Weak intermolecular interactions in gas-phase nuclear magnetic resonance

Gas-phase nuclear magnetic resonance (NMR) spectra demonstrating the effect of weak intermolecular forces on the NMR shielding constants of the interacting species are reported. We analyse the interaction of the molecular hydrogen isotopomers with He, Ne, and Ar, and the interaction in the He-CO(2) dimer. The same effects are studied for all these systems in the ab initio calculations. The comparison of the experimental and computed shielding constants is shown to depend strongly on the treatment of the bulk susceptibility effects, which determine in practice the pressure dependence of the experimental values. Best agreement of the results is obtained when the bulk susceptibility correction in rare gas solvents is evaluated from the analysis of the He-rare gas interactions, and when the shielding of deuterium in D(2)-rare gas systems is considered.     

Chemical tailoring of fullerene acceptors: synthesis, structures and electrochemical properties of perfluoroisopropylfullerenes

High-temperature syntheses of the new C(60)(i-C(3)F(7))(2,4,6) and C(70)(i-C(3)F(7))(2,4) isomers and their characterization by spectroscopic methods, X-ray crystallography, cyclic voltammetry and density functional theory provide compelling evidence that they are superior electron acceptors than trifluoromethylfullerenes.     

Generation of potential energy surfaces in high dimensions and their haptic exploration

A method is proposed for the automated generation of potential energy surfaces in high dimensions. It combines the existing algorithm for the definition of new energy data points, based on the interpolating moving least-squares algorithm with a simulated annealing procedure. This method is then studied in a haptic quantum chemistry environment that requires a fast evaluation of gradients on a potential energy surface with automatic improvement of its accuracy. As an example we investigate the nitrogen binding pathway in the Schrock dinitrogen fixation complex with this set-up.     

A Monte Carlo calculation of thermodynamic properties for the liquid NaCl+KCl mixture

The mixing process NaCl+KCl →(Na, K)Cl in the liquid state at T=1083 K and zero pressure is simulated by means of Monte Carlo calculations. A cubic box with periodic boundary conditions containing 216 ions interacting according to a pair potential of the Huggins-Mayer form served as a model of the system. Thermodynamic properties are calculated for the mixture and the pure salt models and compared with experiments. Radial and angular distribution functions are obtained. A perturbation theory of the heats of mixing of molten salts and the ‘random mixture approximation’ are commented on in the light of the Monte Carlo results.     

Photoredox reactions of Cr(III) mixed-ligand complexes

Irradiation of chromium(III) complexes with oxalate and pyridinedicarboxylate ligands (pda = 2,3-, 2,4-, or 2,5-dicarboxylate) leads to diverse behaviors, dictated by light energy, presence of oxygen and the ligand nature. Irradiation within the MC bands is unaffected by O₂ and results in ligand substitution. The LMCT excitation is effective only when oxalate is coordinated to Cr(III); then electron transfer from oxalate to central ion generates an intermediate, consisted of a Cr(II)species and the C₂O₄^? radicals. The species undergo fast redox reactions dependent on the presence of O₂ and the pda ligand.(1) In anoxic medium the fast outersphere electron transfer from Cr(II) to solvent, generates hydrated electrons and re-oxidizes the chromium centre to Cr^III. Then geminate recombination regenerates substrate, whereas competitive release of the C₂O₄^? radical leads to substitution of one oxalate ligand by two water molecules (aquation induced by the LMCT excitation). In the presence of the pda ligand the outersphere electron transfer is accompanied by the innersphere CT, generating Cr(III) coordinated to two radical ligands: C₂O₄^? and pda^3?; the intermediate releases also e_aq^?, but this reaction is slower than that of the homoleptic oxalate complex. Hydrated electrons are scavenged also by the released radicals. All these processes are completed within microseconds and in consequence, the Cr(III) complexes irradiated in deoxygenated solutions are insensitive to subsequent oxygenation.(2) When UV-irradiation is carried out in oxygenated medium reaction of Cr(II) species with molecular oxygen competes with the outer- or inner electron transfer observed in anoxic medium. Both these pathways result in generation of chromate(VI). Quantum yield of the Cr(VI) production is sensitive to the presence and structure of pda ligand, decreasing within the series 2,3-pda > 2,4-pda > 2,5-pda.     

Solid-state structures and magnetic properties of halide-bridged, face-to-face bis-nickel(II)-macrocyclic ligand complexes: ligand-mediated interchanges of electronic configuration

Dramatic differences are found between the ambient and 100 K X-ray structures of [L(2)Ni2Br2](ClO4)2 (L(2) = alpha,alpha'-bis{(5,7-dimethyl-1,4,8,11-tetraazacyclotetradeca-6-yl)-o-xylene), in which the bromide-bridged, bimetallic, macrocyclic ligand complexes of nickel(II) are held face-to-face and in which each bimetallic complex has a net triplet spin multiplicity. The ambient structure of this complex consists of very highly ordered, infinite chains of alternating R and S isomers in which the identical Ni(II) coordination spheres are near to the average expected for the high- and low-spin Ni(II) coordination sites, and there is appreciable stereochemical strain in the linkage of the macrocyclic ligands to the phenyl ring. In contrast, every other dinickel complex of the 100 K structure is displaced about 40 pm along the infinite chains to form tetrameric repeat units (pairs of dinickel complexes), in which each dinickel complex has well-defined high-spin and low-spin Ni(II) coordination sites; the high-spin sites are adjacent in the tetramers, and the stereochemical strain in the linkage to the phenyl spacer is relaxed. The molecular magnetic moments and structural contrasts are similar for the 100 K structure and the previously reported ambient structure of [L(2)Ni2Br3](ClO4) complex for which the molecular magnetic moments also correspond to a single triplet state per complex. The halide-bridged, monochloro- and monobromo dinickel complexes also have triplet spin multiplicity, and they crystallize with a coordinated perchlorate completing the axial coordination of the high-spin Ni(II) site, while the other Ni(II) site of these halide-bridged complexes has equatorial Ni-N bond lengths typical of low-spin Ni(II) coordination. The bridging halide is sandwiched between the face-to-face macrocyclic ligand Ni(II) moieties and slightly off the Ni-Ni axis in all of the complexes. The temperature dependence of the magnetic moments of the series of complexes indicates that their singlet-triplet energy gaps are small, with zero point energy differences that are generally less than 10(3) cm(-1). The very weak metal-metal electronic coupling, the triplet state spin multiplicity of each dinickel complex, and the averaged high-spin/low-spin coordination environments of the ambient structure implicate a vibronic mechanism for the electronic configurational exchange in the dibromo and tribromo complexes. The single molecular vibrational mode that correlates with the configurational exchange in these complexes includes the concerted motion of the bridging bromide between the Ni(II) centers. Activation of this vibrational mode is sufficient to effect the configurational exchange. These complexes present especially clear examples of the effects of the coupling of nuclear vibrational motions to the interchange of electronic configuration between two different centers.     

Untersuchungen über Perylen und seine Derivate

Ohne ZusammenfassungDie Mikroanalysen wurden im Institut für Pharmazeutische Chemie durch Herrn Dr.Fritz Stimler ausgeführt.Meinem Lehrer Prof. Dr.Alois Zinke sage ich an dieser Stelle für wertvolle Anregungen während meiner Arbeit den herzlichsten Dank.     

Über Monochloraldehyd

Ohne ZusammenfassungNachdem es mir zuerst gelungen ist, das Monochloraldehyd rein zu erhalten, glaube ich mir das Recht der weiteren Untersuchung wahren zu dürfen.Schliesslich sei mir gestattet, auch an dieser Stelle meinem hochverehrten Lehrer, Herrn Prof. A. Lieben für die freundliche Unterstützung, die er mir bei Ausführung dieser Arbeit zu Theil werden liess, meinen Dank auszusprechen.