Mechanistic Insights Into the Dissociative Photoionization Pathways of Ethyl Propionate: A Combined Experimental and Theoretical Approach

Abstract

Ethyl propionate (C₅H₁₀O₂, EP) has been extensively studied in the fields of biofuels and atmospheric chemistry. However, its vacuum ultraviolet (VUV) photoionization has not been investigated. This study examines the photoionization process of EP using tunable VUV synchrotron radiation, coupled with a reflectron time-of-flight mass spectrometer. This method yielded the photoionization mass spectrum of EP and photoionization efficiency (PIE) spectra of 10 identified fragment ions (i.e., C₄H₇O₂^+., C₃H₇O₂^+., C₃H₆O₂^+., C₃H₅O₂^+., C₃H₆O^+., C₃H₅O^+., C₃H₄O^+., C₂H₅O^+., C₂H₅^+., and C₂H₄^+.). The results, interpreted with the aid of high-accuracy theoretical calculations, conclude possible formation mechanisms for each fragment ion. In the dissociation pathway of EP's cation, intramolecular hydrogen shifts and bond cleavage are the predominant processes. The C₃H₇O₂^+. and C₂H₄^+. reaction channels do not arise from one-step bond cleavage, but their reaction energy barriers are influenced by product energy, making them comparable to direct reaction channels. The active reaction sites within the molecules are elucidated using Laplacian bond order (LBO). Rate constants are calculated using RRKM theory, which confirms the kinetic factors governing the EP reaction process. This study provides a detailed understanding of the photoionization and dissociation of the main ions of EP within the 9.35–15.50 eV photon energy range.