Substituent effects in the tautomerization of imidic acids R−C(OH)=NH → R−C(O)NH2: Kinetic implications for the formation of peptide bonds in the interstellar medium

Abstract

A number of amides, RC(O)NH₂, have been detected spectroscopically in the space between the stars. Naturally the study of how these are formed is an important question on the path of chemical evolution from the elements C, H, N, O, P, … to life because the so-called peptide-bond −C(O)−NH− is a key linkage in poly-amino acids or proteins. Both cyanides and water are abundant in the interstellar medium (ISM) and it has been suggested [J Phys Chem A. 2021;126:924-939] that these react on water–ice grains, catalyzed by acid H₃O⁺, to form firstly imidic acids R−C≡ N + H₂O → RC(OH)NH and subsequently to amides. Here we explore the kinetics in the gas-phase of the intramolecular tautomerization reaction of the imidic acids for R = H, HO, NC, H₂N, HC(O), H₃C, HOCH₂, H₂CCH,H₃C(O), H₂NCH₂, C₂H₅, and CN, particularly at low temperatures where quantum mechanical small curvature and quantized reactant states tunneling are dominant. The most reactive imidic acid is H₂NC(OH)NH which goes on to form urea, one of three known amides in the interstellar medium (ISM), which can self-react to form cytosine and uracil two canonical nucleobases in RNA. The thermochemistry ($$, $$, $$, $\{H_T-H_0\}$) of the imidic acids and amides is also reported as well as the tautomerization of sulfur and phosphorus analogs HC(SH)NH and HC(OH)PH.