Chemical shift factors determining

The preceding estimate is based on one-fermion theory, so the observed resonance frequency in a fermion beam may be different as a result of fermion-fermion interaction. Therefore, it is strongly advisable that I be tunable over a wide range to search for the actual resonance pattern. The same experiment can then be repeated in a proton, atomic or molecular beam and the RFR effect should be // 2-dependent with a pattern of resonance determined by the novel chemical shift factor Spin-spin interaction between fermions would split the spectrum as in ordinary NMR, but the RFR fingerprint would be unique. 

The magnitude of the chemical shift depends on the nature of the valence and inner electrons of the nucleus and even on electrons that are not directly associated with the nucleus. Chemical shifts are influenced by inductive effects, which reduce the electron density near the nucleus and reduce the shielding. The orientation of the nucleus relative to tt electrons also plays an important role in determining the chemical shift. A proton located immediately outside a TT-electron system (as in the case of the protons on benzene rings) will be significantly deshielded. In most molecules the chemical shift is determined by a combination of these factors. Chemical shifts are difficult to predict using theoretical principles, but have been weU studied and can usually be easily predicted empirically upon comparison to reference data. 

There are two factors that determine chemical shifts - electron distribution and molecular anisotropy. We have already seen how electronics define chemical shifts in previous sections. When we use Table... 

The 13C NMR sensitivity can sometimes be a problem, but for the kind of samples studied here the effective concentration of monomer units is several molar which does not place excessive demands on present Fourier transform NMR spectrometers. In addition to the sensitivity of the chemical shift to structure (9), the relaxation of protonated carbons is dominated by dipole-dipole interaction with the attached proton (9). The dependence of the relaxation parameters T, or spin-lattice, and Tor spin-spin, on isotropic motional correlation time for a C-H unit is shown schematically in Figure 1. The T1 can be determined by standard pulse techniques (9), while the linewidth at half-height is often related to the T2. Another parameter which is related to the correlation time is the nuclear Overhauser enhancement factor, q. The value of this factor for 13C coupled to protons, varies from about 2 at short correlation times to 0.1 at long correlation... 

Several methods have been developed to determine the chemical shift anisotropies in the presence of small and large quadrupolar broadenings, including lineshape analysis of CT or CT plus ST spectra measured under static, MAS, or high-resolution conditions [206-210]. These methods allow for determination of the quadrupolar parameters (Cq, i)q) and chemical shift parameters (dcs, //cs> <5CT), as well as the relative orientation of the quadrupolar and chemical shift tensors. In this context, the MQMAS experiment can be useful, as it scales the CSA by a factor of p in the isotropic dimension, allowing for determination of chemical shift parameters from the spinning sideband manifold [211],... 

Several other nmr procedures have been used for the determination of fractionation factors. These have advantages in some systems. Instead of determining the effect of the concentration of an exchanging site on the averaged chemical shift, the effect on the averaged relaxation rate of water protons can be used in a very similar way (Silverman, 1981 Kassebaum and Silverman, 1989), For example, addition of the enzyme Co(ii)-carbonic anhydrase to an aqueous solution increases the observed value of XjT because the proton-relaxation rate is the average of that for the bulk solvent (cfl. 0.3 s ) and that for water bound to the cobalt ca. 6x 10 s ). The average is different in an H2O/D2O mixture if the bulk solvent and the Cobound solvent have different deuterium contents, and it has been used to determine a value for the fractionation factor of Co-bound water molecules in the enzyme. 

The above interpretation of the factors indicated by current theory to be important in determining proton and carbon-13 chemical shifts does not offer much encouragement for the optimistic statement that [carbon-13 chemical shifts] should provide a more reliable index of charge than the hydrogen shifts (Fraenkel and Farnum, 1968, p. 251). It is true that the other atom terms are a more serious perturbation on proton than on carbon-13 chemical shifts, and are difficult to evaluate. However it is also apparent that carbon-13 shifts are determined by a number of terms and 0 whose charge dependence may well be in... 

A general comment on the use of the empirical correlation between Si and Sn NMR (and likewise on C/ Si or Sn/ Pb NMR) chemical shifts is in order. The basis for this correlation is that the paramagnetic term Op dominates the chemical shift. According to Ramsay s theory, Op is proportional to the reciprocal energy difference h.E between the magnetically active orbitals and proportional to the expectation value for the electron radii (r )np- Thus, a linear correlation between the 5 Si and 8 Sn implies that the ratio of both determining factors of Op is constant for the all compounds of interest. In particular, it is not clear, however, if the ratio for tetravalent silicon and tin compounds is the same as for trivalent silicon and tin compounds. Therefore, the extension of a correlation based exclusively on the... 

Fig. 17. Determination of Ea and vn for fluorine exchange in SF4 where here is the frequency factor and S is the chemical shift in cps. [Muetterties and Phillips (89).]...

In an attempt to relate calculated results to experimental findings for monomeric, lignin model compounds, preliminary work has compared theoretically determined electron densities and chemical shifts reported from carbon-13 nuclear magnetic resonance spectroscopy (62). Although chemical shifts are a function of numerous factors, of which electron density is only one, both theoretical and empirical relationships of this nature have been explored for a variety of compound classes, and are reviewed by Ebra-heem and Webb (63), Martin et al. (64), Nelson and Williams (65), and Farnum (66). 

A large number of 13C NMR studies on proline derivatives and proline peptides have appeared in the literature [815-830]. As the electron charge density of cis-proline carbons is different from that of franx-prolinc carbons, these isomers can be differentiated by nCNMR spectroscopy [826, 830]. On the basis of calculations Tonelli [831] predicted four conformations for the dipeptide Boc-Pro-Pro-OBzl, three of which could be detected by 13C NMR spectroscopy [826, 830], In proline-containing peptides the stereochemistry of the proline residue plays an important role for the conformation of these oligomers. The 13C chemical shift data of cis and trans proline derivatives, collected in Table 5.29, are useful to determine the stereochemistry of the amino acid-proline bond, e.g. in cyclo-(Pro-Gly)3, melanocyte-stimulating hormone release-inhibiting factor or thyrotropin-releasing hormone. 

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