The prebiotic molecular inventory of Serpens SMM1 I. An investigation of
  the isomers CH$_{3}$NCO and HOCH$_{2}$CN
  Methyl isocyanate (CH$_{3}$NCO) and glycolonitrile (HOCH$_{2}$CN) are isomers
and prebiotic molecules that are involved in the formation of peptide
structures and the nucleobase adenine, respectively. ALMA observations of the
intermediate-mass Class 0 protostar Serpens SMM1-a and ALMA-PILS data of the
low-mass Class 0 protostar IRAS~16293B are used. Spectra are analysed with the
CASSIS line analysis software package in order to identify and characterise
molecules. CH$_{3}$NCO, HOCH$_{2}$CN, and various other molecules are detected
towards SMM1-a. HOCH$_{2}$CN is identified in the PILS data towards IRAS~16293B
in a spectrum extracted at a half-beam offset position from the peak continuum.
CH$_{3}$NCO and HOCH$_{2}$CN are equally abundant in SMM1-a at [X]/[CH$_{3}$OH]
of 5.3$\times$10$^{-4}$ and 6.2$\times$10$^{-4}$, respectively. A comparison
between SMM1-a and IRAS~16293B shows that HOCH$_{2}$CN and HNCO are more
abundant in the former source, but CH$_{3}$NCO abundances do not differ
significantly. Data from other sources are used to show that the
[CH$_{3}$NCO]/[HNCO] ratio is similar in all these sources within $\sim$10\%.
The new detections of CH$_{3}$NCO and HOCH$_{2}$CN are additional evidence for
a large interstellar reservoir of prebiotic molecules that can contribute to
the formation of biomolecules on terrestrial planets. A plausible formation
pathway for HOCH$_{2}$CN is the thermal Strecker-like reaction of CN$^{-}$ with
H$_{2}$CO. The similar [CH$_{3}$NCO]/[HNCO] ratios indicate that these two
species either are chemically related or their formation is affected by
physical conditions in the same way. The relatively high abundances of
HOCH$_{2}$CN and HNCO in SMM1-a may be explained by a prolonged stage of
relatively warm ice mantles, where thermal and energetic processing of HCN in
the ice results in the efficient formation of both species.
