Recent rate constant and product measurements of the reactions C2H3+H-2 and C2H3+H - Importance for photochemical modeling of hydrocarbons on Jupiter

Recently the rate constants of the reactions C2H3 + H-2 --> C2H4 + H and C2H3 + H --> products have been measured by more direct methods than used previously. The new determination for the rate constant of C2H3 + H-2 is lower by a factor of 1000 at room temperature than the rate used by Allen et al. in their methane photochemical model (The relative abundance of ethane to acetylene in the jovian stratosphere, Icarus 100, 527-533 (1992)). A fast rate for this reaction is key to their proposed reaction scheme to convert C2H2 to C2H6 and explain the C2H6/C2H2 ratio on Jupiter. However, their scheme becomes ineffective when a rate constant expression consistent with Fahr et al. is used (Experimental determination of the rate constant for the reaction of C2H3 with H-2 and implications for the partitioning of hydrogen in the atmospheres of the other planets, Icarus 116, 415-422 (1995)). The rate measurement for C2H3 + H by Monks et al. (Kinetics and products of the reaction between H and C2H2 of T = 213 and 298 K, J. Phys. Chem. 99, 17,151-17,159 (1993)) is consistent with recent measurements but they found C2H4 to be a significant product, in addition to C2H2 + H-2 only the latter product channel had been used in methane photochemical models. With the new lower rate constant for C2H3 + H-2, the reaction C2H3 + H --> C2H4 becomes important in determining the C2H6/C2H2 ratio on Jupiter. However, the new reaction scenario is less efficient in converting C2H2 to C2H6 than the one proposed by Alien et al. because it depends in part upon H, which is in scarce supply in the lower stratosphere where C2H3 + H --> C2H4 is important. (C) 1996 Academic Press, Inc.