Meteorites are known to contain nitrogen-substituted polycyclic aromatic hydrocarbons (PANHs) like amino acids, vitamins, and nucleobases, with non-terrestrial isotopic abundances, indicative of their formation in space. Recent studies suggest these molecules may form through barrierless gas-phase radical-neutral reactions, which could be operative in low temperature regions of space. Radical derivatives of pyridine, one of the simplest aromatic nitrogen heterocycles, are suggested to play a part as key intermediates in the formation of larger PANHs. Previous studies indicate that reactions involving the pyridyl radicals and small unsaturated hydrocarbon chains are good candidates for future experimental work because they likely possess submerged barriers along the reaction path. However, the pyridyl radicals have not been studied with high-resolution spectroscopy that will enable future astronomical observations and aid in laboratory studies of their reactivity. Here we show high-level coupled-cluster calculations that determine the relative energies, structures, and spectroscopic parameters for these radicals. We also calculated anharmonic vibrational frequencies and vibrational corrections to rotational constants. To the best of our knowledge, these properties have not previously been calculated at this level of theory. We will discuss the results of these calculations as well as efforts to obtain the rotational spectra of these radicals using Fourier transform microwave spectroscopy. Beyond chemistry in the interstellar medium, these radicals likely play a role in Titan's atmosphere and have been identified as intermediates in the combustion of biomass, providing further motivation for both our computational and experimental work.

Rotational spectroscopy of the pyridyl radicals

ACS Spring Meeting 2021

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