C-13 NMR SPECTROSCOPIC STUDY OF PARA-SUBSTITUENT EFFECTS IN HIGHLY CROWDED 1,1'-DIADAMANTYLBENZYL CATIONS

A series of para-substituted 1,1'-diadamantylbenzyl cations were studied under stable ion conditions at low temperatures in superacid solutions. These studies were informative in probing the effects of ring substituents and their ability to delocalize positive charge from the carbenium center in highly crowded systems. The observed C-13 NMR chemical shifts of the carbocation center in comparison with those of the corresponding cumyl cations indicate the effects of steric crowding of the diadamantyl substituents in minimizing p-pi interactions. This renders the carbocationic C-13 NMR chemical shifts virtually unaffected by para-substituents.