2013ApJ...770L...2F

The ortho-para chemistry of ammonia in the cold interstellar medium is investigated using a gas-phase chemical network. Branching ratios for the primary reaction chain involved in the formation and destruction of ortho- and para-NH₃ were derived using angular momentum rules based on the conservation of the nuclear spin. We show that the "anomalous" ortho-to-para ratio of ammonia (∼0.7) observed in various interstellar regions is in fact consistent with nuclear spin selection rules in a para-enriched H₂gas. This ratio is found to be independent of temperature in the range 5-30 K. We also predict an ortho-to-para ratio of ∼2.3 for NH₂. We conclude that a low ortho-to-para ratio of H₂ naturally drives the ortho-to-para ratios of nitrogen hydrides below the statistical values.