Vibrationally stimulated addition reactions between hydrogen halides and unsaturated hydrocarbons: A negative result

No evidence for enhancement of gas phase addition reactions between hydrogen halides and unsaturated hydrocarbons was observed in cases where one reactant was prepared in a high vibrational state by laser excitation of an overtone transition. The specific reaction systems studied were HCl(ν=5 or 6)+ various unsaturated hydrocarbons, and also C2H2(00500)+HBr or HCl, at room temperature. The reactant states were prepared by one photon-overtone absorption inside a cw dye laser, to an energy level above the activation energy of the corresponding thermal reaction. It is estimated that the rate constant for addition of these vibrationally excited molecules is at least 10--1000 times slower than the rate corresponding to the pre-exponential factor of the associated thermal reaction, depending on the employed rate of collisional quenching. Also, the reaction of C2H2(00500)+HBr did not appear to accelerate even when heated to 3000 C