Chemical shifts contributions

It is known that proton chemical shifts are largely determined by electronic effects The coordinate Si<-D interaction, weakening the covalent silicon bonds (increasing their ionic character), enhances the electron density on the atoms of a hydrocarbon substituent which results, as a rule, in an increase of proton shielding. This is usually accompanied by a drahielding of protons of the donor fragment This chemical shift contribution, however, is often insignificant and masked by other effects, in particular, when the silicon atom has a 7t-donor substituent. 

We can see that each of H and Hb has the same three contributions to its overall chemical shift, and therefore, both protons have potentially identical chemical shifts. The same argument of very slow rotation, which was made for the possible observation of separate methyl proton signals, also applies to Hy and Hb (and Rj and R2). As we saw for the methyl protons, rapid rotation about the carbon-carbon bond of HyHbYC-CR R2X averages the three chemical-shift contributions, and the H /Hb (and Ri/R2) pair is isochronous in achiral and racemic media. If groups X and Y, however, are very large, and bond rotation commensu-rately is slow, separate signals could, in principle, be detected. 

If R = R2 = H, the appearance of H, Hb, H, and H2 depends upon (i) the relative populations of the three conformers (aia b tmd c), (ii) the four coupling con.stants [ /H(a)H(i) H(a)H(2). H(b)H(i). and 7H(b)H(2)]> and (iii) the rate of rotation about the carbon-carbon bond of HyHbYC-CH H2X. Since the bond rotational speed is almost always sufficiently rapid to average chemical-shift contributions (see earlier), the third criterion is not considered further with respect to the averaging of coupling-constant contributions. 

The dipolar interaction and CSA contribute to i7 22K,iT-iK,iTiK,iT2-2K> but not to Hoo. On the contrary, the isotropic chemical shift contributes only to iToo-Hence, in order to achieve zero-order decoupling, all the terms in Eq. 73 must vanish, except iToo- This can be accomplished when, according to the definition of... 

According to the bond polarization theory we get the following major chemical shift contributions ... 

A very interesting application of powder MAS and MQMAS NMR to the investigation of the high-temperature sequence of phase transitions of LiRbS04 appeared.77 The central transition of 87Rb was investigated within 373-488 K, where five phases separated by four phase transitions are expected. The three main phase transitions affect the EFG parameters and the spin-lattice relaxation rate, and a critical behavior was evidenced and related to a damped soft mode. The MQMAS experiment permits to separate the chemical shift contribution from the second-order-induced quadrupolar isotropic shift in a single experiment, and indicates that the evolution of the chemical shift is almost continuous. Two different rubidium sites are clearly evidenced above 438 K, while X-ray could detect them only above 457 K. MQMAS data show a similar electronic environment for these two sites. 

Bertini et report a detailed study of the magnetic properties of myglobin, in which they determined the axial and orthorhombic terms of the paramagnetic susceptibility tensor using a combination of hyperfine chemical shift measurements and static suscetibility measurements (Evans Method). The determination of the magnetic anisotropy provided a measurement of the residual dipolar couplings and also permitted a separation of the contact and pseudocontact chemical shift contributions of resonances of the Fe(III) ligands. 

The total spectral firequency in absolute units is given by the Larmor frequency plus the chemical shift contribution as... 

For paraffins the chemical shift is calculated assuming that the carbon of interest is an alkyl-substituted methane molecule. First, a constant is used which nearly corresponds to that of methane (—1.87 ppm). Then chemical shift contributions are added for each carbon up to five carbons adjacent to the specified carbon. These contributions are described by constants a for the first bonded carbon, for the second carbon, which is two bonds away, / for the third, S for the fourth, and s for the fifth. If there are two a, or other carbons, the constant is multiplied by the appropriate number of adjacent carbons of the type. Values of a through s were obtained by Grant and Paul from a regression analysis of a series of paraffins. The results are shown in Table 6.2. For branched alkanes upheld shifts are observed for branched carbons and carbons next to branches, so corrective terms are required to account for chemical shifts of carbons associated with branching. Quaternary, tertiary, secondary, and primary carbon atoms are designated by 4°, 3°, 2°, and 1°, respectively. In the cor-... 

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