Deuteron quadrupole coupling constants and reorientational correlation times in protic ionic liquids

Abstract

We describe a method for the accurate determination of deuteron quadrupole coupling constants χ_D for N–D bonds in triethylammonium-based protic ionic liquids (PILs). This approach was first introduced by Wendt and Farrar for O–D bonds in molecular liquids, and is based on the linear relationship between the deuteron quadrupole coupling constants χ_D, and the proton chemical shifts δ¹H, as obtained from DFT calculated properties in differently sized clusters of the compounds. Thus the measurement of δ¹H provides an accurate estimate for χ_D, which can then be used for deriving reorientational correlation-times τ_ND, by means of NMR deuteron quadrupole relaxation time measurements. The method is applied to pure PILs including differently strong interacting anions. The obtained χ_D values vary between 152 and 204 kHz, depending on the cation–anion interaction strength, intensified by H-bonding. We find that considering dispersion corrections in the DFT-calculations leads to only slightly decreasing χ_D values. The determined reorientational correlation times indicate that the extreme narrowing condition is fulfilled for these PILs. The τ_c values along with the measured viscosities provide an estimate for the volume/size of the clusters present in solution. In addition, the correlation times τ_c, and the H-bonded aggregates were also characterized by molecular dynamics (MD) simulations.