Rate constants at 297 K for proton-transfer reactions with C 2H 2: An assessment of the average quadrupole orientation theory

Abstract

Rate constants for proton-transfer reactions of the type XH + + C 2H 2 → C 2H 3 + + X (type 1) where X = H 2, D 2, CH 4, CO, N 2, CO 2 and N 2O, and the type C 2H 2 + X − → XH + C 2H − (type 2) where X = H, D, NH 2, CH 3NH, C 2H 5NH, (CH 3) 2N, OH, CH 3O, C 2H 5O and (CH 3) 2CHO have been measured at 297 ± 2 K using the flowing afterglow technique. The measured rate constants are compared with the predictions of classical theories on ion—molecule collisions, viz. the Langevin and Average Quadrupole Orientation theories. The Average Quadrupole Orientation theory provides collision rates generally to within ±5% of the measured reaction rates for reactions of type (1) but underestimates by ca. 10–60% the collision rates for several of the reactions of type (2). The results for reactions of type (2) are interpreted to suggest a small contribution of induced dipole—induced dipole interaction to the collision rate. Trends in k expt/ k AQO have been observed for homologous series of reactions involving alkoxide and amide ions. These trends have raised questions regarding the extent to which charge delocalization may affect the collision rate and the influence of reaction exothermicity on proton-transfer efficiency for these reactions.