Chemical shift, in NMR

The example of B5H9 serves to show how the chemical shift may be used as an aid to determining the stmcture of a molecule and, in particular, in deciding between alternative stmctures. There are many examples in the literature of this kind of application which is reminiscent of the way in which the chemical shift in NMR spectroscopy may be employed. However there is one important difference in using the two kinds of chemical shift. In XPS there are no interactions affecting closely spaced lines in the spectmm, however close they may be. Figure 8.15 illustrates this for the C lx lines of thiophene. In NMR spectroscopy the spectmm becomes more complex, due to spin-spin interactions, when chemical shifts are similar. 

Just as chemical shifts in H NMR aie measured relative to the protons of tetrfflnethylsi-lane, chemical shifts in NMR are measured relative to the carbons of tetramethylsilane. Table 13.3 lists typical chemical-shift ranges for some representative types of carbon atoms. In general, the factors that most affect chemical shifts are... 

Parameters for the calculation of chemical shift in nmr spectra. Chemical shifts in nmr spectra. 

This mechanism arises from the asymmetry of the shielding tensor. Let us recall that the chemical shift in NMR has its origin in the screening (shielding) of the static magnetic field B0 by the electronic distribution at... 

The Cl shift of Mossbauer spectra provides a measure of the total electron density in the nucleus of an atom, i/r(o), relative to some reference standard. For Fe, 8 is quite sensitive to valence, coordination number, or number and type of bonds to its ligands. Unlike chemical shifts in NMR spectra which are frequently dominated by the spin... 

Fig. 5. Scheme of the synthesis of a "model hematoporphyrin and the two possible dimers that can be derived from it. The ester dimer has a very different chemical environment for the two a H, marked x, while in the ether dimer 6, the two a-H would have the same chemical shift in NMR ... 

S 1. Fourth letter in the Greek alphabet hence, used to denote the fourth item in a series (for example, the fourth methylene carbon in a fatty acid). 2. Symbol for thickness. 3. Symbol for chemical shift in NMR. 

Interelectronic interactions that alter how any particular electron in a multi-electronic atom interacts with the nucleus and vice versa. These effects lead to so-called chemical shifts in NMR experiments, thereby providing valuable structural information concerning a molecule s bonding and conformation. 

NUCLEAR MAGNETIC RESONANCE Chemical shifts in NMR experiments, SHIELDING... 

Chemical shifts, in NMR, 4 234-245 Chemical transport reactions, and synthesis of chalcogenide halide compounds, 23 330-332. 364, 368-369... 

The relation between UV- and PES-spectra is quite similar as found for planar aromatic compounds. All chemical shifts in NMR-spectra can well be explained by normal ring-current effects and Van der Waals interactions. Polarographic data do not deviate from those for planar compounds. 

Though these codes failed to characterize uniquely the molecular structure54), they are useful for ordering structures as basis for systematic searches for regularities in molecular data 55), fragment56) and ring S7) search, for recognition of structural similarity 54> in molecules, for the prediction of chemical shift in NMR-data, etc. 

Chemical Shifts in NMR. The first effect is very useful in chemical analysis The nuclear spin transitions are affected by the "hyperfine" I S coupling between electron spin S (for any single electron in the molecule that has density at the nuclear position) and nuclear spin I this is due to the isotropic Fermi contact term ... 

The main parameters derived from ESR spectra are the g-values (ana-lagous to the chemical shift in NMR), the hyperfine coupling constants (like spin-spin coupling constants), line-widths and intensities. Because spectra are usually displayed as first derivatives (Fig. 3.2), line-widths are conveniently measured as the distances between points of maximum and minimum slope (a in Fig. 3.2). Intensities are often quoted as the total extent of each line (/ in Fig. 3.2), but this can be misleading, and it sometimes helps to integrate to give the normal absorption spectrum. 

Solvent effects on the fluorescence spectrum and the effect of protonation on electron density and chemical shift in NMR spectrum of pyridazine were investigated. 

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