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@ARTICLE{Maksic:54979,
      author       = {Maksic, Z. B. and Baric, D. and Müller, T.},
      title        = {{C}lar's sextet rule is a consequence of the sigma-electron
                      framework},
      journal      = {The journal of physical chemistry / A},
      volume       = {110},
      issn         = {1089-5639},
      address      = {Washington, DC},
      publisher    = {Soc.},
      reportid     = {PreJuSER-54979},
      pages        = {10135 - 10147},
      year         = {2006},
      note         = {Record converted from VDB: 12.11.2012},
      abstract     = {A number of condensed PAHs are examined to identify the
                      underlying reasons governing empirical Clar's rule taking
                      benzene as a limiting case. It is found that the so-called
                      Clar's structures are the only minima on the MP2(fc)
                      potential energy hypersurfaces, meaning that other
                      conceivable valence isomers are nonexistent. The influence
                      of the electron correlation energies to the stability of
                      Clar's structures is substantial with predominating
                      influence of the sigma-electrons. However, the contributions
                      arising from the sigma- and pi-electron correlation energies
                      are approximately the same, if Clar's structures are
                      compared with some artificial pi-electron localized or
                      graphite-like delocalized planar systems. Analysis of the
                      Hartree-Fock (HF) energies provides a compelling evidence
                      that the origin of stability of Clar's structures lies in a
                      decrease of the positive T, V(ee) and V(nn) energy terms
                      relative to some characteristic virtual "delocalized" or
                      "localized" model geometries. Partitioning of the mixed and
                      terms in the sigma- and pi-type contributions, by using the
                      stockholder (SHR), equipartitioning (EQP) and standard pi
                      (SPI) schemes, unequivocally shows that the driving force
                      leading to Clar's structures are more favorable sigma-type
                      interactions. All these conclusions hold for the archetypal
                      benzene too, which could be considered as a limiting Clar
                      system. Finally, the boundaries of Clar's hypothesis and
                      some common misconceptions are briefly discussed. Perusal of
                      the geometric parameters and pi-bond orders reveals that
                      there are no benzene rings completely "vacant" or "fully
                      occupied" by the pi-electrons, envisaged by Clar in his
                      picture of condensed benzenoid compounds. Instead, there are
                      six-membered rings with higher and lower total pi-electron
                      density. The bond length anisotropy of the former rings is
                      smaller. It is concluded that Clar's proposition is a useful
                      rule of thumb providing qualitative information on the
                      stability of the PAH systems, which in turn should not be
                      overinterpreted.},
      keywords     = {J (WoSType)},
      cin          = {ZAM},
      ddc          = {530},
      cid          = {I:(DE-Juel1)VDB62},
      pnm          = {Scientific Computing},
      pid          = {G:(DE-Juel1)FUEK411},
      shelfmark    = {Chemistry, Physical / Physics, Atomic, Molecular $\&$
                      Chemical},
      typ          = {PUB:(DE-HGF)16},
      pubmed       = {pmid:16913689},
      UT           = {WOS:000239817900021},
      doi          = {10.1021/jp062917b},
      url          = {https://juser.fz-juelich.de/record/54979},
}