Nuclear Magnetic Resonance: A Spectroscopic Probe to Understand the Electronic Structure and Reactivity of Molecules and Materials

Abstract

This Perspective focuses on the ability of chemical shift to identify and characterize the electronic structure and associated reactivity of molecules and materials. After a general introduction on NMR parameters, we will show selected examples where the chemical shift of various NMR active nuclei has been used to investigate and understand electronic properties, with a particular focus on organometallic compounds and inorganic materials with relevance to catalysis. We will demonstrate how the NMR parameter of probe molecules and ligands can be used to elucidate the nature of active sites and how they can help to understand and predict their reactivity. Lastly, we will give an overview over recent advances in deciphering metal NMR parameters. Overall, we show how chemical shift is a reactivity descriptor that can be analyzed and understood on a molecular level.

Olefin Metathesis Catalysts Generated In Situ from Molybdenum(VI)‐Oxo Complexes by Tuning Pendant Ligands

“…Subsequently, we investigated the three compounds 1 F3‐9 using solution 95 Mo NMR in order to obtain further insight into the electronic structure of these compounds [29,30] . Here, the 95 Mo chemical shift revealed a high sensitivity to subtle changes of the coordination sphere (see Table 2): The general trend is, that more electron withdrawing alkoxide groups result in a higher chemical shift and deshielding of the nucleus with 1 F3 being the most shielded at δ Mo =61 ppm, 1 F6 showing an intermediate chemical shift of δ Mo =100 ppm and 1 F9 being the most deshielded at δ Mo =149 ppm.…”

“…Paramagnetic deshielding originates from couplings between frontier molecular orbitals and occurs when an occupied orbital can couple with an empty orbital of the right symmetry (orthogonal to each other and to the applied magnetic field) and close in energy. [30] By deconvoluting the contributions to δ 11 or the related chemical shielding σ 11 , we find that the contribution of the MoÀ Cl bonding and antibonding orbitals changes only marginally and the main contribution to the change in σ 11 , and by extension δ 11 , comes from the Mo=O bond, more specifically from the π-bonding orbitals. The Mo=O π-bond consists of the Mo=O π x and Mo=O π y orbitals, which extends along the ROÀ MoÀ OR axis and along the axis ClÀ MoÀ Cl, respectively.…”

“…Subsequently, we investigated the three compounds 1 F3-9 using solution 95 Mo NMR in order to obtain further insight into the electronic structure of these compounds. [29,30] Here, the 95 Mo chemical shift revealed a high sensitivity to subtle changes of the coordination sphere (see Table 2): The general trend is, that more To study the origin of the observed chemical shift trend, we calculated and analyzed the individual principal components of the chemical shift tensors. [30,31] The calculated chemical shifts agree well with experiments and the associated principal component of the CS tensors show that the δ 11 component, which extends along the ROÀ MoÀ OR axis (Figure 3a), drives the change in chemical shift.…”

“…[29,30] Here, the 95 Mo chemical shift revealed a high sensitivity to subtle changes of the coordination sphere (see Table 2): The general trend is, that more To study the origin of the observed chemical shift trend, we calculated and analyzed the individual principal components of the chemical shift tensors. [30,31] The calculated chemical shifts agree well with experiments and the associated principal component of the CS tensors show that the δ 11 component, which extends along the ROÀ MoÀ OR axis (Figure 3a), drives the change in chemical shift. This component is oriented perpendicular to the Mo=O and MoÀ Cl bonds and is thus associated with the magnetic couplings between either or both of these two bonds.…”

NMR Crystallography Enhanced by Quantum Chemical Calculations and Liquid State NMR Spectroscopy for the Investigation of Se‐NHC Adducts**

“…For example, experimental and theoretical studies have been devoted to revealing the correlation between molecular orbitals and chemical shift components. [66][67][68] It is worth mentioning that quantum chemistry computation can deliver not only the diagonalized CSA tensor values, but also the orientation of the tensor. For example, analysis of the CSA tensor indicates differences between Fisher and Schrock carbenes.…”