Cesium-133 NMR studies of Cs + salts in various solvents

Abstract

During the past few years a number of invest igat ions have shown that the nuclear magnetic resonances of a lka l i elements can act as very sensi t ive probes of the environment of a l ka l i ions in solut ions. In par t i cu la r a f a i r amount of at tent ion was directed at the sodium-23 and l i th ium-7 magnetic resonances ( I ) . Studies on the heavier nuclei 39K, 87Rb and 133Cs have been much more sparse and, in general, confined to aqueous solut ions (2-5). We wish to report some prel iminary resul ts obtained in a systematic study of the inf luence of solute-solvent and solute-solute in teract ions on the 133Cs magnetic resonance. Cesium chlor ide, bromide, iodide, perchlorate, n i t r a t e , and thiocyanate were of reagent grade qua l i t y and were used as received except for drying. Cesium tetraphenylborate was prepared by a metathetical reaction between cesium chlor ide and sodium tetraphenylborate in tetrahydrofuran-water mixture. Solvents were pur i f ied by previously described techniques ( I ) . The NMR measurements were carr ied out at 7.871MHz and a f i e l d of 1.409 T in a pulsed Fourier transform mode with a Varian DA-60 spectrometer equipped with a wideband probe (6) and an external proton lock. A 0.500 M aqueous CsBr solut ion was used as external reference. The 133Cs sh i f t s are reported re la t i ve to the CS+(aq~., ion at i n f i n i t e d i l u t ion . The sh i f t s are also corrected for the bulk diamagnetic suscep t i b i l i t y d i f ference between water and the nonaqueous solvents. Chemical sh i f t s were measured in water and in eleven nonaqueous solvents, general ly in the 0.001-0.5 M concentration range. In a l l cases the chemical sh i f t s were strongly concentration-dependent indicat ing some contact ion pair formation even in polar solvents of high donic i ty . The large radius and the low charge-to-surface ra t io of the Cs + ion makes i t a rather poorly solvated ion and thus cesium sal ts are more l i ab le to form contact ion pairs than l i th ium or sodium salts in polar solvents such as dimethylformamide. In the case of cesium thiocyanate, chlor ide, bromide and iodide, the 133Cs resonances