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
