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Quadruple bond between two carbon atoms

  1. Nov 23, 2013 #1
    I was wondering if a quadruple bond between two carbon atoms would be possible. I found this on wikipedia about "dicarbon": http://en.wikipedia.org/wiki/Diatomic_carbon, which is related to this question, since what I'm describing is an isomer of dicarbon. Anyone have any ideas?
  2. jcsd
  3. Nov 24, 2013 #2


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    The wikipedia article is not very good. Valence bond theory also predicts a double bond as carbon atoms have two unpaired electrons. It is not a requirement of VB theory for the molecule to obey the octet rule.
    Higher bond orders may be present in electronically excited state of C2.
  4. Nov 24, 2013 #3
    So what do you think about a quadruple bond?
  5. Nov 24, 2013 #4
    Whether bonding in dicarbon is better described as a double or quadruple bond is still being debated, see http://www.ch.imperial.ac.uk/rzepa/blog/?p=10733 .

    Assigning definite integer-valued bond orders to chemical bonds is just a property of the approximative model used to describe bonding.
  6. Nov 25, 2013 #5


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    Whatever else one may or may not say about C2, if it is described with a single closed-shell determinant wave function (i.e., with Hartree-Fock or Kohn-Sham), one can rotate its occupied orbitals into two equivalent doubly-occupied non-standard sigma bond orbitals and two standard doubly occupied pi bond orbitals via orbital localization (Pipek-Mezey-like).

    As the theoretical basis for bond order concept is closely related to this form of MO theory (which, for C2, is not entirely beyond question--it has low lying excited states), one could formally interpret this as a quadruple bond. Or as a double bond or triple bond, depending on how one wishes to interpret the non-standard sigma system.

    In short, it is the perfect fighting ground for bored theorists 8).
  7. Nov 25, 2013 #6


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    Another question is what a number like "bond order" really tells you.
    What we observe are bond lengths and strengths and not bond orders.
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