Hydrogen/deuterium exchange was observed by
2H-NMR spectroscopy at the CH3 groups of the
long-lived alkanoyl cations
CH3CH2CO+
(4), (CH3)2CHCO+
(8), and
(CH3)3CCO+ (13)
when treated with excess DF·SbF5 superacid. The intermediacy of the corresponding
protio(deuterio)acylium dications is suggested to
account
for the exchanges. Under similar conditions, no exchange was
observed in the acetyl ion CH3CO+
(1) in DF·SbF5,
but at the same time its electrophilic reactivity is greatly enhanced
in superacids. The acetyl cation in a superacidic
medium also abstracts tertiary hydrogens of isoalkanes to give
protonated acetaldehyde. Density functional theory
calculations at the B3LYP/6-31G** level were carried out to support the
suggested exchange mechanism as well as
lack of exchange in the acetyl cation. On the basis of the
calculated results, two alternative mechanisms are also
suggested for the Nenitzescu rearrangement of pivaloyl chloride in
isobutane with excess AlCl3.
Density functional theory (DFT) calculations at the B3LYP/6-31G** level were performed to investigate geometries and energies of a series of diprotonated hydrogen halides (H 3 X 2+ ) and gitonic protio methyl-and dimethylhalonium dications (CH 3 XH 2 2+ and (CH 3 ) 2 XH 2+ ). On the basis of computed energies, proton affinities and related thermodynamic parameters were also calculated. A hydrogen/deuterium exchange experiment of dimethylbromonium ion, (CH 3 ) 2 Br + , with an excess of 4:1 DF/SbF 5 superacid indicated lack of exchange at the methyl group supporting protonation of the nonbonded electron pair on the bromine leading to the involvement of gitonic protodimethylbromonium dication. A similar hydrogen/deuterium exchange experiment was also carried out on dimethylchloronium ion, (CH 3 ) 2 Cl + .
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