High-energy irradiation of matrix isolated acetic acid yields a ·CH3⋯CO2 complex: A spectroscopic and <i>ab initio</i> study

The mechanism of the chemical transformations of isolated small organic molecules induced by high-energy radiation is of basic interest for astrophysics and astrochemistry. In this work, we first applied a combination of electron paramagnetic resonance (EPR) and Fourier-transform infrared (FTIR) spectroscopy to identify the products of the radiation-induced transformations of isolated CH3COOH and CD3COOH molecules. As revealed by EPR, ·CH3 (or ·CD3) is the principal primary radical generated from acetic acid in solid argon and xenon, while the FTIR results suggest that this radical is trapped mainly in the form of the ·CH3⋯CO2 radical-molecule complex. The assignment of this previously unknown complex was based on the complexation-induced shifts of the absorption bands corresponding to CH3OPLA and CO2bend vibration modes, confirmed by analysis of the kinetic curves, photochemical behavior, and comparison with the results of ab initio computations at the spin-unrestricted coupled-cluster singles, doubles, and perturbative triples level of theory. Most likely, the complex in matrices adopts the geometry close to the theoretically predicted structure with Cs symmetry stabilized by the C⋯C and O⋯H interactions. It was suggested that the complex could be produced via the intermediate formation of a CH3COOH+· radical cation deprotonating to the CH3COO· radical, which promptly decomposed to ·CH3 + CO2 fragments. We believe that the results obtained in this study may contribute to a better understanding of the processing of acetic acid molecules in astrophysically relevant ices under high-energy irradiation and give a valuable insight into the understanding of weak intermolecular interactions involving radicals relevant to atmospheric chemistry, combustion, and carbon dioxide conversion.