Self-Association of Water and Water−Solute Associations in Chloroform Studied by NMR Shift Titrations

Association constants and the corresponding free energies Δ G were obtained by nonlinear least-square fitting of the water 1 H NMR shift changes observed at different concentrations. The only until now available earlier value (Shaw, J.-H. L.; Wang, S. M.; Li, N. C. J. Phys. Chem. 1973, 77, 236) is shown to be based on erraneous evaluations and on inaccurate data. Analysis for the self-association of water in chloroform based on a 1:1 dimer model yields Δ G = −3.5 kJ/mol; based on a trimer model one obtains an almost equally good fit with Δ G = −15.4 kJ/mol. The Δ G value in acetonitrile as solvent is with +9 kJ/mol positive (vide infra) as are all Δ G values observed for water−solute associations in chloroform with DMF, DMSO, diphenyl sulfoxide, and benzene, with the exception of HMPT (hexamethylphosphotriamide) and a special diamide designed to take up water by 2 hydrogen bonds. The complexation induced shifts, determined simultaneously from the NMR titrations, are between 3 and 4.5 ppm. They are similar to those observed with hydroxy compounds and show that the simulations are based on realistic models. No correlation is found between the CIS and the corresponding Δ G values. The only upfield shift with −4.4 ppm is found for the benzene−water complex, in agreement with theoretically derived structures.