Ammonia deuteration mechanism in interstellar conditions

Ion trap study of D₃⁺ + NH₃ reaction

¹ Department of Surface and Plasma Science, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, 180 00 Prague, Czech Republic
² Alexandru Ioan Cuza University of Iași, Iași, Romania

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 6 February 2026
Accepted: 18 March 2026

Abstract

Aims. We investigate the reaction mechanism of deuterium transfer in the D₃⁺ + NH₃ reaction. The distinction between proton hop and full scrambling mechanisms is critical for accurate modeling of deuterated ammonia isotopologs.

Methods. The reaction rate coefficients and product branching fractions were measured in a cryogenic 22-pole radiofrequency ion trap between 110 and 300 K. The D₃⁺ ions were stored in helium buffer gas at number densities of 10¹³−10¹⁴ cm⁻³ and reacted with ammonia at controlled number densities of 10¹⁰−10¹¹ cm⁻³.

Results. The reaction proceeds with reaction rate coefficients decreasing from (5.4 ± 1.1) × 10⁻⁹ cm³s⁻¹ at 149 K to (3.9 ± 0.8) × 10⁻⁹ cm³s⁻¹ at 293 K, close to the capture rate coefficient predicted by the average dipole orientation theory. The measured reaction rate coefficients exhibit a systematic isotope effect compared to the analogous H₃⁺ + NH₃ system, with D₃⁺ reacting slightly more slowly. The product branching fractions reveal that NH₃D⁺ is the dominant product throughout the temperature range, with a branching fraction greater than 99.5% at low temperatures. This observation confirms that the reaction proceeds via the proton hop mechanism rather than full scrambling.