Angular dependence of the electron-acceptor character of alkylthio groups in organic radicals

In the radical anions and nitroxides from p-alkylthionitrobenzenes, bulky alkylthiosubstituents produce a substantial decrease of the nitrogen hyperfine splitting, this being taken as an indication that they behave as a-electron acceptors. The temperature dependence of the coupling constant at the 13C of the alkyl group directly bonded to the sulfur atom shows that these radicals experience progressive deviation from planarity with increasing size of the alkyl. The electron-acceptor character of the alkylthio group has therefore been explained in terms of electron transfer from the a orbital of the nitrobenzene moiety containing the unpaired electron to the u* vacant orbital of the S-R bond. INDO calculations, without the inclusion of the sulfur 3d orbitals, support such an interpretation.