| Abstract
| - Although deuterium isotope fractionation in cold, dark interstellar clouds is reasonably well understood via gas-phase chemistry, there are some discrepancies between observation and theory. For example, the observed abundance ratio between the deuterated species C 3HD and the cyclic molecule C 3H 2 is significantly higher than most theoretical values unless the exchange reaction C 3H $_{3}^{+}$ + HD $\longrightarrow$ C 3H 2D + + H 2 is efficient. In this paper, we report quantum chemical and dynamical calculations on this reaction, which show it to possess a large activation energy barrier and to be very slow at all normal interstellar temperatures.
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