Direct D-atom incorporation in radicals: An overlooked pathway for deuterium fractionation

TL;DR

This study investigates direct D-H exchange in radicals at low temperatures, revealing efficient deuterium enrichment pathways in hydrocarbon radicals relevant to astrochemistry.

Contribution

It uncovers a novel D-atom incorporation pathway in radicals, demonstrating efficient deuterium enrichment and providing insights into astrochemical deuterium fractionation mechanisms.

Findings

Efficient D atom enrichment in C3H2 isomers at 40K or below.

Cyclic C3HD is the only observed deuterated isomer, consistent with energy considerations.

Detection of isotopomers of deuterated propyne suggests stabilization of specific adducts.

Abstract

Direct D-H exchange in radicals is investigated in a quasi-uniform flow employing chirped pulse mm-wave spectroscopy. Inspired by the H-atom catalyzed isomerization of C3H2 reported 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 C3H2 isomers occurring at 40K 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 + C3H3 adducts. 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.