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  • 1985-1989  (10)
  • 1
    Electronic Resource
    Electronic Resource
    Silicon and germanium clusters. A theoretical study of their electronic structures and properties (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 3301-3310 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Silicon and germanium clusters containing three to seven atoms have been studied with the pseudopotential MO-LCAO method followed by configuration interaction procedure. Si and Ge clusters have very similar electronic structures and consequently analogous physico-chemical properties but differ substantially from small carbon clusters. Linear structures are clearly less favorable than more compact structures. On the other hand, some planar geometries possess considerable stability. The Si and Ge clusters which are sections of the diamond-type crystal lattice are less stable than clusters which can be considered as segments of closed-packed lattices or as steps in pentagonal crystal growth. The reason is that the majority of atoms in small clusters are surface atoms which cannot assume the tetrahedral coordination characteristic of Si and Ge bulk atoms. The appearance of typical bulk properties is expected only for very large Si and Ge clusters with small surface atoms/bulk atoms ratio.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.450262
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  • 2
    Electronic Resource
    Electronic Resource
    Alternancy symmetry: A unified viewpoint (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 1722-1735 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A general formulation of the alternancy symmetry adaptation for the semiempirical Pariser–Parr–Pople (PPP) type Hamiltonians is presented at both the spin-orbital and spin-adapted many-electron levels. The derivation of the general form of the alternancy symmetry conjugation operators is based solely on the tight-binding approximation for the short range one-particle part of the Hamiltonian considered. It starts by a simple formulation of the desired invariance properties of the PPP type Hamiltonian. Using algebraic properties of the unitary group generators and of their particle number nonconserving extensions, it leads to a completely explicit and general form for the alternancy symmetry conjugation operators. In this way the prior descriptions, which become special cases of this general formulation, are interrelated and unified. The spin and quasispin character of certain components of these operators are also pointed out and explicitly derived. The spin-adapted version is based on the unitary group formulation of the valence bond-type approach. A completely general formulation is given which applies to many-electron states of an arbitrary multiplicity of neutral systems, either of the normal kind with an identical number of starred and nonstarred sites, or of the radicaloid character with different numbers of starred and nonstarred sites. An explicit form of the dependence of the relative phase factor of the alternancy symmetry conjugation operator on the total spin, total electron number and on the number of starred and nonstarred sites is also given. General rules for the construction of spin and alternancy symmetry-adapted states are illustrated on a few simple examples. Finally, a brief discussion of the implications of the alternancy symmetry is presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449359
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    Investigation of the electronic and geometric structure of small Li anionic clusters with quantum chemical CI procedure (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 5657-5662 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Geometric structure of Li−n (n=2–9) clusters is determined with the analytical gradient minimization procedure of the Hartree–Fock energy using a relatively small AO basis set. The energies of the Li−n clusters for the SCF optimized geometries are calculated with the multireference diexcited configuration interaction method. The geometries of Li−n (n〈5) differ appreciately from those of neutral as well as of cationic Li clusters. Linear geometries of Li− trimer and Li− tetramer can be ascribed to the electrostatic repulsion. The atomization energy of Li−n clusters as function of cluster nuclearity shows a generally increasing tendency. The calculated electron affinities of Lin clusters exhibit features which are in details analogous to the features of the adiabatic electron affinities determined experimentally for Cun clusters.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454526
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  • 4
    Electronic Resource
    Electronic Resource
    Ab initio configuration interaction study of mixed BeLik clusters (k=1–9) (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 5794-5802 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Electronic and geometric structure of mixed neutral BeLik and cationic BeLi+k clusters (k=1–9) has been investigated employing ab initio configuration interaction quantum chemical methods. The values of atomization energies per atom for BeLik with k〈7 and for BeLi+k with k〈8 increase strongly with the cluster size. The binding energy per atom remains nearly unchanged for neutral BeLik with k=7–9 and cationic BeLi+k clusters with k=8 and 9. The quantities which measure cluster stability towards fragmentation processes predict high stabilities for BeLi6 and BeLi+7. The geometries of cluster, their stabilities as well as other properties can be easily interpreted as simple consequences of the nodal properties of the cluster MOs. The connection between the results obtained from this work and from the superatom model has been pointed out. The specific nature of the chemical bonding in mixed clusters obtained from quantum chemical investigations has been analyzed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455555
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  • 5
    Electronic Resource
    Electronic Resource
    Ab initio configuration interaction study of the electronic and geometric structures of small sodium cationic clusters (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 4861-4866 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The favorable geometries of small sodium clusters Na+n (n=3–9) are determined with the analytical gradient method in the framework of the ab initio SCF approximation. The transition from the planar towards three-dimensional cluster geometries is understood in terms of some basic quantum theoretical concepts. The binding energies per atom for Na+n clusters calculated with the MRD CI procedure increase, in general, as a function of n. Nevertheless, the atomization energy per atom as well as the ionization potential as functions of the nuclearity n exhibit well developed oscillations for even–odd n. Consequently, the fragmentation energy for the channel Na+n→Na+n−1+Na shows strong oscillations as well. This explains large abundances of cationic clusters with odd nuclearity found in some detection devices. The possible consequences for the fragmentation process of Na+n is discussed. It is found that the process Na+n→Na+n−2+Na2 is favorable for Na+5, Na+7, and Na+9.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455681
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  • 6
    Electronic Resource
    Electronic Resource
    Theoretical study of the interaction of carbon monoxide with palladium clusters: relations between surface and organometallic chemistry (1987)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 91 (1987), S. 2658-2664 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100294a039
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  • 7
    Electronic Resource
    Electronic Resource
    Theoretical aspects of metal atom clusters (1986)
    s.l. : American Chemical Society
    Chemical reviews 86 (1986), S. 539-587 
    Details
    ISSN: 1520-6890
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/cr00073a004
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  • 8
    Electronic Resource
    Electronic Resource
    Ab initio MRD CI investigation of the optical spectra of C4 and C5 clusters (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 1066-1073 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ground and excited electronic states of linear and rhombic C4 and linear C5 clusters have been studied with ab initio single reference and multireference configuration interaction calculations. The spectrum of linear C4 is characterized by the existence of low-lying Π states at 1–1.5 eV above the 3Σ−g ground state. In rhombic C4, which has very similar ground state energy as the linear form, the first allowed transition is found at 2.4 eV. The optical spectrum of linear C5 exhibits some similarities with that of linear C3: in both molecules the ground state is 1Σ+g and the lowest allowed transition, 1Πu←1Σ+g, is about 3 eV above the ground state.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454276
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  • 9
    Electronic Resource
    Electronic Resource
    Neutrale und geladene Diradikale, Zwitterionen, Trichter auf der S1-Hyperfläche und Protonentranslokation; ihre Bedeutung für den Sehvorgang und andere photochemische und photophysikalische Prozesse (1987)
    Weinheim : Wiley-Blackwell
    Zeitschrift für die chemische Industrie 99 (1987), S. 216-236 
    Details
    ISSN: 0044-8249
    Keywords: Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Um die Struktur von Photoprodukten verstehen zu können, ist es wesentlich, diejenigen Geometrien zu kennen, bei denen Moleküle aus angeregten Zuständen in den Grundzustand (S0) zurückkehren. In Frage kommen dabei vor allem Geometrien, die lokalen Minima auf den Potentialhyperflächen der S1- (energieärmstes angeregtes Singulett) oder T1-Zustände (energieärmstes Triplett) oder konischen Durchschneidungen („Trichter“, „Funnels“) der Hyperflächen der S0- und S1-Zustände entsprechen. Obwohl für ausreichend kleine Moleküle derartige Geometrien heute bereits, wenn auch mit großem Aufwand, numerisch bestimmt werden können, wären qualitativere, damit aber zugleich allgemeinere Aussagen über diejenigen Geometrien photochemischer Systeme, bei denen sich die Hyperflächen der S0- und S1-Zustände nähern, von Interesse. Viele dieser Geometrien sind mit einem diradikaloiden Charakter des Systems verknüpft, so daß es sinnvoll ist, die Eigenschaften von Diradikalen und verwandten Spezies eingehender zu untersuchen. Nach einem Überblick über das Zweielektronen-Zweiorbitale-Modell für Moleküle mit Diradikaloid-Geometrien werden die Bedingungen für eine Berührung von S0- und S1-Hyperflächen formuliert. Gestützt werden die Ergebnisse des einfachen Modells durch die umfangreicher ab-initio-Konfigurationswechselwirkungs(CI)rechnungen für die Verdrillung von Ethylen im polarisierenden Feld einer positiven Ladung, für die Verdrillung um geladene Doppel-bindungen und um Einfachbindungen zwischen π-Donor und π-Acceptor, sowie durch die Ergebnisse ähnlicher Rechnungen für „push-pull“-substituierte Cyclobutadiene. In einigen Fällen werden nahezu entartete S0-, S1- und T1-Zustände vorhergesagt. Die naheliegenden Konsequenzen, die aus diesen Befunden für die mechanistische Beschreibung der cis-trans-Isomerisierung, der Bildung von TICT-Zuständen (Twisted Intramolecular Charge Transfer), der Protonentranslokation und - möglicherweise - des ersten Schritts beim Sehvorgang zu ziehen sind, werden zusammengefaßt. Darüber hinaus sind die Ergebnisse wichtig für das Verständnis der Regiospezifität von Singulett-Photocycloadditionen.
    Additional Material: 16 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/ange.19870990308
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  • 10
    Electronic Resource
    Electronic Resource
    Neutral and Charged Biradicals, Zwitterions, Funnels in S1, and Proton Translocation: Their Role in Photochemistry, Photophysics, and Vision (1987)
    Weinheim : Wiley-Blackwell
    Angewandte Chemie International Edition in English 26 (1987), S. 170-189 
    Details
    ISSN: 0570-0833
    Keywords: Free radicals ; Zwitterions ; Diradicals ; Quantum chemistry ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A knowledge of the geometries at which excited molecules return to the electronic ground state (S0) is essential for the understanding of the structures of photoproducts. Particularly good candidates are geometries corresponding to local minima on the S1 (lowest excited singlet) and T1(lowest triplet) surfaces, as well as S0-S1 conical intersections (funnels). Given sufficient effort, such geometries can nowadays be found numerically for small enough molecules. Still, it is interesting to ask whether more approximate, but also more general, statements can be made concerning the geometries at which the S0 and S1 surfaces closely approach each other. Since many of these are biradicaloid geometries, it is logical to examine the properties of biradicals and related species at some length. After reviewing the two-electron two-orbital model for molecules at biradicaloid geometries, we formulate the conditions under which the S0 and S1 surfaces touch. The results obtained for the simple model are supported by ab initio large-scale configuration interaction (CI) calculations for the twisting of ethylene in the polarizing field of a positive charge and for the twisting of charged double bonds and π-donor-to-π-acceptor single bonds, and by similar calculations for “push-pull” perturbed cyclobutadienes, some of which are predicted to have nearly degenerate S0, S1, and T1 states. The likely consequences of these results for the detailed description of the mechanisms of cis-trans isomerization, the formation of twisted internal charge-transfer (TICT) states, proton translocation, and possibly of the initial step in vision, as well as for the understanding of the regiospecificity of singlet photocycloaddition, are summarized.
    Additional Material: 16 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/anie.198701701
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