Thanks Bill for clarifying the notion of aromatic bonds I think the C-C bond shared by the benzene and imidazole ring should be close to a double bond distance
http://en.wikipedia.org/wiki/Benzimidazole How come calculation from monomer sketcher and the BZI entry in the library has a distance close to a single bond? Joe On Thu, Sep 24, 2009 at 12:36 PM, William G. Scott <wgsc...@chemistry.ucsc.edu> wrote: > > On Sep 23, 2009, at 5:03 PM, Zheng Zhou wrote: > >> Is there a clear definition of delocalized bond or armatic bond? > > > Hi Zheng: > > Aromatic bonds only occur in planar cyclic molecules that have 4n +2 pi > electrons. Aromatic interactions give energetic stabilization beyond that > observed for delocalized conjugated systems, for example, the aromatic > stabilization of benzene is the energy difference between real benzene and a > hypothetical set of resonance structures representing delocalized > 1,3,5-cyclohexatriene. Similarly, anti-aromatic interactions occur in planar > cyclic molecules having 4n pi electrons; they are destabilized relative to > their hypothetical cycloalkene counterparts. (If you allow for perpendicular > aromatic interactions, Hoffmann and Goldstein in 1971 demonstrated that 4n > pi electrons in that case are also stabilizing, due to the different > topology of the pi electron interactions, something that may be of relevance > to aromatic side chain clusters.) > > HTH, > > Bill >
