Isomeric Identification of the Nitroindole Chromophore in Indole + NO Organic Aerosol.

Oxidation of indole by nitrate radical (NO) was previously proposed to form nitroindole, largely responsible for the brown color of indole secondary organic aerosol (SOA). As there are seven known nitroindole isomers, we used chromatographic separation to show that a single nitroindole isomer is produced in the indole + NO reaction and definitively assigned it to 3-nitroindole by comparison with chromatograms of nitroindole standards. Mass spectra of aerosolized 3-nitroindole particles were recorded with an aerosol mass spectrometer and directly compared to mass spectra of SOA from smog chamber oxidation of indole by NO in order to help identify peaks unique to nitroindole (/ 162, 132, and 116). Quantum chemical calculations were done to determine the energetics of hypothesized indole + NO intermediates and products. The combination of these data suggests a mechanism, wherein a hydrogen atom is first abstracted from the N-H bond in indole, followed by isomerization to a carbon-centered radical in the 3-position and followed by addition of NO. Alternative mechanisms involving a direct abstraction of a H atom from a C-H bond or a NO addition to the ring are predicted to be energetically unfavorable from large barriers for the initial reaction steps.