ADDITION OF OH TO ACETYLENE AND CONSECUTIVE REACTIONS OF THE ADDUCT WITH O-2

The reaction of hydroxyl with acetylene (proceeding mainly by addition), and reactions of the adduct with O-2 are studied, using the VUV flash photolysis/resonance fluorescence technique in Ar at 127 mbar and temperatures between 294 and 353 K and between 16 and 260 mbar at room temperature. The rate constant for the reaction of OH with acetylene is determined to be k(T) = (1.2 +/- 0.2).10(-12) exp(-(204 +/- 41) K/T) cm(3) s(-1) at 127 mbar in good agreement with literature data, and the pressure dependence is confirmed. Although the adduct is thermally stable, biexponential decays of OH are observed throughout, even at the lowest levels of O-2 attained. This can be explained by the occurrence of two reaction channels of the adduct with O-2 one of them regenerating OH. The rate constants derived, using the analytical solution of the corresponding rate equations for OH and the adduct, are partly degenerate. The OH-regenerating channel predominates with a rate constant of (42 +/- 5).10(-13) cm(3) s(-1) at room temperature with a slightly negative temperature coefficient, independent of pressure (where the error Limits include the uncertainty introduced by the degeneracy). Upper limits between 17.10(-13) cm(3) s(-1) at 16 mbar and 6.4.10(-13) cm(3) s(-1) at 260 mbar are obtained for the reaction of the adduct with O-2 not leading back to OH at room temperature which are equivalent to an upper limit for isomerization and fragmentation of the adduct of (1.0 +/- 0.1).10(-13) cm(3) s(-1), independent of pressure. No reactions of the adduct with acetylene can be detected at room temperature (k <10(-14) cm(3) s(-1)).