Chloroform Anion Fragmentation in Liquid Methylcyclohexane:  <i>t</i><sup>-0.6</sup> Simulation of the Geminate Ion Kinetics

Bühler, Rolf E.; Domazou, and Anastasia S.; Katsumura, Yosuke

doi:10.1021/jp984760w

Cited by 7 publications

(23 citation statements)

References 9 publications

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“…For an application, see the paper on CHCl 3 . 5 C. If the product of a geminate ion recombination is an optical absorber (E p > 0) in the λ range studied, then the intercept of the t -0.6 linear plot is not just the free ion yield but, due to p > 0 and the large G tot value, corresponds to a higher value and the slope is much lower (may even be negative): 1 In this case the quotient slope/intercept does not yield the mobility value. For an application of this result, see the formation of the solvent separated ion pair (CCl 3 + ||Cl -) solv from CCl 3 + + Clin a solution of CCl 4 in MCH.…”

Section: Theory For Data Analysismentioning

confidence: 99%

Geminate Ion Kinetics for Hexa-, Penta- and Tetrachloroethane in Liquid Methylcyclohexane (MCH): Effect of the Anion Lifetimes

et al. 2003

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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.

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Section: Theory For Data Analysismentioning

confidence: 99%

Geminate Ion Kinetics for Hexa-, Penta- and Tetrachloroethane in Liquid Methylcyclohexane (MCH): Effect of the Anion Lifetimes

et al. 2003

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“…In MCH solutions the initial process usually corresponds to the mechanism of the high mobility cations: with the precursor cation M +* relaxing (or isomerizing) into the high mobility radical cation MCH + ( k r ) and, in parallel, fragmenting ( k f ) into the diffusional cation of methylcyclohexene (MCHene + ). ,,,− The solvated electron is scavenged by the solute Tetra (T) to form its anion Tetra - (or T - ), which is expected to have a long fragmentation lifetime . The solute Tetra might also influence the relative yields of MCH + and MCHene + .…”

Section: Resultsmentioning

confidence: 99%

“…Scheme 1 represents the expected mechanism, where k r = k 0 + k 2 [T] includes a possible concentration dependence. It corresponds to the finding with chloroform as solute . To test Scheme 1, the chemical systems of Table were studied in detail with both isomers.…”

Section: Resultsmentioning

confidence: 99%

“…The anion decay is so slow that the final t -0.6 linearity (δ 2 , ID 2 ) for the mixed geminate ions (MCH + ,MCHene + /Cl - ) is not available. If the δ 2 values are taken from the CHCl 3 system, then the resulting ID 2 values are too low, from what is known from previous experiments. ,,, The ID 2 values (free ion yield) therefore were calculated from the known MCH + and MCHene + spectra 8 for the particular MCH + /MCHene + ratio (Figure ). The δ 2 values from the CHCl 3 system then were too large to simulate the systems with [T] < 0.1 M. δ 2 was reduced until a complete fit for all concentrations (5.3−530 mM Tetra) was reached (see Final Simulation).…”

Section: Resultsmentioning

confidence: 99%

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

Bühler

Quadir

2003

J. Phys. Chem. A

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In the context of studies on the influence of the anion lifetime on the geminate ion kinetics, 1,1,1,2- and 1,1,2,2-tetrachloroethane (1112-Tetra and 1122-Tetra) were studied as solutes in liquid methylcyclohexane (MCH) at low temperatures (133−183 K). The two isomers serve as examples of long anion lifetime. The analysis of the pulse radiolysis data was based on the t -0.6 semiempirical law for geminate ion kinetics. The visible band with λmax = 450 or 430 nm is shown to be due to the anion of 1112-Tetra or 1122-Tetra, respectively. Its kinetics relates to three consecutive geminate pairs of ions, due to two ionic reactions: (a) The fast process represents the cationic mechanism: the precursor cation M+* relaxes (or isomerizes) to the high mobility ion MCH+ (k r) and simultaneously fragments (k f) to a diffusional methylcyclohexene+ (MCHene+). The total M+* decay (k tot = k r + k f) produces mixed cations (MCH+,MCHene+). (b) The slow process is due to the anion fragmentation (k -) from Tetra- to Cl- + R•, with τ- = 13.7 or 20.0 μs (143 K) for 1112- or 1122-Tetra, respectively. The fragment radical R• is freed too late to allow scavenging of positive charge. The three geminate pairs of ions are (M+*/Tetra-), (MCH +,MCHene+/Tetra-), and (MCH+,MCHene+/Cl-). All ions (except Cl-) contribute to the optical absorption. The rate constants k tot and k - are both independent of the concentration of Tetra. For k tot this means that M+* decays in a fixed ratio of k f to k r. This is in contrast to previous findings with N2O or CHCl3 but corresponds to our recent proposal that M+* appears to represent some isomer of MCH+ in a higher energy state (or of higher ionization potential). The anion fragmentation rate for 1112-Tetra is k -(143 K) = (7.3 ± 0.6) × 104 s-1 with E act = 17.8 kJ/mol and log A = 11.2. For 1122-Tetra it is k -(143 K) = (5.0 ± 1.0) × 104 s-1 with E act = 16.9 kJ/mol and log A = 10.9. The free ion intercepts, from the t -0.6-simulations, reveal for all geminate pairs with Tetra- a strong dependence on the Tetra concentration [T], eventhough complete electron scavenging was ascertained. This is explained by the formation of dimer anions through an equilibrium T- + T ⇌ . The absorption at 450 nm (or 430 nm) then is due to (most likely a charge resonance transition (T ← T-)). For the initial geminate pair (M+*/Tetra-), the free ion intercept was smaller than the one for Tetra- alone (actually ). As this result was based on the assumption that M+* and MCH+ have the same mobility, this now reveals that the precursor cation M+* must have at least a 9 times higher mobility than MCH+.

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“…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 .…”

Section: Introductionmentioning

confidence: 99%

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

Borovkov

2018

J. Phys. Chem. B

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The probability of formation of spin-correlated secondary radical ion pairs (RIPs) in diluted solutions of charge acceptors in irradiated haloalkanes is believed to be extremely low due to the dissociative attachment of excess electrons to solvent molecules. Contrary to this, it has been found that spin-correlated RIPs can be formed upon irradiation in some liquid chloroalkanes with yield sufficient to observe the recombination fluorescence of the RIP's partners. This allowed the study of primary radical cations (RCs) as well as radical ionic states of molecules dissolved in haloalkanes using the method of time-resolved magnetic field effect (TR MFE) in radiation-induced fluorescence. With this method, the magnetic resonance characteristics of the solvent RCs in a series of liquid haloalkanes were examined for the first time. For the 1,2-dichloroethane RC, the rate of scavenging by solute molecules and the dominant mechanisms of paramagnetic relaxation were determined. Polysulfone and poly(ethyl methacrylate) were used to demonstrate that due to their high dissolving ability, chloroalkanes can be exploited as solvents to study the magnetic resonance characteristics of radical ionic states of polymeric molecules in solutions with the TR MFE method.

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Chloroform Anion Fragmentation in Liquid Methylcyclohexane: t^-0.6 Simulation of the Geminate Ion Kinetics

Cited by 7 publications

References 9 publications

Geminate Ion Kinetics for Hexa-, Penta- and Tetrachloroethane in Liquid Methylcyclohexane (MCH): Effect of the Anion Lifetimes

Geminate Ion Kinetics for Hexa-, Penta- and Tetrachloroethane in Liquid Methylcyclohexane (MCH): Effect of the Anion Lifetimes

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

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

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Chloroform Anion Fragmentation in Liquid Methylcyclohexane: t-0.6 Simulation of the Geminate Ion Kinetics

Cited by 7 publications

References 9 publications

Geminate Ion Kinetics for Hexa-, Penta- and Tetrachloroethane in Liquid Methylcyclohexane (MCH): Effect of the Anion Lifetimes

Geminate Ion Kinetics for Hexa-, Penta- and Tetrachloroethane in Liquid Methylcyclohexane (MCH): Effect of the Anion Lifetimes

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

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

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Chloroform Anion Fragmentation in Liquid Methylcyclohexane: t^-0.6 Simulation of the Geminate Ion Kinetics