Nuclear Magnetic Resonance Chemical Shift of the Water Proton in Aqueous Tetrabutylammonium Butyrate Solutions

Water proton chemical shifts have been measured for aqueous solutions of tetrabutylammonium butyrate and tetrabutylammonium bromide as a function of salt concentration and temperature. Large, negative (downfield) salt shifts are reported for Bu(4)NBut solutions at temperatures below 33 degrees C. The chemical shift becomes more negative with increasing molality up to about 0.5 m: at higher compositions, the chemical shift levels or becomes less negative. These results suggest that hydrophobic hydration cages formed under the influence of Bu(4)NBut have their maximum size at approximately 0.5 m and that at higher concentrations overlap of these cages must occur as suggested by earlier thermodynamic investigations. The observed downfield molal chemical shifts are much larger than those estimated from individual ionic contributions, suggesting a mutual enhancement or linking together of hydration cages even in dilute solution.