The first Astromolecule of the Month for 2023 is the ammonium cation, in its pure protium isotopic form, NH+
4, and in its singly deuterated form, NH₃D⁺. Although it is expected to be abundant, the ammonium cation is a challenge to detect due to its high symmetry.

As a result of its symmetry, NH+
4 has no dipole moment and thus no pure rotational spectrum. It was detected in interstellar sources and in comet 67P by alternative approaches.

ISM  While NH+
4 has no dipole moment, NH₃D⁺ has a dipole moment because the r₀ bond length of the N–D bond differs from the N–H bond lengths. The symmetry is reduced from T_d to C_3v, and one of the principle rotation axes aligns with the remaining C₃ rotation axis. Cernicharo and co-workers detected deuterated ammonium toward Orion-IRc2 and B1-bs in 2013 using the IRAM 30m telescope. The dectection of the 1₀-0₀ rotational transition utilized new data measured by Doménech et al. that were subsequently confirmed with high resolution measuments by Stoffels et al.

COMET  The ammomium cation was detected in the coma of comet 67P/Churyumov-Gerasimenko via its mass spectrum by Beth and co-workers with the Rosetta Spectrometer for Ion and Neutral Analysis (ROSINA) on the Rosetta probe. A number of ammonium salts have subsequently been identified in comet 67P.