Fragmentação de ésteres em fase gasosa por impacto de elétrons

Abstract

This work presents an experimental/theoretical investigation into the ionization and fragmentation of three esters: methyl formate, methyl acetate, and ethyl acetate. Time-of-flight mass spectrometry was applied across a wide range of electron impact energy, from 12 eV to 800 eV. To convert the fragment cross-sections obtained relative to the not dissociated molecular ion and determine the total absolute ionization cross-section the Binary-Encounter-Bethe methodology was used. Both experimental and theoretical results were compared with available data. It was found that the molecular structure and dissociation pathways significantly alter the production of various common fragments among the molecules. An increase in both partial and total absolute cross-sections was observed as a function of electron impact energy, which is strongly dependent on the chain length. The extension of the molecular chain implies a greater number of valence electrons and, therefore, more molecular orbitals and excited levels reached by ionization participating in the dissociation process. The data allowed for a comparison between the maximum total ionization cross-section and the volumetric polarizability, revealing a linear behaviour as proposed in the literature for organic compounds, which suggests the validity of the present calculations. This study is of interest to astrochemistry, particularly astrobiology. Specifically, the interstellar medium (ISM) contains organic molecules, including esters essential for the evolution and chemical diversity of a stellar system. The matter within the ISM, found in heterogeneously distributed gas and dust clouds, is processed by ionizing radiation, including electron incidence, forming complex molecules based on carbon, hydrogen, and oxygen.