Carbon nuclei data analysis

A general equation can be derived that describes the variation in direction of the valence electron density about the nucleus. The distortion from sphericity caused by valence electrons and lone-pair electrons is approximated by this equation, which includes a population parameter, a radial size function, and a spherical harmonic function, equivalent to various lobes (multipoles). In the analysis the core electron density of each atom is assigned a fixed quantity. For example, carbon has 2 core electrons and 4 valence electrons. Hydrogen has no core electrons but 1 valence electron. Experimental X-ray diffraction data are used to deri e the parameters that correspond to this function. The model is now more complicated, but gives a better representation of the true electron density (or so we would like to think). This method is useful for showing lone pair directionalities, and bent bonds in strained molecules. Since a larger number of diffraction data are included, the geometry of the molecular structure is probably better determined. 

However, the direct measurement of carbon-13 is clearly preferable. The basic problem was that, compared with proton NMR, the sensitivity of C-NMR was lower by a factor of about 6000, but this was overcome with the introduction of Fourier transform NMR [127]. Conventional frequency NMR is extremely inefficient since only one frequency is observed at any given instant. However, using a short pulse radiofrequency, all the nuclei in a sample can be excited simultaneously. The absorption of individual frequency components by each nucleus are detected by the receiver and these are abstracted by Fourier transformation using data acquisition and processing equipment. The use of C-NMR as a tracer, especially in biosynthetic studies, has now become a very versatile tool. Not only can the ultimate fate of carbon-13 be determined, but unlike its radioactive counterpart, carbon-14, the location of the label and the molecular structure can often be assigned without the necessity for degradation of a complex compound. As a cross check, the level of carbon-13 incorporation may frequently be confirmed by mass spectrometric analysis. 

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