  Many molecules exhibit multiple rotational isomers (conformers) that
interconvert thermally and are difficult to isolate. Consequently, a precise
characterization of their role in chemical reactions has proven challenging. We
have probed the reactivity of specific conformers using an experimental
technique based on their spatial separation in a molecular beam by
electrostatic deflection. The separated conformers react with a target of
Coulomb-crystallized ions in a trap. In the reaction of Ca$^+$ with
3-aminophenol, we find a twofold larger rate constant for the \textit{cis}-
compared to the \textit{trans}-conformer (differentiated by the O-H bond
orientation). This result is explained by conformer-specific differences in the
long-range ion-molecule interaction potentials. Our approach demonstrates the
possibility of controlling reactivity through selection of conformational
states.
