Jürgen Fabian

MO-LCAO Calculations of Polymethines@@@MO-LCAO-Berechnungen an Polymethinen: I. One-electron approximation for streptopolymethines@@@I. Lsungen fr Streptopolymethine in Linelektronennherung

Idealized polymethines are defined as conjugated chain molecules with complete bond equilization and charge alternation. A constant π-bond order along the chain with N atoms has been derived for three types of polymethines which differ by the nature of the heteroatomic end groups. Corresponding to the number of π-electrons they are termined (N + 1)π-, Nπ- or (N − 1)π-polymethines. This classification embraces closed shell as well as open shell systems. The relations for the energy eigenvalues, coefficients and derived quantities have been obtained in closed form. The results for the different classes of polymethines are strongly related. Their molecular properties should be similar.Idealized polymethines are defined as conjugated chain molecules with complete bond equilization and charge alternation. A constant π-bond order along the chain with N atoms has been derived for three types of polymethines which differ by the nature of the heteroatomic end groups. Corresponding to the number of π-electrons they are termined (N + 1)π-, Nπ- or (N − 1)π-polymethines. This classification embraces closed shell as well as open shell systems. The relations for the energy eigenvalues, coefficients and derived quantities have been obtained in closed form. The results for the different classes of polymethines are strongly related. Their molecular properties should be similar.

Multiple Minima Hypersurfaces of Water Clusters for Calculations of Association Energy

Multiple minima of water cluster hypersurfaces are explored to find thermodynamic properties by means of the corresponding partition functions of their canonical distributions. The combination of semiempirical quantum chemical procedures for calculating the cluster energies in local minima of supermolecules and the statistical thermodynamics approach for both the evaluation of macroscopic association energies and the possible reduction by average of absolute errors intrinsic to the parametrized Hamiltonian are shown to provide an appropriate model for comparison between experimental and theoretical results. The method can explicitly take into account environmental effects due to intermolecular interaction. Water trimer and tetramer association energies of 10.9 and 14.1 kJ/mol obtained from virial coefficient calculations compare very well to the values of 10.5 and 16.4 kJ/mol, respectively, found for the theoretical association energies in this paper. c 2000 John Wiley & Sons, Inc. Int J Quantum Chem 79: 8-16, 2000

An ab initio study of the structure and electronic spectrum of 1,2-dithiete and 1,2-dithiin

Abstract The ground-state geometries of 1,2-dithiete and 1,2-dithiin were optimized at both the Hartree/3-Fock and MP2 levels with the 6-31G* basis set. In contrast to 1,2-dithiete, 1,2-dithiin is nonplanar with C 2 symmetry. Subsequent calculations on the first excited singlet states of each molecule were carried out using the variation-perturbation CIPSI method, in order to elucidate the main features of the electronic absorption spectrum.

Porphyrins and Phthalocyanines

Important naturally occurring pigments such as haemin and chlorophyll and synthetic pigments such as metal phthalocyanines owe their color to large cyclic conjugated systems. They represent tetradentative ligands which are chromophorically derived from porphin.

Electronic excited states of azo compounds: Strong ππ* fluorescence of bis-4,4′-diethylaminoazobenzene

Abstract The low-temperature absorption and luminescence spectra of bis-4,4′-diethylaminoazobenzene (BDAAB) in 2-methyltetrahydrofurane (MTHF) have been measured. On the basis of the spectral characteristics, this luminescence is interpreted as the fluorescence from the first excited singlet state (ππ*). A comparison of the spectral and photophysical properties of BDAAB with those of other para-substituted trans-azobenzenes enables us to draw some conclusions concerning the structure and fluorescence of these systems.

What is the basic chromophore of a dye

Taking the 4,5-diphenyl-substituted 2-dimethylamino-3-azacyclopentadienone (4) as an example its colour has been discussed in relation to different basic chromophores (parent chromophores). The parent chromophore can be defined either as the structure which has nearly the same absorption wavelength as the dye under consideration or as the substructure which is electronically most closely related to the dye. In the case of4 both considerations lead to different results. Based on the correlation diagram between the electronic states involved in the electronic excitation the long-wavelength absorption of4 is better understood in terms of the merocyanine parent chromophore than in terms of a betainic substructure, which should have the same colour as4 and is the parent chromophore according toKlessinger. This conclusion is drawn fromPPP configuration analysis of the wavefunctions of4 relative to different molecular substructures. The molecular geometry predicted by the geometry-optimized MINDO/3 and QCFF/Pi calculations provided additional arguments in favour of the cyclomerocyanine structure of4. In respect to the colour4 belongs to a new series of cyclic-conjugated chromophoric systems.

DFT- AND POST-HF-STUDY ON STRUCTURE AND ELECTRONIC EXCITATION OF ACYCLIC AND CYCLIC SULFUR DIIMIDES

A series of representative organic acyclic and cyclic sulfur diimides were studied by Hartree–Fock-plus-correlation ab initio quantum chemistry and by density functional theory using Beckes three-parameter functional along with the LYP functional. The widely used formula representation –NSN– for these compounds suggests octet expansion of sulfur. This is not confirmed by theory. Although d-polarization functions significantly improve the numerical results, sulfur d-orbitals are hardly occupied. The calculated electronic charge distribution derived by population analysis and by the atoms-in-molecules topological theory favors charge separation resulting in a more or less ylidic structure with –NS+–N−– and –N−=S+–N– resonance contributors. This structure does not exclude relatively short SN bond lengths. The characteristics of the bonds in the parent structures is preserved in some non-Kekule-type NSN heterocyles. Strong SN bond charge separations of organyl sulfur diimides are accompanied by short SN bond distances and narrow S0/T1 and S0/S1 energy gaps. The experimentally well known naphtho[1,8-cd][1,2,6]thiadiazine and the unknown 3,4-dimethylene-1,2,5-thiadiazole belong to this series. The calculations confirm that 1,2,3-thiadiazole is the diaza analog of thiophene rather than the vinylene-bridged sulfur diimide, while more complex heterocycles such as benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole take an intermediate position between classical and non-classical structures. Two closely related minimum structures are defined on the DFT Born–Oppenheimer energy surface of naphtho[1,8-cd;4,5-c′d′]bis[1,2,6]thiadiazine, but so far only one compound is experimentally known. The lowest energy structure is the quinoid form corresponding to the experimentally known compound.

On the reaction of dithiocarbamates with nitrogen-containing derivates of oxalic acid

SummaryThe action of bisimidochlorides of oxalic acid on dithiocarbamates produces 2-thioxo-3-aryl(alkyl)-4,5-diiminothiazolidines by cycloacylation. The molecular structure of 2-thioxo-3-(4-methoxyphenyl)-4,5-bis(phenylimino)-thiazolidine is confirmed by X-ray crystal structure analysis.ZusammenfassungBei der Einwirkung von Bisimidchloriden der Oxalsäure auf Dithiocarbaminate entstehen durch Cycloacylierung 2-Thioxo-3-aryl(alkyl)-4,5-diiminothiazolidine. Die Molekülstruktur von 2-Thioxo-3-(4-methoxyphenyl)-4,5-bis(phenylimino)thiazolidin wird durch Röntgenkristallstrukturanalyse bestätigt.

THE ELECTRONIC STRUCTURE AND PHYSICAL PROPERTIES OF THIONITROSO COMPOUNDS—A QUANTUM CHEMICAL STUDY

Abstract Semiempirical and nonempirical quantum chemical methods have been used to describe the electronic structure and physical properties of thionitroso compounds unknown so far. Predictions on geometry, electron distribution, ionization potential, electron affinity, UV-VIS spectrum and the S 0−T 1−energy splitting of these compounds have been made. First experimental results on thionitroso compounds are critically examined.

Theoretical study of molecular structure and spectra of HNS

Abstract Using complete active space self-consistent field electronic wavefunctions the three-dimensional potential energy and electric dipole functions for the lowest two singlet and triplet states of HNS have been generated and used for the evaluation of the spectroscopic constants, anharmonic frequencies and radiative transition probabilities. The electronic excitation energies have been calculated by a multi-reference configuration interaction approach.