Nuclear Magnetic Resonance Studies of Solvation of Halides: 19F Studies of Solvent and Counterion Effects on Chemical Shift

Abstract

We report new measurements adding to the literature on solvent and counter ion dependence of 10F− chemical shift. These are compared to earlier results for 35Cl−, 79Br−, and 127I−. The observations support the following points: (1) there are few systematic differences between behavior of anion shifts in dilute and moderately concentrated aqueous solution, (2) anion shifts commonly show larger sensitivity to solvent than counter ion over convenient ranges of variation so that "solvent shift" may be easily measured. Using the concept of collision effects on shifts developed by Richards and his collaborators, it is possible to separate, in many cases, the large effects arising from contact, or primary solvation sphere, collisions from the smaller effects of outer sphere interactions.We find no preferential solvation of F− by H2O in mixtures with CH3OH; at most, small preference over C2H5OH and formamide, and a small opposite preference in favor of diols. However, water appears strongly preferred in the primary solvation shell over acetone, acetonitrile, dioxane, and (in contrast to other halides), dimethylsulfoxide. Solvation by amines involving both outer and inner sphere site occupation is considered.The inter-anion effects on fluoride ion chemical shifts tend to undermine the concept of additivity of ionic effects on shifts.