Assignment of the Second Singlet in Carbonyl

Marked increases in extinction going from crystalline to melted samples are observed for the electronic absorption at $\text{∼1850 Å}$ (second singlet) in various appropriately selected aliphatic ketones. These changes probably indicate that the transition moment in the crystal is perpendicular to the plane of incidence. Taken together with the crystal geometry this leads to assignment of the second singlet as ${\mathrm{n\to \sigma }}_{\mathrm{CO}}\mathrm{*.}$ The polarized spectrum of 9‐heptadecanone is extended from 56 to 65 kK and extinction coefficients are included. A dichroic ratio anomaly observed in the polarized spectrum is interpreted as involving different amounts of hypo‐ (hyper‐) chromism for parallel and perpendicular polarization directions.