ION-PAIRS (CFCL2-TO-CL-)SOLV FROM GEMINATE ION RECOMBINATION IN METHYLCYCLOHEXANE WITH CFCL3 - FORMATION, REACTIVITY, AND STABILITY(PARALLEL)

Pulse irradiated samples of CFCl3 in liquid methylcyclohexane (MCH) at low temperatures exhibit an optical absorption with lambda(max) at 43 5 nm. From extensive scavenger studies with variation of dose, tempera ture, and solution composition, it is obvious that the 435-nm species has both a cation and an anion as precursor and behaves itself as a po sitive species. It is assigned to the cation CFCl2+, produced by charg e transfer from the solvent radical cation to the fragment radical CFC l2. from CFCl3- anion dissociation. The absorption is attributed to a charge-transfer band between CFCl2+ and a solvent (or solute) molecule . Except for the very early time, the CFCl2+ cation disappears at lowe r doses by first-order kinetics up to four half-lives dependent on tem perature, much slower than expected from geminate ion recombination, w ith k(l)(143 K) = (4.5 +/- 0.6) X 10(3) s-1. The corresponding Arrheni us parameters are very low (E(act) = 12.6 +/- 1.0 kJ/mol, log A = 8.2 +/- 0.4). It is concluded that CFCl2+ and Cl- on recombination are for ming an ion pair with the CFCl2+-absorber remaining unperturbed within the ion pair. The ion pair reacts with quadricyclane (Q), methyltetra hydrofuran (MTHF), and also with solvent radicals, the latter initiati ng a dose effect on the ion pair lifetime for higher doses. The rate c onstant with Q at 143 K is (8.7 +/- 0.4) X 10(5) M-1 s-1. The ion pair reactivity, as well as the very low preexponential factor, is difficu lt to explain in the context of contact ion pairs. Together with furth er arguments, it is concluded that the ion pair (CFCl2+ parallel-to Cl -)solv most likely must be solvent separated.