Spin-Orbit Coupling in the 3A2—1A1 Transition of Formaldehyde

Abstract

An approximate quantitative calculation is given for the effect of spin-orbit coupling in perturbing the 3A2 state of H2CO. The spin-orbit coupling mixes the 3A2 state with some higher singlet state, and allows the 3A2←1A1 transition to have nonvanishing probability. The calculations show that the 3A2←1A1, np→π2 transition of H2CO should be perturbed principally by the 1A1←1A1, π1→π2 transition. The bands of the 3A2←1A1 transition should therefore have the structure of a transition which is allowed by orbital symmetry with an oscillating electronic transition moment along the C–O axis (parallel bands). The oscillator strength, f, is calculated to be f3A2→1A1≅1.5×10−7. The natural radiative lifetime τ for the 3A2→1A1 phosphorescence transition is calculated to be τ≅1×10−2sec. The calculated value for τ is in agreement with the experimental values for the related molecules (CH3)2CO, (C6H5)2CO, and C6H5COCH3, insofar as a comparison can be made. It is further shown that the spin-orbit perturbation in the π→π* transitions of aromatic hydrocarbons should be about one thousand times less effective than the spin-orbit perturbation in the n→π* transitions of carbonyl compounds. This is in agreement with the few pieces of experimental data which have been reported thus far.