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

Abstract

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.