Chemical orientation values

Legend RWj and RW2 = chemical orientation values 1 and 2 (explanation in Section 5.8) ... 

Chemical Orientation Value (from German Richtwert )... 

By comparison, a saturated methine carbon (C-H) has a CSA of only 25 ppm because the mobility of electrons around the carbon nucleus is much less in an sp3-hybridized carbon and depends much less on the orientation of the C-H bond with respect to B0. In solution-state NMR we only see the isotropic chemical shift, < iso, and the fixed-position chemical shifts and the CSA value are obtained from solid-state NMR measurements. Although CSA does not affect chemical shifts in solution, it does contribute to NMR relaxation and can be exploited to sharpen peaks of large molecules such as proteins in solution. For large molecules, such as proteins, nucleic acids, and polymers, or in viscous solutions, molecular tumbling is slow and CSA broadens NMR lines due to incomplete averaging of the three principle chemical shift values on the NMR timescale. Like isotropic chemical shifts, CSA in parts per million is independent of magnetic field strength B0 but is proportional to B0 when expressed in hertz. Because linewidths are measured in... 

Figure 15.4. The l3Cspectraofa single crystal of calcium formate in two orientations with respect to B(). In the top spectrum the crystal is oriented such that all eight distinguishable chemical shift tensors give different chemical shift values. In the lower spectrum the crystal is oriented such that Bo lies in a symmetry plane of the crystal structure, causing pairwise degeneracies of the chemical shift values.

Interpretation of 2 D NM R data obtained for cortistatin B (11) showed that it had the same skeleton as 10, but contained a hydroxyl group on C-16 with the (3 configuration. The carbon chemical shift value of 8 214.4 and the IR absorption at 1740 cm indicated the existence ofa ketone at C-16 in cortistatin C (12). Cortistatin D (13) was found to be a 17-hydroxy analog of cortistatin C, which was confirmed by "C NMR deuterium shift experiments as well as HMBC data. Analysis of NOESY data for 13 revealed the orientation of hydroxyl group at C-17 as a [13]. 

The shielding of the magnetic field felt at the nucleus is altered by the presence of electrons. If the electrons have directional character the chemical shift is also directional. Subsequently chemical shift values will be dependent on the orientation of the molecule with respect to the applied field and therefore be regarded as anisotropic. The chemical shift was found to be dependent on (3 cos2 -l)/2, where 0 is the angle between the magnetic field and the molecule-fixed direction. 

As a consequence, in a sample where there are many differently oriented molecules there will be a range of chemical shift values giving a very wide signal (Fig. 3.2.10). 

The latter definition follows a chemical orientation that it is also present in ISO 11843-1 in effect, ISO defines calibration as the complete set of operations which estimates under specified conditions the calibration function The calibration function is understood as the functional relationship between the expected value of a response variable and the value of the net state variable". It is worth observing note 3 of the ISO guide as it establishes that the calibration function is a conceptual feature that cannot be determined a priori but only estimated through calibration. 

The analysis of the chemical shift values and NOESY spectra of dimers provided important information about the tendency to accommodate their bicycUc moiety in an antiparallel stacked orientation (Figs. 26 and 27). This observation was considered to be very important for Dimer-l -l -Major in which the tetrahedral geometry of both the indene C-1 atoms appeared to force the indene moieties to stack... 

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