The Long Wave-Length Spectra of Aldehydes and Ketones Part II. Conjugated Aldehydes and Ketones

Abstract

An interpretation of the long wave‐length absorption regions of conjugated aldehydes and ketones is given. The strong regions can be assigned to transitions of the N→V₁ type known to produce the intense long wave‐length absorption in polyenes. The weak absorption regions, which are characteristic of the carbonyl group, can be explained as arising from the type of transition previously proposed to explain the weak carbonyl absorption in saturated aldehydes and ketones. This transition involves the excitation of a loosely bound electron occupying a nonbonding orbital lying in the molecular plane and across the C–O direction, to an excited molecular orbital with a node in this plane. In conjugated molecules several of these excited MO's exist and transitions to them all are predicted. The longest wave‐length carbonyl absorption represents a transition to the lowest of these MO's and the observed shift of this absorption toward longer wave‐lengths with each addition to the number of conjugated bonds in the chain can be explained by the fact that this lowest excited MO is reduced in energy as the length of the chain increases. The longest wave‐length absorption for quinones and for aromatic aldehydes and ketones can be explained by the same process as that producing this absorption in chain molecules. The long wave‐length spectra of molecules containing conjugated C=S groups are shown to be similar to those for the analogous molecules containing C=O groups. The absorption from the C=S group is observed at longer wave‐lengths because the orbital from which excitation comes is less firmly bound than in C=O.