J. Phys. Chem. B 1998, 102, 3371-3378
3371
Reversible Hole Trapping in Liquid Cyclohexane† I. A. Shkrob,* A. D. Liu, M. C. Sauer, Jr., K. H. Schmidt, and A. D. Trifunac Chemistry DiVision, Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: September 25, 1997; In Final Form: February 5, 1998
Mobile solvent holes in cyclohexane can be reversibly trapped by methylcyclohexane (∆G° ) -0.11 eV), 1,1-dimethylcyclopentane (∆G° ) -0.20 eV), trans-1,2-dimethylcyclopentane (∆G° ) -0.25 eV), and 2,3dimethylpentane (∆G° ) -0.21 eV). The two dimethylcyclopentanes are identified as “special” impurities responsible for bimodal scavenging kinetics of the solvent holes. The mechanism of the reversible trapping is shown to be charge transfer. Neither the difference in the ionization potentials ∆IPliq of the solvent and the solute, nor the driving force ∆G° of the scavenging reaction are found to correlate with the rate constants of this charge transfer. The proposed explanation is that these rate constants are controlled by the height of the activation barrier that can be estimated from the difference in the vertical IP for the solute and adiabatic IP of the solvent. The correlation of the rate constants with this difference suggests that electron transfer involving mobile holes occurs much faster than the relaxation time of the solute radical cations. This work gives the first data on adiabatic IP for saturated hydrocarbons in solution.
Introduction
Experimental Section
Ionization of cyclohexane (CH) results in the formation of a solvent hole (CH•+) whose lifetime (>200 ns) and high mobility (µh ∼ 2 × 10-2 cm2/Vs) makes possible observation of these species by transient conductivity.1-7 The solvent hole exhibited several peculiarities; among them was the fact that different experiments yielded quite different estimates of their lifetime (for a recent review see ref 1). Recently, we have demonstrated, using laser-induced conductivity and pulse radiolysis, that the discrepancy originated in the bimodality of hole scavenging in liquid cyclohexane.3-5 The bimodality was, in turn, explained through a reversible charge transfer reaction between the mobile holes and a shallow-trap hydrocarbon impurity (which we called a “special impurity”).5 We have now determined that at least three alkanes give rise to such bimodal decay; we continue to refer to them as “special impurities” (SI) for convenience. Such charge transfer reactions are of interest, for two reasons. First, though equilibria between the solvent hole and solutes were postulated to explain the results on radiolysis of alkane mixtures as early as 1960s,8 reVersible charge transfer has not been observed directly. Second, from the thermodynamics of such equilibria it might be possible to determine relative adiabatic ionization potentials (IPs) of hydrocarbons in the liquid phase, as Ausloos and co-workers did in the gas phase.9 After an extensive search for the chemical identity of the SI in CH, we determined that of the known impurities in CH, both 1,1- and trans-1,2-dimethylcyclopentane (1,1-DMCP and trans1,2-DMCP) cause bimodal decay of CH•+. Methylcyclohexane can also trap the cyclohexane holes reversibly, although this trapping does not result in bimodal kinetics, and the addition of 2,3-dimethylpentane (2,3-DMP), a possible impurity, gives the same type of bimodal scavenging kinetics as observed for 1,1- and trans-1,2-DMCP. In this paper we examine these reactions in some detail.
Materials. Cyclohexane (B&J brand) was obtained from VWR Scientific and passed successively through several 1-m columns of activated silica gel. This treatment reduces the olefinic and aromatic impurity to the level undetectable by a Perkin Elmer 8500 gas chromatograph equipped with a flame ionization detector (
