RATE CONSTANTS FOR THE REACTIONS OF CN WITH HYDROCARBONS AT LOW AND ULTRA-LOW TEMPERATURES

The pulsed laser-photolysis (PLP), time-resolved laser-induced fluores cence (LIF) technique has been used to study the reactions of the CN r adical with CH4, C2H6, C2H4, C3H6 and C2H2 at low and ultra-low temper atures. Using a cryogenically cooled cell, rate constants have been de termined for all five reactions at temperatures down to 160 K. The PLP -LIF method has also been implemented in a CRESU (cinetique de reactio n en ecoulement supersonique uniforme) apparatus providing rate consta nts for the reactions of CN with C2H6, C2H4 and C2H2 at temperatures d own to 25 K. The rate constants for CN + C2H4 and CN + C2H2 increase m onotonically as the temperature is lowered from room temperature to 44 K but the values at 25 K are lower than those at 44 K. Remarkably, th e rate constant for CN + C2H6 increases below 75 K, reaching its large st value at 25 K. It is tentatively suggested that this behaviour may reflect the transient formation of an energised van der Waals complex which facilitates the subsequent abstraction of an H atom.