23Na nuclear magnetic resonance linewidth studies of sodium tetraethylaluminate: complexes with hexamethylphosphoramide in benzene

The linewidth of the 23Na resonance in sodium tetraethylaluminate in benzene has been measured as a function of the ratio of [donor]: [Na+] using the strong complexing agent hexamethylphosphoramide. In this system changes in the sodium linewidth arise primarily from changes in the nuclear quadrupole relaxation at the sodium nucleus which in turn arise from changes in the electric field gradient. As the ratio of hexamethylphosphoramide to sodium ion is increased up to 3: 1 the 23Na linewidth progressively decreases due to an increase in cation–anion distance in the ion pair. After a ratio of 4:1 is reached, essentially no linewidth change is noted, indicating a stable solvation number of 4 for hexamethylphosphoramide. The results of this study indicate that this is a new and promising approach to the determination of solvation numbers of the sodium ion.