Probing a Hydrogen‐π Interaction Involving a Trapped Water Molecule in the Solid State

Abstract

The detection and characterization of trapped water molecules in chemical entities and biomacromolecules remains a challenging task for solid materials. We herein present proton-detected solid-state Nuclear Magnetic Resonance (NMR) experiments at 100 kHz magicangle spinning and at high static magnetic-field strengths (28.2 T) enabling the detection of a single water molecule fixed in the calix[4]arene cavity of a lanthanide complex by a combination of three types of noncovalent interactions. The water proton resonances are detected at a chemical-shift value close to zero ppm, which we further confirm by quantum-chemical calculations. Density Functional Theory calculations pinpoint to the sensitivity of the proton chemical-shift value for hydrogen-π interactions. Our study highlights how proton-detected solid-state NMR is turning into the method-of-choice in probing weak non-covalent interactions driving a whole branch of molecular-recognition events in chemistry and biology.

The effect of methyl group rotation on 1H–1H solid-state NMR spin-diffusion spectra

“…We recently investigated a calixarene–lanthanide complex, in which we also observed negative cross peaks in spin-diffusion spectra. 11 In this case, the negative cross peaks involved methyl groups and deshielded aromatic as well as imino protons. The observation that (i) methyl groups are involved and (ii) the 1 H chemical-shift differences between the methyl group and proton pairs for which such cross peaks have been observed is rather large (>4.6 ppm), led us to study their origin in order to disentangle the role of coherent and incoherent cross-relaxation (NOE-type) contributions to the observed peaks.…”

“…8 Combining fast MAS with very high static magnetic-field strengths, such as the recently commercially available 28.2 T superconducting NMR magnets, 9 is expected to further decrease homogeneous linewidths, as illustrated by first examples. 10,11…”

“…1 First reports on the potential presence of negative cross peaks in 1 H– 1 H 2D solid-state NMR spectra potentially caused by NOE effects have been published very recently. 4,11,27 These examples all involve methyl groups that can potentially have fast rotational correlation times that are required to obtain negative cross peaks. They comprise an active pharmaceutical ingredient, 4 a lanthanide–calixarene complex and β-aspartyl l -alanine 27 for which such negative cross-peaks can be envisioned between methyl groups and high frequency-shifted chemical entities.…”

High-field and fast-spinning 1H MAS NMR spectroscopy for the characterization of two-dimensional covalent organic frameworks

“…It is thus likely that the 1 H MAS NMR spectroscopy of 2D COFs will become an important tool for their structure characterization. Moreover, these developments will increasingly enable the application of various 1 Hdetected 2D correlation experiments, like 1 H-1 H, 1 H- 13 C, and others as described [50][51][52][53] and reviewed 54 in the recent literature, thus paving the way to the future in-depth structural characterization of 2D COFs.…”

Opportunities and Challenges in Applying Solid‐State NMR Spectroscopy in Organic Mechanochemistry

“…[ 105 ] With the advent of fast MAS, also proton‐detected heteronuclear correlation experiments have been reported. [ 106 ]…”