RATE COEFFICIENTS FOR THE ENDOTHERMIC REACTIONS C-2)+H-2(D-2)-]CH+(CD+)+H(D) AS FUNCTIONS OF TEMPERATURE FROM 400-1300 K((P)

We have measured the bimolecular rate coefficients for the reactions o f C+(P-2) with H-2 and D-2 as functions of temperature from 400 to 130 0 K using a high temperature flowing afterglow apparatus. The temperat ure dependences of these rate coefficients are accurately fit by the A rrhenius equation, with activation energies equal within experimental uncertainty to the reaction endothermicities. Internal energy dependen ces have been deduced by combining the present data with previous drif t tube and ion beam measurements. We found that reactant rotational en ergy and translational energy an equally effective in surmounting the energy barrier to reaction, and that vibrational excitation of the neu tral reactant to the upsilon=1 state enhances the rate coefficients by a factor of similar to 1000 for the reaction with H-2 and by similar to 6000 for the reaction with D-2 at temperatures of 800 and 500 K, re spectively. This vibrational enhancement is larger than the enhancemen t that would be produced if the same amount of energy were put into tr anslational and/or rotational modes of the reactants. In addition, rat e coefficients have been derived for the three-body association reacti on of C+(P-2) with H-2 in a helium buffer over the temperature range 3 00-600 K. (C) 1997 American Institute of Physics.