Deuteron Quadrupole Coupling Constants and Reorientation Correlation Times of Cations in Amino Acid Ionic Liquids

The deuteron quadrupole coupling constants and reorientation correlation times of imidazolium cations in amino acid ionic liquids (AAILs) from NMR spin-lattice relaxation times, T₁, are determined. A proper determination of the reorientation correlation times τ_c of the acidic C(2)─H groups on the imidazolium rings requires knowledge of reliable NMR deuteron quadrupole coupling constants χ_D. For that purpose, a relation between the deuteron quadrupole coupling constants χ_D and the proton chemical shifts δ¹H from density functional theory (DFT) calculated properties of differently sized clusters of the AAILs is first derived. Using this relation, the simple measurement of proton chemical shifts δ¹H provides an accurate estimate for χ_D, which can then be used for determining the reorientation correlation times τ_c. This method is applied to AAILs including the same imidazolium cation but differently functionalized anions. The obtained χ_D values range between 180 and 210 kHz, depending on the differently strong cation-anion interaction further enhanced by hydrogen bonding. The resulting reorientation correlation times τ_c indicate that the extreme narrowing condition is fulfilled for this type of IL. Using the Stokes-Einstein-Debye relation, the correlation times τ_c and the additionally measured viscosities η provide an estimate for the volume/size of the clusters present in solution.