Solvent structure effects on electron reactivity in isomeric butanol/water mixtures

Abstract

The nearly diffusion controlled reaction rate constants k2 of solvated electrons with efficient electron capturers in butanol/water mixtures are examined according to the Smoluchowski, Debye, and trap-depth models. The k2 values depend on the diffusion rates of the reactants and the solvation energies of the electrons. Both these factors are affected by the rigidity of the alkyl group on the alcohol. Increased rigidity decreases the diffusion rate, which tends to decrease k2, and decreases the electron solvation energy, which tends to increase k2. Rate constants of efficient electron capturing solutes are dominated by the former, those of inefficient capturers by the latter. The free energy of activation ΔG‡13 for electron capture by an inefficient electron scavenger (solute) is empirically related to Er/ f(μ), where Er is a representative optical absorption energy of the solvated electrons (related to the solvation energy) and f(μ) is a function of the dipole moment of the scavenger. It is also related to the polarizability of the solvent molecules. Capture by an inefficient solute has a large negative entropy of activation attributed to solvent rearrangement about the reaction site.

Solvent effects on the reactivity of solvated electrons with ions in tert-butanol/water mixtures

“…For charged solutes, ~ they used eqs. [6] and [7] to calculate r,: where r, is the distance at which the coulombic interaction between the ions equals the thermal agitation energy kBT, and ro is the center-to-center separation of the solvated reactant ions in contact. Schwarz used ro = 1.1 nm and the static value of E of the bulk solvent in eqs.…”

Solvent structure effects on solvated electron reactions with ions in 2-butanol/water mixed solvents

“…The Smoluchowski-Debye-Stokes-Einstein equation for the rate constant k2 of a bimolecular reaction between charged or polar species is given by (16) where K = probability that a reactant encounter-pair will react, q = solvent viscosity (kg/m s), and rd and r, are the effective radii for mutual diffusion and reaction, respectively. Rearrangement of [6] gives the dimensionless ratio of reaction parameters Kr,/rd, which is a sort of reaction efficiency parameter:…”

Solvent effects on the reactivity of solvated electrons with in C1 to C4 alcohols

“…However, reactivity is also often dependent on the solvation energy of the reactants, especially of e; (13,14,24). The mean free energy of solvation of electrons in water, -162 kJ/mol at 298 K (25), is similar to the optical absorption energy EA,,, = 166 kJ mol (26).…”

“…This increase in reaction rate accompanied by a decrease in diffusion rate points to the behavior of a reactant encounter pair. The value of k,(e; +NO&) also increases with decreasing trap depth of e;, as represented by the optical absorption energy E, (13,14). However, there is lack of overall correlation in all C, to C, alcohols, including secondary and tertiary alcohols (9,10).…”

“…Electrons with lower optical absorption energies tend to be more reactive (13,14,24), so the energy E, half way up the low energy side of the absorption band is chosen for correlations with reaction rate constants ( Table 1). Figure 1A shows correlations of k, (e; + NO;,) with solvent viscosity q .…”