Observability of Paramagnetic NMR Signals at over 10 000 ppm Chemical Shifts

Abstract

We report an experimental observation of 31P NMR resonances shifted by over 10 000 ppm (meaning percent range, and a new record for solutions), and similar 1H chemical shifts, in an intermediate‐spin square planar ferrous complex [tBu(PNP)Fe‐H], where PNP is a carbazole‐based pincer ligand. Using a combination of electronic structure theory, nuclear magnetic resonance, magnetometry, and terahertz electron paramagnetic resonance, the influence of magnetic anisotropy and zero‐field splitting on the paramagnetic shift and relaxation enhancement is investigated. Detailed spin dynamics simulations indicate that, even with relatively slow electron spin relaxation (T1 ≈10−11 s), it remains possible to observe NMR signals of directly metal‐bonded atoms because pronounced rhombicity in the electron zero‐field splitting reduces nuclear paramagnetic relaxation enhancement.

A dysprosium single molecule magnet outperforming current pseudocontact shift agents

“…As a result, the solution 1 H NMR spectra of 1 at room temperature reveal PCSs exceeding ±1000 ppm, in agreement with ab initio calculations based on the X-ray structure. Hyperfine shifts of the order of 10 000 ppm have been recently reported for H and P atoms coordinated to an iron( ii ) center, 26 but the major contribution is a through-bond Fermi contact that quickly decays a few bonds away from the paramagnetic centre. On the contrary, the magnetic anisotropy of 1 induces through-space pseudochemical shifts that can be felt by 1 H nuclei at distances >10 nm from the Dy 3+ centre, paving the way towards a new generation of NMR tags for protein structure determination.…”