Chemical shift of nucleus

The anisotropy in the chemical shift of nucleus i (aia) is related to the chemical shift tensor and the elements of the order matrix, as shown in Equation 2.7. 

This sequence of events can be represented schematically in a circle (Fig. 7.8) with the rotation rate ( 2i) in the center. This is simply the rotation of the two opposed (antiphase) vectors in the x -y plane at the frequency determined by the chemical shift of nucleus I. The Sz part just goes along for the ride because the antiphase relationship is retained throughout. Stand up with your arms outstretched at your sides your right arm represents the 1H net magnetization vector (13C = a) on the +x axis, and your left arm represents the... 

Evolution Indirectly measure the chemical shift of nucleus A. 

The second step in processing the 2D data is to perform a second Fourier transform on each of the columns of the matrix. Most of columns will represent noise, but when we reach a column which falls on an F2 peak, transformation of the t FID gives a spectrum in F, with a peak at the chemical shift of nucleus A (Fig. 9.15). The final 2D spectrum is a matrix of numerical values that has a pocket of intensity at the intersection of the horizontal line F = 2a and the vertical line F2 = 2b and has an overall intensity determined by the efficiency of transfer of magnetization from nucleus A to nucleus B. This efficiency tells us something about the relationship (/ value or NOE intensity) between the two nuclei... 

Using Goodman s notation, suppose you have experimental NMR data on diastereomers A and B and the computed NMR shifts for structures a and b. The key is deciding whether A correlates with a or b and the same for B. Goodman proposed three variants on how to compare the chemical shift differences, and we will discuss here the simplest version, called CPI. The difference in the chemical shift of nucleus I between the two diastereomers is... 

Since is a powerftil radiofi equency field, it causes saturation of nucleus A, i.e. ftie change of spin state of nucleus A is very rapid. Under these conditions, nucleus X does not exhibit signals corresponding to interaction wifti the two spin states of A but only a single time-averaged line is observed and this resonance signal therefore represents the true chemical shift of nucleus X. 

Total concentration of polymer ionic groups (monomol dm" ) Concentration of simple salt Degree of substitution of ionic dextrans Chemical shift of nucleus A (ppm)... 

In contrast to the HMQC and the HSQC experiment, transversal magnetization of the nucleus 2, which is not detected, will be created in the beginning of the polarization transfer experiment (Figure Ic). In the following evolution time this magnetization is modulated with the chemical shift of nucleus 2. Since nucleus 1 is detected, the overall sensitivity in this experiment is related to the factors therefore, the choice of the detected nucleus is... 

This expression for the frequency differences, however, still depends on the magnetic field Bq. In order to have a common unit at all Bq fields, the chemical shift of nucleus i is defined by... 

Let us now consider the two terms of this equation separately for their evolution during t2. The operation of the chemical shift of nucleus i on the in-phase term of eq. A6-21d gives... 

The secondary isotope effect, AY(X), is the difference between the chemical shift of nucleus Y for the two types of molecules substituted with either the light or the heavy isotope, n is the number of bonds between the isotope and the nucleus in question (Y). Y could typically be or The light and heavy pairs are... 

The chemical shift of a nucleus depends in part on its spatial position in relation to a bond or a bonding system. The knowledge of such anisotropic effects is useful in structure elucidation. An example of the anisotropic effect would be the fact that axial nuclei in cyclohexane almost always show smaller H shifts than equatorial nuclei on the same C atom (illustrated in the solutions to problems 37, 47, 48, 50 and 51). The y-effect also contributes to the corresponding behaviour of C nuclei (see Section 2.3.4). 

Energy differences between conformations of substituted cyclohexanes can be measured by several physical methods, as can the kinetics of the ring inversion processes. NMR spectroscopy has been especially valuable for both thermodynamic and kinetic studies. In NMR terminology, the transformation of an equatorial substituent to axial and vice versa is called a site exchange process. Depending on the rate of the process, the difference between the chemical shifts of the nucleus at the two sites, and the field strength... 

Another important component of Table 18 is substituent chemical shift (SCS) datadenvedforeachoftheSlentnes The SCS is simply thedifferencein F-NMR chemical shifts of the substituted compounds and that of unsubstituted fluoroben zene (-113 5 ppm in CDCI3) These values numerically represent the mfluence a substituent has on the shieldmg or deshieldmg of the fluorine nucleus and depend upon substituent position o, m, orp) Fluonne chemical shifts can be predicted for polysubstituted fluorobenzene systems simply by addmg the SCS value of each substituent to a base value of -113 5 ppm... 

By using a system of measurement in which NMR absorptions are expressed in relative terms (parts per million relative to spectrometer frequency) rather than absolute terms (Hz), it s possible to compare spectra obtained on different instruments. The chemical shift of an NMR absorption in 8 units is constant, regardless of the operating frequency of the spectrometer. A H nucleus that absorbs at 2.0 8 on a 200 MHz instrument also absorbs at 2.0 8 on a 500 MHz instrument. 

Chemical shift (Section 13.3) The position on the NMR chart where a nucleus absorbs. By convention, the chemical shift of tetramethylsilane (TMS) is set at zero, and all other absorptions usually occur downfield (to the left on the chart). Chemical shifts are expressed in delta units. 5, w here 1 5 equals 1 ppm of the spectrometer operating frequency. 

The application of NMR spectroscopy to tacticity determination of synthetic polymers was pioneered by Bovey and Tiers.9 NMR spectroscopy is the most used method and often the only technique available for directly assessing tacticity of polymer chains. "2 7 8 0JI The chemical shift of a given nucleus in or attached to the chain may be sensitive to the configuration of centers three or more monomer units removed. Other forms of spectroscopy (e.g. TR spectroscopy l2 lJ) are useful with some polymers and various physical properties (e.g. the Kerr effect14) may also be correlated with tacticity. 

Some of the most important 2D experiments involve chemical shift correlations between either the same type of nuclei (e.g., H/ H homonu-clear shift correlation) or between nuclei of different types (e.g., H/ C heteronuclear shift correlation). Such experiments depend on the modulation of the nucleus under observation by the chemical shift frequency of other nuclei. Thus, if H nuclei are being observed and they are being modulated by the chemical shifts of other H nuclei in the molecule, then homonuclear shift correlation spectra are obtained. In contrast, if C nuclei are being modulated by H chemical shift frequencies, then heteronuclear shift correlation spectra result. One way to accomplish such modulation is by transfer of polarization from one nucleus to the other nucleus. Thus the magnitude and sign of the polarization of one nucleus are modulated at its chemical shift frequency, and its polarization transferred to another nucleus, before being recorded in the form of a 2D spectrum. Such polarization between nuclei can be accomplished by the simultaneous appli-... 

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