Zinc-67 NMR study of zinc ions in water and in some nonaqueous and mixed solvents

Solutions of zinc salts in water, methanol (MeOH), dimethylformamide (DMF) and binary mixtures of water with the two nonaqueous solvents were studied by zinc-67 NMR measurements. Anhydrous zinc nitrate solutions in DMF and MeOH show upfield, concentration independent, chemical shifts at –27 and –19 ppm, respectively, vs. the aqueous solution standard. Addition of DMF or MeOH to an aqueous solution of a zinc salt results in a diamagnetic shift but for the addition of acetonitrile a paramagnetic shift results. In all cases the signal was broadened very considerably, e.g., in ZnCl₂ solution the linewidth increased from 40 to 600 Hz in going from water to 35% aqueous MeOH. Both⁶⁷Zn and¹³C NMR failed to show any complexation of Zn²⁺ ion by crown ethers in aqueous solution. A gradual addition of EDTA, of diaza-18-crown-6 or of tetraazacyclotetradecane resulted in an immediate broadening of the⁶⁷Zn signal which became undetectable when one equivalent of a ligand was added.