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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Theoretical chemistry accounts 47 (1978), S. 193-203 
    ISSN: 1432-2234
    Keywords: VB method
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract As a first step in the development of a semiempirical VB method,ab initio VB calculations were performed to obtain potential energy curves for the molecules HF and BeH2 and the energy profile of the collinear exchange reaction F + H2 → HF + H. The applicability of the method is discussed with particular emphasis on the calculation of integrals over OAO's, the choice of valence structures to be included in the CI scheme and the interpretation of the wave function in terms of OAO's.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Theoretical chemistry accounts 5 (1966), S. 251-265 
    ISSN: 1432-2234
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Abstract The results of PPP-calculations for cyanines, merocyanines, and aminoquinones agree well with the experimental electronic and vibrational spectra. The spectrum of 2,5-diaminobenzoquinone-1,4 must be considered as a p-benzoquinone-spectrum which under the influence of substituents has undergone a redshift. Also the charge distribution of this diaminoquinone is only slightly different from that of the quinone. These results tend to verify the assignment of the IR-spectra of the aminoquinones as given by Draber and Wallenfels and show that in the case of the aminoquinones the quinone character is so strong that it does not appear to be necessary to formulate them as a quadrupole-merocyanine.
    Abstract: Résumé Les résultats de calculs PPP pour des cyanines, des merocyanines et des aminoquinones sont en bon accord avec les spectres électroniques et vibrationnels expérimentaux. Le spectre de la 2,5 diamino benzoquinone-1,4 doit être interprété comme un spectre de p-benzoquinone déplacé vers les grandes longueurs d'onde sous l'influence du substituant. De même la répartition des charges de cette diamino quinone se distingue très peu de celle de la quinone. Ces résultats confirment le classement des spectres I.R. des aminoquinones dû à Draber et Wallenfels et montrent que le caractère quinonique des aminoquinones est si marqué qu'il paraît superflu de les considérer comme des merocyanines-quadrupolaires.
    Notes: Zusammenfassung Die Ergebnisse von PPP-Rechnungen für Cyanine, Merocyanine und Aminochinone stehen in guter Übereinstimmung mit den experimentellen Elektronen- und Schwingungsspektren. Das Spektrum des 2,5-Diaminobenzochinons-1,4 muß als unter Substituenteneinfluß langwellig verschobenes p-Benzochinonspektrum gedeutet werden. Auch die Ladungsverteilung dieses Diaminochinons unterscheidet sich nur sehr wenig von derjenigen des Chinons. Diese Ergebnisse stützen die Zuordnung der IR-Spektren der Aminochinone von Draber und Wallenfels und zeigen, daß in den Aminochinonen der Chinoncharakter so ausgeprägt ist, daß es unnötig erscheint, sie als Quadrupol-merocyanine zu formulieren.
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  • 3
    Electronic Resource
    Electronic Resource
    Springer
    Theoretical chemistry accounts 67 (1985), S. 485-490 
    ISSN: 1432-2234
    Keywords: Potential energy hypersurfaces of H3 ; three-centre three-electron problem ; unpolar radical reactions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Using an EHT type Hamiltonian the Heitler-London-VB treatment of the three-centre three-electron problem is simplified such that the interesting features of the potential hypersurfaces of arbitrary three-centre three-electron systems can be obtained simply from the knowledge of atomic orbital energies and overlap integrals. The method is applied to the hypersurfaces of the H3 system and extensions of the discussion of general radical reactions are indicated.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1432-2234
    Keywords: Graphical representation of potential hypersurfaces ; MO and HL models ; Radical reactions ; Three-centre three-electron system ; VB calculations
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract A graphical representation of the potential energy hypersurfaces of the three-centre three-electron problem is proposed which is sufficient to fully describe the system. This method is used to visualize the hypersurfaces of the ground and the first excited doublet state of H3, obtained from VBCI calculations as well as from a judiciously parametrized HMO model and a semiempirical HL model. On the basis of these results three prototype radical reactions are discussed and the performance of the semiempirical models is assessed by comparison with theab initio results, in order to see which features of the models are essential for a correct description of radical reactions. The importance of overlap within the MO approach and of electron interaction within the semiempirical HL method is thus revealed.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1520-6904
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 0894-3230
    Keywords: Organic Chemistry ; Physical Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: MNDOC-CI results are reported for the photochemical α-cleavage reaction of carbonyl compounds. Potential energy surfaces for some excited singlet and triplet states and cross-sections along the bitopic reaction path show that the activation energy of the α-cleavage reaction is determined by the relative energetic position of the lowest two triplet states. The reactivity of aliphatic ketones depends only on the degree of branching at the α-carbon, because the triplet barrier is due to an (avoided) crossing of two surfaces corresponding to an nπ* and ππ* excitation respectively, which correlates with the stability of the leaving alkyl radical. For conjugated carbonyl compounds, on the other hand, the position of the crossing point is independent of the 3nπ* excitation energy, but if the 3ππ* surface is below the 3nπ* surface all the way along the reaction coordinate, the reaction is determined by the barrier on the 3ππ* surface and any additional stabilization of the 3ππ* state increases the barrier.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 0894-3230
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: It is shown by MP2(fc)/6-31G**//HF/6-31G* calculations on model systems that benzenes fused to carbocycles and possessing a β-hydroxy substituent exhibit a characteristic electrophilic regioselectivity, which is a linear function of the size of the annelated ring. This directive property, which determines the susceptibility of various positions within the aromatic fragment towards electrophilic substitution, is rationalized in terms of the degree of matching of two π-electron localization patterns, one occurring in the ground state of the molecule and the other in the transition structure (Wheland σ-complex formed by protonation). The overwhelming influence, however, is exerted by the OH group, which substantially activates its ortho positions. The role of hyperconjugation seems to be small but not negligible. The relevance of the present result in interpreting the Mills-Nixon effect is briefly discussed.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
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  • 8
    ISSN: 0749-1581
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: CC coupling constants of ten bicyclic hydrocarbons and ten cyclic ethers or carbonyl compounds were determined from the satellites of broad band decoupled spectra or from INADEQUATE measurements. The influences of valence angles, dihedral angles, hybridization and heterosubstitution on geminal and vicinal CC coupling constants are discussed in terms of the algebraic additivity of the contributions from individual coupling paths within the framework of a multiple path mechanism.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 24 (1986), S. 1016-1019 
    ISSN: 0749-1581
    Keywords: C,C coupling constants ; INADEQUATE ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 13C NMR spectra of 24 unsaturated hydrocarbons (mono- and dienes) were investigated by means of the INADEQUATE pulse technique. All coupling constants larger than 1.5 Hz could be determined with an accuracy of ± 0.2 Hz.
    Additional Material: 6 Tab.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 25 (1987), S. 456-461 
    ISSN: 0749-1581
    Keywords: 13C NMR ; one- and two- bond C,C coupling constants ; experimental and calculated values ; acetylenes and allenes ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: One- and two-bond C,C coupling constants and the signs of ten of the two-bond coupling constants were determined from the INADEQUATE and SLAP spectra of four acetylenes and one allene. The geminal C,C coupling constants involving sp-hybridized carbons are found to be positive and to correlate linearly with the corresponding s character products. In contrast, INDO-SCPT calculations yield negative values for these coupling constants, and no correlation with the s character products. A separation of those parts of the coupling information transmitted by the σ and π electron framework, based on INDO-FPT-PRMO calculations, reveals some possible reasons for the poor performance of the INDO-SCPT method for systems with π-electrons.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
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