SELF-DIFFUSION COEFFICIENTS OF PAIRED IONS

The self-diffusion coefficients of the cation and anion of tetrabutyla mmonium tetrahydridoborate, (C4H9)(4)N+BH4-, were measured simultaneou sly at several concentrations in CDCl3 solution using an NMR stimulate d echo experiment incorporating pulsed field gradients. A nonionic spe cies of similar size and shape to the tetrabutylammonium ion, tetrabut ylsilane, was included as an internal reference. In the first direct m easurement of self-diffusion of both cation and anion in an ion pair, it was observed that self-diffusion of BH4- is only slightly faster th an that of the (C4H9)(4)N+ ion, with the self-diffusion rate of tetrab utylsilane being significantly greater than that of either BH4- or (C4 H9)(4)N+. These results indicate that the tight ion pair is the primar y diffusive species, and the diffusion of free BH4- is not an importan t contributor to the measured self-diffusion coefficient of the anion. The aggregation states of the paired ions were estimated from ratios of the selfdiffusion coefficients of the tetrabutylammonium ion and te trabutylsilane.