Hydrogen Abstraction of Acetic Acid by Hydrogen Atom to Form Carboxymethyl Radical •CH₂C(O)OH in Solid <i>para</i>-Hydrogen and Its Implication in Astrochemistry

Abstract

Acetic acid CH3C(O)OH attracts significant attention in interstellar chemistry because it is considered to be a potential precursor for the formation of amino acids, such as glycine and alanine. The reaction NH2 + •CH2C(O)OH within the ice mantles and on an ice surface is considered to be responsible for the formation of glycine from CH3C(O)OH. However, detailed experimental investigations are scarce. We took advantage of the unique properties of para-hydrogen (p-H2), which serves as a quantum-solid matrix host and a medium for hydrogen-atom tunneling reactions, to investigate the reaction between CH3C(O)OH and H atoms. On photolysis at 365 nm of a CH3C(O)OH/Cl2/p-H2 matrix to produce Cl atoms and subsequent infrared irradiation to produce H atoms by promoting Cl + H2 (v = 1) → H + HCl, new lines at 3581.6, 1681.0, 1455.6, 1350.7, 1180.9, 968.4, 898.4, 766.4, and 579.5 cm–1 were observed and assigned to the carboxymethyl radical, •CH2C(O)OH; the observed new spectrum agrees satisfactorily with IR intensities and scaled harmonic vibrational wavenumbers predicted with the B3LYP/aug-cc-pVTZ method. The observation of only •CH2C(O)OH as a product of H + CH3C(O)OH indicates that the reaction proceeds through H-abstraction at the CH3 moiety, whereas H-abstraction from the OH-site and other H-addition channels are unfavorable. Further H-abstraction from •CH2C(O)OH was unobserved. This previously neglected H-abstraction from CH3C(O)OH is expected to be a major channel for the formation of •CH2C(O)OH on a grain surface and bulk ice and to play an important role in the formation of glycine and other complex organic molecules (COM) in the interstellar media.