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

We have previously shown that, in the case of extremely short lived chlorocarbon anions (of CCl 4 , CFCl 3 ) in liquid MCH solutions, one observes the formation and decay of the solvent separated ion pairs (R + ∥Cl - ) s o l v , instead of the geminate ion recombination between the solvent cation (MCH + ) and the fragment anion (Cl - ). For longer lived anions (CHCl - 3) 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 - ) s o l v . 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 D f a s t /D d i f f (competition of the high mobility approach of MCH + (with D f a s t ) toward Cl- and the diffusional escape of R* (with D d i f f ), away from Cl-). It is shown that η reduces by a factor of about 6 from 133 to 295 K in parallel to D f a s t /D d i f f 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 CCl 4 (τ i p = 111 μs), followed by Hexa (τ i p = 22.7 μs) and Penta (τ i p = 5.3 μs). If no IP is detectable, IP formation is still possible, but τ i p « τ - (probably true for CHCl 3 ). For all IPs so far found (list of 7 given) the IP decay rate constant k i p is characterized by a very low preexponential Arrhenius factor of log A 8-10. For CCl 4 , 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.