Kinetics and mechanisms of the acid-base reaction between NH₃ and HCOOH in interstellar ice analogs

Interstellar complex organic molecules are commonly observed during star formation, and are proposed to form through radical chemistry in icy grain mantles. Reactions between ions and neutral molecules in ices may provide an alternative cold channel to complexity, as ion-neutral reactions are thought to have low or even no-energy barriers. Here we present a study of the kinetics and mechanisms of a potential ion-generating, acid-base reaction between NH₃ and HCOOH to form the salt NH${}_4^{+}$HCOO⁻. We observe salt growth at temperatures as low as 15 K, indicating that this reaction is feasible in cold environments. The kinetics of salt growth are best fit by a two-step model involving a slow "pre-reaction" step followed by a fast reaction step. The reaction energy barrier is determined to be 70 ± 30 K with a pre-exponential factor 1.4 ± 0.4 × 10⁻³ s⁻¹. The pre-reaction rate varies under different experimental conditions and likely represents a combination of diffusion and orientation of reactant molecules. For a diffusion-limited case, the pre-reaction barrier is 770 ± 110 K with a pre-exponential factor of ∼7.6 × 10⁻³ s⁻¹. Acid-base chemistry of common ice constituents is thus a potential cold pathway to generating ions in interstellar ices.