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I'm currently using Clayden's organic chemistry textbook, and when he shows MO diagrams of functional groups of organic molecules, he sometimes treated it as if only orbitals on two atoms are interacting. For example, the carbonyl chemistry is reduced to looking at the interaction between C and O orbitals. (For conjugated systems the MO diagrams he created make more sense to me)

However, as far as I know, MO theory is used to construct wavefunctions that spread over the whole molecule. Hence, how is it justified that we are combining orbitals centered on a pair of atom rather than all atoms in the molecule? What is that powerful assumption that allows us to apply MO theory locally at functional groups?

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    $\begingroup$ It's 1:30 am here and I don't have the desire to run calculations and generate images right now, but the simple answer is that although they are indeed a simplification, they tend to resemble the actual thing closely enough, so are "good enough" for organic chemists. At least, that's true for the carbonyl case. For other situations like C-H σ or σ* orbitals in ethane, it gets a bit messier because of symmetry properties and then it becomes necessary to talk about weird things like en.wikipedia.org/wiki/Localized_molecular_orbitals. $\endgroup$ – orthocresol Dec 23 '19 at 1:35
  • $\begingroup$ @orthocresol Thank you for your quick response and I would love to see your answer when you have time! By the way, can you point to some reference paper or textbook where this simplification is explicitly discussed and justified? That will certainly be the most helpful along with calculation results. $\endgroup$ – Macrophage Dec 23 '19 at 1:50
  • $\begingroup$ I don't know if it will have what you're looking for, but the go-to book for MO theory in organic chemistry is Ian Fleming's Molecular Orbitals and Organic Chemical Reactions. $\endgroup$ – orthocresol Dec 23 '19 at 1:57
  • $\begingroup$ Thanks, I will check it out when possible! $\endgroup$ – Macrophage Dec 23 '19 at 1:58

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