MULTINUCLEAR NMR-SPECTROSCOPY AND SEMIEMPIRICAL MNDO-PM3 QUANTUM-CHEMICAL INVESTIGATIONS OF THE COMPOUNDS C6H5XY (X=S, SO, SO2 - Y=CF3, CH3)

The compounds C6H5XCF3 and C6H5XCH3 (X = S, SO, SO2) have been investi gated by multinuclear H-1, C-13, O-17, F-19 and S-33 NMR spectroscopy and quantum chemical methods (MNDO in the PM3 parametrization). Compou nds with XCF3, SOCH3 and SO2CH3 groups have been characterized with ef fective non-planar conformations. A nearly planar effective conformati on has been found for C6H5SCH3. As the two lone pairs (LP) of sulphur in the sulphides are essentially non-equivalent with their orbital ene rgies differing considerably, only one LP (with a higher energy level) can participate in the p-pi conjugation. In compounds with XCF3, SOCH 3 and SO2CH3 groups, the electron-withdrawing pi-inductive effect is t he main factor determining the pi-charge distribution in the benzene r ing. The p-pi conjugation is essentially distorted in C6H5SCF3 due to its various conformations. Electron-donating p-pi conjugation only con tributes considerably to the pi-charge distribution in the benzene rin g in C6H5SCH3. An interpretation of the NMR spectra of the considered compounds is given on the basis of the quantum chemical calculations.