  The kinetics, dynamics and conformational specificities for the ionic
Diels-Alder reaction (polar cycloaddition) of maleic anhydride with
2,3-dibromobutadiene radical ions have been studied theoretically using
multisurface adiabatic reactive molecular dynamics. A competition of concerted
and stepwise reaction pathways was found and both the s-cis and s-trans
conformers of of the diene are reactive. The analysis of the minimum dynamic
path of the reaction indicates that both, rotations and vibrations of the
reactant molecules are important for driving the system towards the transition
state. The rates were computed as $k = 5.1 \times 10^{-14}$ s$^{-1}$ for the
s-cis and $k = 3.8 \times 10^{-14}$ s$^{-1}$ for the s-trans conformer of
2,3-dibromobutadiene at an internal temperature of 300 K. The present results
are to be contrasted with the neutral variant of the title system in which only
the gauche conformer of the diene was found to undergo a considerably slower,
concerted and mostly synchronous reaction driven by the excitation of
rotations. The results presented here inform detailed experimental studies of
the dynamics of polar cycloadditions under single-collision conditions in the
gas phase.
