A. Mangini

Substituent effect of the sulphinyl group in electrophilic aromatic substitution

Partial rate factors have been determined for molecular chlorination in nitromethane and protodesilylation in H2SO4–AcOH mixtures of methyl phenyl and diphenyl sulphoxide. The electron-activating effect observed in molecular halogenation is interpreted in terms of polarization of the sulphoxide group in the transition state, induced by the electrophilic reagent. In H2SO4–AcOH mixtures Krel for protodesilylation at the para-position of methyl phenyl sulphoxide is very low, possibly because of hydrogen bonding between the sulphinyl group and the acidic medium. For methyl phenyl sulphide, sulphoxide, and some other compounds the ground state electron densities at the para-position have been evaluated from their 13C n.m.r. spectra.

Carbon-13 nuclear magnetic resonance spectra and electronic densities of symmetrical naphthyridines

The 13C n.m.r. spectra of the four symmetrical naphthyridines have been obtained and assignment of the signals performed by means of heteronuclear decoupling experiments and empirical additivity methods. The shifts were found to depend linearly upon the total charge density on the carbon atoms as calculated by self-consistent molecular orbital INDO and CNDO approximations.

Hindered rotation in xanthenyl-type radicals

The e.s.r. spectra of the 9-phenylxanthenyl, 9-phenylthioxanthenyl, and 9-phenylselenoxanthenyl radicals have been interpreted; unambiguous assignments of the hyperfine splitting constants have been made by use of a number of selectively deuteriated derivatives. Although the theoretical models used to interpret the experimental data failed to explain all the details of the spin-density distribution, they allowed certain conclusions to be drawn with respect to the stereochemistry of the molecules.The results show that the phenyl groups do not rotate freely, but are locked at an angle of ca. 70–80° degrees to the heterocyclic plane.

Thiophen-containing radicals. Anions of isomeric dithienylethylenes

Alkali-metal reduction of some thienylethylene derivatives at –80° yields the corresponding radical anions whose e.s.r. spectra have been interpreted by the use of deuteriated derivatives. The results indicate that phenyl and thienyl groups are locked in a definite position, on the e.s.r. time scale; rotational isomers were detected in the case of the 1,2-di-(2-thienyl)ethylene. The experimental hyperfine splitting constants could be reproduced satisfactorily by molecular orbital calculations, assuming, in the p-model framework, twisting angles of 40° for the 3-thienyl, and 0° for the 2-thienyl and phenyl groups. An analogous theoretical approach, which takes into account the participation of the 3d orbitals of sulphur, was found to disagree with the experimental results.

Liquid crystal proton magnetic resonance spectrum of thieno[2,3-b]thiophen

The 100 MHz 1H n.m.r. spectra of the title compound in the nematic phase of two liquid crystals have been obtained and interpreted in terms of chemical shifts, indirect coupling constants, and direct dipolar couplings. The sign of the J values, the ratio of the interprotonic distances, and the orientation of the molecule with respect to the magnetic field have been determined.