Direct spectroscopic identification of elusive dehydrogenated radicals: CH2OCHO and CH3OCO radicals from methyl formate oxidation.

The atmospheric degradation of volatile organic compounds (VOCs) begins through oxidation by hydroxyl or nitrate radicals, producing highly reactive dehydrogenated radicals. While the stability-and thus, the reactivity-of these radicals critically depends on their isomeric form, directly monitoring individual isomers under laboratory conditions remains a major analytical challenge. Here, we combine high-level ab initio calculations to unambiguously identify radicals via their structures, energies, and internal dynamics with a very sensitive high-resolution millimeter wave spectroscopy (150-500 GHz) setup to resolve and characterize the two isomeric forms of dehydrogenated methyl formate: the formyloxymethyl radical (CH2OCHO) and the methoxy carbonyl radical (CH3OCO). Our results demonstrate that millimeter wave spectroscopy enables the direct, isomer-specific detection of medium-sized dehydrogenated VOCs-a critical advancement for oxidation experiments and environmental monitoring. This integrated approach paves the way for real-time tracking of radical intermediates under diverse atmospheric conditions, with broad implications for understanding VOC degradation pathways.