Ionic Reactant Orientation Inhibits Ion-Neutral Reactions at Low Temperatures.

Barrierless ion-molecule reactions are critical to interstellar chemistry, particularly at low temperatures. Their rate coefficients are often estimated using classical capture theory, which neglects the spatial orientation of molecular ions. Here, we investigate the reaction between sympathetically cooled BeD⁺ and O₂ using a linear quadrupole ion trap combined with a high-resolution time-of-flight mass spectrometer. Isotopic substitution confirms BeOD⁺ and O were the only products. The measured rate coefficient, k = (5.4 ± 1.2) × 10^-11 cm³/s at the collision energy of 97 K (E/k_B), is approximately 15 times lower than that predicted by the classical capture model. Master equation modeling based on electronic structure calculations up to CCSDT-(Q) accurately reproduces the experiment. Unlike the point-charge approximation in capture theory, our results show that reactivity is strongly constrained by the steric requirements of the ionic reactant: only a small subset of orientations between reactants leads to a low-energy barrier that allows the reaction to proceed at low temperatures. These findings underscore the necessity to include steric effects in capture models for accurate predictions.