The geometry and the inversion potential function of formaldehyde in the Ã1A 2 and ã3A 2 electronic states
The formaldehyde molecule (H 2CO) is nonplanar in the excited Ã1A 2 and ã3A 2 electronic states. We have developed the "semirigid invertor" Hamiltonian to calculate the rotation-inversion energy levels of such a molecule, and used least-squares fitting to determine the inversion potential function and geometrical parameters from the data for H 2CO and D 2CO in both these states. In the model the bond lengths and HCH angle are allowed to vary as the molecule inverts and this semirigid flexibility improves the fit. We confirm previous experimental findings that the barrier to inversion in the triplet state is more than twice that of the singlet state; these results do not accord with ab initio calculations.