Cycloalkane holes

In the early studies, the cycloalkane holes were viewed as molecular radical cations that undergo rapid resonant charge transfer. At any given time, the positive charge was assumed to reside on a single solvent molecule and, once in 0.5-2 psec, to hop to a neighboring molecule. The low activation energy was explained by the similarity between the shapes of cycloalkane molecules and their radical cations [60]. 

We turn to the chemical behavior of cycloalkane holes. Several classes of reactions were observed for these holes (1) fast irreversible electron-transfer reactions with solutes that have low adiabatic IPs (ionization potentials) and vertical IPs (such as polycyclic aromatic molecules) (2) slow reversible electron-transfer reactions with solutes that have low adiabatic and high vertical IPs (3) fast proton-transfer reactions (4) slow proton-transfer reactions that occur through the formation of metastable complexes and (5) very slow reactions with high-IP, low-PA (proton affinity) solutes. 

Similarly short lifetimes are expected for branched alkanes, such as isooctane [22]. Due to these lifetime limitations, the chemical behavior of cycloalkane holes is understood in more detail than that of the solvent holes in other hydrocarbon liquids. 

From conductivity studies, it is known that the cycloalkane holes rapidly react with various solutes, typically by electron or proton transfer [7-19]. 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 [4,5,6,20,23-25] 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 [4-25]. 

RadiolyticaUy-generated solvent holes have initial excess energy of several electron-volts. It is generally believed that these excited species relax to the "ground" state on a picosecond time scale or even faster [37,38,53]. Nevertheless, some authors suggest that certain excited cycloalkane holes have lifetimes in nanoseconds [54,55]. Such suggestions are not completely... 

The mechanism for the high mobility. In the early studies, the high-mobility cycloalkane holes were viewed as radical cations that undergo rapid resonant charge transfer [8] ... 

Rate constants for cyclohexane holes may be found in references [7,8,11,13,14,17], for decalin holes - in references [8,9,12,14,26], for methyl-cyclohexane holes - in references [12,122], for squalane holes - in references [24,30]. The data on the temperature dependence of rate constants of scavenging for the four cycloalkane holes are in reference [10]. For these holes, most of the rate constants were measured by determining the decay kinetics of the transient conductivity signals as a function of the solute concentration. The preferable way of studying the scavenging reactions is by detection of the excess dc conductivity following the "hole injection" reactions (10) and (11) [10-13,26]. In cyclohexane, the determination of the rate constants is complicated by the fact that the solvent hole is in equilibrium with an impurity in the solvent [11]. 

We forewarn the reader that the formation of high-mobility holes is not peculiar to these four cycloalkanes For instance, cyclooctane [61], squalane [62,63,64], and CCI4 [65] also yield such holes. However, in these other liquids, the holes are unstable and, consequently, more difficult to study (the lifetimes are 5-20 nsec vs. 1-3 psec). This explains why convincing demonstrations for the occurrence of high-mobility holes are slow to come. For example, squalane (by virtue of its high viscosity) has been frequently... 

Class (1) reactions were observed in all four cycloalkanes. The highest rate constants were observed for reactions of cyclohexane hole with low-IP aromatic solutes, (3-4.5) x 10" sec at 25°C [75]. In these irreversible reactions, a solute radical cation is generated. Class (2) reactions were observed for reactants 1,1-dimethylcyclo-pentane, trans-l, 2-dimethylcyclopentane, and 2,3-dimethyl-pentane in cyclohexane [74], trans-dtcaXm, bicyclohexyl, and Ao-propylcyclohexane in methylcyclohexane [69], and benzene in cis-... 

Among the simple cycloalkanes, we first discuss electron transfer from the three-to eight-membered cycloalkane prototypes to electron holes generated by radiolysis in different matrices, giving rise to the simple cycloalkane radical cations. Because of the significant interest they have attracted, the electron-transfer reactions of cyclopropane and, to a lesser extent, cyclobutane derivatives will be treated separately. Finally, electron transfer from some bicyclic hydrocarbons and the resulting radical cations will be discussed in a separate section (Section 2.4). 

In contrast to the radical cations of strained-ring cycloalkanes, the cyclopentane radical cation, c-CsHio , formed by electron transfer to radiolysis-induced holes in halocarbon matrices, had a simpler spectrum. A triplet with uh = 2.5 mT (2H) was attributed to a localized species with Cj symmetry. The unpaired electron was assigned to a W-shaped cr-orbital, involving C5-C1-C2, and the two equatorial protons at C5 and C2 [80, 88, 89]. At temperatures above 77 K, all ring protons become equivalent, most probably as a result of processes such as ring inversion, or pseudo-rotation around the C5-axis [89]. 

Electron-transfer reactions of higher cycloalkanes were also studied. Electron transfer from C-C7H14 to unstable holes generated by radiolysis in Freon-113 gave rise to a stable radical cation, c-Ci A f its spectrum was interpreted in terms of a twisted chair form with C2 symmetry [37]. Finally, radiolysis of c-CgHie in a Freon-113 matrix generated a Jahn-Teller-active radical cation, c-CgHie, with three sets of non-equivalent protons [37]. A detailed discussion of these species exceeds the scope of this review. 

It has long been speculated that the high-mobility solvent holes exist in hydrocarbons other than the four cycloalkanes. Recently, high-mobility solvent holes were observed in 2,6,10,15,19,23-hexamethyltetracosane (squalane) [24] and in cyclooctane [27]. In the squalane, rapid electron-transfer reactions of solvent holes with low-IP solutes were observed using transient absorbance spectroscopy and magnetic resonance [24]. Fast diffusion and high-rate... 

In cyclohexane and decalins, reaction (1) is endothermic by 0.1-0.4 eV [60] and it seems reasonable that the excitation of the hole may facilitate the proton transfer. Fragmentation of matrix-isolated hydrocarbon radical cations upon excitation with 2-4 eV photons was observed by EPR (see review [61]). For cycloalkanes, the main photoreaction is reaction (3). For radical cations of methyl-branched alkanes, the loss of CH4 was also observed, while the radical... 

Solvent holes in neat cycloalkanes were generated by multiphoton ionization (3 x 4 eV or 2 x 5 eV) of the solvent at fluxes in excess of 0.01 J/cm [15]. In a typical experiment, the laser-induced dc conductivity was measured as a function of the delay time with resolution better than 3 ns. A similar setup was used to observe the dc conductivity in pulse radiolysis with fast 16 MeV electrons [14]. The decay kinetics of solvent holes in cyclohexane and decalins were consistent with the value of =1 for multiphoton laser ionization. For cyclohexane, a lower ratio of fh =0.5 was needed to account for the kinetics observed in pulse radiolysis. (Note that these ratios refer to the situation at ca. 10 ns after the ionization event the conductivity signal of the holes cannot be measured at earlier time). To be consistent with the observations, the simulations required a higher value for the mobility //jj of the... 

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