Spontaneous Generation of Alkoxide Radical from Alcohols on Microdroplets Surface

Abstract

Water microdroplets have been demonstrated to exhibit extraordinary chemical behaviors, including the abilities to accelerate chemical reactions by several orders of magnitude and to trigger reactions that cannot occur in bulk water. One of the most striking examples is the spontaneous generation of hydroxyl radical from hydroxide ions. Alcohols and alkoxide ions, being structurally similar to water and hydroxide ions, might exhibit similar behavior on microdroplets. Here, we report the spontaneous generation of alkoxide radicals from alcohols (RCH₂OH) in aqueous microdroplets through quantum chemical calculations, quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations, ab initio MD simulations, and mass spectrometry. Our results show that an electric field (EF) on the order of 10^-1 V/Å and partial solvation at the air-water interface jointly promote the dissociation of RCH₂OH into RCH₂O^- and H₃O⁺ ions. QM/MM MD simulations indicate that RCH₂O^- can be ionized to produce RCH₂O⋅ radicals on the microdroplet surface. Furthermore, partial solvation and the EF collaboratively catalyze the isomerization of the RCH₂O⋅ radical into a more stable tautomer, R⋅CHOH. This study highlights the molecular mechanisms underlying the widespread generation of radicals at the microdroplet surface and provides insights into the importance of fundamental alcohol chemistry in the atmosphere.

Keywords: AIMD; Air-water Interface; Alcohol Oxidation; Mass Spectrometry; Microdroplet.