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  • 1
    ISSN: 1432-2234
    Keywords: Key words: Dipole moments – Molecular electrostatic potential
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract.  To achieve an improved description of solvation effects in the newly derived version MSINDO of the SINDO semiempirical formalism an appropriate parameterization of dipole moments and electrostatic potentials was undertaken. The mean error of the dipole moment could be reduced by an appropriate choice of orbital exponents. It is also shown that the approximation of the molecular electrostatic potential (MESP) in the asymptotic density model (ADM) implemented in MSINDO results in a superior description compared to the previous SINDO1 implementation. The accuracy is demonstrated for a selected number of small molecules with carbon, nitrogen and oxygen atoms.
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  • 2
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    Electronic Resource
    Springer
    Theoretical chemistry accounts 16 (1970), S. 95-106 
    ISSN: 1432-2234
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Zusammenfassung Linderbergs Formel zur Berechnung des Resonanzintegrals wird weiter untersucht. Konsistenzforderungen für dieα-Parameter führen zu einer Erklärung der Wolfsberg-Helmholz-Form fürβ. Die einfache Annahme, daßβ dem Überlappungsintegral proportional ist, wird verbessert. Die Abhängigkeit vonα- undβ-Werten vom Atomabstand in einem zweiatomigen Molekül wird in dieser Näherung angegeben und diskutiert.
    Abstract: Résumé Etude de la formule proposée par Linderberg pour le calcul de l'intégrale de résonance. Des nécessités de cohérence pour les paramètresα conduisent à une explication de la formule de Wolfsberg-Helmholz pourβ. L'hypothèse simple selon laquelleβ est proportionnel à l'intégrale de recouvrement est améliorée. On présente en la discutant la dépendance à la distance des valeurs de α et deβ dans une molécule diatomique dans cette approximation.
    Notes: Abstract The formula proposed by Linderberg for the evaluation of the resonance integral is further investigated. Consistency requirements for theα parameters lead to an explanation of the Wolfsberg-Helmholz form forβ. The simple assumption thatβ is proportional to the overlap integral is improved. The dependency of a andβ values on the distance in a diatomic molecule in this approximation is presented and discussed.
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Theoretical chemistry accounts 20 (1971), S. 411-412 
    ISSN: 1432-2234
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
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  • 4
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    Electronic Resource
    Springer
    Theoretical chemistry accounts 23 (1971), S. 93-97 
    ISSN: 1432-2234
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Formulas for non-nearest neighbor β parameters over orthogonal orbitals are derived and discussed for the 2pπ-orbitals of benzene and cis- and trans-butadiene.
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  • 5
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    Electronic Resource
    Springer
    Theoretical chemistry accounts 25 (1972), S. 10-16 
    ISSN: 1432-2234
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Zusammenfassung Die Gesamtenergie der niedrigsten 1 Π u, 3 Π u, 1 Π g, 3 Π g Zustände des H2 wurde in einem erweiterten Hartree-Fock Verfahren mit einer Doppelkonfiguration minimisiert. Die Exponenten der Basisfunktionen 1s, 2s, 2pσ, 2pπ, 3dπ wurden unabhängig für alle vier Zustände bei verschiedenen Kernabständen optimisiert. Die Ergebnisse zeigen eine starke Zustands- und Abstandsabhängigkeit der Exponenten. Diese können nicht allgemein durch atomare oder Gleichgewichtswerte dargestellt werden. Die Einzelheiten des Optimisierungsverfahrens werden angegeben.
    Abstract: Résumé Minimisation de l'énergie totale des plus bas états 1 Π u, 3 Π u, 1 Π g, 3 Π g de H2 dans un procédé Hartree-Fock à deux configurations. Les exposants des fonctions de la base: 1s, 2s, 2pσ, 2pπ, 3dπ ont été optimisés séparément pour les quatre états pour différentes distances internucléaires. Les résultats montrent que les exposants dépendent fortement de l'état et de la distance. Ils ne peuvent pas être généralement représentés par des valeurs atomiques et des valeurs d'équilibre. Description des détails du processus d'optimisation.
    Notes: Abstract The total energy of the lowest 1 Π u, 3 Π u, 1 Π g, 3 Π g states of H2 was minimized in an extended Hartree-Fock procedure employing a double configuration wavefunction. The exponents of the basis functions 1s, 2s, 2pσ, 2pπ, 3dπ were optimized separately for all four states at various internuclear distances. The results show that the exponents are strongly state and distance dependent. They cannot be generally represented by atomic or equilibrium values. The details of the optimization process are presented.
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  • 6
    ISSN: 1432-2234
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Zusammenfassung Verschiedene Approximationsstufen (Hartree-Fock, Konfigurationenwechselwirkung und Doppelkonfigurationen-Hartree-Fock-Methode) werden für ausgedehnte und begrenzte Exponentenoptimisierung von Atomorbitalen der Wellenfunktionen verglichen. Die Potentialkurven für die niedrigsten 1 Π u, 3 Π u, 1 Π g, 3 Π g Zustände des Wasserstoffmoleküls werden angegeben. Die Form der Kurven im Rahmen der besten Näherung, d. h. mit Doppelkonfiguration, stimmen im wesentlichen mit früheren aufwendigeren Rechnungen überein. Der Einfluß der verschiedenen Approximationen wird auch an einigen Einelektroneneigenschaften studiert: Ladungsverteilung der Wellenfunktion längs und senkrecht zur Molekülachse, Quadrupolmoment und Verteilung der Rumpfenergie. Unterschiede erscheinen zur Arbeit von Zemke et al. [1], die einen größeren Basissatz mit begrenzter Optimisierung verwandten, bei den Π g Zuständen, wo die π-Orbitale sehr diffus sind. Die Unterschiede betreffen Größe und Lage der Minima und Maxima der Potentialkurven sowie beträchtliche Änderungen in solchen Einelektroneneigenschaften, die stark von der räumlichen Verteilung der Orbitale abhängen.
    Abstract: Résumé Comparaison de différents niveaux d'approximation (Hartree-Fock, interaction de configuration et Hartree-Fock à deux configurations) pour des optimisations étendues et limitées des orbitales atomiques de base. Calcul des courbes d'énergie potentielle pour les plus bas états 1 Π u, 3 Π u, 1 Π g, 3 Π g de la molécule d'hydrogène. Pour la fonction d'onde la plus raffinée: H.F. à deux configurations, la forme des courbes est en accord avec les résultats obtenus dans des travaux précédents plus complexes et plus coûteux. On étudie aussi l'influence des diverses approximations sur plusieurs propriétés monoélectroniques: distribution de charge le long de l'axe moléculaire et perpendiculairement à celui-ci, moment quadrupolaire et distribution de l'énergie d'attraction de coeur. On trouve des différences avec le travail de Zemke et al. (1), qui utilisent une plus grande base partiellement optimisée, pour les états Π g où les orbitales π sont très diffuses. Les différences concernent la grandeur et la position des extrema des courbes de potentiel, ainsi que des variations importantes des propriétés monoélectroniques qui dépendent fortement de la distribution spatiale des orbitales.
    Notes: Abstract Various levels of approximation (Hartree-Fock, configuration interaction and double-configuration Hartree-Fock method) are compared for extensive and limited exponent optimization of the atomic orbitals of the wavefunctions. The potential energy curves for the lowest-lying 1 Π u, 3 Π u, 1 Π g, 3 Π g states of the hydrogen molecule are presented. The shapes of the curves on the highest level of approximation, i.e. with the optimal double-configuration wavefunction, are basically in agreement with previous, more sophisticated and time-consuming work. The influence of the various approximations is also studied for several one-electron properties: charge distribution of the wavefunction along and perpendicular to the molecular axis, quadrupole moment and core attraction energy distribution. Differences arise to the work of Zemke et al. [1], who used a limited exponent optimization with a larger basis set, in the Π g states where the π orbitals are very diffuse. The differences concern magnitude and location of minima and maxima of potential curves, as well as considerable changes in one-electron properties which depend strongly on the spatial distribution of the orbitals.
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  • 7
    Electronic Resource
    Electronic Resource
    Springer
    Theoretical chemistry accounts 31 (1973), S. 63-73 
    ISSN: 1432-2234
    Keywords: Atomic charges ; Dipole moments
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract A new definition of atomic charges in molecules is presented which conserves charge and dipole moment. It contains the Mulliken and Löwdin definitions as special cases of zero and first order truncations of commutator expansions. The definition allows for a systematic improvement of charges paralleling the improvement of the basis set in the LCAO approximation. We have tested the definition in thirteen selected diatomics and polyatomics in optimal minimal Slater basis set SCF calculations by means of 4G-level Gaussian expansions. The results suggest that the proposed definition is better than either Mulliken's or Löwdin's definition.
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  • 8
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    Electronic Resource
    Springer
    Theoretical chemistry accounts 34 (1974), S. 213-224 
    ISSN: 1432-2234
    Keywords: Cyclopropyl cation ; Allyl cation ; Ethylene dimerization ; Cyclobutane ; Symmetry governed reactions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Two symmetry governed reactions, the electrocyclic transformation of planar cyclopropyl cation to allyl cation and the dimerization of ethylene to cyclobutane, are examined using a modified INDO method. Results for the cyclopropyl-allyl cation reaction agree well with previously publishedab initio results, and are much improved over previously published CNDO results. The symmetry-allowed disrotatory path is predicted to be significantly favored over the forbidden conrotatory transition. For the ethylene-cyclobutane system two surprising results are predicted within the constraints imposed upon the reaction path: first, that the entire reaction should occur within a small range in the separation of the two ethylene molecules as they approach one another, and second, that the symmetry-forbidden [2 s +2 s addition should be slightly favored over the symmetry-allowed [2 s +2 a addition. Since the Woodward-Hoffmann rules deal exclusively with changes in electronic energy, it is suggested that they should be applied with some caution to reactions in which changes in nuclear repulsion are quite large during the reaction process.
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  • 9
    Electronic Resource
    Electronic Resource
    Springer
    Theoretical chemistry accounts 35 (1974), S. 98-98 
    ISSN: 1432-2234
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
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  • 10
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    Electronic Resource
    Springer
    Theoretical chemistry accounts 35 (1974), S. 381-382 
    ISSN: 1432-2234
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
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