1H and 13C NMR measurements are reported for the CDCl 3 and CD 2Cl 2 solutions of [La(18-crown-6)(NO 3) 3] (I), [Pr(18-crown-6) (NO 3) 3] (II), [Ce(18-crown-6)(NO 3) 3] (III), and [Nd(18-crown-6)(NO 3) 3] (IV) complexes. Temperature dependencies of the 1H NMR spectra of paramagnetic II-IV have been analyzed using the dynamic NMR (DNMR) methods for six-site exchange. Two types of conformational dynamic processes were identified (the first one is conditioned by interconversion of complex enantiomeric forms and pseudorotation of a macrocycle molecule upon the C 2 symmetry axis; the second one is conditioned by macrocycle molecule inversion). Application of exchange spectroscopy (2D-EXSY) of DNMR for investigation of this dynamic system (II-IV) simplifies the assignment of the NMR signals and represents the first experimental study of multisite exchange. In the present work, the methodology of paramagnetic 4f (Ce, Pr, and Nd) probe applications for the study of free-energy, enthalpy, and entropy changes in chemical exchange processes, as well as the advantages of this method in a comparison with DNMR studies of diamagnetic substances, is discussed. In particular, as a result of paramagnetic chemical shifts in 4f complexes, the range of measurable rate constants expands considerably compared to the analogous range in diamagnetic compounds. Coordination compounds investigated in the paper represent new types of thermometric NMR sensors and lanthanide paramagnetic probes for in situ temperature control in solution.
ASJC Scopus subject areas
- Inorganic Chemistry
- Physical and Theoretical Chemistry