High Mobility Ions in Cycloalkanes. Transient Dc Conductivity

Abstract

Transient dc conductivity is used to observe high-mobility ions in laser photolysis and pulse radiolysis of cyclohexane and decalins. While in decalins the scavenging of the dc conductivity signal from these ions by solutes is a pseudo-first-order reaction, in cyclohexane the behavior is more complex and is indicative of the involvement of two species. This bimodality is rationalized in terms of a dynamic equilibrium between the conformers of the solvent radical cation. A similar equilibrium between the isomer radical cations of decalin is needed to account for the results on photoinduced conductivity in the mixtures of cis-and trans-decalin. In both of these cases, the mechanism of the equilibrium is the reversible charge transfer.

Radiation chemistry of organic liquids: Saturated hydrocarbons

“…For addition of methycyclohexane to cyclohexane, the equilibrium (12) is reached so rapidly that the decay kinetics are exponential. Similar rapidly-set equilibria exist for highmobility holes in mixtures of cis-and trans-decalins [14,38] and the decalin holes and benzene [26].…”

“…These high-mobility cations are shown to be cycloalkane solvent holes with unusually long natural lifetimes (0.2 µs to 5 µs). This long lifetime and the high mobility of the cycloalkane holes makes it possible to study their reactions using microwave [7][8][9] or direct current (dc) [10][11][12][13][14][15][16][17][18][19] conductivity, an option that does not exist for other hydrocarbons. Indeed, in room-temperature paraffins, the solvent holes have natural lifetimes ranging from 1 ns (C 5 ) to 33 ns (C 16 ) [20,21] due to rapid dissociation of the C-C or C-H bond(s) and deprotonation, RH^+ + RH ---" R^ + RH 2 +…”

“…Dilution of cycloalkanes with high-IP alkanes (or higher-IP cycloalkanes) results in the decrease in the hole mobility that correlates with the mole fraction of the cycloalkane in the mixture: the hopping rate decreases when the density of the like molecules decreases [8,14]. The activation energies for the hole mobility, as estimated from conductivity data range from -(3±1) kJ/mol for trans-decalin and cyclohexane to (6-7) kJ/mol for methylcyclohexane and cis-decalin [7,8,10].…”

“…Since the formation of "satellite ions" is partly due to spur chemistry, in the laser multiphoton ionization of neat hydrocarbons [14][15][16][17][18] RH + n hˆ ---" RH^+ + e - (9) (that yields isolated electron-hole pairs only) the yield of "satellite ions" would be much reduced. Another way of generating the solvent holes is by a valence-band electron transfer to the photoexcited aromatic radical cation ("hole injection") [10][11][12][13]26] A^+ + hˆ ---" A^+ * (10)…”

“…In neat trans-decalin, the reaction rates for high-IP solutes correlate poorly with the solute IP and PA [9,12]. In binary mixtures of cis-and trans-decalin, the mobility of the solvent hole linearly scales with the fraction of trans-decalin (for which µ h is 4.5 times higher than in cis-decalin) [14]. While for most of the solutes the rate constants also scale linearly with the hole mobility, for aliphatic alcohols the rate constant systematically decreases with the fraction of trans-decalin approaching the value of (5-6)x10 9 M -1 s -1 in neat trans-decalin [12,14].…”