Electronic Structures of Molecules X. Aldehydes, Ketones and Related Molecules

Electron configurations for the normal states of H₂CO, CH₃HCO, Cl₂CO are explicitly given, also for the low excited states of H₂CO. The structures, ionization potentials, and longest wavelength electronic band spectra of these and other related or analogous molecules (saturated aldehydes, ketones, thioaldehydes, thioketones, etc.) are interpreted in relation to these configurations. In particular is it shown that the minimum ionization potential of $=\mathrm{C}=\mathrm{O}$ or $=\mathrm{C}=\mathrm{S}$ corresponds to removal of a nonbonding 2p electron from the O atom or a nonbonding 3p from the S atom, unless the groups attached to the C contain other unusually easily ionized electrons. Similarly, the longest wavelength band system, commonly attributed to the C=O (or C=S) double bond, corresponds to excitation of the nonbonding 2po or 3ps to an excited orbital which is largely but probably not quite wholly localized in the C=O or C=S bond, and which has C↔O or C↔S antibonding power, i.e., loosens the bond somewhat. This excitation process is responsible for color in the $=\mathrm{C}=\mathrm{S}$ compounds. The C=O (or C=S) bond is a true double bond in the sense that the binding is effected essentially by two pairs of C–O (or C–S) bonding electrons. The C=O bond is essentially the same also in saturated monobasic acids RCOOH or at least in their esters.