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  • Theoretical, Physical and Computational Chemistry  (3)
  • 1995-1999  (3)
  • 1
    Electronic Resource
    Electronic Resource
    New aspects in the theory of π electron systems on the basis of quantum statistical considerations (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 69 (1998), S. 727-752 
    Details
    ISSN: 0020-7608
    Keywords: π electron systems ; quantum statistics ; electronic counting rules ; quantum Monte Carlo simulations ; model Hamiltonians ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electronic properties of π systems are studied under special consideration of the quantum statistical constraints experienced by a fermionic ensemble. In a many-electron basis of atomic occupation numbers these constraints decompose into a formal on-site constraint and a formal intersite constraint. The on-site constraint can be equated with the Pauli exclusion principle (PEP) while the intersite constraint can be equated with the Pauli antisymmetry principle (PAP). Under special molecular topologies the intersite constraints of fermion ensembles are suppressed. In this case the conventional fermionic statistics coincides with a mixed quantum statistics with fermionic on-site and bosonic intersite properties. Such a mixed statistics is realized in the π subspace of polyenes, (4n+2) Hückel annulenes (n=0, 1, 2,…) and the odd spin space of (2n+1) annulenes (n=1, 2, 3,…) if the π electron hoppings are restricted to nearest-neighbor centers. We discuss the topological conditions to conserve this statistical peculiarity at least approximately in two-dimensional (2D) π topologies. The quantities “aromaticity” and “antiaromaticity,” widely used in the chemical literature, are traced back to quantum statistical, topological, and molecular size considerations. The competition between the quantum constraints PEP and PAP, on the one hand, and the strength of the two-electron interaction in a given π Hamiltonian, on the other, is analyzed on the basis of Pariser-Parr-Pople (PPP), Hubbard (Hu), and simple Hückel molecular orbital (HMO) calculations. The influence of the PAP is reduced with increasing correlation strength while the influence of the PEP does not depend on this coupling parameter. The numerical results have been derived by Green's function quantum Monte Carlo (GF QMC) simulations.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 727-752, 1998
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    BaxC60 fullerides: π Electronic peculiarities of the C60 molecule and their consequences for the solid state (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 333-373 
    Details
    ISSN: 0020-7608
    Keywords: alkaline-earth doped C60 fullerides ; C60 π electronic structure ; band structure properties ; quantum statistics of electronic systems ; Pauli antisymmetry principle ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electronic structure of BaxC60 fullerides was studied theoretically under special consideration of π electronic effects in the C60 molecule. Band structure data were derived by an intermediate neglect of differential overlap (INDO) crystal orbital (CO) approach. Different electronic configuration were evaluated in the Ba-doped C60 fullerides. BaxC60 solids with x=0, 3, 4, 6 are insulators. For a Ba5C60 model extrapolated from the crystal structure of Ba6C60, a finite band gap is also predicted. For a Ca5C60-like structure of Ba5C60, a quasi-degeneracy between a metallic configuration and an insulating Mott-like state was found. With an increasing Ba-to-C60 charge transfer (CT), sizable changes in the π system of C60 occur. In the neural molecule and for not too high an electron count, the π electrons form more or less electronically isolated hexagon-hexagon (6-6) “double” bonds with only minor hexagon-pentagon (6-5) “double-bond” admixtures. In the vicinity of C6012-, the 6-6 bonds have lost most of their double-bond character while it is enhanced for the 6-5 bonds. In highly charged anions, the π electron system of the soccer ball approaches a configuration with 12 decoupled 6π electron pentagons. For electron numbers between C60 and C6012-, the net π bonding is not weakened. The INDO CO results of the BaxC60 solids are supplemented by INDO MO and ab initio (3-21 G* split-valence basis) calculations of molecular C60 and some highly charged anions. Ab initio geometry optimizations show that the bond alternation of C60 with short 6-6 and long 6-5 bonds is inverted in C12-60. The high acceptor capability of C60 is explained microscopically on the basis of quantum statistical arguments. In the π electron configurations of C60 and C6012-, the influence of the Pauli antisymmetry principle (PAP) is minimized. The quantum statistics of (π) electron ensembles with a deactivated PAP is of the so-called hard-core bosonic (hcb) type. In these ensembles, the on-site interaction is fermionic while the intersite interaction is bosonic. Energetic consequences of the quantum statistical peculiarities of π systems are explained with the aid of simple model systems; we selected annulenes and polyenes. Computational tools in this step are Green's function quantum Monte Carlo (GF QMC) and full configuration interaction (CI) calculations for the π electrons of the model systems. These many-body techniques were combined with a Pariser-Parr-Pople (PPP) Hamiltonian.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 333-373, 1997
    Additional Material: 19 Ill.
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  • 3
    Electronic Resource
    Electronic Resource
    A theoretical study of bond energies in model Si-H-Cl molecules using density functional approaches for representing Si surface chemistry (1997)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 18 (1997), S. 2075-2085 
    Details
    ISSN: 0192-8651
    Keywords: silicon ; ab initio ; density functional ; molecular orbital calculations ; effective core potentials ; surface chemistry models ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: The reliability of density functional theory (DFT) methods for calculating Si(SINGLE BOND)2H, Si(SINGLE BOND)Cl, and Si(SINGLE BOND)Si bond energies is examined in reactions involving molecules and small clusters representing various surface sites appropriate for Si surface chemistry. Results are presented for systematic studies using a valence double-zeta polarization basis for both all-electron calculations and valence-electron calculations employing effective core potentials (ECPs). All-electron DFT results are comparable to much more demanding MP4, G2, and MC-SCF-CI calculations for computed bond energies. Whereas the use of ECPs introduces systematic energy differences of ca. 3-5 kcal/mol compared to AE results, depending on the type of bond involved, the use of ECPs for carrying out calculations on larger clusters is discussed where AE calculations become more computationally demanding. The convergence of Si bond energies as a function of replacing hydrogens with silyl groups is examined. In constructing models to describe etching processes involving Cl species on Si surfaces, the need for incorporating differences in thermochemistries for one-, two-, and three-coordinate Si surface sites is emphasized. Comparisons of semiempirical approaches for thermochemistries of Si-containing species find these methods somewhat less reliable for obtaining reliable bond energies compared to computationally more demanding DFT and ab initio correlated models.   © 1997 John Wiley & Sons, Inc.   J Comput Chem 18: 2075-2085, 1997
    Additional Material: 1 Ill.
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