Photodecomposition of Formaldehyde; Stability of the HCO Radical

Observations on the photodecomposition of mixtures of normal and deuterated formaldehydes give reliable data for calculating the stability of the HCO radical. The relative occurrence of the intramolecular split and the H-atom dissociation reaction in the primary photochemical act is determined by the deviation of the apparent equilibrium constant for the hydrogen isotope product species from the actual constant. It is shown that a wavelength of 3650 A is effective for breaking the first CH bond in formaldehyde, and this leads to 27 kcal as a lower value for the heat of dissociation of HCO to H and CO. The intramolecular split reaction occurs not only at 3650 A, but also at 3130 A, and is an important if not the major primary photochemical process.