The reaction between NH3+and CH3COOH: a possible process for the formation of glycine precursors in the interstellar medium

Abstract

Context. The formation of glycine is strongly relevant to our understanding of the interstellar medium and is most accuretely studied computationally.Aims. We carry out a theoretical study of the reactions between the radical cation of ammonia and CH3 COOH/CH2 COOH as possible processes leading to glycine derivatives.Methods. The gas-phase reactions were theoretically studied using ab initio methods. We employed the second-order Moller-Plesset level in conjunction with the cc-pVTZ basis set. In addition, the electronic energies were refined by means of single-point calculations at the CCSD(T) level on the MP2/cc-pVTZ geometries with the aug-cc-pVTZ basis set.Results. We report accurate potential energy surfaces for the reactions considered in this work. The different intermediate species as well as the most relevant transition states for these reactions are characterized.Conclusions. Formation of protonated glycine from the reaction of NH3 + with acetic acid is an exothermic (–9.15 kcal/mol at CCSD(T) level) barrier free process. However, the results obtained indicate that the hydrogen-transfer process forming NH4 + and CH2 COOH is clearly the dominating path, in agreement with the experimental evidence. The formation of ionized glycine from the reaction of product CH2 COOH with NH3 + is a quasi-isoenergetic (2.03 kcal/mol at CCSD(T) level) barrier free process that leads to a highly stable intermediate: protonated glycine.