THE NATURE OF FORMALDEHYDE IN ITS LOW-LYING EXCITED STATES

Long path length, high resolution spectra in the near ultraviolet of H2CO and D2CO at −78 °C. have been obtained, and further strong evidence in support of a non-planar excited state is presented. The double-headed bands to the long wavelength side of the singlet–singlet system are shown to belong to the singlet–triplet transition. A vibrational analysis and partial rotational analysis shows the excited triplet state to be very similar to the excited singlet state. The larger C—O stretching frequency in the triplet state may characterize the multiplicity of these states in the simple aliphatic aldehydes and ketones. It is found that the C—O bond order in both states is very nearly 1.5, and striking similarities between the bonding in these states and the C—O bonding in formic acid are pointed out. The similarity between the singlet and triplet excited states may not extend to chemical behavior because of spin correlation rules, but there are no general reasons why the triplet state should be more reactive than the singlet.