Isotropic chemical shielding

Fig.3 The dependences on the dihedral angles(< >,i /), of the isotropic chemical shielding constant for the L-alanine residue Cp- (a)and Ca-(b) carbons in peptides. Chemical shielding calculations were carried out using the GIAO-CHF method with 4-31G ab initio MO basis set. The 4-31G optimized geometries for the model molecules, N-acetyl-N -methyl-L-alanineamide, were employed.

In MAS-NMR spectra, only the isotropic chemical shielding and indirect interactions survive, and (for I>l/2 nuclei), the second-order nuclear electric... 

Fig. 19. Correlation between solid-state Co isotropic chemical shifts ( ) and the isotropic chemical shielding constants (criso) calculated at 6-311 -t- G /B3PW91 level. All the calculations were performed on the bare complexes using experimental geometries. Data were taken from refs 53 and 167, and Table 1.

The APRR (adiabatic polarization transfer under rotational resonance conditions) experiment is essentially an polarization transfer experiment, with the difference that the spinning rate is not kept constant but is ramped adiabatically through the condition (Fig. 15). This leads to a more complete transfer and a more broadband resonance condition, rendering the experiment less sensitive to distributions of isotropic chemical shielding in disordered systems. 

Eor molecules that are axially symmetrical, with along the principal axis of symmetry, = (parallel component), and O22 = ( 33 = (perpendicular component). These anisotropic chemical shifts are observed in solid samples and liquid crystals, whereas for small molecules in solution, rapid tumbling averages the shift. The average, isotropic chemical shielding (which would be comparable to the solution chemical shift) is given by the trace of the shielding tensor or... 

Experimentally, the isotropic chemical shielding cta of a given nucleus A is generally expressed as the chemical shift with respect to a chosen reference signal cTref, such as tetramethylsilane (TMS) for H spectra ... 

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