Low‐Temperature Experiments on the Formation of Deuterated C 3 H + 3

Abstract

Many deuterated molecules have been discovered in inter- and circumstellar regions. In some cases, the observed abundances can be explained with simple thermodynamic models; often, however, isotope enrichment is more complicated. This has been seen recently in detailed low-temperature experiments performed for the fundamental systems H3+/H2D + and CHn+/CHn-1D+. An unsolved problem is to explain the large abundance of C3H 2 and larger hydrocarbons and their deuterated variants observed in cold, dark interstellar clouds. In this work a variable-temperature 22 pole trap is utilized for closely scrutinizing various ion-molecule reactions that may contribute to the formation of C3H2D+ or C3HD2+ and, via dissociative recombination, C3HD. The experimental study of the promising candidate C 3H3- + HD ? C3H 2D+ + H2, which has already been excluded by theory, corroborates that this exothermic H-D exchange does not occur at all. A careful analysis of the data reveals that the 15 K rate coefficient is smaller than 4 ? 10-16 cm3 s-1. In contrast, quite efficient routes have been found in the low-temperature experiments, starting with C3+ and proceeding via deuterated C 3H+ to C3H2+ and C 3H3+. Formation of C3D+ in C3+ HD collisions is 6 times faster than assumed in astrochemical models (k = 9.3 ? 10-10 cm3 s -1). Surprisingly, direct production of C3HD+ via radiative association has also been observed (kr = 6.0 ? 10-11 cm3 s-1). Reactions of partly or fully deuterated C3H+ + H2 collision system are strongly dependent on temperature and the hydrogen ortho to para ratio. In addition, it shows very complicated isotope effects. For example, in C 3H+ + HD collisions the formation of C3HD + (k = 4.6 ? 10-10 cm3 s-1) dominates over the H-D exchange (k = 5.6 ? 10-11 cm 3 s-1) and radiative association C3H 2+ (kr = 3.2 ? 10-11 cm 3 s-1). The reactions of C3H2+ + H2 are very slow for all isotope combinations. Although several questions remain open and more low-temperature experiments are needed, it is recommended that the new values are included in astrochemical databases, since they are fundamental to the correct description of the carbon chemistry in interstellar clouds. ? 2005. The American Astronomical Society, All rights reserved.