Kinetics and mechanisms of the acid-base reaction between NH$_3$ and HCOOH in interstellar ice analogs

TL;DR

This study investigates the low-temperature kinetics and mechanisms of an acid-base reaction between NH₃ and HCOOH in interstellar ice analogs, revealing a feasible pathway for ion formation in cold space environments.

Contribution

It provides the first detailed kinetic analysis of NH₃ and HCOOH reacting in ices at 15K, demonstrating ion formation via a two-step process with quantifiable energy barriers.

Findings

Salt growth observed at temperatures as low as 15K.

Reaction energy barrier determined to be 70 ± 30K.

Pre-reaction step involves diffusion and orientation effects.

Abstract

Interstellar complex organic molecules (COMs) 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 a the kinetics and mechanisms of a potential ion-generating acid-base reaction between NH$_{3}$ and HCOOH to form the salt NH$_{4}^{+}$HCOO$^{-}$. We observe salt growth at temperatures as low as 15K, 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 $\pm$ 30K with a pre-exponential factor 1.4 $\pm$ 0.4 x 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 $\pm$ 110K with a pre-exponential factor of $\sim$7.6 x 10$^{-3}$ s$^{-1}$. Acid-base chemistry of common ice constituents is thus a potential cold pathway to generating ions in interstellar ices.