Oxygen-17 nuclear magnetic resonance studies of the second coordination sphere of chromium(III) in aqueous solution

Abstract

The oxygen-17 bulk water relaxation times and chemical shifts have been measured at 8 MHz on several aqueous solutions of Cr(H/sub 2/O)/sub 6//sup 3 +/ and on a solution of CrF(H/sub 2/O)/sub 5//sup 2 +/. Analysis of the NMR data has yielded the average lifetime of water molecules in the second coordination sphere. Further, the data have been analyzed to yield the relative contributions of each NMR relaxation mechanism to the overall relaxation. The average lifetime of water molecules in the second coordination sphere of Cr(H/sub 2/O)/sub 6//sup 3 +/ has been calculated at 298/sup 0/K to be 1.0 x 10/sup -10/ sec. The activation parameters are: ..delta..H/sup + +/ = 2.8 kcal and ..delta..S/sup + +/ = -0.8 eu. A few measurements were made on a sample of CrF(H/sub 2/O)/sub 5//sup 2 +/. From these data it was concluded that the average lifetime for water molecules in the second coordination sphere of CrF(H/sub 2/O)/sub 5//sup 2 +/ is not greatly different from the lifetime of water in the second coordination sphere of Cr(H/sub 2/O)/sub 6//sup 3 +/. These lifetimes are longer than the calculated lifetime for self diffusion in water and consequently are thought to reflect the electrostatic attraction between the charge on the Cr(III) and the dipole of the water molecules. (auth)