Chemical shift origins

It can be seen that, in all cases, relaxation rates are directly proportional to (Aa). Because Aa reflects the anisotropy of the shielding tensor and because the chemical shift originates from the shielding effect, the terminology Chemical Shift Anisotropy is used for denoting this relaxation mechanism. Dispersion may be disconcerting because of the presence of Bq (proportional to cOq) in the numerator of and R2 (Eq. (49)). Imagine that molecular reorientation is sufficiently slow so that coo 1 for all considered values of coo from (49), it can be seen that R is constant whereas R2 increases when Bq increases, a somewhat unusual behavior. 

The H and C chemical shifts reported in this review are referenced to tetramethylsUane (TMS) as the internal standard on the 8 scale (ppm). Liquid NH, has been used as the external standard for the " N chemical shifts, with the following values employed to correct the chemical shifts originally reported relative to other commonly used standards liquid NH3 (0.0 ppm), 1 M urea in DMSO (77.0 ppm), 1 M HNO, in H2O (377.3), liquid MeNO, (381.7 ppm). Liquid nitromethane and 1 M urea in DMSO-Jg are frequently used as secondary external standards for measurements in coaxial systems or in separate experiments. A more detailed discussion can be found in recently published review articles on nitrogen NMR spectroscopy. " 2 ... 

Chemical shifts originally quoted relative to benzene. The conversion to the r scale is made by adding 2.73 ppm to the quoted values. 

Empirical additive group contributions to the experimental phosphorus chemical shift, originally suggested by Van Wazer et were moderately successful, as were attempts to rationalize the shift data for several tertiary, secondary phosphines.More recently Grim et al. showed that the observed chemical shift for 1209 tertiary and secondary phosphines and quaternary phosphonium salts could be predicted with reasonable certainty using nine empirical group contributions (a ) for organic substituents. 

Additional structural effects on NMR chemical shifts originate from the spatial proximity of groups that are not directly connected. In particular, these effects are observed when hydrogen atom is changed to a substituent at the y-carbon (i.e. C -C-C-H C -C-C-X). If X=Me, and the spatial proximity is enforced by slruetural constrains or conformational energies, the upfield shift at C is 6ppm. For conformalionally flexible systems, the observed upfield shift reflects the relative abundance of such spatially close conformations (e.g. gauche or eclipsed) at the equilibrium. 

The experimental finding that the NMR resonance frequency of a particular nucleus depends on the chemical environment - the "chemical shift" - still forms the basis for a majority of applications of NMR spectroscopy. The chemical shift originates from the fact that in an atom or molecule the local magnetic field B sensed by the nucleus is different from the applied static magnetic field, B. The difference between the two fields is usually expressed by the equation... 

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