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Aromatic Stabilization (aromatic + stabilization)

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Chemistry100%

Terms modified by Aromatic Stabilization

  • aromatic stabilization energy
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    Selected Abstracts

    Chemical graph theory and n -center electron delocalization indices: A study on polycyclic aromatic hydrocarbons

    JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 10 2007
    Marcos Mandado
    Abstract Relations between aromaticity indices derived from chemical graph theory and those based on 6-center electron delocalization are investigated for a series of polybenzenoid hydrocarbons. Aromatic stabilization obtained by means of the effective scaled electron delocalization is highly correlated to the resonance energy, RE, obtained both from SCF MO calculations and conjugated ring circuits model. Local aromaticity of benzene rings is discussed using two different criteria, in one of them aromaticity is just given by the cyclic , -electron conjugation of the ring, whereas terms involving more than one ring are also considered in the other one. Indices derived from chemical graph theory and those obtained from the 6-center electron delocalization give rise to the same local aromaticity. Moreover, 6-center electron delocalization provides more quantitative information. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007 [source]

    Aromatic stabilization in heterofullerenes C48X12 (X,=,N, P, B, Si),

    JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 10 2003
    Zhongfang Chen
    Abstract B3LYP density functional calculations were performed on two S6 symmetrical isomers (I and II) of C48X12 (X=N, B, P, Si) heterofullerenes, and their global and local aromaticity were evaluated by nucleus-independent chemical shifts (NICS). Despite the unfavorable heteroatom repulsive interactions, isomer II is more stable than I owing to the combination of global and local aromaticity. The latter arises from the presence of triphenylene units in isomer II. The aromatic stabilization effects found in this study should be taken into account when predicting the most stable isomers of heterofullerenes. The same is true for predictions of the isomers of fullerene adducts such as C60Cl12. Copyright © 2003 John Wiley & Sons, Ltd. [source]

    Is Cyclopropane Really the ,-Aromatic Paradigm?

    CHEMISTRY - A EUROPEAN JOURNAL, Issue 38 2009
    Wei Wu Prof.
    Abstract Dewar proposed the ,-aromaticity concept to explain the seemingly anomalous energetic and magnetic behavior of cyclopropane in 1979. While a detailed, but indirect energetic evaluation in 1986 raised doubts,"There is no need to involve ,,-aromaticity',",other analyses, also indirect, resulted in wide-ranging estimates of the ,-aromatic stabilization energy. Moreover, the aromatic character of "in-plane", "double", and cyclically delocalized ,-electron systems now seems well established in many types of molecules. Nevertheless, the most recent analysis of the magnetic properties of cyclopropane (S. Pelloni, P. Lazzeretti, R. Zanasi, J. Phys. Chem. A2007, 111, 8163,8169) challenged the existence of an induced ,-ring current, and provided alternative explanations for the abnormal magnetic behavior. Likewise, the present study, which evaluates the ,-aromatic stabilization of cyclopropane directly for the first time, fails to find evidence for a significant energetic effect. According to ab initio valence bond (VB) computations at the VBSCF/cc-PVTZ level, the ,-aromatic stabilization energy of cyclopropane is, at most, 3.5,kcal,mol,1 relative to propane, and is close to zero when n -butane is used as reference. Trisilacyclopropane also has very little ,-aromatic stabilization, compared to Si3H8 (6.3,kcal,mol,1) and Si4H10 (4.2,kcal,mol,1). Alternative interpretations of the energetic behavior of cyclopropane (and of cyclobutane, as well as their silicon counterparts) are supported. [source]

    ,,, Energy Separation in Homodesmotic Reactions

    CHEMPHYSCHEM, Issue 12 2005
    Georg Hohlneicher Dr.
    Abstract A well-established quantity for specifying the aromaticity or antiaromaticity of cyclic conjugated molecules is the so-called aromatic stabilization energy (ASE), which can be derived,either experimentally or theoretically,from appropriate homodesmotic reactions. To gain further insight into the origin of aromaticity, several schemes have been devised to partition ASE into nuclear and electronic as well as , and , contributions, some of which have resulted in contradictory statements about the driving force of aromatic stabilization. Currently, these contradictions have not been resolved and have resulted in a confusing distinction between two different types of aromaticity: extrinsic and intrinsic aromaticity. By investigating different homodesmotic reactions we show that, in contrast to ASE itself, the individual contributions that enter the ASE can strongly depend on the type of reaction. Caution is therefore advised if conclusions or physical interpretations are derived from the individual components. The contradictions result from the fact that some reactions suffer from an imbalance in the number of interaction terms at the two sides of the reaction equation. The concept of isointeractional reactions is introduced and results in the elimination of the imbalance. For these reactions, the contradictions disappear and the distinction between intrinsic and extrinsic aromaticity becomes unnecessary. As far as the ,,, partitioning is concerned, several schemes proposed in the literature are compared. Contradictory results are obtained depending on the partitioning scheme and reaction used. In this context, it is demonstrated that for the partitioning of the electron,electron interaction, the scheme introduced by Jug and Köster is the one that is most theoretically grounded. [source]