The radical cation Diels-Alder reaction of the Id-butadiene radical cation
with ethylene, yielding the cyclohexene radical cation, was studied by B3LY
P hybrid functional and QCISD(T)//QCISD calculations using the 6-31G* basis
set. The intermediates and transition states involved in three different m
echanisms, a concerted C-s-symmetric and a stepwise unsymmetric anti [4 + 2
] pathway and a stepwise unsymmetric out-gauche pathway leading to vinylcyc
lobutane, have been considered. The synchronous C-s-symmetric pathway is pr
evented by a pseudo-Jahn-Teller distortion and is 19 kcal/mol higher in ene
rgy than the stepwise pathways. The stepwise anti pathway was found to be t
he lowest-energy pathway with an activation energy of 0.3 kcal/mol relative
to the initially formed ion-molecule complex. The gauche-out pathway, lead
ing to vinylcyclobutane, is 3.5 kcal/mol higher in energy than the anti pat
hway, leading to cyclohexene. In contrast to earlier calculations by Bauld
at the MP2/6-31G*//3-21G level of theory, an ion-molecule complex was found
to be part of the reaction pathway and no in-gauche intermediate could be
located. The transition structures and intermediates involved in both stepw
ise pathways closely resemble the previously described species involved in
the stepwise mechanism of the neutral Diels-Alder reaction.