NMR chemical shielding analysis

Experimentally, the isotropic chemical shielding cta of a given nucleus A is generally expressed as the chemical shift with respect to a chosen reference signal cTref, such as tetramethylsilane (TMS) for H spectra  

Increasing chemical shift 5a is generally plotted leftward ( downfield ) of the TMS signal, so that chemical shielding Ga increases toward the right ( upheld  

The default NCS analysis output for ethanol is shown in 1/0-7.2 for nuclei 1-7. The rows display localized contributions due to L-type NBOs and their NL-type corrections (from the corresponding NLMO delocalization tads), which sum to the total isotropic shielding at the bottom of the column. 

The formal theory of NMR shielding is beyond the scope of the present discussion. However, one can expect in a general way that shielding depends on the size and shape of occupied orbitals in proximity to the nucleus. The size-dependence (r radial weighting) emphasizes the contribution of electrons closest to the nucleus, while the angular momentum shape-dependence (s, p, d. .. type) emphasizes the contribution of s-character (peaked at the nucleus) versus p-character (noded at the nucleus) for orbitals centered on the nucleus of interest. 

Compared to the experimental H-NMR ethanol spectrum for the neat liquid as summarized in Table 7.1, the calculated shieldings for molecular ethanol raise additional interesting questions, such as follows  

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Wang, J. M., Wolf, R. M., Caldwell, J. W, Kollman, P. A., 8 Case, D. A. (2004). Toward direct determination of conformations of protein building units from multidimensional NMR experiments. V. NMR chemical shielding analysis of N-formyl-serinamide, a model for polar side-chain containing peptides. Journal of Computational Chemistry, 24,1157. 

The starting point for NCS analysis is the GIAO expression for the anisotropic NMR chemical shielding tensor components Oafi as a sum of unperturbed (u) and induced (i) contributions,... 

The results of two-parameter correlations of the 13C and 15 N NMR chemical shifts of 2-substituted 5(6)-nitrobenzimidazoles with the induction and resonance constants (a, ak) are given in Table 3.26 [688, 689], The electronic influence of substituents on the chemical shifts of carbons (and protons) in positions 4 and 7 are mainly transmitted by a resonance mechanism, C-4 being more sensitive than C-7 to the substituent effect. For positions 5 and 6 a slightly smaller contribution from the resonance component to the total transmission of substituent effects is observed. However, analysis of correlation between 5 15 N and substituent parameters indicates an approximately equal influence of the induction and resonance substituent effects on the nitro group shielding (see Table 3.26) [688, 689], When other sets of substituent constants (F and R, c, and ck, etc.) are used in the correlation the percentage ratios of resonance and inductive contributions remain unchanged. 

NMR techniques as a detailed knowledge of all the NMR parameters, such as chemical shielding anisotropy and quadrupole tensor parameters, as well as their distributions, is necessary to draw detailed conclusions on the local environment and long-range order of V atoms. However, the SATRAS analysis of V MAS spectra coupled with the results of V 3Q-MAS and 5Q-MAS studies (Figure 5.15) can provide this information and demonstrate the formation of a V—P—Ti compound, in contrast to previous hypotheses for such systems. 

Analysis of the apparently competing factors which cause these differences between cyclic and acyclic systems is incomplete. The effect of substituents on the 17o NMR chemical shifts of simple cyclohexenones (31-33) has been investigated [67]. Both a and P alkyl groups cause shielding of the cyclohexenone carbonyl signal (cf. 32 and 33 with 31). 

Pb NMR has been used to investigate 20 lead(IV) tetracarbox-ylates both in solution and in the solid state. From an analysis of the range of, and trends in, the NMR chemical shifts, circumstantial evidence has been presented for augmentary coordination by Lewis basic ortho-aromatic substituents of the carboxylate groups. The Pb CSA parameters have been extracted from analysis of the spinning sideband manifolds of the CPMAS spectra. Thirteen of the 20 CSA analyses performed show effectively axial CSA tensors. A simple shielding model which rationalizes this typical CSA pattern has been presented... 

Many solid-state NMR studies of oriented polymer fibers or film other than silk have been described. Orientation-dependent chemical shielding tensors especially serve as probes with which the relative orientations of specific bond vectors can be determined [10]. This analytical method can be applied to obtain structural information from oriented polyamide fibers such as poly (p-phenylene terephthalamide) (PPTA) [11], poly(m-phenylene isophthalamide) (PMIA) and poly(4-methyl-m-phenylene terephthalamide) (P4M-MPTA) fibers without isotope labeling of the samples [12] (Chapter 12). Oriented carbonyl carbon labeled poly (ethylene terephthalate) (PET) films have also been analyzed with this method [13] (Chapter 14). Especially, more quantitative structural information will be obtained for a locally ordered domain which has been recognized as an amorphous domain in X-ray diffraction analysis in heterogeneous polymer samples. 

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