Correlations among Electronic Transitions for Carbonyl and for Carboxyl in the Vacuum Ultraviolet

Three more or less structured bands (at 55, 60, and 68 kK in propionaldehyde) are located in compounds containing carbonyl. These bands are tentatively assigned, respectively, as n′→π; n→σ* (n→3s); and π→π*. The π→π* is tentatively located in compounds containing carboxyl, e.g, at about 67 kK in formic acid and acetic acid, and is followed as it red shifts in methyl formate and finally to 58 kK in formamide. The band is considered as perturbed carbonyl with some charge transfer until the perturbation becomes too strong—as it evidently is in dimethyl formamide where the band comes at 51 kK. Here one sees (from the absence of a Brooker deviation) that the molecule must be regarded as allylic. Other bands in carboxyl are tentatively identified as being analogous to the 55- and 60-kK bands in propionaldehyde, though the possibility of a second π→π* absorption in the region studied cannot be ruled out. The spectra of formic acid and acetic acid dimer are presented and interpreted.