Quantum chem. calcns. on 2-methyl-4-(p-fluorophenyl)-2,3-dihydrobenzo[1,5]thiaazatropolone and 2-methyl-4-(p-fluorophenyl)-2,3,4,5-tetrahydrobenzo[1,5]thiaazatropolon e were performed by means of the MNDO/2 method. The geometries of the compounds were taken from crystal structure data. Although the geometries of I and II are similar, the MO's of the 2 mols. differ significantly with respect to the coefficients of AO's in the MO's. There is no interaction between the p-orbitals of the 2 benzene rings in mol. II, but there is interaction between the p-orbitals (HOMO and MO's closed to HOMO) of the 2 aromatic rings in mol. I The interaction is through the C:N bond. The result is consistent with NMR measurements.

Key words: CRYSTAL STRUCTURE DETERMINATION, X-RAY DIFFRACTION ANALYSIS, COMPUTER APPLICATIONS, NITROGEN HETEROCYCLICS, CONJUGATION, QUANTUM CHEMISTRY, BENZENE P, CHEMICAL BONDS, MOLECULAR ORBITAL THEORY, SULFUR HETEROCYCLICS COMPOUNDS