Geminate Ion Kinetics for Hexa-, Penta- and Tetrachloroethane in Liquid Methylcyclohexane (MCH): Effect of the Anion Lifetimes

Abstract

We have previously shown that, in the case of extremely short lived chlorocarbon anions (of CCl4, CFCl3) in liquid MCH solutions, one observes the formation and decay of the solvent separated ion pairs (R+||Cl-)solv, instead of the geminate ion recombination between the solvent cation (MCH+) and the fragment anion (Cl-). For longer lived anions (CHC ) there was no formation of ion pairs (IPs) observable. To evaluate the correlation between IP formation and anion lifetime τ-, three new chlorocarbon solutes (RCl), hexachloroethane (Hexa), pentachloroethane (Penta), and tetrachloroethane (Tetra), were studied, for which a coarse lifetime classification from positronium studies suggested that Hexa- and Penta- should be short lived (IP formation possible) and Tetra - long lived (no IP formation). The results in this paper agree with this expectation and confirm the correlation with τ-. With anion lifetimes of 250 and 150 ns for Hexa- and Penta- at 143 K, ion pairs were observed, whereas Tetra- with τ- = 13.7 μs decayed too late to yield IPs. The solvent separated ion pairs are formed through charge transfer (CT) from MCH+ to the fragment radical R• from the anion decay: MCH+ + R· · ·Cl-−η → (R+||Cl-)solv. The IP absorption is due to the CT band of (R+ ← MCH), and the stability relates to the complexing with the solvent. The efficiency η for CT reduces with time and therefore correlates to the anion lifetime: the later R• is freed, the lower η. It also correlates with the ratio of Dfast/Ddiff (competition of the high mobility approach of MCH+ (with Dfast) toward Cl- and the diffusional escape of R• (with Ddiff), away from Cl-). It is shown that η reduces by a factor of about 6 from 133 to 295 K in parallel to Dfast/Ddiff reducing from 400 to 10. It is concluded that the high mobility of the solvent cation is a requirement for positive CT from MCH+ to R•. The IP formation therefore gains importance at very low temperature; however, it loses importance at room temperature. The IP lifetime at 143 K is longest for CCl4 (τip = 111 μs), followed by Hexa (τip = 22.7 μs) and Penta (τip = 5.3 μs). If no IP is detectable, IP formation is still possible, but τip ≪ τ- (probably true for CHCl3). For all IPs so far found (list of 7 given) the IP decay rate constant kip is characterized by a very low preexponential Arrhenius factor of log A ≈ 8−10. For CCl4, Hexa, and Penta, the log A values are 9.0 ± 0.2, 8.3 ± 0.3, and 10.4 ± 0.4, respectively. Simulation of the complete mechanism is rather complex, but it is carefully analyzed with schemes of various complexity, particularily with and without the cation mechanism, related to the precursor M+* of the high mobility cation MCH+. It is shown that the details of the cationic mechanism are covered up by the strong absorption from the ion pair IP, if formed.