Semiempirical Molecular Orbital Calculations. II. The Nonbonding Orbital, the Electronic Spectrum, and the Spin—Orbit Coupling in X2CO Molecules

Phosphorescence lifetimes have been calculated for the T1→S0(π*→n) transition in formaldehyde and some of its halogen substituents. The wavefunctions used were obtained by a charge self-consistent Wolfsberg—Helmholz method. The so-called n orbital on the oxygen atom is mixed with carbon and halogen p orbitals to a considerable extent. As a consequence, one-center terms on the halogens have to be considered in the evaluation of spin—orbit coupling matrix elements. The results indicate a considerable internal heavy-atom effect on the (π*→n) phosphorescence lifetimes of formaldehyde derivatives. The available experimental evidence agrees with this conclusion.