Anion of Tetrachloroethane (Tetra): Fragmentation and Geminate Ion Kinetics in Liquid Methylcyclohexane (MCH)

Abstract

In the context of studies on the influence of the anion lifetime on the geminate ion kinetics, 1,1,1,2- and 1,1,2,2-tetrachloroethane (1112-Tetra and 1122-Tetra) were studied as solutes in liquid methylcyclohexane (MCH) at low temperatures (133−183 K). The two isomers serve as examples of long anion lifetime. The analysis of the pulse radiolysis data was based on the t-0.6 semiempirical law for geminate ion kinetics. The visible band with λmax = 450 or 430 nm is shown to be due to the anion of 1112-Tetra or 1122-Tetra, respectively. Its kinetics relates to three consecutive geminate pairs of ions, due to two ionic reactions: (a) The fast process represents the cationic mechanism: the precursor cation M+* relaxes (or isomerizes) to the high mobility ion MCH+ (kr) and simultaneously fragments (kf) to a diffusional methylcyclohexene+ (MCHene+). The total M+* decay (ktot = kr + kf) produces mixed cations (MCH+,MCHene+). (b) The slow process is due to the anion fragmentation (k-) from Tetra- to Cl- + R•, with τ- = 13.7 or 20.0 μs (143 K) for 1112- or 1122-Tetra, respectively. The fragment radical R• is freed too late to allow scavenging of positive charge. The three geminate pairs of ions are (M+*/Tetra-), (MCH +,MCHene+/Tetra-), and (MCH+,MCHene+/Cl-). All ions (except Cl-) contribute to the optical absorption. The rate constants ktot and k- are both independent of the concentration of Tetra. For ktot this means that M+* decays in a fixed ratio of kf to kr. This is in contrast to previous findings with N2O or CHCl3 but corresponds to our recent proposal that M+* appears to represent some isomer of MCH+ in a higher energy state (or of higher ionization potential). The anion fragmentation rate for 1112-Tetra is k-(143 K) = (7.3 ± 0.6) × 104 s-1 with Eact = 17.8 kJ/mol and log A = 11.2. For 1122-Tetra it is k-(143 K) = (5.0 ± 1.0) × 104 s-1 with Eact = 16.9 kJ/mol and log A = 10.9. The free ion intercepts, from the t-0.6-simulations, reveal for all geminate pairs with Tetra- a strong dependence on the Tetra concentration [T], eventhough complete electron scavenging was ascertained. This is explained by the formation of dimer anions through an equilibrium T- + T ⇌ . The absorption at 450 nm (or 430 nm) then is due to (most likely a charge resonance transition (T ← T-)). For the initial geminate pair (M+*/Tetra-), the free ion intercept was smaller than the one for Tetra- alone (actually ). As this result was based on the assumption that M+* and MCH+ have the same mobility, this now reveals that the precursor cation M+* must have at least a 9 times higher mobility than MCH+.