Triplet-State Zero-Field Splitting Calculations in Aromatic Hydrocarbons and Heterocycles

Zero-field splitting in the lowest triplet states of aromatic hydrocarbons and heterocycles is calculated using several molecular orbital schemes. The most reliable results are obtained using closed-shell SCF molecular orbitals with configuration interaction between all triplet states singly excited relative to the ground state. Slater-type atomic orbitals are used and the two-center Coulomb integrals involving these orbitals are evaluated exactly. Results calculated using best Slater-type orbitals are consistently higher than experimental values, but good agreement with experiment is obtained when two-zeta of four-zeta atomic orbitals are used.