Electron-induced processes in ellagic acid molecules via gas-phase resonance electron attachment and electron transfer following photoexcitation in solution

Abstract

The low-energy (0–14 eV) resonance electron interaction with gas-phase ellagic acid (EA) molecules is studied using dissociative electron attachment (DEA) spectroscopy. Photoinduced electron transfer reactions with solvated EA are studied using the chemically induced dynamic nuclear polarization (CIDNP) technique. Molecular negative ions EA˙⁻, the most abundant species generated by thermal electron attachment to EA, autodetach their extra electrons within 200 µs, allowing us to estimate the adiabatic electron affinity of EA as 1.3 eV—a value in excellent agreement with that predicted at B3LYP/6-31+G(d) level. In an intriguing observation, the slow (microsecond timescale) cleavage of a single O–H bond, resulting in [EA – H]⁻ fragments, can tentatively be explained by the H-atom roaming across the molecular framework or by the statistical accumulation of the energy required to overcome the potential barrier along the reaction coordinate. In contrast to a variety of polyphenolic molecules, [EA – 2H]˙⁻ is not formed at thermal electron energies, despite this decay being energetically favorable, likely due to competition with single H-atom abstraction. Fully deprotonated EA (present in solution at pH > 10 as [EA – 4H⁺]⁴⁻) can attach solvated electrons to produce [EA – 4H⁺]˙⁵⁻ radicals, consistent with the high electron-accepting ability of isolated EA. However, deprotonated EA can also donate electrons to the model electron acceptor, 2,2′-dipyridyl, generating [EA – 4H⁺]˙³⁻ radicals, with no further decomposition observed in the present CIDNP experiments, in agreement with the limited fragmentation seen in gas-phase DEA applied to intact EA. The present findings could be important for understanding the biological effects produced by EA, namely, its synergism with radiotherapy and its antibacterial activity, both likely associated with electron-driven processes.