Shielding, Chemical Shift

Here we have introduced three second-rank tensors, the rotational magnetic moment tensor T2(G) and the proton and fluorine screening tensors, T2(rotational magnetic moment and chemical shift (shielding) tensors, a further term which should be included in the Zeeman Hamiltonian describes the diamagnetic susceptibility. This term takes the form... 

P. Mansfield, M. J. Orchard, D. C. Stalker and K. H. B. Richards, Symmetrized multipulse nuclear-magnetic-resonance experiments in solids measurement of the chemical-shift shielding tensor in some compounds. Phys. Rev., 1973, B7, 90-105. 

Recheck and make sure the experimental values of the NMR parameters viz. chemical shifts, shielding tensors, coupling constants, etc. used in modelhng studies are correct... 

Chemical shift is measured on a ppm scale relative to a standard of known frequency. TMS is the practical frequency standard for H, C, and Si. In principle, the absolute frequency reference is the electron-free nucleus. For theoretical work, the shift of the neutral diamagnetic atom is used as it may be calculated accurately. Alternatively, the absolute scale may be established independently of the NMR experiment from the rotational splitting constant measured by microwave spectroscopy on a reference gas, e.g., CO for C and In Figure 7, the chemical shift/shielding scale for C is shown together with the terms used to describe changes in shift. 

With this introduction to the origin of NMR signals and how spectrometers work, we return to consider further aspects of chemical shift, shielding and deshielding, and signal splitting. 

Another theoretical method, extensively exploited in chemical shift shielding calculation is gauge-including atomic orbitals (GIAO) [177] approach. That technique provides reasonable chemical shifts values for various nuclei. It should be noted that this approach does not take into account the periodicity of the crystal lattice and calculations are carried out for isolated molecules (gas phase). If analysis of intermolecular interactions is target of the project or there is its important part, then arbitrarily constructed cluster model has to be built up. 

Applications including chemical shifts, shielding effects and spin-spin coupling... 

Figure Bl.11.7. chemical shifts in [10]-paracyclophane. They have values on either side of the 1.38 ppm found for large polymethylene rings and, thus, map the local shielding and deshielding near the aromatic moiety, as depicted in the upper part of the figure.

The methods listed thus far can be used for the reliable prediction of NMR chemical shifts for small organic compounds in the gas phase, which are often reasonably close to the liquid-phase results. Heavy elements, such as transition metals and lanthanides, present a much more dilficult problem. Mass defect and spin-coupling terms have been found to be significant for the description of the NMR shielding tensors for these elements. Since NMR is a nuclear effect, core potentials should not be used. 

Instead of measuring chemical shifts m absolute terms we measure them with respect to a standard—tetramethylsilane ( 113)481 abbreviated TMS The protons of TMS are more shielded than those of most organic compounds so all of the signals m... 

Problem 13 3 in the preced ing section was based on the chemical shift difference be tween the proton in CHCI3 and the proton in CHBrs and Its relation to shielding... 

The decreased shielding caused by electronegative substituents is primarily an inductive effect and like other inductive effects falls off rapidly as the number of bonds between the substituent and the proton increases Compare the chemical shifts of the pro tons m propane and 1 mtropropane... 

Table 13 1 collects chemical shift information for protons of various types The beginning and major portion of the table concerns protons bonded to carbon Within each type methyl (CH3) protons are more shielded than methylene (CH2) protons and meth ylene protons are more shielded than methme (CH) protons These differences are small as the following two examples illustrate... 

Acetylenic hydrogens are unusual in that they are more shielded than we would expect for protons bonded to sp hybridized carbon This is because the rr electrons circulate around the triple bond not along it (Figure 13 9a) Therefore the induced magnetic field is parallel to the long axis of the triple bond and shields the acetylenic proton (Figure 13 9b) Acetylenic protons typically have chemical shifts near 8 2 5... 

Annulene satisfies the Huckel (4n+2) tt electron rule for aromaticity and many of its proper ties indicate aromaticity (Section 11 20) As shown in Figure 13 10a [18]annulene contains two different kinds of protons 12 he on the ring s periphery ( out side ) and 6 reside near the middle of the molecule ( inside ) The 2 1 ratio of outside/inside protons makes it easy to assign the signals in the NMR spectrum The outside protons have a chemical shift 8 of 9 3 ppm which makes them even less shielded than those of benzene The six inside protons on the... 

The induced field of a carbonyl group (C=0) deshields protons in much Ihe same way lhal a carbon-carbon double bond does and Ihe presence of oxygen makes il even more eleclron wilhdrawmg Thus protons attached to C=0 m aldehydes are Ihe leasl shielded of any protons bonded to carbon They have chemical shifts m Ihe range 8 9-10... 

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