  Recent advances in experimental methodology enabled studies of the
quantum-state and conformational dependence of chemical reactions under
precisely controlled conditions in the gas phase. Here, we generated samples of
selected gauche and s-trans 2,3-dibromobutadiene (DBB) by electrostatic
deflection in a molecular beam and studied their reaction with Coulomb crystals
of laser-cooled $\mathrm{Ca^{+}}$ ions in an ion trap. The rate coefficients
for the total reaction were found to strongly depend on both the conformation
of DBB and the electronic state of $\mathrm{Ca^{+}}$. In the
$\mathrm{(4p)~^{2}P_{1/2}}$ and $\mathrm{(3d)~^{2}D_{3/2}}$ excited states of
$\mathrm{Ca^{+}}$, the reaction is capture-limited and faster for the gauche
conformer due to long-range ion-dipole interactions. In the
$\mathrm{(4s)~^{2}S_{1/2}}$ ground state of $\mathrm{Ca^{+}}$, the reaction
rate for s-trans DBB still conforms with the capture limit, while that for
gauche DBB is strongly suppressed. The experimental observations were analysed
with the help of adiabatic capture theory, ab-initio calculations and reactive
molecular dynamics simulations on a machine-learned full-dimensional potential
energy surface of the system. The theory yields near-quantitative agreement for
s-trans-DBB, but overestimates the reactivity of the gauche-conformer compared
to the experiment. The present study points to the important role of molecular
geometry even in strongly reactive exothermic systems and illustrates striking
differences in the reactivity of individual conformers in gas-phase
ion-molecule reactions.
