Effect of pressure upon the equilibrium : tight ion-pairs⇌loose ion-pairs

Abstract

The effect of pressure (up to ∼5000 atm) upon the equilibrium, tight ion-pairs⇌loose ion-pairs, was investigated for sodium and lithium fluorenyl in THF at about 22°C. Pressure shifts the equilibrium towards the loose pairs, ΔV being –24.4 and –15.6 ml for the sodium and lithium salts, respectively. The ΔV values correlate with the relevant ΔS values, thereby indicating that both are reflecting the increase in the number of immobilized solvent molecules arising from the conversion of one pair into the other.

The linear extinction coefficients at λ_max of either pair substantially increase with pressure, viz., by about 60 % when the pressure rises from 1 to 5000 atm. Apparently, the absorption curves become sharper at higher pressures, probably because the rotation of a pair and its soft vibrations are hindered.