THE PROTIOTETRAMETHYLAMMONIUM DICATION (CH3)(3)NCH4(2- HYDROGEN() )DEUTERIUM EXCHANGE AND CALCULATIONAL STUDIES - SEARCH FOR THE PARENT PROTIOAMMONIUM DICATION NH5(2+1)/

The protonation of ammonium ions NR4+ (R = H, CH3) to form the corresp onding protioammonium dications HNR42+ (R = H, CH3) was investigated b y hydrogen/deuterium exchange experiments and by ab initio molecular o rbital calculations. Hydrogen/deuterium exchange was observed by H-2 N MR spectroscopy at the CH3 groups of tetramethylammonium ion N(CH3)(4) (+) when reacted with excess DF:SbF5 superacid. On the basis of calcul ational results, the intermediacy of the corresponding C-H-protonated protiotetramethylammonium dication (CH3)(3)NCH42+ is suggested to acco unt for the observed exchange. Under similar conditions, no exchange w as observed in the parent ammonium ion NH4+ in FSO3D:SbF5 superacid, a lthough the NH52+ dication was calculated to be kinetically stable wit h a considerable barrier to deprotonation. The pentafluoroammonium dic ation NF52+ was also investigated theoretically, and the results were compared to those of neutral NF5.