CAPTURE CALCULATIONS FOR THE RATES OF IMPORTANT NEUTRAL-NEUTRAL REACTIONS IN DENSE INTERSTELLAR CLOUDS - C+C2H2 AND CN+C2H2

We have used long-range capture theories with a range of anisotropic potentials to calculate the rate coefficients for the neutral reactions C + C2H2 and CN + C2H2 as functions of temperature in the range 10-300 K. Both reactions are of probable importance in the gas-phase chemistry of dense interstellar clouds. The C + C2H2 reaction has been studied in the laboratory at room temperature, while the CN + C2H2 reaction has been studied in the laboratory at temperatures as low as 25 K. Our results are only in partial agreement with the measured values indicating (a) that short-range and/or statistical effects must in general be taken into account and (b) that the long-range theories are likely to be most accurate at low temperatures.