Carbon-13 nuclear magnetic chemical shifts

Inasmuch as establishing the carbon framework of a molecule overcomes a major obstacle to structure determination, carbon nuclear magnetic resonance ( C-NMR) spectroscopy is potentially the most powerful spectroscopic implement for natural products characterization. The broad range of chemical shifts are valuable in determining the presence of different functional groups (49, 84, 85, 235, 321,... 

The section on Spectroscopy has been retained but with some revisions and expansion. The section includes ultraviolet-visible spectroscopy, fluorescence, infrared and Raman spectroscopy, and X-ray spectrometry. Detection limits are listed for the elements when using flame emission, flame atomic absorption, electrothermal atomic absorption, argon induction coupled plasma, and flame atomic fluorescence. Nuclear magnetic resonance embraces tables for the nuclear properties of the elements, proton chemical shifts and coupling constants, and similar material for carbon-13, boron-11, nitrogen-15, fluorine-19, silicon-19, and phosphoms-31. 

Fig. 22. Carbon-13 nuclear magnetic resonance of carbonyl carbon chemical shifts on complexation of K.+.

If one wishes to obtain a fluorine NMR spectrum, one must of course first have access to a spectrometer with a probe that will allow observation of fluorine nuclei. Fortunately, most modern high field NMR spectrometers that are available in industrial and academic research laboratories today have this capability. Probably the most common NMR spectrometers in use today for taking routine NMR spectra are 300 MHz instruments, which measure proton spectra at 300 MHz, carbon spectra at 75.5 MHz and fluorine spectra at 282 MHz. Before obtaining and attempting to interpret fluorine NMR spectra, it would be advisable to become familiar with some of the fundamental concepts related to fluorine chemical shifts and spin-spin coupling constants that are presented in this book. There is also a very nice introduction to fluorine NMR by W. S. and M. L. Brey in the Encyclopedia of Nuclear Magnetic Resonance.1... 

Csaszar, A. G. 1992. Conformers of Gaseous Glycine, J. Am. Chem. Soc. 114,9568-9575. de Dios, A. C., J. G. Pearson, and E. Oldfield. 1993. Chemical Shifts in Proteins An Ab Initio Study of Carbon-13 Nuclear Magnetic Resonance Chemical Shielding in Glycine, Alanine, and Valine Residues, J. Am. Chem. Soc. 115, 9768-9773. 

As we shall see, all relaxation rates are expressed as linear combinations of spectral densities. We shall retain the two relaxation mechanisms which are involved in the present study the dipolar interaction and the so-called chemical shift anisotropy (csa) which can be important for carbon-13 relaxation. We shall disregard all other mechanisms because it is very likely that they will not affect carbon-13 relaxation. Let us denote by 1 the inverse of Tt. Rt governs the recovery of the longitudinal component of polarization, Iz, and, of course, the usual nuclear magnetization which is simply the nuclear polarization times the gyromagnetic constant A. The relevant evolution equation is one of the famous Bloch equations,1 valid, in principle, for a single spin but which, in many cases, can be used as a first approximation. 

The fourth chapter in this volume, contributed by Helmut Duddeck, is an exceptionally thorough survey of substituent effects on carbon-13 nuclear magnetic resonance (NMR) chemical shifts. Organic chemists and others who are routinely dependent on 13C NMR for structure elucidation and for information about stereochemistry will welcome the summary presented here. Although... 

The H and nuclear magnetic resonance (NMR) chemical shift of all the parent structures are fully reported in CHEC-II(1996) <1996CHEC-II(6)447>. Since then, the complete proton and carbon chemical shift assignments have been made for 2- and 3-formyl, acetyl, or methyl phenoxathiin <1996PJC36>. 

In an attempt to relate calculated results to experimental findings for monomeric, lignin model compounds, preliminary work has compared theoretically determined electron densities and chemical shifts reported from carbon-13 nuclear magnetic resonance spectroscopy (62). Although chemical shifts are a function of numerous factors, of which electron density is only one, both theoretical and empirical relationships of this nature have been explored for a variety of compound classes, and are reviewed by Ebra-heem and Webb (63), Martin et al. (64), Nelson and Williams (65), and Farnum (66). 

---