As. Domazou et al., ION-PAIRS (CFCL2-TO-CL-)SOLV FROM GEMINATE ION RECOMBINATION IN METHYLCYCLOHEXANE WITH CFCL3 - FORMATION, REACTIVITY, AND STABILITY(PARALLEL), Journal of physical chemistry, 98(11), 1994, pp. 2877-2882
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.