What experimental evidence supports resonance?

Equal bond lengths and intermediate bond energies, along with characteristic spectroscopic signals such as nuclear magnetic resonance chemical shifts and infrared frequencies, are the main experimental evidence for resonance in organic molecules.

Think of resonance like a picture that cannot be drawn with just one simple line drawing, so chemists say the real molecule is a blend of several drawings called resonance structures; for example, benzene is often shown with alternating single and double bonds on paper but experiments measure all six carbon–carbon bonds as the same length, halfway between a single and a double bond, which tells us the electrons are shared across the whole ring instead of sitting in one place. Bond energies also come out between expected single and double values, which means the energy of the real molecule is stabilized by this delocalization of electrons. Spectroscopy gives more evidence: nuclear magnetic resonance shows hydrogen and carbon environments that match a delocalized structure, and infrared peaks shift to positions consistent with partial bond orders rather than pure single or double bonds.

A common mistake is thinking resonance structures are real, separate molecules rather than just contributors to one actual structure, so remember the real molecule is a weighted average and not flipping between drawings; another error is assuming resonance always increases stability equally in all cases, when actually the extent of stabilization depends on how well electrons can delocalize, so compare possible resonance forms and count formal charges to judge which contribute more; students also sometimes ignore spectroscopic data and rely only on drawings, but measuring bond lengths and energies or checking nuclear magnetic resonance and infrared results usually resolves doubts quickly.

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Experimental Evidence for Resonance

1. Use resonance structures to explain bonding
2. Compare measured bond lengths
3. Expect bond lengths between single and double
4. Check bond energies for intermediate values
5. Use NMR to identify environments
6. Use IR for bond order information

Common Resonance Mistakes and Fixes

1. Treating resonance structures as separate molecules
2. Assuming equal stabilization from resonance
3. Ignoring spectroscopic and bond measurements

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