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  • 1
    ISSN: 1573-904X
    Keywords: danazol ; isodanazol ; impurity profiling ; identification ; isolation ; HPLC ; NMR spectroscopy ; HPLC-UV
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract We report on a new isomeric impurity of danazol. This impurity designated as isodanazol was detected by reversed-phase high-performance liquid chromatography (HPLC) and thin-layer chromatography (TLC). Its structure was determined after separation by preparative HPLC. Mass spectrometry revealed the isomeric nature of the impurity while the UV spectrum indicated profound difference in the isoxazole moieties. The structure of the isomeric isoxazole ring in isodanazol was determined by NMR spectroscopy using COSY, HETCOR and NOE measurements. The difference between the U V spectra of danazol and isodanazol is explained on the basis of the difference between the aromaticities of their isoxazole rings supported by quantum chemical calculations. The quantitative determination of the impurity down to the 0.05% level can be performed by HPLC, gas chromatography and TLC densitometry.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Liebigs Annalen 1999 (1999), S. 2119-2125 
    ISSN: 1434-193X
    Keywords: Polymorphism ; Quinoxalines ; X-ray analysis ; Quantum chemical calculations ; Molecular mechanical calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Synthesis of 2(1H)-quinoxalinone O-(2′-quinoxalinyl)oxime (3) results in two easily distinguishable crystal forms with different colours, providing a rare example of colour polymorphism. The crystal structures of the two forms have been determined and compared. Intermolecular interactions and light-absorption characteristics of the polymorphs have been analysed by a mixed quantum chemical/molecular mechanical method. In one of the polymorphic forms the crystal field stabilises relatively high energy conformations by enhanced electrostatic interactions. The difference in the light absorption is found to be caused partly by a conformational difference of the molecules. The crystal field increases the difference in the absorption wavelengths and shift them towards higher values.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 53 (1995), S. 485-493 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The energy of weakly overlapping group functions can be written as a series according to the powers of the (σ - I) matrix, where σ is the molecular overlap matrix and I is the unit matrix [1,2]. This power series of the energy is studied by investigating the importance of different order terms to obtain accurate energies and to predict equilibrium bond lengths. It is found that the series is truncated advantageously at an even-order term. Approximate formulas for the first- and second-order terms are proposed in order to reduce computational work. Numerical examples are presented to illustrate the effect of these terms to the energy. The relation of the projection energy to the approximate first- and second-order terms is also discussed. It is found that, by choosing appropriate projection factors, the projection energy corrects the zeroth-order energy more efficiently than does the first-order term. The inclusion of the approximate second-order term represents a slight improvement with respect to the use of the projection energy at the expense of some extra computation. © 1995 John Wiley & Sons, Inc.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 57 (1996), S. 361-367 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The orthogonal group function approach, as based on the Huzinaga equation, is extensively applied in reduced basis frozen-core calculations. Although the theory is developed for orthogonal electronic groups, the use of reduced basis sets prevents strict orthogonality and the formalism is complemented to take, partially, into account nonorthogonality (projection factors, projection energy). In the present article, an alternative to this approach, based on the nonorthogonal formalism, is proposed. An orbital equation is derived from the Adams-Gilbert equation and the energy is evaluated according to a recent proposal based on the power-series expansion of the overlap energy. A comparative overview of the orthogonal and nonorthogonal formalisms is presented and the results of reduced basis frozen-core calculations as obtained with the two methods are compared. It is found that the nonorthogonal formulation predicts equilibrium geometrical parameters in some cases similarly and, in other cases, slightly better than does the orthogonal one. Based on this observation and on the fact that the nonorthogonal formulation is exempt from empirical parameters (projection factors), it is concluded that the nonorthogonal formalism represents an appealing alternative in reduced basis frozen-core calculations. © 1996 John Wiley & Sons, Inc.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 19 (1998), S. 38-50 
    ISSN: 0192-8651
    Keywords: quantum mechanics ; molecular mechanics ; self-consistent Madelung potential ; polar molecular crystals ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: The self-consistent Madelung potential (SCMP) approach for calculating molecular wave functions for a subunit embedded in a symmetrical environment constituted by the copies of the subunit is implemented with semiempirical NDDO model Hamiltonians and supplemented with empirically parameterized dispersion-repulsion interaction potentials. This model yields sublimation enthalpies in good agreement with available experimental data for a series of molecular crystals, including imidazol, benzimidazole, urea, urethane, dicyaneamide, formamide, uracil, cytosine, maleic anhydride, succinic anhydride, and 1,3,5-triamino-2,4,6-trinitro-benzene. The SCMP-NDDO method, which avoids difficulties concerning the parametrization of charges in the molecular mechanics force fields, is proposed mainly for the treatment of molecular crystals with large unit cells. It might be particularly useful where important charge reorganization is expected under the effect of the crystal field. Charge distributions, obtained by the SCMP and the simple dielectric cavity self-consistent reaction field models, are compared and analyzed.   © 1998 John Wiley & Sons, Inc.   J Comput Chem 19: 38-50, 1998
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
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