Simultaneous determination of deuteron quadrupole coupling constants and rotational correlation times: the model case of hydrogen bonded ionic liquids

Abstract

We show that deuteron quadrupole coupling constants (DQCCs), and reorientational correlation times of molecular bonds N–D that are involved in hydrogen bonding, can be determined from NMR T₁ relaxation time experiments simultaneously. For this purpose, we used trialkylammonium-based protic ionic liquids (PILs) as model compounds. They exhibit high viscosities and wide liquid ranges that allow measurements far beyond the extreme narrowing region (ω₀τ_c ≪ 1). The T₁ minima already occur at temperatures significantly above room temperature. We obtain reasonable DQCCs for the liquid phase if anisotropic motion is considered. The DQCCs are very small due to attractive Coulomb interaction between the cation and anion, which is further enhanced by hydrogen bonding. The DQCCs strongly depend on the interaction strength of the anion but are independent of the alkyl chain length of the trialkyl ammonium cations pointing to the exclusive cation–anion interaction along the hydrogen bond.