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

Computational Chemistry Group, Universidad de Valladolid, Departamento de Química Física Facultad de Ciencias, 47005 Valladolid, Spain e-mail: [laural;predondo;vmrr;alargo;cbb]@qf.uva.es

Received: 13 January 2010
Accepted: 22 March 2010

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 CH₃COOH/CH₂COOH 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 CH₂COOH is clearly the dominating path, in agreement with the experimental evidence. The formation of ionized glycine from the reaction of product CH₂COOH 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.