Residual Dipolar and Quadrupolar Couplings

In anisotropic environment (e.g. in stretched or compressed polymer gels or liquid crystalline phases) molecules are partially oriented, and otherwise "hidden" interations such as the direct dipole-dipole interaction between two nuclear spins or the electric quadrupole interaction between I > 1/2 nuclei and the electric field gradient (EFG) become visible. Both depend on the orientation of the internuclear vector and the EFG, respectively, and hence carry structural information. By comparison of experimenal residual dipolar couplings (RDCs) with values back-calculated from a model structure, certain structures (conformations, configurations) can be excluded.

A. Doppler, L.D.M. Nicholls, C. Golz, M. Alcarazo, M. John, Magn. Reson. Chem. 2019, 57, 961-967.

⁷Li residual quadrupolar couplings (RQCs) present an extremely poweful method to study the aggregation of organolithium compounds. In hexameric oder tetrameric aggregates the high symmetry averages the EFG to zero, and no quadrupolar splitting is observed. In contrast, dimeric und monomeric compounds show splitting into a triplet (I = 3/2).

A.-C. Pöppler, H. Keil, D. Stalke, M. John, Angew. Chem. Int. Ed. 2012, 51, 7843-7846.

In low-symmetric Li compounds the binding motif and degree of solvation can be determined by a quantitative analysis of ⁷Li RQCs.



F. Rüttger, T. Patten, J. Kretsch, A. Krawczuk, D. Stalke, M. John, Chem. Eur. J. 2023, 29, e202203995.


Slice-Selective NMR Spectroscopy

Slice-selective excitation is fundamental to most present-day MRI methods, but can also be used in standard high-resolution NMR: Applying bandselective rf pulses at variable frequency offsets in the presence of a z-gradient yields a series of NMR spectra of thin (~1 mm) horizontal slices of the sample. Through referencing with solutions of known concentrations, local concentrations can be determined. One possible application is a "single-shot" NMR titration, where the signal of the first component (Li⁺) is obtained spatially resolved over a concentration gradient of the "titrated" component (12-Krone-4).

T. Niklas, D. Stalke, M. John, Chem. Commun. 2015, 51, 1275-1277.

Further applications are swelling of polymers in organic solvents and the determination of the homogeneity of anisotropic (oriented) samples, reaction monitoring, where two reagents diffuse towards each other in the sample tube, and concentration determinations in two-phase systems.



A.-C. Pöppler, S. Frischkorn, D. Stalke & M. John, ChemPhysChem 2013, 14, 3103-3107.


Structure, Dynamics and Aggregation of Cyanine Dyes

Cyanines such as the medically relevant Indocyanine Green (ICG) show (light-induced) trans-cis-Isomerizations and O₂ and light-induced dimerization. NMR is a highly suitable but yet little applied method to study these processes.



F. Rüttger, S. Mindt, C. Golz, M. Alcarazo, M. John, Eur. J. Org. Chem. 2019, 4791-4796.