Tm-DOTA as responsive relaxation and shift probe for NMR local temperature monitoring at high magnetic fields

Paramagnetic properties of Na[Tm(DOTA)] complex were investigated by 1H NMR under magnetic fields of 4.70, 11.75, 14.09 and 18.79 T. The Curie-Spin contribution to the transverse relaxation rates is found to be predominant at low temperatures (below 300 K). The experimental paramagnetic LIS values are well fitted by linear dependence on reciprocal temperature and have height temperature sensitivity of 1.9 ppm/K at 277 K. The obtained results indicate that the coordination compound Na[Tm(DOTA)] is the most shifting and also sensitive relaxation paramagnetic probe for in situ monitoring of local temperature in liquid media. At elevated temperature (above 310 K) it wasobserved a molecular dynamics related to intramolecular interconversion between two isomers: the square-anti-prismatic (SAP) and the twisted-square-anti-prismatic (TSAP) conformers. The free energy of activation of SAP-TSAP interconversion is found to be 65 ± 2 kJ mol−1.