Some H/D exchange reactions involved in the deuteration of interstellar molecules

The forward and reverse rate coefficients, k/sub f/ and k/sub r/, for the reactions (1) D/sup +/+H/sub 2/arrow-right-leftH/sup +/+HD, (2) H/sup +//sub 3/ +HDarrow-right-leftH/sub 2/D/sup +/+H/sub 2/, and (3) CH/sup +//sub 3/+HDarrow-right-leftCH/sub 2/D/sup +/+H/sub 2/ have been determined in the laboratory. From the data, it is calculated that k/sub f/ for each of these reactions will reach its collisional limiting value at interstellar cloud temperatures and that k/sub r/(1) is so small at low temperatures that it represents a negligible loss of HD in interstellar clouds. From the measured values of k/sub f/(2) and k/sub r/(2), a value for the enthalpy change ..delta..H/sup 0/ in the reaction has been estimated and is seen to be in reasonable agreement with the value calculated from a consideration of the zero point energies and the rotational energy contents of the reactant and product species. Also the variation of ..delta..H/sup 0/ with temperature is established which has important implications in the determination of the electron densities in interstellar clouds. ..delta..H/sup 0/ has also been determined for reaction (3), and again the most appropriate values for use in interstellar chemical calculations are indicated. The large value of ..delta..H/sup 0/ implies that k/sub r/ (3) will be umimportant and that CH/sub 2/D/sup +/ will be available to participate in the synthesis of deuterated interstellar molecules. The collisional association rate coefficient for the production of CH/sub 4/D/sup +/ from CH/sub 2/D/sup +/+H/sub 2/ has also been determined, and from this we have estimated the corresponding radiative association rate coefficient. Thus the possible role of CH/sub 4/D/sup +/ in interstellar molecular synthesis is discussed briefly.