High-Mobility Ions in Cyclohexane. A Transient Absorption Study

Abstract

Transient absorption kinetics in radiolysis of N2O-saturated cyclohexane has been studied (0.1−100 ns; 300−800 nm). The spectra indicate the involvement of at least three cations (ions I, II, and III), only one of them having abnormally high mobility. Ion II is probably the cyclohexene radical cation, and ion III might be the dimer olefin ion. These two ions absorb as much as ion I at 450−500 nm. While ion II and ion III are scavenged by ethanol and triethylamine with a rate constant of ≈1010 mol-1 dm3 s-1, the scavenging of ion I proceeds with rate constants of ≈9 × 1010 and 2.3 × 1011 mol-1 dm3 s-1, respectively. The spectrum of ion I is similar to the spectrum of the radical cation of cyclohexane isolated in low-temperature matrices. We were not able to observe the absorption from ion I at delay times longer than 50 ns. A corresponding fast growth of the absorption from solute radical cations of pyrene and perylene was observed. The data (simulated using continuum-diffusion and Monte Carlo approaches) indicate that the scavenging constant is ≈4 × 1011 mol-1 dm3 s-1; the lifetime of the precursor of the aromatic radical cations is ≈30 ns. This short lifetime cannot be explained by a reaction with radiolytic products or by homogeneous recombination, and it seems to be incompatible with identification of the long-lived high-mobility ions observed in conductivity experiments as the radical cation of cyclohexane. A mechanism in which the mobile radical cation is in equilibrium with a normally-diffusing ion is examined in an attempt to resolve this conundrum.

Radiation chemistry of organic liquids: Saturated hydrocarbons

“…The kinetic data on delayed fluorescence [49] and transient absorbance [6,20,25] in radiolysis of cyclohexane supported this conclusion. It was concluded that the lifetime of the solvent hole is only 20-30 ns [6,25], or 10 times shorter than the lifetime of the high-mobility cations observed in the conductivity experiments. Only recently was it determined what causes the appearance of the lifetime-limited kinetics in the pulse radiolysis experiments: Some impurities in cyclohexane reversibly trap the solvent holes reducing their lifetime [11][12][13][14]25].…”

“…These scavenging reactions establish the identity of the high-mobility cations as the solvent holes: Rapid generation of aromatic radical cations (A^+) in reactions of the holes with aromatic solutes (A) was observed using pulse radiolysis -transient absorption spectroscopy [4,5,6,20,[23][24][25] and, more recently, using pulse-probe laser-induced dc conductivity [26]. Rapid decay of the conductivity and transient absorbance signals from the cycloalkane holes was also observed .…”

“…Second, the "satellite ions" could be generated in scavenging reactions of the solvent holes with radiolytic products in multiple-pair spurs [25,[61][62][63][64][65]. The olefins are formed upon the fragmentation of excited solvent molecules generated in recombination of short-lived electronhole pairs [1] RH^+ + e ----" 1,3 RH * (5) 1 RH * ---" olefin + H 2…”

Multimer Radical Ions and Electron/Hole Localization in Polyatomic Molecular Liquids: A Critical Review

“…Class (iii) reactions include proton-transfer reactions of solvent holes in cyclohexane and methylcyclohexane [71,74,75]. The corresponding rate constants are 10-30% of the fastest class (i) reactions.…”

“…Extremely slow class (iv) reactions were observed for scavenging of (a) cyclohexane hole by cyclopropane [60] and (b) cyclohexane and decalins holes by O 2 [75]. H atom transfer from the hole to O 2 , and H 2 -transfer from cyclopropane to the hole were suggested as the possible reaction mechanisms.…”