Anton Gradišek, Department of Solid State Physics, Jožef Stefan Institute, Ljubljana, Slovenia

Investigating Molecular Dynamics by NMR: From Liquid Crystals to Borohydrides

NMR is a powerful tool to study dynamic processes both in liquid and in solid systems. Dynamic processes can be tracked using various approaches, such as direct diffusion measurements, NMR spectra, and nuclear spin-lattice relaxation time. In the latter case, it is beneficial to perform measurements in various magnetic fields since this enables us to access processes that take place on different timescales. I will present some recent studies that we carried out at Jožef Stefan Institute.

In liquids, the most typical dynamic process is molecular self-diffusion. In liquid crystals that consist of large molecules it is also possible to monitor molecular rotations and reorientations, and, furthermore, collective motions in the ordered phases, such as order director fluctuations.

In solids, molecular motions include jumping of atoms between the bonds and rotations of some internal groups. Protonic conductor CsH₅(PO₄)₂ consists of layers of PO₄ tetrahedra, connected by hydrogen bonds. Proton NMR spectra and spin lattice relaxation measurements uncover two dynamic processes: around room temperature, hydrogen jumps inside the bond in a double-well potential. At elevated temperatures, hydrogen starts hopping between the bonds, where it is assisted by the rotations of the PO₄ tetrahedra – this constitutes the mechanism of proton conductivity. LiZn₂(BH₄)₅ system consists of a complex network of BH₄ tetrahedra, connected by Zn and Li atoms. Our study unveiled that there are two types of tetrahedra in the system, each with different properties. We were able to determine activation energies for rotations of the tetrahedra around different axis which depend on the position of the tetrahedra between Zn or Li atoms. As a part of work in progress, I will present some preliminary results on systems that contain BF₄ instead of BH₄. In these systems, dynamics is expected to be similar, though the activation energies are bigger due to the much-heavier BF₄ unit.