  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.
