Observation of ¹H-¹³C and ¹H-¹H proximities in a paramagnetic solid by NMR at high magnetic field under ultra-fast MAS

The assignment of NMR signals in paramagnetic solids is often challenging since: (i) the large paramagnetic shifts often mask the diamagnetic shifts specific to the local chemical environment, and (ii) the hyperfine interactions with unpaired electrons broaden the NMR spectra and decrease the coherence lifetime, thus reducing the efficiency of usual homo- and hetero-nuclear NMR correlation experiments. Here we show that the assignment of ¹H and ¹³C signals in isotopically unmodified paramagnetic compounds with moderate hyperfine interactions can be facilitated by the use of two two-dimensional (2D) experiments: (i) ¹H-¹³C correlations with ¹H detection and (ii) ¹H-¹H double-quantum single-quantum correlations. These methods are experimentally demonstrated on isotopically unmodified copper (II) complex of l-alanine at high magnetic field (18.8 T) and ultra-fast Magic Angle Spinning (MAS) frequency of 62.5 kHz. Compared to ¹³C detection, we show that ¹H detection leads to a 3-fold enhancement in sensitivity for ¹H-¹³C 2D correlation experiments. By combining ¹H-¹³C and ¹H-¹H 2D correlation experiments with the analysis of ¹³C longitudinal relaxation times, we have been able to assign the ¹H and ¹³C signals of each l-alanine ligand.