Stealing from a distant neighbor: an unexpectedly fast long-span peroxy radical hydrogen-shift reaction in a long-chain diether

Abstract

Gas-phase autoxidation is an atmospheric chemistry reaction mechanism capable of transforming volatile organic compounds (VOCs) into highly oxygenated organic molecules (HOMs) that contribute to secondary organic aerosol (SOA) formation and growth. The key steps in this mechanism are intramolecular hydrogen shift (H-shift) reactions in organic peroxy radicals (RO₂). For acyclic saturated molecules, these H-shift reactions are generally sufficiently slow that they cannot compete with atmospheric bimolecular reactions with NO_x species, except for the 1,5 and 1,6 H-shifts, occurring via transition states (TS) of six- and seven-membered rings. Here, we report a surprisingly fast long-range H-shift reaction in a RO₂ formed in the photo-oxidation of a volatile diether. In 1,2-diethoxyethane (1,2-DEE), we observe experimentally a 1,8 H-shift reaction that occurs with a rate coefficient on the order of ∼1 s⁻¹ at 294 K – a rate that outcompetes all other RO₂ unimolecular chemistry in the system and will, under most atmospheric conditions, outcompete bimolecular processes as well. Theoretical calculations indicate that activation of the C–H bond by an α-oxyl substituent and weaker transannular strain in the 1,8 H-shift transition state, combined with inductive deactivation of C–H bonds by a β-oxyl group at the abstraction site of competing 1,5 and 1,6 H-shifts, enable the longer-span 1,8 H-shift to be competitive. Our findings broaden the recognized reactivity of functionalized RO₂ and highlight the potential for structurally diverse VOCs to undergo unexpected autoxidation pathways, producing HOMs at greater yield and with higher molecular complexity than previously anticipated.