Freezing of molecular rotation in a paramagnetic crystal studied by $^{31}$P NMR

TL;DR

This study uses $^{31}$P NMR to investigate molecular rotation freezing in a paramagnetic crystal, revealing temperature-dependent spectral changes and quantifying activation energies for PF$_6$ reorientation modes.

Contribution

It provides a detailed analysis of molecular rotation freezing in a specific compound using NMR and theoretical modeling, highlighting the anisotropy and activation energies involved.

Findings

Freezing of PF$_6$ rotation modes observed via spectral moments.

Activation energies for reorientation modes quantified as 250 K and 1400 K.

Anisotropy of spectral moments matches experimental data.

Abstract

We present a detailed $^{31}$P nuclear magnetic resonance (NMR) study of the molecular rotation in the compound [Cu(pz)$_{2}$(2-HOpy)$_{2}$](PF$_{6}$)$_{2}$, where pz = C$_4$H$_4$N$_2$ and 2-HOpy = C$_5$H$_4$NHO. Here, a freezing of the PF$_6$ rotation modes is revealed by several steplike increases of the temperature-dependent second spectral moment, with accompanying broad peaks of the longitudinal and transverse nuclear spin-relaxation rates. An analysis based on the Bloembergen-Purcell-Pound (BPP) theory quantifies the related activation energies as $E_{a}/k_{B}$ = 250 and 1400 K. Further, the anisotropy of the second spectral moment of the $^{31}$P absorption line was calculated for the rigid lattice, as well as in the presence of several sets of PF$_6$ reorientation modes, and is in excellent agreement with the experimental data. Whereas the anisotropy of the frequency shift and enhancement of nuclear spin-relaxation rates is driven by the molecular rotation with respect to the dipole fields stemming from the Cu ions, the second spectral moment is determined by the intramolecular interaction of nuclear $^{19}$F and $^{31}$P moments in the presence of the distinct rotation modes.