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The push-pull characters of a large series of donor-acceptor substituted azo dyesù71 structures in allùhave been quantified by the NN double bond lengths, dNN, the 15N NMR chemical shift differences, ;;15N, of the two nitrogen atoms and the quotient, ;*/;, of the occupations of the antibonding ;*, and bonding ; orbitals of this partial NN double bond. The excellent correlation of the occupation quotients with the bond lengths strongly infers that both ;*/; and dNN are excellent parameters for quantifying charge alternation in the push-pull chromophore and the molecular hyperpolarizability, ;0, of these compounds. By this approach, selected compounds can be appropriately considered as viable candidates for nonlinear optical (NLO) applications.
The through space NMR shieldings (TSNMRS) of dodecahedrane C20H20, of the isomeric hydrocarbons C20H12, of the ions C20H122+ and C20H122- of the fluxional fullerene C20 and of its dication C202+ have been ab initio calculated employing the NICS concept on basis of MP2/6-31G* geometries and visualized as iso-chemical-shielding/deshielding surfaces (ICSSs). TSNMRS values were employed to study the exohedral magnetic properties of the compounds studied. Hereby, the curved It-conjugation in the compounds studied could be quantified.
Identification of benzenoid and quinonoid structures by through-space NMR shieldings (TSNMRS)
(2010)
A new approach to analyze multi-component Saturation Transfer Difference (STD) NMR spectra by combining the STD and the DOSY experiment is proposed. The resulting pulse sequence was successfully used to simplify an exemplary multi- component protein/substrate system by means of standard DOSY processing methods. Furthermore, the same experiment could be applied to calculate the ratio of saturated substrate molecules and its saturation rate in the case of competitive interactions. This ratio depends on the strength of this interaction between the substrates and the protein, so that this kind of information could be extracted from the results of our experiment.
Intramolecular deactivation processes in complexes of salicylic acid or glycolic acid with Eu(III)
(2010)
The complexation of Eu(III) by 2-hydroxy benzoic acid (2HB) or glycolic acid (GL) was investigated using steady- state and time-resolved laser spectroscopy. Experiments were carried out in H2O as well as in D2O in the temperature range of View the MathML source. The Eu(III) luminescence spectra and luminescence decay times were evaluated with respect to the temperature dependence of (i) the luminescence decay time ;, (ii) the energy of the View the MathML source transition, (iii) the width of the View the MathML source transition, and (iv) the asymmetry ratio calculated from the luminescence intensities of the View the MathML source and View the MathML source transition, respectively. The differences in ligand-related luminescence quenching are discussed. Based on the temperature dependence of the luminescence decay times an activation energy for the ligand-specific non-radiative deactivation in Eu(III)-2HB or Eu(III)-GL complexes was determined. It is stressed that ligand-specific quenching processes (other than OH quenching induced by water molecules) need to be determined and considered in detail, in order to extract speciation- relevant information from luminescence data (e.g., estimation of the number of water molecules nH2O in the first coordination sphere of Eu(III)). In case of 2HB, conclusions drawn from the evaluation of the Eu(III) luminescence are compared with results of a X-ray structure analysis.
4-Alkyl-2,2,6,6-tetramethyl-1,4,2,6-oxaazadisilinanes RN[CH2Si(Me)2]2O [R = Me (1), i-Pr (2)] were synthesized by two methods which provided good yields up to 84%. Low temperature NMR study of compounds (1) and (2) revealed a frozen ring inversion with the energy barriers of 8.5 and 7.7 kcal/mol at 163 and 143 K, respectively, which is substantially lower than that for their carbon analog, N-methylmorpholine. DFT calculations performed on the example of molecule (1) showed that N-Meax conformer to exist in the sofa conformation with the coplanar fragment C-Si-O-Si-C, and its N-Meeq conformer in a flattened chair conformation.
3,4-Dihydro-2-H-pyran and oxalyl chloride react, depending on the conditions, to keto esters, a pyran-3- carboxylic acid or derivatives thereof, or to an hitherto unknown bicyclic acetal containing a vinyl chloride moiety. The structure of the latter product has been unambiguously elucidated by single-crystal X-ray structure analysis. A mechanism for its formation is proposed.
Near edge X-ray absorption. ne structure and X-ray photoelectron spectroscopy have been employed to follow the reversible trans to cis isomerization of tetra-tert-butyl-azobenzene (TBA) adsorbed on Au(111). For one monolayer the molecules adopt an adsorption geometry characteristic of the trans-TBA isomer. The azo-bridge (N = N) is aligned nearly parallel to the surface and the phenyl rings exhibit a planar orientation with a small tilt angle <= 4 degrees with respect to the surface normal. Illumination of the molecular layer at 455 nm triggers the trans to cis isomerization which is associated with a pronounced change of the geometrical and electronic structure. The N1s to pi* transition of the central azo-bridge shifts by 0.45 +/- 0.05 eV to higher photon energy and the transition dipole moment (TDM) is tilted by 59 +/- 5 degrees with respect to the surface normal. The pi-system of one phenyl ring is tilted by about 30 degrees with respect to the surface normal, while the second ring plane is oriented nearly perpendicular to the surface. This reorientation is supported by a shift and broadening of the C-H resonances associated with the tert-butyl legs of the molecule. These findings support a configuration of the photo-switched TBA molecule on Au(111) which is comparable to the cis-isomer of the free molecule. In the photo-stationary state 53 +/- 5% of the TBA molecules are switched to the cis configuration. Thermal activation induces the back reaction to trans-TBA.
Metal-ion-induced self-assembly in aqueous solution of the rigid ligand 1,4-bis(2,2':6',2 ''-terpyridine-4'-yl)benzene (1) with Fe(OAc)(2) and Ni(OAc)(2) is investigated with viscosimetry, SANS, and AFM. Ligand 1 forms extended, rigid-rod like metallo-supramolecular coordination polyeectrolytes (MEPEs) with a molar mass of up to 200 000 g mol(-1) under the Current experimental conditions. The molar mass depends oil concentration, stoichiometry, and time. By spin-coating MEPEs oil a solid surface, we call image the MEPEs in real space by AFM. Both AFM and SANS confirm the extended rigid-rod-type structure of the MEPEs. As a control experiment, we also studied the flexible ligand 1,3-bis[4'-oxa(2,2':6',2 ''-terpyridinyl)]propane (2). Ligand 2 does not form extended macro-assemblies but likely ringlike structures with three 10 four repeat units. Finally, we present it protocol to control the stoichiometry during self-assembly using conductometry, which is of paramount importance to obtain meaningful and reproducible results.
Metal ion induced self-assembly of the rigid ligand 1,4-bis(2,2':6',2 ''-terpyridine- 4'-yl) benzene (1) with Fe(II), Co(II), Ni(II) and Zn(II) acetate in aqueous solution results in extended, rigid- rod like metallosupramolecular coordination polyelectrolytes (MEPE-1). Under the current experimental conditions the molar masses range from 1000 g mol(-1) up to 500 000 g mol(-1). The molar mass depends on concentration, stoichiometry, metal-ion and time. In addition, we present viscosity measurements, small angle neutron scattering and AFM data. We introduce a protocol to precisely control the stoichiometry during self-assembly using conductometry. The protocol can be used with different terpyridine ligands and the above-mentioned metal ions and is of paramount importance to obtain meaningful and reproducible results. As a control experiment we studied the mononuclear 4'- (phenyl)2,2':6',2 ''-terpyridine (3) complex with Ni(II) and Zn(II) and the flexible ligand 1,3- bis[4'-oxa(2,2': 6',2 ''-terpyridinyl)] propane (2) with Ni(II) acetate (Ni-MEPE-2). This ligand does not form extended macroassemblies but likely ring-like structures with 3 to 4 repeat units. Through spin- coating of Ni-MEPE-1 on a solid surface we can image the MEPEs in real space by AFM. SANS measurements of Fe-MEPE-1 verify the extended rigid-rod type structure of the MEPEs in aqueous solution.