SELF-ASSOCIATION OF WATER AND WATER-SOLUTE ASSOCIATIONS IN CHLOROFORMSTUDIED BY NMR SHIFT TITRATIONS

Association constants and the corresponding free energies Delta G were obtained by nonlinear least-square fitting of the water H-1 NMR shift changes observed at different concentrations. The only until now avai lable earlier value (Shaw, J.-H. L.; Wang, S. M.; Li, N. C. J. Phys. C hem. 1973, 77, 236) is shown to be based on erraneous evaluations and on inaccurate data. Analysis for the self-association of water in chlo roform based on a 1:1 dimer model yields Delta G = -3.5 kJ/mol; based on a trimer model one obtains an almost equally good fit with Delta G = -15.4 kJ/mol. The Delta G value in acetonitrile as solvent is with 9 kJ/mol positive (vide infra) as are all Delta G values observed for water-solute associations in chloroform with DMF, DMSO, diphenyl sulfo xide, and benzene, with the exception of HMPT (hexamethylphosphotriami de) and a special diamide designed to take up water by 2 hydrogen bond s. 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 base d on realistic models. No correlation is found between the CIS and the corresponding Delta G values. The only upfield shift with -4.4 ppm is found for the benzene-water complex, in agreement with theoretically derived structures.