Paramagnetic shielding term

For nuclei other than hydrogen, such as 13C, the paramagnetic shielding term [Pg.110]

Upheld shifts may also arise from substitution of an atom (e.g. H) by its heavier isotope (,e.g. 2H). The upheld shift follows from a lower potential energy of the ground state, hence an increased AE in eq. (3.4), and a reduction of bond distance. The influence of both effects will be to decrease the paramagnetic shielding term. Deuterium isotope shifts depend on the degree of deuteration and reach values of up to 1.5 ppm (Table 3.1). 

Parallel-spin pairing at a carbon atom causes a decrease in the paramagnetic shielding term and, consequently, the carbon resonates at higher field. 

Bulky //-substituents force carbons to resonate at lower field due to orbital contraction, which causes an increase in the paramagnetic shielding term. 

These findings lend support to Deverell s electronic distortion theory (22) which conveys the idea that orbital distortion affects both the electric field gradient and the paramagnetic shielding term according to ... 

There are also reports on substantial deviations from proportionality between chemical shifts and the inverse of the longest wavelength. Yamasaki et al. (225) have found that sulphur-coordinated cobalt complexes do not obey this linear relation. Tris(ethylxanthato)-cobalt(iii) as well as other CoS -type complexes are far more shielded than one would expect on the basis of the UV absorption maxima, clearly showing the importance of the radial parameter in the paramagnetic shielding term (which has been considered in the Lucken... 

UB n.m.r. chemical shifts for all possible isomers of monochloro-, monobromo-, and monoiodo-decaborane(14) have been determined using the nB-nB double-resonance technique.25 The data for 5-chloro- and 6-iodo-decaborane(14) were reported for the first time. All trends in the chemical shifts are dominated by the influence of the 2p orbital size on the paramagnetic shielding term (crp). 

The Stephens and Buckingham theory shows that the paramagnetic shielding term (ap) is dependent on AE (the energy separation of the ground and excited states), Rh (the metal-hydrogen intemuclear distance), and k, the exponent of the d orbitals. The latter was taken to be constant over any particular row of transition metals. 

The dominance of the paramagnetic shielding term in determining CO in the absence of large anisotropy effects is now generally accepted. Metal-carbonyl bonding, as monitored by NMR must therefore be explained in... 

We now turn to the important paramagnetic term a. In the original Pople LCAO MO approximation the local paramagnetic shielding term for an atom A takes the form... 

It can be argued that only the paramagnetic shielding term should vary appreciably with the environment see Section 6.1. 

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