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  • 1
    Electronic Resource
    Electronic Resource
    How to Derive Force Field Parameters by Genetic Algorithms: Modelling tripod-Mo(CO)3 Compounds as an Example (1998)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1998 (1998), S. 693-702 
    Details
    ISSN: 1434-1948
    Keywords: Genetic Algorithms ; Force field calculations ; Refinement of force field parameters ; Tripod metal compounds ; Conformational analysis ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Force field parameters used to describe the conformation of coordination compounds involving transition metals are generally derived by a trial-and-error procedure, until a somehow satisfying agreement between the calculated and observed conformations of a few members of a class of related compounds is reached. It is shown in this paper that a more general and less biased alternative is available, applicable to many structures at a time. Genetic Algorithms will effectively optimize force field parameters in an automatic way, on the basis of a potentially exhaustive set of all the structural data available for a given class of compounds. The feasibility of this procedure has been demonstrated by the derivation of force field parameters describing the conformational behaviour of tripod-Mo(CO)3 compounds [tripod= RCH2C(CH2X)(CH2Y)(CH2Z), X,Y,Z = PR′R′′)] by simultaneous optimization based on the structure of ten individual molecules. With the force field parameters relevant to the organic part of these compounds taken from MM2*, the parameters involving contributions from the Mo center were refined. The agreement between observed and calculated structures is characterized by an rms deviation of around 0.3 Å for the ten structures contained in the data base. To assess the validity of this approach, the conformational space of CH3C(CH2PPh2)3Mo(CO)3 was explored exhaustively. A contour diagram representing the relative energy of the molecule with respect to the rotational positions of its phenyl groups was found to effectively reproduce the scatter of these conformational parameters as earlier derived from an analysis of 82 relevant compounds. - As a further assessment, the conformational space of CH3C[CH2P(o-Tol)2]3Mo(CO)3, which was not included in the data base, has been analyzed. It is found that the structure corresponding to the global energy minimum corresponds to that observed in the crystal with an rms deviation of only 0.3 Å. The novel approach to problems of this type - Genetic Algorithms had not previously been applied in this context - thus appears promising.
    Type of Medium: Electronic Resource
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    Paper (German National Licenses)
  • 2
    Electronic Resource
    Electronic Resource
    How to Predict Conformations Accessible to a Molecule in Solution: Validation of a Force Field-Based Prediction of NOE Distances by Comparison with the Experimental Data for the Series of Compounds CH3C[CH2P(Bzl)R]3Mo(CO)3 (R = Ph, m-Xyl) (1998)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1998 (1998), S. 1641-1653 
    Details
    ISSN: 1434-1948
    Keywords: tripodMolybdenum(CO)3 compounds ; Conformational ensembles in solution ; Modelling NOE contacts ; MM2* Force field calculations ; Packing forces ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: NMR-NOE analysis of the three compounds (RRS/SSR)-CH3C(CH2PPhBz)3Mo(CO)3 (1), (RRR/SSS)-CH3C(CH2PPhBz)3Mo(CO)3 (2), and (RRR/SSS)-CH3C[CH2P(m-Xyl)Bz]3Mo(CO)3 (3), leads to experimental values for a number of intramolecular H···H contacts in each case. By the very nature of the NOE method, and with the type of molecules studied here, these values have to be understood as a Boltzmann weighted average over all the conformations accessible to the molecules in solution. - A Boltzmann weighted force field approach is used to predict these values on the basis of sets of force field parameters derived earlier for this class of compounds. The agreement between observed and calculated NOE distances is highly satisfactory in each case (rms = 0.2 Å to 0.3 Å). By a statistical analysis it is shown that the predictions made by applying this Boltzmann weighted force field approach are highly significant. There is only a chance of 3 in 100 000 that this quality of prediction might be obtained by chance. This is taken as a validation, albeit indirect, of the energy scale reproduced by the force field parameters as developed. The methods described give a detailed insight into the conformational flexibility of molecules of the type studied. Solid state structures of the molecules are also correctly modelled by the force field used.
    Additional Material: 14 Ill.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Dinuclear Cobalt(II) Complexes of Pyrazole Ligands with Chelating Side ArmsDedicated to Prof. Dr. Walter Siebert on the occasion of his 60th birthday. (1997)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 130 (1997), S. 605-613 
    Details
    ISSN: 0009-2940
    Keywords: Pyrazolate complexes ; Dinuclear complexes ; Bridging ligands ; Cobalt ; Conformational analysis ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of pyrazole-based potential ligands bearing polydentate amine substituents in the 3- and 5-positions of the heterocycle has been synthesized [3,5-bis(R2NCH2)-pyzH R2N = Me2N(CH2)3NMe (2aH), [Me2N(CH2)3]2N (2bH), (Et2NCH2CH2)2N (2cH)]. Upon reaction with two equivalents of CoCl2 they form complexes LCo2Cl3 (3a-c; L = 2a-c, respectively) which are shown crystallographically to contain a dinuclear metal core bridged by both the pyrazolate unit and a chlorine atom, with each cobalt center carrying a further terminal chlorine atom. Two of the ligand side arms in 3b, c are dangling, thus leading to five-coordination of the cobalt(II) centers in all cases. Addition of two equivalents of NaBPh4 to solutions of 3b, c induced coordination of the formerly dangling side arms to the metal centers by substitution of the terminal chlorine atoms. The resulting compounds [LCo2Cl](BPh4)2 (4b, c, respectively) were characterized by X-ray structure analyses. They can be viewed as dinuclear linked versions of tran-type complexes [(tran = tris(aminoalkyl)amine] with distorted trigonal-bipyramidal coordination spheres around cobalt(II). Conformational analyses employing force-field calculations were carried out for 4b, c in order to rationalize the conformations observed in the solid state with regard to the accessible conformational space.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19971300511
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  • 4
    Electronic Resource
    Electronic Resource
    Conformation of tripod Metal Templates in CH3C(CH2PPh2)3MLn (n = 2, 3): Neural Networks in Conformational AnalysisDedicated to E. Weiss on the occasion of his 70th birthday. (1996)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 129 (1996), S. 745-757 
    Details
    ISSN: 0009-2940
    Keywords: Conformational analysis ; Triphos ligands ; Neural networks ; Factor analysis ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The conformational space spanned by tripod metal templates CH3C(CH2Ph2)3M is analysed on the basis of the solid-state structures of 82 tripodCo templates in compounds tripodCoL2 and tripodCoL3. Systematic analysis, including the techniques of conformational space group scatter graphs, principal-component analysis, and partial least squares, reveals a series of basic regularities: The torsion of the phenyl groups is strongly linked to the torsional skew of the bicyclooctane-type framework of the chelate cage. For one sense of this skew there are two classes of low-energy conformation that differ by the helicity of the phenyl arrangement and by the degree of torsional skew in the chelate backbone. From the scatter graphs it is evident that a change in helicity may occur by one- or by two-ring flip mechanisms. The basic regularities found by the above methods are also evident from the analysis of the same data by a neural network approach. The fact that these regularities are found for tripodCoL2 and tripodCoL3, irrespective of the widely different coligands L and crystal environments, means that the conformation of the tripod metal templates is governed by the forces acting within them and not so much by the forces imposed on them by their individual chemical or crystal environment. It is shown that the classifications, although derived from a data basis only containing Co compounds, are characteristic for tripod metal templates irrespective of the specific metal involved.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19961290704
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  • 5
    Electronic Resource
    Electronic Resource
    How to Predict Activation Barriers - Conformational Transformations of Compounds CH3C(CH2PPh2)3-n[CH2P(oTol)2]nMo(CO)3 (n = 1-3): Force Field Calculations versus NMR Data (2000)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 2000 (2000), S. 597-615 
    Details
    ISSN: 1434-1948
    Keywords: Tripod molybdenum tricarbonyl compounds ; Conformational analysis in solution ; Dynamic NMR ; Force field calculations ; Activation barriers ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Tripod metal entities tripodM are sterically congested systems. The conformations adopted by compounds CH3C(CH2PPh2)3-n[CH2P(oTol)2]nMo(CO)3 (n = 1: 1, n = 2: 2, n = 3: 3) will thus be largely determined by the repulsive forces acting in these molecules. The steric demand of the o-tolyl groups impedes their free rotation and enantiomerization processes referring to the compounds as a whole are sufficiently slow to permit their analysis by NMR techniques. Through a combination of line-shape analysis, EXSY methods, and coalescence experiments, the ΔG
    Additional Material: 20 Ill.
    Type of Medium: Electronic Resource
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  • 6
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    Sensors, Vol. 18, Pages 4068: Designing a Robust Kelvin Probe Setup Optimized for Long-Term Surface Photovoltage Acquisition (2018)
    MDPI
    In: Sensors
    Details
    Publication Date: 2018
    Description: We introduce a robust low-budget Kelvin probe design that is optimized for the long-term acquisition of surface photovoltage (SPV) data, especially developed for highly resistive systems, which exhibit—in contrast to conventional semiconductors—very slow photoinduced charge relaxation processes in the range of hours and days. The device provides convenient optical access to the sample, as well as high mechanical and electrical stability due to off-resonance operation, showing a noise band as narrow as 1 mV. Furthermore, the acquisition of temperature-dependent SPV transients necessary for SPV-based deep-level transient spectroscopy becomes easily possible. The performance of the instrument is demonstrated by recording long-term SPV transients of the ultra-slowly relaxing model oxide strontium titanate (SrTiO 3 ) over 20 h.
    Electronic ISSN: 1424-8220
    Topics: Chemistry and Pharmacology , Electrical Engineering, Measurement and Control Technology
    Published by MDPI
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