Alkali-metal cryptates in nitromethane. Thermodynamic parameters for complex formation and for transfer among different solvents

Abstract

Enthalpies of complexation of alkali-metal cations and cryptand 222 in nitromethane have been measured calorimetrically at 298 K. The single-ion enthalpy for the transfer of alkali-metal cations from propylene carbonate (PC) to nitromethane (NM) were derived and single-ion ΔH^°_t values for these cations in transfers from water are reported.

Enthalpies for the transfer of alkali-metal cryptates from water to NM have been obtained from direct calorimetric measurements of the enthalpies of solution of metal-ion cryptates in NM and in water. Combination of these data with single-ion ΔH^°_t values for the corresponding anions yielded ΔH^°_t[M⁺222] from water to NM.

Entropies of complexation between alkali-metal cations and cryptand 222, obtained from free energy and enthalpy data in NM are reported. The thermodynamic parameters ΔG_c°, ΔH_c° and ΔS_c° suggest that the process of complexation between the alkali-metal cations and cryptand 222 in dipolar aprotic solvents is strongly influenced by the solvation energy of the cation in the respective solvent. The thermodynamic parameters for the metal-ion cryptates in their transfers from PC to NM are compared with corresponding data for the transfer to other dipolar aprotic solvents and it is shown that, within experimental error, small variations are found in the transfer of the metal-ion cryptates among dipolar aprotic solvents.