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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 5645-5655 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An effective Hamiltonian in a basis of spin- and space-symmetry adapted configuration state functions (CSF), which includes information from Kohn–Sham density functional theory (DFT), is used to calculate configuration interaction (CI) wave functions for the electronic states of molecules. The method emphasizes on states of multiconfigurational character which cannot be represented by conventional DFT. The CI matrix elements are constructed empirically by using the exact operator and corrections from DFT. Both the optimized KS orbitals from the parent determinant and the corresponding KS potential from the parent state density are used. Depending on their energy gap the CI off-diagonal elements between CSF are exponentially scaled to zero to avoid double counting of electron correlation. The selection of the most important CSF describing nondynamical correlation effects and the use of an approximate resolution of the identity (RI) for the evaluation of the two-electron integrals allows a very efficient DFT/MRCI treatment of molecules with several hundreds of electrons. As applications, the prediction of excitation energies for singlet and triplet states of organic molecules and transition metal complexes, the calculation of electronic circular dichroism spectra and investigations of the energetics of diradicals are presented. It is found, that the new DFT/MRCI approach gives results of high accuracy (rms errors for relative energies <0.2 eV) comparable to those from sophisticated ab initio treatments. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Journal of the American Chemical Society 114 (1992), S. 10542-10547 
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ground and first excited states of the organic dyes quinizarin (Q) and tetrahydroxyphenylporphyrin (TOHP) chemisorbed on γ-Al2O3 were investigated with semiempirical PM3 and multireference CI methods. Q/γ-Al2O3 and TOHP/γ-Al2O3 represent promising materials for frequency domain optical data storage (FDOS) based on persistent spectral hole burning (PSHB), since it is possible to detect spectral holes at high temperatures (77 K). To find out whether differences in the electronic structure between the free and the adsorbed molecules account for the hole-burning properties, we performed calculations of Q–Al complexes with various coordination spheres. The calculated vertical excitation energies for the first electronically excited state of the investigated structures are in good agreement with experimental data. Analysis of the molecular orbitals shows that complexation of the investigated molecules with Al affects the electronic structure of the dyes only little. Furthermore, the character of the first excited state is conserved. The change of the dipole moment between the ground and the first excited states is about the same for the free and for the complexated systems. Therefore, it is concluded that hole burning of dye molecules chemisorbed on γ-Al2O3 at high temperatures is not due to a change of the electronic structure of the adsorbed dye molecules compared to the free dye molecules, but has to be attributed to the special interactions of Q with the γ-Al2O3 surface. © 1995 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Chirality 10 (1998), S. 147-153 
    ISSN: 0899-0042
    Keywords: Chemistry ; Organic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The synthesis of the chiral 9,12-dimethyl-4-oxa[7]paracyclophane 3 was achieved by the dithia route with subsequent sulfone pyrolysis. The conformational flexibility of the oxamethylene bridge with local Cs symmetry is evident from low temperature NMR experiments. Experimentally, an activation enthalpy of 11.4 kcal/mol is found for this process, which is in good agreement with the rotational barrier derived from semiempirical AM1 calculations (10.2 kcal/mol). Ab initio Hartree-Fock geometry optimizations have been performed for 3 and the corresponding hydrocarbon 9,12-dimethyl[7]paracyclophane 4. The distance of the O-atom to the center of the benzene ring in the structure of 3 is only 2.784 Å, i.e., significantly closer than the analogous C4-benzene ring distance in the slightly more strained 4 (3.112 Å). The enantiomeric separation of (±3) has been achieved by HPLC and the circular dichroism (CD) spectrum is reported. Ab initio all-electron DFT/SCI calculations of the CD spectrum are in good agreement with the experimental data and reveal the importance of p(O-atom)→ π* charge-transfer type excited states at relative low energies (6.6 eV, 188 nm) responsible for an intense negative CD band. The benzene type π → π* states are energetically lowered by 0.4-0.8 eV due to the boat-type deformation of the benzene moiety. By comparison of theoretical and experimental CD data the absolute configuration of 3 is assigned as (-)-(S). Chirality 10:147-153, 1998. © 1998 Wiley-Liss, Inc.
    Additional Material: 5 Ill.
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  • 5
    ISSN: 1434-193X
    Keywords: Cycloadditions ; Photochemistry ; Cage compounds ; Quinones ; Density functional calculations ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Thermal addition of o-benzoquinones to barrelenes (bicyclo[2.2.2]octatrienes) produces tetracyclic trienediones of type 1. These multichromophoric molecules offer a plethora of photochemical reactions of which the most interesting are (1) rearrangement and bisdecarbonylation of the enedione chromophore and (2) [2π + 2π] intramolecular cycloaddition of the face-to-face double bonds. Theoretical methods (semiempirical as well as density functional) were also used to investigate the spin multiplicity diversity of these reactions. The diketone 7 (bicyclo[2.2.2]octa-5,7-diene-2,3-dione) was investigated using He(I) photoelectron spectroscopy.
    Additional Material: 8 Ill.
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  • 6
    ISSN: 1434-193X
    Keywords: Chirality ; Circular dichroism ; Cycloadditions ; Fullerenes ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of regioisomeric trisadducts of C60 (5-11, 14-18) having the positional relationships e and trans-n (n = 2-4) resulting from successive additions of malonates or bis(oxazolines) to [6,6]-double bonds of the fullerene framework has been isolated and characterized. The new adducts 8-11 and 15-17 represent examples of unprecedented addition patterns. The addition patterns of the new trisadducts with C2, Cs or C1 symmetry have been assigned on the basis of the known positional relationships of the addends in their precursor bisadducts, and those of the C2- or Cs-symmetric representatives have been verified by analysis of their NMR spectra. The absolute configurations of the adducts with inherently chiral addition patterns could be determined either by comparison of the calculated and experimental CD spectra of the bis(oxazoline) adducts or with knowledge of the absolute configurations of the chiral bisadduct precursors containing bis(oxazoline) addends. The CD spectra of the pairs of diastereomers with an enantiomeric addition pattern fA -15/fC -15, fA -16/fC -16, and fA -18/fC -18 show mirror image behavior and pronounced Cotton effects.
    Additional Material: 6 Ill.
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  • 7
    ISSN: 1434-193X
    Keywords: Cyclophanes ; Dilution principle ; Heterocycles ; Medium membered rings ; Ring strain ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The N-protected 1,10-diaza[2.2]metacyclo- and 1,10-diaza-[2]metacyclo[2](2,6)-pyridinophanes 4 and 5 have been synthesized for the first time using high dilution conditions. The free secondary diamine 7 was obtained from 4 by alkaline hydrolysis. - Quantum chemical calculations support the hypothesis that 7 is one of the most strained anti-hetera[2.2]metacyclophanes known. This is also evident from the 1H-NMR spectrum of 7, which shows one of the strongest high field shifts for the intraannular protons in comparison to other anti-[2.2]metacyclophanes.
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  • 8
    ISSN: 1434-193X
    Keywords: Chirality ; Circular dichroism ; Cyclophanes ; Theoretical calculations ; Strained molecules ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The chiroptical properties of planar-chiral and helical molecules without stereogenic centres and with well-defined structure, strain, electronic properties and chirality are investigated by experimental and theoretical circular dichroism spectroscopy. We report on the synthetic and theoretical achievements made during the past 10 years in the field of rigid [2.2]metacyclophane propellers, as well as in the area of [n]para- and [n]metacyclophanes containing aliphatic parts like adamantane or unsaturated bridges. Results for carbo and hetero helicenes and helicene-related molecules like twisted phenanthrenes are presented. It is demonstrated how reliable theoretical calculations using configuration interaction and time-dependent methods in combination with density functional theory today are and how useful the interplay between theory and experiment stimulates the development of chiral molecules optimized for this purpose. Even for large molecules it is routinely possible to assign Cotton effects to electronic transitions, to evaluate chromophore contributions, to determine absolute configurations and conformational equilibria, and to discover intramolecular charge transfer effects.
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  • 9
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Chemie in unserer Zeit 33 (1999), S. 367-372 
    ISSN: 0009-2851
    Keywords: Chemistry ; Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 10
    ISSN: 0009-2940
    Keywords: Indigo ; Calculations, AM1 ; cis/trans Isomerization ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: AM1-SCF and AM1-SDCI calculations have been carried out for indigo and three conformers of N,N′1-diacetylindigo in their cis and trans configurations. The molecular structure of trans- N,N′1-diacetylindigo was determined by X-ray diffraction. The calculated ground-state data obtained within the SCF approximation (relative energies, structural parameters) are in excellent agreement with the corresponding experimental values. Potential energy curves (PEC) for the isomerization reaction cis/trans in several electronic states were obtained from AM1-SDCI calculations. For N,N′1-diacetylindigo the ground state PEC has a maximum at a torsion angle about the central C—C bond of 90°, whereas the S1- and T11-state PEC's show minima at the corresponding geometry. For indigo itself significant barriers to the photoisomerization in the S1- and T11-excited states are predicted because of increased NH…OC hydrogen bonding. The results provide an explanation for the phototropic behaviour of N,N′1-diacetylindigo and the photostability of indigo. The calculated enthalpies of activation for the isomerization process cis → trans in the ground state compare well with available experimental data, thus rendering feasible the prediction of the thermal stability of cis isomers not yet synthesized.
    Additional Material: 4 Ill.
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