We explore herein the interconnection between the collective intermolecular solvent modes(CIM) and ultrafast reaction rate, assuming that frequency-dependent solvent friction controls the rate of suchreactions. We attempt to find a possible explanation for the observed near-insensitivity of ultrafast reactionrates (for example, charge transfer reaction) to the medium viscosity. Results are presented here by employingan analytical scheme that estimates the high-frequency solvent frictional response. Representative roomtemperature reaction media considered here are an ionic liquid (BMIMPF6, η~310cP), a dipolar solvent(ethanol, η~1:09cP) and a deep eutectic solvent (Acetamide+ LiBr, η~1950cP). It is found that the wavenumber and frequency-dependent rotational friction, ГR(ƙ,ᴢ), estimated by using the available experimental dielectric relaxation (DR) data for the ionic liquid and the deep eutectic solvent (DES), cannot predictthe viscosity independence of ГR(ƙ,ᴢ) at high frequency. Missing dispersion in the DR data of the DES appears to be critical and incorporation of this missing amplitude via collective solvent intermolecular modescentered around 100 cm-1 markedly improves the high-frequency behaviour of ГR(ƙ,ᴢ): Subsequently, thecalculated ГR(ƙ,ᴢ) for these solvents at high frequency exhibits near-insensitivity to medium viscosity andexplain the viscosity independence of ultrafast reaction rates.
Comparison of the frequency dependent rotational friction among three different solvent systems considered – normal liquid (ethanol), deep eutectic solvent (acetamide/LiBr) and ionic liquid ([BMIM][BF6]). Note the high frequency friction values for these solvents are very similar, although their zero frequency values differ quite considerably because of widely different viscosities.
Volume 135, 2023
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