Ammonia deuteration mechanism in interstellar conditions

Aims . We investigate the reaction mechanism of deuterium transfer in the D 3 + + NH 3 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 D 3 + ions were stored in helium buffer gas at number densities of 10 13 −10 14 cm −3 and reacted with ammonia at controlled number densities of 10 10 −10 11 cm −3 . Results . The reaction proceeds with reaction rate coefficients decreasing from (5.4 ± 1.1) × 10 −9 cm 3 s −1 at 149 K to (3.9 ± 0.8) × 10 −9 cm 3 s −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 H 3 + + NH 3 system, with D 3 + reacting slightly more slowly. The product branching fractions reveal that NH 3 D + 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.