Chemical shielding interactions, quadrupolar nuclei

The most common interactions are dipolar coupling between two spins, chemical shielding and quadrupolar interaction for spins I> 1. The chemical-shielding interaction arises from the motion of electrons around a nucleus induced by the external applied static magnetic field. This motion generates local magnetic fields that modify the total field experienced by the nucleus, and are characteristic of the local chemical environment of the nuclei. All nuclei with a spin /> 1 possess a quadrupolar moment Q that interacts with the EFG Vrs — d2 V/dXfdx at the nuclear site. The EFG tensor Vrs is a symmetric tensor with zero trace Tr( E) = A E = 0 (from Laplace equation). 

Since the short relaxation times associated with a quadrupolar nucleus drastically reduce the time delay to be applied in an NMR experiment between two pulses, measuring times are short or, in other words, distinct NMR signals can often be detected with a limited time spent down to micromolar concentrations. Along with this apparent advantage, quadrupolar nuclei provide information in addition to the classical parameters chemical shift (or shielding) and nuclear spin-spin coupling constants. Variations in linewidths for quadrupolar nuclei are another sensitive quantity allowing for the evaluation of the electronic and the steric situation in the first coordination sphere of a vanadium compound, its periphery, its (local) symmetry and its interaction with the matrix, i.e. counter-ions, solvent molecules and other constituents present in solution. 

Under anisotropic conditions, NMR lineshapes for a quadrupolar nucleus are dominated by chemical shielding and (first and second order) quadrupolar interactions. Dipolar interaction is usually a minor contribution only. First-order quadrupole interaction lifts the degeneracy of the allowed 21 (i.e. seven in the case of V / = V2) Zeeman transitions as shown in Figure 3.7, giving rise to seven equidistant lines, viz. a central line (mj = + V2 -V2. unaffected by quadrupole interaction) and six satellite lines. The overall breadth of the spectrum is determined by the size of the nuclear quadrupole coupling constant Cq the deviations from axial symmetry and hence the shape of the spectral envelope are governed by the asymmetry parameter. Static solid-state NMR thus provides additional parameters, in particular the quadrupole coupling constant, which correlates with the electronic situation in a vanadium compound. [ 1 The central component reflects the anisotropy of the chemical shift. 

Rapid molecular motions in solutions average to zero the dipolar and quadrupolar Hamiltonian terms. Hence, weak interactions (chemical shift and electron-coupled spin-spin couplings) are the main contributions to the Zeeman term. The chemical shift term (Hs) arises from the shielding effect of the fields produced by surrounding electrons on the nucleus ... 

NMR spectra cannot be measured in solids in the same way in which they are routinely obtained in solutions because NMR lines from solids are too broad. In solution all interactions apart from chemical shift and indirect coupling are averaged to zero by thermal motions of molecules. Magnetic interactions in the solid state are described by a Hamiltonian H [1], which is a sum of several contributions Zeeman interaction (the same as in solution), direct dipole-dipole interaction, magnetic shielding (giving chemical shifts), scalar spin-spin coupling to other nucleus, and for nuclei with / >1/2 also quadrupolar interactions. 

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