Detection of HNCOH+ in the interstellar medium

Only five out of the $\sim$350 molecules detected in the interstellar medium (ISM) carry the isocyanate functional group (NCO), the essential substructure of peptide bonds in proteins. Among them are isocyanic acid (HNCO) and its most stable N-protonated isomer H$_2$NCO$^+$. Previous studies have predicted that the O-protonated isomer HNCOH$^+$ should be present in the ISM; yet this species has remained elusive. We search for the metastable HNCOH$^+$ towards the Galactic Center molecular cloud G+0.693-0.027, where H$_2$NCO$^+$ has already been detected. We use an ultra-deep broadband spectral survey carried out towards G+0.693-0.027 with the Yebes 40-m, IRAM 30-m and APEX telescopes covering nearly 150 GHz of bandwidth between 31 and 275 GHz. We report the first detection of HNCOH$^+$ in the ISM with a column density of $N$ = (9.9 $\pm$ 0.5)$\times$10$^{12}$ cm$^{-2}$, which corresponds to a molecular abundance relative to H$_2$ of (7.3$^{+1.3} _{-1.0}$)$\times$10$^{-11}$. With the measured H$_2$NCO$^+$ column density of $N$ = (2.6 $\pm$ 0.3) $\times 10^{12}$ cm$^{-2}$, this yields a HNCOH$^+$/H$_2$NCO$^+$ ratio of 3.8$^{+0.5} _{-0.4}$. This result is consistent with recent kinetic calculations that favour the formation of HNCOH$^+$ over H$_2$NCO$^+$. The detection of HNCOH$^+$ in the ISM broadens the inventory of molecules known to contain the isocyanate functional group and reinforces recent works showing that isomer abundances in the ISM do not necessarily correlate with thermodynamic stability.