Dynamical processes and order behind the pronounced 1H NMR paramagnetic relaxation enhancement induced in [P6,6,6,14]3[GdCl6]-based ionic liquid mixtures

NMR relaxometry is a technique that allows the molecular dynamics study of chemical compounds across a broad time scale, ranging from slow translational diffusion or collective motions to fast rotations. The present work is a 1 H NMR relaxometry and diffusometry study of systems based on [P 6,6,6,14 ][Cl] and [P 6,6,6,14 ] 3 [GdCl 6 ] ionic liquids, complemented by X-ray diffractometry measurements. The use of the X-ray profiles enabled the determination of an almost temperature independent interdigitated disposition of the cations that is at the origin of local order fluctuations. This structure also affects the paramagnetic relaxation enhancement, which is very significant for these systems and is achieved at very low metal concentrations (around 1mM). The present study provides a comprehensive analysis that is consistent for all the analyzed systems and across the different experimental techniques, despite the experienced technical challenges related to extremely short relaxation times. Additionally, the Electrochemical Impedance Spectroscopy profiles of the neat [P 6,6,6,14 ][Cl] sample were explained by an analogous equivalent circuit model that allowed for a global analysis consistent with the diffusometry and X-ray diffractometry results. The representation of the real and imaginary parts of the impedance allowed for a visual deconvolution of the contributions of the different circuit blocks considered in the model. • This study employs a comprehensive, multi-technique approach to investigate the structural, molecular dynamics, and impedance properties of phosphonium-based ionic liquid systems. By integrating NMR relaxometry, NMR diffusometry, molecular dynamics simulations, X-ray diffractometry, and electrochemical impedance spectroscopy, we provide a detailed and complementary perspective on these complex materials, offering new insights into their fundamental properties and potential applications. • We demonstrate the ability to assess structural properties related to the local order of phosphonium-based ionic liquid systems using NMR relaxometry. Notably, this insight is achieved despite the absence of liquid crystalline behaviour, highlighting the sensitivity of NMR relaxometry to subtle ordering effects in these complex materials. • We investigate the paramagnetic relaxation enhancement induced by a gadolinium-based magnetic ionic liquid across varying metal concentrations and temperatures. The observed trends align with structural and molecular dynamics insights, reinforcing the coherence of our multi-technique approach in characterizing these complex systems.