Solvation of Na+ in N,N-dimethylformamide + methanol solutions. A nuclear magnetic relaxation study using dynamic solvent isotope effects

Abstract

Inter- and intramolecular nuclear magnetic quadrupole relaxation measurements have been used to study the system methanol (CH3OH)+ N,N-dimethylformamide (DMF)+NaI at 25°C. The dynamic behavior of the solvent molecules was investigated, throughout the composition range of the binary mixtures, by means of14N relaxation of DMF and2H of methanol-d1 (CH3OD). The intermolecular relaxation of23Na+ in pure DMF was used to obtain information about the symmetry of the solvent electric dipole arrangement in the solvation sphere of the ion. The investigation of preferential solvation around Na+ in the binary mixtures was carried out by means of23Na+ relaxation measurements using, for the first time, both the CH3OH/CD3OD and the DMF/DMF-d7 dynamic isotope effect. The results show that, throughout the composition range, there is preferential solvation by DMF. Furthermore, the use of the isotope effects of both components allowed for the first time a basic check of the reliability of the method since we obtained two independent sets of data for the composition of the Na+ solvation shell in the mixtures. The consistency of the two separate data sets demonstrates that the application of the dynamic isotope effect represents a powerful tool in preferential solvation studies.