Benzenium ions formed by the interaction of the phenyl cation with methanol
and methyl fluoride as well as the transition states for their rearrangeme
nt were optimized at the B3LYP/6-31G(d,p) level of theory. Among the reacta
nt complexes [C6H5. CH3X](+) corresponding to the different sites of the ph
enyl cation attack on the CH3X molecules (X = OH,F) the addition complex (O
-protonated anisole) was the most stable one for X = OH, followed by the CH
insertion complex (ipso-protonated benzyl alcohol), while for X = F the CF
insertion complex had the lowest energy. Addition and insertion complexes
may transform into the ring protonated isomers. Barrier heights for subsequ
ent proton migrations in the arenium ions studied were in the 1-23 kcal/mol
range and those for methyl migrations were in the 5-28 kcal/mol range. All
these rearrangements were allowed, since their barriers were substantially
lower than the complexation energy (67 kcal/mol for X = OH and 42 kcal/mol
for X = Fl. The global minimum was para-protonated anisole for X = OH and
meta-protonated ortho-fluorotoluene for X = F. These theoretical prediction
s were compared with experimental results in studies of nucleogenic phenyl
cation reactions with methanol and methyl fluoride. Low barriers for methyl
migrations, especially that of CH3 shift from ipso- to ortho-positions rel
ative to F in the phenyl cation-methyl fluoride encounter complex, relieved
objections to proton shifts in arenium ions as a mechanism of the observed
H/T scrambling.