=20 Mark, i= n a paper by Verdal and Hudson, Chemical Physics Letters 434 (2007) 241=E2= =80=93244, the Mills Nixon effect is described the change in reactivity cau= sed by the localization of the benzene double-bonds. If the localizat= ion of pi bonds changes reactivity, why cannot other functional groups that= alter the distribution of aromatic bonds in a ring still be regarded as Mi= lls Nixon? The reactivity must change if the electronic density is affected= (Fukui, 1982). Thus, why cannot any functional group that affects the subd= ivision of bonds in a benzene be called Mills-Nixon?
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=20 Sergio
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=20 ----- Origina= l Message -----
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=20 From: Mark Zo= ttola mzottola**gmail.com
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=20 Sent: 09/26/1= 2 03:36 PM
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=20 To: Manzetti,= Sergio
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=20 Subject: CCL:= Mills-Nixon effect
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=20 The = Mills-Nixon effect, that of bond fixation - does not depend on any annulati= on onto a benzene ring or any aromatic system. Rather it is a breakin= g of symmetry of the pi system to avoid the destabilization of an anti-arom= atic system, i;e., those systems with 4n pi electrons. THis apparentl= y also extends to molecules with 4n+2 pi electrons containing an anti-aroma= tic substructure.
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=20 Biph= enylene is an appropriate example. Two benzene rings are bonded toget= her each at the 1,2 positions, defining a 4-membered ring between them. &nb= sp;Considering the resonance contributing structures, one sees that the 4-m= embered ring can contain 2 pi bonds - and is therefore a cyclobutadienyl su= bstructure. As the entire molecule contains 12 pi electrons, the full= y conjugated system would be anti-aromatic as well. The Mills-Nixon e= ffect breaks the overall symmetry of the system by lengthening the bonds be= tween the two benzene rings. That bond elongation isolates the two be= nzene rings and avoids the destabilization of a fully-conjugated anti aroma= tic system. Therefore, the Mills-Nixon effect disrupts the conjugatio= n of the system to achieve stability.
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=20 When= one attaches substituents to an aromatic system, there is no change in the= number of pi electrons in the ring system. Therefore attaching a met= hoxy group to benzene does not change the aromaticity of the benzene system= . Similarly, adding a nitro group to cyclobutadiene does not change t= he number of pi electrons in the system (4) and therefore the system remain= s anti-aromatic. There will be changes to the bond lengths in any cyc= lic, conjugated system by the addition of any substituent, be it methoxy, n= itro, fluoro, amino, etc. Those changes reflect the perturbation of t= he MO energy levels by the substituent, but they are not disrupting the con= jugation in the overall system. Therefore, these are not an aspect of= the Mills-Nixon effect.
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=20 On = Wed, Sep 26, 2012 at 6:46 AM, Sergio Manzetti sergio.manzetti!^!gmx.com <owner-chemistry*ccl.net> wrote:
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=20 Dear CCLrs= , does the Mills-Nixon effect unilaterally depend on the annelation of four= , five or six-membered rings onto benzene giving and alternation of reactiv= ity, or does the effect also encomapss:
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=20 - linking of ANY functional group to an aromatic unit?
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=20 Thanks
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=20 Sergio<= /span>
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