The Chemical Shift and Shielding

Circulation of electrons around a nucleus can be viewed as being similar to the flow of an electric current in an electric wire. From physics, we know that the flow of a current through a wire induces a magnetic field. In an atom, the local diamagnetic current generates a secondary, induced magnetic field that has a direction opposite that of the applied magnetic field. 

FIGURE 5.10 Diamagnetic anisotropy—the diamagnetic shielding of a nucleus caused by the circulation of valence electrons. 

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The shift from TMS for a given proton depends on the strength of the applied magnetic field. In an applied field of 1.41 Tesla the resonance of a proton is approximately 60 MHz, whereas in an applied field of 2.35 Tesla (23,500 Gauss) the resonance appears at approximately 100 MHz. The ratio of the resonance frequencies is the same as the ratio of the two field strengths  

The chemical shift in 5 units expresses the amount by which a proton resonance is shifted from TMS, in parts per million (ppm), of the spectrometer s basic operating frequency. Values of dfor a given proton are always the same irrespective of whether the measurement was made at 60 MHz (Bo = 1-41 Tesla) or at 100 MHz (Bq = 2.35 Tesla). For instance, at 60 MHz the shift of the protons in CHsBr is 162 Hz from TMS, while at 100 MHz the shift is 270 Hz. However, both of these correspond to the same value of S(2.70 ppm)  

VARIATION OF H EXCESS NUCLEI WITH OPERATING FREQUENCY 

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Hg(CN)2 in the solid state has a structure (I42d neutron diffraction), completely different from that of Cd(CN)2 Almost-linear molecules (r(Hg—C) 201.9, r(C—N) 116.0pm (corrected for thermal motion) a(C—Hg—C) 175.0°) are arranged such that four secondary bonds N" Hg (274.2 pm) yield the often-occurring 2 + 4 coordination around Hg.103 Analysis of the 199Hg MAS NMR spectrum of Hg(CN)2 has yielded the chemical shift and shielding tensor parameters.104... 

Nnclear Magnetic Moments 106 Absorption of Energy 107 The Mechanism of Absorption (Resonance) 109 Popnlation Densities of Nnclear Spin States 111 The Chemical Shift and Shielding 112 The Nnclear Magnetic Resonance Spectrometer 114... 

Chemical Shifts and Shielding Effects.—Phosphorus-31. The positive shifts (<5p) which are reported in this chapter are upfield from 85 % phosphoric acid. 

The complications that arise in solid state NMR spectra as compared to NMR spectra in solution are the consequence of the fixed orientation of the sample relative to the external magnetic field Bq. Mainly three interactions are responsible for the enormous linebroadening that can be observed for solid powder samples. These are (1) the shielding or chemical shift, including the chemical shift or shielding anisotropy CSA or Acr Hqsa) (2) homo- and/or heteronuclear dipole-dipole coupling (ffoo) and (3) in addition, for nuclei with spin >1/2, the quadrupolar interactions (Hq). 

The HREIMS of 2,7-dibromocarbazole (388) was identical to that of 3,6-dibromocarbazole (386). Differences were identified in the chemical shifts and splitting pattern of the H-NMR spectrum. Thus, a deshielded, two-proton doublet at 7.87 with ortho-coupling (/=8.3 Hz), assignable to the C-4 and C-5 protons, indicated the location of two bromine atoms at the C-2 and C-7 positions of the carbazole nucleus. This was also evident from the shielded ortho- and meto-coupled (/=8.3, 1.5 Hz) two-proton double doublet at S 7.36, assignable to the C-3 and C-6 protons. Further, a mutually meto-coupled (/= . 5 Hz) two-proton doublet at S 7.57, assignable to the C-1 and C-9 protons, supported the location of the two bromine atoms ortho to these carbons. Based on these spectral analyses and the HREIMS identical to that of... 

Both the chemical shift and the relaxation rate may serve as an experimental observable in studies directed towards obtaining structural and thermodynamic information on cation binding. There are very few such studies in the literature making use of direct observation of alkaline earth resonances. The choice of the two possible experimental parameters depends on the sensitivity of either quantity to environmental changes. From early studies of the concentration dependence of shieldings in aqueous magnesium salts (104)... 

The interaction between the orbital magnetic moment of the electrons and the nuclear moment can also be separated into an isotropic part (the chemical shift) and a traceless symmetric part (the shift anisotropy). The equation for the shift tensor contains two terms, usually called the dia- and paramagnetic contribution, but only the sum of the two corresponds to a physical quantity. Actually, the theory (76) is concerned with the shielding important difference between the two... 

More recently it has been shown that in addition to Pi the nephe-lauxetic ratio )3 [)3 = B/Bg, Bg is the B value for the free ion in the gas phase and is equal to 1120 cm for Co(III) (173) ] has an effect on the shielding experienced by the cobalt nucleus. Juranic (179, 180) and Bramley et al. (173) found that there is a linear relationship between the chemical shift and )3 vi. The correlation is rather good for cubic complexes (Oh symmetry of donor atoms) but does not hold as well for distorted complexes (such as the tris-chelates). Co NMR data are shown in Table III. 

Chemical Shifts and Shielding Effects.—Phosphorus-31. The sign convention used for expressing shifts in this Report is not the same as was used in earlier volumes. Positive chemical shifts are now downfield from 85 % phosphoric acid, and are given without the appellation p.p.m. Since both conventions are in use, it remains necessary to state the sign convention used in each paper published. 

Although this theory predicts the temperature dependence of the metal chemical shifts, it also predicts, for example, that an isotope shift should be independent of the remoteness of substitution, since only the vibrational frequencies of the whole molecule are considered. In practice a large dependence of the isotope on the position of substitution is observed experimentally. A theory which successfully explains both the intrinsic temperature dependence of the chemical shift and the observed isotope shifts is based on the expansion of the nuclear shielding as a function of powers of displacement coordinates. The intrinsic temperature-dependent nuclear shielding can be expressed as ... 

Chemical Shifts and Shielding ElSects.—Phosphorus-31. In this section, positive P chemical shifts (dp) are upheld from 85% phosphoric acid. A linear relationship between dp and vapour pressure for elemental phosphorus in the gas phase has been reported. Solvent effects have also been examined. dp compounds. The n.m.r. parameters of various protic and deuteriated... 

It should be noted that the negative sign of the shielding constant cr indicates deshielding, and so shielding variations can be compared with the experimental chemical shift 8 where a positive sign denotes deshielding. Such a chemical shift map successfully predicts the chemical shifts and conformations of the L-alanine (L-Ala) residues in polypeptides and proteins. For example, the experimental chemical shift for the a-helix form appears at lower frequencies by —5.5 ppm from that for the j8-sheet form. The calculated map reasonably predicts the experimental data. 

Molecular structures may be deduced from the chemical shifts and integrated intensities. These shifts are relatively constant, except when they are influenced by hydrogen bonding, and may, to some extent, be predicted in terms of short- and long-range shielding by other parts of the molecule. 

Chemical Shifts and Shielding Effects.—Chemical shifts are usually given without the appellation p.p.m. The scope of n.m.r. pulsed methods has been reviewed. ... 

As explained briefly in Chapter 1, variation in how much an NMR-active nucleus is shielded by its electron cloud from the applied magnetic field determines the frequency of the photons that induce transitions between allowed spin states. Because the resistance of the electron cloud to the applied field varies in direct proportion to the applied field strength, the frequency units cancel and we are left with a unitless quantity called the chemical shift and denoted 6. 

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