Interatomic shielding

This NMR technique has been applied to a series of alkanediammonium ions, and the results (induced shifts of proton resonances) are summarized in Fig. 2. It may be seen that the shielding region extends for approximately 4.5 methylene units, or 6 A, which coincidentally is the interatomic distance axially spanning the cavity of cucurbituril. Similar induced chemical shift effects are found in CNMR spectra, and UV spectral perturbations are noted upon encapsulation of certain aromatic-ring bearing ammonium ions (particularly 4-methyl-benzylamine). Conclusive evidence for internal complexation with cucurbituril has been secured by crystallography [3]. 

The complexes of tetraalkylstannanes SnR4 with tetracyanoethylene (TCNE)85 are more stable than those with oxygen. The Sn—C bond serves as a a -donor in this case. The Kq values of such complexes are also defined predominantly by the degree of shielding of the donor centre. With change of R these constants diminish in the series Me >> Et > i- Pr > i-Bu > f-Bu. The steric hindrance to the complexation of Me4- PbEt with TCNE is smaller in comparison with the isostructural tin compounds as expected on the basis of the larger Pb interatomic Pb-C distance85. 

The concepts developed for compounds of tetrahedral silicon do not explain many experimental peculiarities of Si NMR spectroscopy in the case of penta-coordina-tion. Thus, for example although the interatomic Si... N distance in 1-chloro-methylhomosilatrane is much greater than in 1-chloromethylsilatrane, the shielding of silicon in the former is greater (—85.1 ppm) than in the latter (—77.2 ppm). Even in a narrow series of 1-substituted silatranes the coordination shifts A8 . 

Figure 2.43 The full optimized Leu-enkephalin structure with enlarged parts of both systems. Selected interatomic distances are indicated. (A) The plots show the correlation of experimental isotropic chemical shift values (5 so) and calculated nuclear shielding values ( 7 so). (B)The plots represent the correlation of experimental chemical shift tensor values Sii) and calculated nuclear shielding parameters of the enkephalin peptides. (C) The correlations of the experimental versus the computed parameters are shown for Leu-enkephalin. Reprinted from Ref. [96]. Copyright 2014 American Chemical Society.

Shielding by electron cloud is taken into account by multiplying the scattering cross-section an E,d) by a shielding factor F E,0). The shielding factor is obtained by solving the scattering equations for the shielded interatomic potential ... 

The aim of this section is to provide an overview about the development and application of solid state NMR spectroscopic techniques for the study of molecular structures and dynamics on the molecular and intermolecular length scale (lA-lOA). In particular, anisotropic magnetic nuclear spin interactions like chemical shielding anisotropy (CSA), magnetic dipolar interaction and quadrupolar interaction are used as probes for interatomic distances and orientations of molecular groups, i.e. structures, and changes of these interactions are monitored and used as a measure of dynamic processes inside the system. 

Shannon, R.D. (1976). Revised Effective Ionic Radii and Systematic Studies of Interatomic Distances in Halides and Chalcogenides, Ada Cryst. A, Vol.32, pp.751-767 Slater, J.C. (1930). Atomic Shielding Constants, Phys. Reo. Vol.36, pp. 57-64 Stoklosa, A Zajfcki, J. Kurek, S.S. (2004). Effective Nuclear Charge of the Ion, Materials Science-Poland, Vol. 22. No.l, 35-45... 

X 10 ppm amagaH. The coefficients a, G and g arise from two-, three- and four-body interactions, respectively. At low densities the shielding constant depends linearly on density, whereas at high pressures many-body collisions become important as well and cause deviation from linearity. The second virial coefficient arises from the Xe-Xe pair interactions (with the potential V(r), r is the interatomic separation) and can be presented... 

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