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

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.