Nuclear spin effects in ammonia chemistry

Aims. We investigate whether the nuclear-spin state of H2 (ortho or para state) modifies the kinetics of two key reactions in ammonia formation in the interstellar medium: NH2+ + H2 → NH3+ + H and NH3+ + H2 → NH4+ + H. Methods. The reaction rate coefficients were measured in a cryogenic 22-pole radiofrequency ion trap over 15-270 K using normal H2 (ortho-to-para ratio 3 : 1) and para-enriched H2 (99.5% para-H2) produced by catalytic conversion at ≈15 K. Results. For NH2+ + H2, the reaction rate coefficients are in the range 10−10-10−9 cm3 s−1 with a weak negative temperature dependence. For NH3+ + H2, the reaction rate coefficients exhibit an Arrhenius dependence with an activation energy of ≈90 meV at high temperatures (>100 K), combined with an increasing value towards low temperatures (10-100 K) due to quantum tunneling. In the studied range of temperatures, the reaction rate coefficients obtained with para-enriched and normal H2 are indistinguishable within the experimental uncertainty for both reactions.