Solvent hole migration

The reader may notice that only saturated hydrocarbons (with a possible exception of CCI4) have been observed to yield rapidly migrating solvent holes. As mentioned above, part of this bias is explained by the fact that the holes are usually short-lived, so their dynamic properties are difficult to study. However, in many liquids (such as aromatic hydrocarbons and sc CO2), the solvent holes are relatively stable, yet no rapid hole hopping is observed. In such liquids, the solvent hole has a well-defined dimer cation core with strong binding between the two halves (in the first place, it is this dimerization that... 

The rate constants, kj2, of the forward reaction (12) are an order of magnitude lower than those of the class (i) reactions, though some of the holetrapping solutes have comparably low adiabatic IPs. The values of kiz did not correlate with the observed ACP of reaction (12). An explanation was proposed that the rate constants are controlled by the height of the activation barrier determined by the difference in the vertical IP of the solute and the adiabatic IP of the solvent [11]. This suggests that electron transfer to the rapidly-migrating solvent hole (as it passes by the scavenger molecule) is much faster than the relaxation time of the solute radical cations. 

Once formed, the twisted cation-radical become surrounded by the chair-form neutral molecules. It does not transfer the hole to the neighboring chair-form neutral molecules until this twisted cation-radical acquires the chair configuration in a spontaneous manner. As a result, fast migration of the hole, that is, the one-electron transfer from the neutral solvent molecule to its cation-radical, is detained. Hummel and Luthjens (1986) considered the idea that conformation dynamics is involved in the electron transfer. Scheme 5.20 illustrates this dynamics. 

Both the styrene monomer and the neutral dimer can trap a migrating positive hole or positive charge from solvent radical-cations (solventt) or related cationic species, which leads to the formation of radical cations, dimer cations, and bonded dimer cations. 

This result suggests that the methylcyclohexane holes are coupled to the solvent, forming a polaron. This coupling makes the charge migration of methylcyclohexane holes in cyclohexane as efficient as in neat methylcyclohexane. From the critical concentration of methylcyclohexane, the delocalization radius was estimated as ca. 1 nm, or 4 to 5 molecular diameters [10]. 

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