“…In the meanwhile we have published details of the t -0.6 kinetic law, particularily to include overlapping ionic reactions, − and we were able to identify the solvent radical cation MCH + and its reaction with positive scavengers (e.g., norbornadiene) . We also detected the existence of a precursor to MCH + , called M + *, some excited state of MCH + .…”
Section: Introductionmentioning
confidence: 88%
“…Theoretical support for the t -0.6 rate law was recently given by Barczak and Hummel . From the many semiempirical theories to simulate the nonhomogeneous, geminate ion kinetics, only the t -0.6 rate law is able to explain the experimental results in methylcyclohexane. , …”
The chloroform anion in liquid methylcyclohexane (MCH) fragments during the time scale of geminate ion recombination (143 K e T e 193 K). Its lifetime can only be determined if the geminate ion kinetics can be calculated. The t -0.6 semiempirical law is used. Because CHCl 3 quenches M + *, the precursor of the solvent radical cation MCH + , much less effectively than N 2 O, 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 reactions of the M + * decay, which are forming mixed ion pairs (MCHene + , MCH + /X -). It is found that mixed pairs are still describable by the t -0.6 linearity, yet the mobility factor δ (slope of t -0.6 ) is now λ-dependent. Complete simulation of the ionic mechanism yields the following results:(1) The fragmentation rate constant for CHCl 3is k 1 (143 K) ) (3.6 ( 0.3) × 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 143 K, 153 K, and 173 K, the absorption coefficients of the CHCl 3 band were determined and a Lorentzian line shape fitted: λ max ) 470 nm, max ) 1900 ( 30 M -1 s -1 and a width of hwhm ) 28 ( 2 nm.(3) Assuming that the M + * fragmentation (k frag ) and the natural relaxation to MCH + (k 0 ) are the same as in N 2 O-saturated MCH, the quenching rate constant at 143 K may be derived: k 2 (M + * + CHCl 3 f MCH + ) ) (7.7 ( 2.6) × 10 6 M -1 s -1 . Quenching by CHCl 3 therefore is about four times slower than by N 2 O; the yield of the olefinic cation (MCHene + ) is strongly increased relative to the MCH + yield. Furthermore, the ratio quenching/fragmentation with chloroform as solute is found to increase with temperature, suggesting that fragmentation at room temperature might have much less importance.
“…In the meanwhile we have published details of the t -0.6 kinetic law, particularily to include overlapping ionic reactions, − and we were able to identify the solvent radical cation MCH + and its reaction with positive scavengers (e.g., norbornadiene) . We also detected the existence of a precursor to MCH + , called M + *, some excited state of MCH + .…”
Section: Introductionmentioning
confidence: 88%
“…Theoretical support for the t -0.6 rate law was recently given by Barczak and Hummel . From the many semiempirical theories to simulate the nonhomogeneous, geminate ion kinetics, only the t -0.6 rate law is able to explain the experimental results in methylcyclohexane. , …”
The chloroform anion in liquid methylcyclohexane (MCH) fragments during the time scale of geminate ion recombination (143 K e T e 193 K). Its lifetime can only be determined if the geminate ion kinetics can be calculated. The t -0.6 semiempirical law is used. Because CHCl 3 quenches M + *, the precursor of the solvent radical cation MCH + , much less effectively than N 2 O, 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 reactions of the M + * decay, which are forming mixed ion pairs (MCHene + , MCH + /X -). It is found that mixed pairs are still describable by the t -0.6 linearity, yet the mobility factor δ (slope of t -0.6 ) is now λ-dependent. Complete simulation of the ionic mechanism yields the following results:(1) The fragmentation rate constant for CHCl 3is k 1 (143 K) ) (3.6 ( 0.3) × 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 143 K, 153 K, and 173 K, the absorption coefficients of the CHCl 3 band were determined and a Lorentzian line shape fitted: λ max ) 470 nm, max ) 1900 ( 30 M -1 s -1 and a width of hwhm ) 28 ( 2 nm.(3) Assuming that the M + * fragmentation (k frag ) and the natural relaxation to MCH + (k 0 ) are the same as in N 2 O-saturated MCH, the quenching rate constant at 143 K may be derived: k 2 (M + * + CHCl 3 f MCH + ) ) (7.7 ( 2.6) × 10 6 M -1 s -1 . Quenching by CHCl 3 therefore is about four times slower than by N 2 O; the yield of the olefinic cation (MCHene + ) is strongly increased relative to the MCH + yield. Furthermore, the ratio quenching/fragmentation with chloroform as solute is found to increase with temperature, suggesting that fragmentation at room temperature might have much less importance.
“…Perhaps, the best supported claim for such a long-lived excited solvent hole is found in the study of geminate pair dynamics in methylcyclohexane at 143 K [55,56].…”
mentioning
confidence: 99%
“…For example, the twist-boat to chair transition in room-temperature cyclohexane occurs on a microsecond time scale. Perhaps, the best supported claim for such a long-lived excited solvent hole is found in the study of geminate pair dynamics in methylcyclohexane at 143 K [55,56].…”
mentioning
confidence: 99%
“…Using transient absorption spectroscopy, it was concluded that the (high-mobility) solvent hole [56] has a (high-mobility) precursor with the natural lifetime of 300 ns [55]. The decay of the precursor can be accelerated upon addition of N 2 O; without the quencher, the precursor either fragments yielding methycyclohexene radical cations (90 %) or relaxes (10%) [55].…”
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
