The Formate Anion in Interstellar Ices and Laboratory Analogs

Weak infrared (IR) bands near 1384 and 1353 cm^–1 (λ = 7.225 and 7.391 μm) have been observed in spectra of dark, cold molecular clouds for nearly 30 years. In 2000, we used several lines of spectroscopic and chemical evidence to assign these two features to the formate anion, HCOO^–. Here we examine a recently proposed reassignment of the same two peaks to formamide and acetaldehyde. We find that radiolysis of ices made of the more-abundant interstellar ice compounds (e.g., H₂O, CO, CH₃OH) does not lead to an IR-detectable level of formamide and that only with a significant variation of ice composition from interstellar values can acetaldehyde be made in a detectable amount. We conclude that the interstellar IR features reported near 1384 and 1353 cm^–1 can be confidently assigned to HCOO^–, placing formate alongside cyanate (OCN^–) as the only two anions yet identified in interstellar ices. Suggestions made earlier to aid in IR spectral assignments in laboratory and extraterrestrial ices are repeated and emphasized.