Chloroform anion fragmentation in liquid methylcyclohexane: t(-0.6) simulation of the geminate ion kinetics

The chloroform anion in liquid methylcyclohexane (MCH) fragments during the time scale of geminate ion recombination (143 K less than or equal to T le ss than or equal to 193 K). Its lifetime can only be determined if the gemi nate ion kinetics can be calculated. The t(-0.6) semiempirical law is used. Because CHCl3 quenches M+*, the precursor of the solvent radical cation MC H+, much less effectively than N2O, the fragmentation of M+* to produce the methylcyclohexene cation (MCHene(+)) has to be considered by theory. The t (-0.6) simulation therefore was modified to include the two parallel reacti ons of the M+* decay, which are forming mixed ion pairs (MCNene(+), MCH+/X- ). It is found that mixed pairs are still describable by the t(-0.6) linear ity, yet the mobility factor delta (slope of t(-0.6)) is now lambda-depende nt. Complete simulation of the ionic mechanism yields the following results : (1) The fragmentation rate constant for CHCl3- is k(1)(143 K) = (3.6 +/- 0.3) x 10(6) s(-1) with E-act = 4.6 +/- 0.5 kJ/mol and logA = 8.2 +/- 0.2. The lifetime of 280 ns is substantially larger than expected from gas phase data. (2) By applying the known G(fi) values to the free ion spectra at 14 3 K, 153 K, and 173 K, the absorption coefficients of the CHCl3- band were determined and a Lorentzian line shape fitted: lambda(max) = 470 nm, epsilo n(max) = 1900 +/- 30 M-1 s(-1) and a width of hwhm = 28 +/- 2 nm. (3) Assum ing that the M+* fragmentation (k(frag)) and the natural relaxation to MCH (k(0)) are the same as in N2O-saturated MCH, the quenching rate constant a t 143 K may be derived: k(2)(M+* + CHCl3 --> MCH+) = (7.7 +/- 2.6) x 10(6) M-1 s(-1). Quenching by CHCl3 therefore is about. four times slower than by N2O; the yield of the olefinic cation (MCHene(+)) is strongly increased re lative to the MCH+ yield. Furthermore, the ratio quenching/fragmentation wi th chloroform as solute is found to increase with temperature, suggesting t hat fragmentation at room temperature might have much less importance.