Information from chemical shifts

Some chemical information from chemical shifts, line shapes, etc. Not often fully exploited. 

ID spectrum ID Information from chemical shifts, coupling constants, integrals... 

Information from chemical shifts 4.5.1 General principles... 

These are the most widely studied NMR spectra, the proton being the nucleus with the highest sensitivity. Information from chemical shifts, spin-spin coupling and peak areas enables the structural features of organic compounds to be recognized and their identities established. 

In principle all the X-ray emission methods can give chemical state information from small shifts and line shape changes (cf, XPS and AES in Chapter 5). Though done for molecular studies to derive electronic structure information, this type of work is rarely done for materials analysis. The reasons are the instrumental resolution of commercial systems is not adequate and the emission lines routinely used for elemental analysis are often not those most useftil for chemical shift meas-ure-ments. The latter generally involve shallower levels (narrower natural line widths), meaning longer wavelength (softer) X-ray emission. 

NMR provides one of the most powerful techniques for identification of unknown compounds based on high-resolution proton spectra (chemical shift type integration relative numbers) or 13C information (number of nonequivalent carbon atoms types of carbon number of protons at each C atom). Structural information may be obtained in subsequent steps from chemical shifts in single-pulse NMR experiments, homo- and heteronuclear spin-spin connectivities and corresponding coupling constants, from relaxation data such as NOEs, 7) s 7is, or from even more sophisticated 2D techniques. In most cases the presence of a NOE enhancement is all that is required to establish the stereochemistry at a particular centre [167]. For a proper description of the microstructure of a macromolecule NMR spectroscopy has now overtaken IR spectroscopy as the analytical tool in general use. 

The 13C NMR spectra of compounds Via, b, and c were more informative. The chemical shifts of carbon resonances in 13C NMR spectra have been used as a measure of the electron densities of the carbon atoms being studied (55), and so a comparison of the carbinyl carbon resonances of compounds Via, b, and c with those of the alcohols from which they were... 

Finally, NMR-derived distance information as well as information about dihedral angles (obtained from chemical shifts) is incorporated into structure calculations performed using molecular dynamics and simulated annealing programs such as CNS (48) and XPLOR-NIH (49) to calculate the protein stmcture. 

The experimental methods presented in Part A permit us to determine the number of each type of carbon present in a molecule and the spatial relationships between protons, which are critical to the elucidation of the two- and, perhaps, three-dimensional structure of that molecule. As we have seen, this new information, combined with that obtained from chemical shifts, coupling constants, and other spectroscopic methods, might be sufficient to determine... 

Thus, the chemical shift itself is an incredibly rich and precise source of information. Indeed, these shifts can be thought of as a unique fingerprint of the molecule under the conditions of the measurement. The obvious question that arises is why a 3D structure cannot be derived from chemical shifts. In theory, there is no reason why this should not be possible however, in practice, the ab initio models required for accurately calculating the above contributions in a dynamic and complex system such as a protein, in particular when solvent effects are considered, are beyond our current computational capabilities. All is not lost however, as the fact that proteins consist of a limited number of residue types that are connected through the same repetitive bonding structure allows for derivation of vastly simplified empirical models for approximating the abovementioned complex contributions. 

---