X-Ray Evidence

From WikidChem

From the last slide, we know that the vibration of the benzene molecule between single and double bonds is a single minimum - a resonance and not an equilibrium structure.

Another way to test this is to use X-ray diffraction. One bond of the benzene molecule was lengthened artificially by adding fusing a second ring to the aromatic ring. The atomic positions show that this bond was indeed lengthened. The other bonds, however, do not alternate short-long-short-long-short. Instead, they conform to a single, new bond length. Similarly the difference density map shows similar electron density in all bonds. This again illustrates the resonance of the benzene molecule.