Determination of Carbon-13 Chemical Shielding Tensor in the Liquid State by Combining NMR Relaxation Experiments and Quantum Chemical Calculations

Abstract

Based on multifield NMR relaxation measurements and quantum chemistry calculations, a strategy aiming at the determination of the chemical shielding tensor (CST) in the liquid state is described. Brownian motions in the liquid state restrict the direct observation of CST to a third of its trace (isotropic shift), and even if CST can be probed indirectly through some spin relaxation rates (specific longitudinal relaxation rates, dipolar chemical shift anisotropy (CSA) cross-correlation rates), an insufficient number of experimental parameters prevents its complete determination. This lack of information can be compensated by using quantum chemical calculations so as to obtain the molecular CST orientation even if a relatively modest level of computation is used. As relaxation parameters involve a dynamic part, a prerequisite is the determination of the molecular anisotropic reorientation which can be obtained independently from dipolar cross-relaxation rates. A polycyclic molecule exhibiting a well-characterized anisotropic reorientation serves as an example for such a study, and some (but not all) carbon-13 chemical shielding tensors can be accurately determined. A comparison with solid-state NMR data and numerous chemical quantum calculations are presented.

Determination of 15N Chemical Shift Anisotropy from a Membrane-Bound Protein by NMR Spectroscopy

“…Structural dependence of 1 H and 13 C chemical shifts are well studied by experimental as well as computational approaches both in solids and solutions. 10,14,34–40 15 N CSA, on the other hand, is relatively less known as it is influenced by many factors such as torsion angles, hydrogen bonding interactions (both intra and inter residue), intra-residue angles, the chemical nature of a side chain, electrostatic interactions and solvent interactions. 23,41–47 Many attempts have also been made towards theoretical determination of 15 N-CSA using quantum chemical methods by several groups in the recent past.…”

A Concerted Approach for the Determination of Molecular Conformation in Ordered and Disordered Materials

“…Up to now there are only a few NMR investigations in disordered condensed phases which apply computational models for structure proposal. [26,[31][32][33] We now extend the use of ab initio calculations to a concerted approach for structural investigations in arbitrary ordered and disordered materials. This approach is based on a combination of isotope labeling, ab initio calculations and solid-state NMR spectroscopy.…”

Simulation of radio‐frequency field inhomogeneity effects: application to pulse trains aimed at the determination of CSA–dipolar interference terms

“…From such interference terms, it is indeed possible to determine the characteristics of shielding tensors in the liquid state. 2 The influence of CSA-dipolar interference terms on NMR experiments is well understood. 3 In particular, as far as transverse relaxation 4 is concerned, and provided that the two nuclei involved are J coupled, this phenomenon manifests itself by a differential broadening of the two lines in the corresponding doublet.…”