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

Abstract

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 (18.8T) and ultra-fast Magic Angle Spinning (MAS) frequency of 62.5kHz. 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.