Isotope Effects in the Association Reactions of Methyl and Ethyl Iodide Cations

Rate coefficients for production of stabilized dimeric parent cations at 295 K have been determined in CH3I, CD3I, CH3I—CD3I mixtures, C2H5I, CH3CD2I, CD3CH2I, and C2D5I. These processes are the most rapid reported for association reactions, the various individual values falling within the limits 0.33 × 10−24 (CH3I) and 10.1 ×10−24 cm6 molecule−2 sec−1 (C2H5I). The temperature dependence of the stabilization coefficients in CH3I and CD3I was also measured over the range 220 ± 3 to 320 ± 1 K, as well as the efficiencies of other third bodies in the stabilization process. The differences observed for the variously labelled analogues are interpreted in terms of vibrational level (energy) depression upon deuteration, which affects the intrinsic lifetime of the collision complex.