Dynamic Aspects of Bond Alternation in Cyclic Polyenes

Abstract

The theoretical shapes of the potential curves for bond alternation in ground and excited states in cyclic polyenes are given. It is shown that the current MO model tends to give a rather small barrier to tunneling between the two alternating nuclear configurations, and that the barrier disappears in the excited states. If the potential curves do differ between ground and excited states, peculiar progressions should appear in the spectrum. An analysis is presented of the low‐temperature spectrum of C₁₈H₁₈ that shows no such effect. It is suggested that the success that bond alternation has had in interpreting various phenomena is due to the fact that it makes the electrons less free than current models would have, but that this effect may stem from other subtler causes.