Ion-ion-solvent interactions in solution. 8. Spectroscopic studies of the lithium perchlorate/N,N-dimethylformamide system

Solvation interactions and ion association in solutions of LiClO4 in N,N-dimethylformamide (DMF) have been studied by using infrared spectra, Raman spectra, and NMR spectra for the nuclei 7Li, 13C, 14N, 17O, and 35Cl. The behavior of the OCN deformation vibration, the 13C NMR, and the 17O signal of the carbonyl support the proposition that solvation is through the carbonyl oxygen. A quantitative study of the OCN deformation vibrational band in the Raman spectra yielded a solvation number of 3.6 for the Li+ ion with an indication that severe crowding and solvent-solvent interactions exist for the solvated cation. The solvation interactions are particularly strong and ion association is not significant below 2 M salt concentration. Above this concentration, evidence for both solvent-shared and contact ion pairs has been obtained. Consistent evidence from the perchlorate stretching vibrational band in the Raman spectra and 35Cl, 7Li, and 17O NMR supported a set of sequential equilibria (Li+)s + ClO4- ⇌ (Li+(DMF)ClO4-)s ⇌ (Li+·ClO4-)s (K1, K2). Values of K1 of 0.06 dm3 mol-1 at 2.5 M and 0.2 dm3 mol-1 at 3.5 M were obtained while K2 appeared to have almost the same numerical values as K1 above 3.5 M. The behavior of the carbonyl stretching vibrational band was found to be complex and not in full accord with previous descriptions based on the infrared spectra