Quadruple bond between two carbon atoms

  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. DrDu

    DrDu 4,452
    Science Advisor

    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. So what do you think about a quadruple bond?
     
  5. 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. cgk

    cgk 509
    Science Advisor

    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. DrDu

    DrDu 4,452
    Science Advisor

    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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