 level calculations, that the hexane ggg conformer is more stable than the gtg conformer. Because this is unexpected and if true might have a significant impact on force field parametrisations, we have applied Hartree–Fock and post-HF methods to evaluate the relative stability of these conformers. We find that at levels higher than MP2 the gtg conformer is more stable than the ggg conformer, in agreement with the conventional idea that each additional gauche bond causes a further decrease in stability of the conformer. DFT methods were also applied, but although DFT methods including gradient corrections show correct qualitative behaviour, quantitatively the relative energies are far off compared to the post-HF results.

On the high-resolution X-ray photoelectron spectrum of poly(ethylene terephthalate) : the sensitivity to trans-gauche conformation

A recently suggested dependence of the C 1s core-level binding energy shift on the conformation of the glycol unit in PET is confirmed on the basis of ab initio calculations. The joint result shows that core-level X.p.s. spectroscopy may be employed to study polymer conformational properties.

A COMBINED EXPERIMENTAL FAR-INFRARED AND COMPUTATIONAL DENSITY FUNCTIONAL APPROACH APPLIED TO ALUMINUM-ALKYL COCATALYSTS

Abstract The structure of aluminiummethyl(chloride) complexes is investigated by experimental far-infrared spectroscopy and by density functional theory calculations. The vibrational frequencies and relative intensities are reproduced well using the B3-LYP based density functional method (DFT). The computed spectra allow for a unique identification of the configuration of the aluminiummethyl(chloride) complexes. The results show that far-IR spectra may be accurately predicted for medium-sized molecules using DFT calculations with moderate basis sets size.

Trans–gauche energy difference from Raman spectrum of gaseous pentane

Abstract The trans–gauche energy difference derived from the Raman spectrum of gaseous pentane is rediscussed for the case when the explicit temperature dependence is not available. In that case differences in Raman scattering cross-section and degeneracy of the conformational energy levels needs to be taken into account. Using ab initio derived Raman cross-sections, a good value for the trans–gauche energy difference is obtained. This method can be applied to systems which do not allow for study over a wide temperature range such as polymers.

Structure and reactivity of aluminium amidinates in olefin polymerisation

Using density functional theory calculations we have studied ethylene polymerisation activity of experimentally studied aluminium amidinate homogeneous catalysts. We confirm an earlier picture for the amidinates that finite temperature effects need to be considered to rationalize catalytic activity. Static calculations do not provide any explanation for the experimentally observed activity. The tendency to form dinuclear complexes of various kinds makes this class of species rich in chemistry. The inadequacy of quantum calculations to provide accurate barriers for β-hydrogen transfer for the real experimental systems, however, renders these systems also as computationally difficult and a challenge for future studies.

Activity of Aluminium‐Based Complexes in Olefin Polymerization. A Case of Breakdown of a Simple Picture to Rationalize Homogeneous Catalysis

Using Density Functional Theory type calculations we have studied ethylene polymerization activity of several experimentally studied, aluminium‐based homogeneous catalysts. This has led us to a qualitative picture that captures the experimental findings reported so far and that provides an atomistic account of the factors involved in the catalytic activity of the aluminium complexes for olefin polymerization. We confirm an earlier picture for the amidinates, i.e., that finite temperature effect needs to be considered to rationalize catalytic activity. This picture, however, requires further justification, as aluminium‐based olefin polymerization catalysis has turned out to be much more complex than anticipated for a simple metal. The tendency to form dinuclear complexes of various kinds and the inadequacy of quantum calculations to provide accurate barriers for β‐hydrogen transfer for the real experimental systems makes this class of species rich in chemistry but also difficult to predict.