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The conformer distribution of normal-propyl cyanide is investigated using broadband chirped pulse rotational spectroscopy in the millimeter-wave regime coupled with buffer gas cooling. Here we explore the relative abundances of the anti and gauche conformers following room-temperature gas-phase injection into a 25 K buffer gas cell and compare to that which is observed following temperature-programmed desorption from an ice surface, similar to the slow warm-up experienced by ice grains as they approach warmer regions within the interstellar medium. The conformer distributions observed in the gas phase from room-temperature injection are then used to determine their relative energies, an important parameter needed to interpret the isomer and conformer abundances derived from astronomical observations. We find the gauche conformer to be the most stable species by ∼97 ± 21 cm−1. We further examine the relative conformer abundances following ice desorption, which are distinct from those following the gas-phase introduction. The ratios measured off the ice correspond to a conformer temperature of ∼56 K, which is much lower than their sublimation temperature of 170 K. 

Abstract Direct D-H exchange in radicals is investigated in a quasi-uniform flow employing chirped-pulse millimeter-wave spectroscopy. Inspired by the H-atom catalyzed isomerization of C₃H₂reported in our previous study, D-atom reactions with the propargyl (C₃H₃) radical and its photoproducts were investigated. We observed very efficient D-atom enrichment in the photoproducts through an analogous process of D addition/H elimination to C₃H₂isomers occurring at 40 K or below. Cyclic C₃HD is the only deuterated isomer observed, consistent with the expected addition/elimination yielding the lowest energy product. The other expected addition/elimination product, deuterated propargyl, is not directly detected, although its presence is inferred by the observations in the latter part of the flow. There, in the high-density region of the flow, we observed both isotopomers of singly deuterated propyne attributed to stabilization of the H+C₃H₂D or D+C₃H₃adducts. The implications of these observations for the deuterium fractionation of hydrocarbon radicals in astrochemical environments is discussed with the support of a monodeuterated chemical kinetic model.