Solid‐State NMR Correlation Experiments and Distance Measurements in Paramagnetic Metalorganics Exemplified by Cu‐Cyclam

Abstract

We show how to record and analyze solid-state NMR spectra of organic paramagnetic complexes with moderate hyperfine interactions using the Cu-cyclam complex as an example. Assignment of the (13)C signals was performed with the help of density functional theory (DFT) calculations. An initial assignment of the (1)H signals was done by means of (1)H-(13)C correlation spectra. The possibility of recording a dipolar HSQC spectrum with the advantage of direct (1)H acquisition is discussed. Owing to the paramagnetic shifting the resolution of such paramagnetic (1)H spectra is generally better than for diamagnetic solid samples, and we exploit this advantage by recording (1)H-(1)H correlation spectra with a simple and short pulse sequence. This experiment, along with a Karplus relation, allowed for the completion of the (1)H signal assignment. On the basis of these data, we measured the distances of the carbon atoms to the copper center in Cu-cyclam by means of (13)CR2 relaxation experiments combined with the electronic relaxation determined by EPR.

Bipyridinium‐bis(carboxylate) Radical Based Materials: X‐ray, EPR and Paramagnetic Solid‐State NMR Investigations

“…By using fast magic‐angle spinning, 13 C and even 1 H NMR analysis can identify and characterize organic matter with paramagnetic centers. This has been shown in particular for metal‐organic systems 14–16. Figure 5 compares, among others, the 1 H spectrum of bp4pc · 2H 2 O and H‐bp4pc under 60 kHz MAS.…”

“…A further understanding of the paramagnetic center can be achieved by correlation to 13 C spectra and the comparison with DFT calculations of the shifts of both nuclei. Figure 6 (A) shows a 1 H‐ 13 C dipolar INEPT correlation spectrum optimized for paramagnetic organic molecules at 40 kHz MAS 15,16. Note that in Figure 5 the 1 H signals are slightly less shifted by the higher sample temperature because of the faster spinning (Curie's law).…”

“…On the other hand, there has been substantial progress over the last years in dealing with this perturbation, particularly with very fast magic‐angle spinning (MAS) 14. Moreover, for several cases of organic compounds complexing paramagnetic metal ions, it was shown that one can exploit the hyperfine interaction in terms of better hydrogen resolution because of the signal dispersion, correlation of the hyperfine shift to the structure by means of DFT calculations,15 and the measurement of distances by electron‐nuclear relaxation experiments 16. Radicals are used in NMR analysis of solutions, which is commonly more advanced, as tags to give additional constraints for the structure determination of biomolecules, and for signal enhancement by dynamic nuclear polarization (DNP).…”

Solid‐State NMR Characterization of Paramagnetic Bis(L‐valinato)copper(II) Stereoisomers – Effect of Conformational Disorder and Molecular Mobility on 13C and 2H Fast Magic‐Angle Spinning Spectra

“…Despite the very impressive progress in the field of quantum chemical calculations, including the cluster6a and the projector augmented wave/gauge (PAW/GIPAW)6b approaches, the precise theoretical calculation of the total observed shift, which includes both diamagnetic contributions from paired electrons and hyperfine contributions from unpaired electrons, has still not been properly solved for solid‐state paramagnetic molecules. However, encouraged by the good correlation between the experimental and quantum chemically calculated solid‐state MAS NMR chemical shifts,2c,6c–6f we performed quantum chemical density functional theory (DFT) calculations of the 13 C Fermi contact contributions in the studied bis( L ‐valinato)copper(II) complexes to examine their applicability to assist signal assignments.…”

Proton-detected fast-magic-angle spinning NMR of paramagnetic inorganic solids

“…This improvement is ascribed to the shorter recoupling times made possible with faster MAS, which is notably not efficient in the 30 kHz range. 19 …”