Shielding contributions

As indicated, the possibility of orbital decomposition is retained in the diamagnetic and paramagnetic terms in eqs.(29,30). The value for the paramagnetic shielding contribution extracted via eq.(30) does not, of course, correspond to a well-defined basis set, but is at the same level of numerical quality as the LORG calculation used for the total shielding. 

Shabab (45) attributed the a-SCS to a subtle balance of both the paramagnetic and diamagnetic (40-42) shielding contributions in which the latter increases more rapidly than the former with increasing atomic number of X. Thus, the net observed effect becomes less deshielding as the atomic number of X increases (45). 

TABLE 6.1. Paramagnetic Shielding Contributed by Neighboring Atoms (ppm) ... 

Apart from pyrrole, the chemical shifts for the [3-protons increase with decreasing electronegativity of the heteroatom. In contrast, the chemical shifts of the a-protons do not display any obvious regularity, probably due to paramagnetic shielding contributions which will become more important with increasing availability of 7-orbitals. The chemical shift of the pyrrole N-H is solvent dependent. 

The neighbor anisotropy term <7 of eq. (3.2) plays an important role in proton shielding, permitting, for example, a distinct differentiation between aromatic and olefinic protons due to the ring current effect. However, this contribution is small in 13C NMR (<2 ppm). A comparison of the methyl carbon shieldings in methylcyclohexene and toluene shows that the ring current effect often cannot be clearly separated from other shielding contributions ... 

Base Proton Complexation Shifts The complexation shifts of certain nucleic acid base resonances of poly(dA-dT) on formation of the daunomycin neighbor exclusion complex reflect the shielding contribution due to the anthracycline ring less the contribution from one neighboring base pair which is displaced following intercalation. Thus, the adenosine H-2 resonance remains unperturbed (Figure 27) while the thymidine exchangeable H-3 proton... 

Quantum mechanical calculations of 33S nuclear quadrupole coupling constants are not an easy matter (not only for the 33S nucleus, but for all quadrupolar nuclei). Indeed, the electric field gradient is a typical core property, and it is difficult to find wave functions correctly describing the electronic distribution in close proximity to the nucleus. Moreover, in the case of 33S, the real importance of the Sternheimer shielding contribution has not been completely assessed, and in any case the Sternheimer effect is difficult to calculate. 

Bernd Tesche (1994) has verified in the meantime that approximately the same values are valid for H4. These values agree remarkably well with those from the (old) multiple-pulse line-narrowing experiment and one might be tempted to conclude that this is it. It would be an illusion to expect still better agreement because the calculation was done for an isolated molecule, whereas the measurement includes intermolecular shielding contributions. These contributions can be estimated (but not more than that) on the basis of a point dipole model (Post, 1978 Avara-mudhan and Haeberlen, 1979). For malonic acid we expect that these contributions do not exceed 0.5 ppm for the various elements of the a tensor. 

Our interest is mainly focused on ring current effects in these quasi-annulenes. However, it is well known that deviations from electroneutrality at a carbon atom affects the shielding at that carbon and at the nearby protons. This latter effect is electrostatic in origin and is determined principally by the component of the electric field in the direction of the C—H bond. It is necessary to separate out the shielding contributions of such electrostatic effects, in order to make a meaningful comparison between the ring current effects in the ions and the neutral annulenes. Such a separation, however, is fraught with difficulties. In... 

The shifts of some organolithium compounds have been compared to their parent hydrocarbons, and in each case there is a down-field shift of the a-carbon of the lithium derivative. The downfield shift indicates that extensive rehybridization had occurred at the a-carbon atom. However, opposing this downfield shift there is a shielding contribution that is proportional to the negative charge on the a-carbon atoms consequently the shifts to low field in the phenyl-methyl series are variable (Table VIII). 

Let us start with the field-free SO effects. Perturbation by SO coupling mixes some triplet character into the formally closed-shell ground-state wavefunction. Therefore, electronic spin has to be dealt with as a further degree of freedom. This leads to hyperfine interactions between electronic and nuclear spins, in a BP framework expressed as Fermi-contact (FC) and spin-dipolar (SD) terms (in other quasirelativistic frameworks, the hyperfine terms may be contained in a single operator, see e.g. [34,40,39]). Thus, in addition to the first-order and second-order ct at the nonrelativistic level (eqs. 5-7), third-order contributions to nuclear shielding (8) arise, that couple the one- and two-electron SO operators (9) and (10) to the FC and SD Hamiltonians (11) and (12), respectively. Throughout this article, we will follow the notation introduced in [58,61,62], where these spin-orbit shielding contributions were denoted... 

A ring-current model, used to calculate the magnetic anisotropy of a cyclopropane ring, permits estimates of the shielding contribution of a cyclopropane ring to the chemical shifts of neighbouring protons. Illustrations include 3a,5a-, 3/3,5 -, and 5j8,7)5-cyclosteroids. 

The residual chemical shielding contributions are suppressed by the supercycle shown in Fig. 26. The average Hamiltonian of the DRAWS scheme (Eq. (107)) consists of zero-quantum and double-quantum terms with similar magnitudes (Eqs. (108) and (109)) and shows a rather strong dependence on the angles connecting PAS and RAS of the dipolar couphng tensor. 

Fig. 28. (a) The basic BABA pulse sequence over one rotation period, (b) Supercycle for the elimination of chemical shielding contributions over two rotation periods. ... 

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