SEMIEMPIRICAL CALCULATIONS ON SULFUR-CONTAINING HETEROCYCLES

The PPP procedure (neglect of the penetration integrals, constant parameters, idealized geometry, p model for the sulfur, and limited number of singly excited states in the configuration interaction) provides a satisfactory interpretation of the singlet-singlet absorptions of heterocyclic sulfur-containing compounds in the ultraviolet and visible spectral regions. The authors suggest the values US = 20 eV, βCS = 0.7βCC, and γSS = γCC as parameters for σ-bivalent sulfur and employ the Mataga-Nishimoto relation for the two-center repulsion integrals. A reliable evaluation of the calculated charge distribution in the ground state would require additional experimental data.