Quadruple bond between two carbon atoms

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Discussion Overview

The discussion revolves around the possibility of a quadruple bond between two carbon atoms, particularly in the context of dicarbon (C2). Participants explore theoretical frameworks, bond orders, and the implications of different bonding models.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions the feasibility of a quadruple bond between carbon atoms, referencing dicarbon and its isomers.
  • Another participant critiques the Wikipedia article on dicarbon, suggesting that valence bond theory predicts a double bond due to carbon's two unpaired electrons, while noting that higher bond orders may exist in electronically excited states of C2.
  • A participant highlights the ongoing debate regarding whether bonding in dicarbon is better described as a double or quadruple bond, emphasizing that bond order assignments depend on the model used.
  • One contributor discusses the interpretation of bond order within molecular orbital theory, suggesting that C2 can be viewed as having a quadruple bond under certain theoretical frameworks, while also acknowledging the existence of low-lying excited states.
  • Another participant raises a question about the significance of bond order, arguing that observable properties like bond lengths and strengths are more relevant than bond orders themselves.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether a quadruple bond is possible or how to best describe the bonding in dicarbon. Multiple competing views remain regarding the interpretation of bond orders and the theoretical models applied.

Contextual Notes

The discussion reveals limitations in the models used to describe bonding, including the dependence on theoretical frameworks and the unresolved nature of bond order interpretations.

pierce15
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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?
 
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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.
 
So what do you think about a quadruple bond?
 
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.
 
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).
 
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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