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 CCl 4 , CFCl 3 ) 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 (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 Hexaand Pentashould be short lived (IP formation possible) and Tetralong 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 Hexaand Pentaat 143 K, ion pairs were observed, whereas Tetrawith 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 f (R + ||Cl -) solv . The IP absorption is due to the CT band of (R + 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 fast /D diff (competition of the high mobility approach of MCH + (with D fast ) toward Cland the diffusional escape of R • (with D diff ), away from Cl -). It is shown that η reduces by a factor of about 6 from 133 to 295 K in parallel to D fast /D diff 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 ip ) 111 µs), followed by Hexa (τ ip ) 22.7 µs) and Penta (τ ip ) 5.3 µs). If no IP is detectable, IP formation is still possible, but τ ip , τ -(probably true for CHCl 3 ). For all IPs so far found (list of 7 given) the IP decay rate constant k ip 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.

Spin-Correlated Radical Ion Pairs Generated in Liquid Haloalkanes Using High-Energy Radiation

“…In a study by Saeki and colleagues performed with the picosecond radiolysis technique, experimental data, within the framework of the model used, suggested a τ A of 1.5 ns. This is in drastic contrast with other data reported, indicating values of τ A in femtoseconds. In this context, data for CH 2 Cl 2 , and CHCl 3 , are also of interest, in which the decay time of the solvent radical anion was shown to exceed that for CCl 4 .…”

“…At first glance, it seems that the formation of such pairs under these conditions is improbable. Indeed, in a diluted solution, the ionization of the solvent is the dominating process − The excess electron is generally accepted to vanish in a few picoseconds via a dissociative attachment (reaction ) with a haloalkane molecule − Thus, even if the solution contains electron acceptors in a low concentration, reaction prevents the formation of secondary radical anions (RAs) as the electron affinity of the halogen atoms exceeds 3 eV, which is much higher than that of the hydrocarbon molecules. This property of haloalkanes is frequently exploited in pulse radiolysis experiments to distinguish between optical absorption bands related to the positive or negative charge carriers in irradiated liquids. ,, …”

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

“…In consequence, the δ 2 values should also be independent of [T]: there is no change of the ratio MCH + /MCHene + with [T]. The mobility value δ 2 only depends on λ due to ε (MCH + ) and ε (MCHene + ) (see eq 6 of ref ).…”

“…It also covers combinations of ionic reactions (parallel or consecutive, for positive and/or negative ions) and is able to simulate ion recombinations leading to products, which are optical absorbers. In this paper the mathematical equations needed are quoted from the adjacent paper, referring to eqs 1−12.…”

“…In the adjacent paper (and its preceding one) the effect of the anion lifetime on the geminate ion kinetics is discussed for chlorocarbons in liquid methylcyclohexane (MCH). In the context of that paper, tetrachloroethane (Tetra) served as an example of long anion lifetime.…”