Calculation of the formation enthalpies of charge-transfer complexes with iodine from the binding constants at 298.15 K

There is a continuing interest in charge-transfer complex chemistry, particularly in their formation thermodynamics. Nowadays, the binding constants and Gibbs energies of formation are available for many complexes. However, the experimental determination of the complexation enthalpies is a more difficult procedure, as it usually requires the establishment of the temperature dependence of the binding constant. Our recent studies of the compensation relationship in the thermodynamics of solvation in hydrogen-bonded systems opened the way to the calculation of the enthalpies of specific (i.e., localized donor-acceptor) interactions (Delta int(sp)H) directly from the Gibbs energies at 298.15 K (Delta int(sp)G). The calculation procedure is based on the following equation: Delta int (sp)G/(kJ & sdot;mol- 1) = 0.66 & sdot;Delta int(sp)H/(kJ & sdot;mol- 1) + 2.5n (n corresponds to the coordination number of a solute). In the present work, we checked whether the above relation can be applied to the complex formation between halogens (primarily iodine) and organic electron donors. For 152 complexes with aromatics, nitrogen, oxygen, sulfur, selenium, and phosphor-containing compounds, the formation enthalpies were estimated using the data on equilibrium constants at 298.15 K. In most cases, agreement with the experimental binding enthalpies within the expected measurement error was observed (RMS =1.4 kJ & sdot;mol- 1, AD = -0.4 kJ & sdot;mol- 1).