Chemical shift anisotropy aromatic carbon

In an organic solid representative broadenings are 150 ppm for aromatic carbon chemical shift anisotropy and 25 kHz (full width at half-height) for a rather strong carbon-proton dipolar interaction. At a carbon Larmor frequency of 15 MHz, the shift anisotropy corresponds to 2.25 kHz. In high magnetic fields the forms of the respective Hamiltonians are... 

As regards the phenyl ring motions, interesting features are obtained from measurements of 13C chemical shift anisotropy of protonated and unpro-tonated aromatic carbons. The chemical shift parameters are orthogonal to each other, with o and 072 in the phenyl plane and <733 bisecting the phenyl plane. 

It can be seen in Fig. 14 that, as the temperature increases, there is a decrease in the chemical shift anisotropy (<733-an) of protonated carbons, indicating an increase in the mobility of the aromatic rings at higher tern-... 

Fig. 14 Chemical shift anisotropy (< 33— 11) for the protonated (O) aromatic carbons and unprotonated ( ) aromatic carbons in PET (from [12])...

As seen from Fig. 15, in the case of unprotonated aromatic para carbons, a phenyl ring yr-flip does not change the orientation of the chemical shift anisotropy tensor components and, thus, this yr-flip does not affect the NMR response of such para carbons, whereas phenyl ring oscillations do. 

In contrast, for protonated ortho and meta aromatic carbons, both phenyl ring yr-flips and oscillations affect the orientation of the chemical shift anisotropy tensor components and, consequently, their NMR response. The change in the amplitude of the oscillations with temperature for protonated and unprotonated aromatic carbons is shown in Fig. 16. For the two types of carbons the amplitudes of the oscillations are quite similar. 

Fig. 15 Effect of a phenyl ring yr-flip on the orientation of the chemical shift anisotropy tensor components for unprotonated and protonated aromatic carbons...

The temperature dependence of the phenyl ring motions has also been studied by 13C NMR at two different frequencies, 22.6 MHz [32] and 62.9 MHz [34], through the chemical shift anisotropy of the aromatic proto-nated carbons. 

C chemical shift anisotropies of the unsaturated carbons. The chemical shift parameters used in the Ar-Al-PA for the unprotonated and proto-nated aromatic carbons, as well as for the carbonyl groups, are shown in Fig. 80. 

As regards the aromatic rings in MT, the chemical shift anisotropy of the unprotonated aromatic para carbons are only sensitive to oscillations, whereas the protonated aromatic ortho and meta carbons reflect both oscillations and phenyl ring yr-flips. The oscillation amplitudes of these two types of carbons are shown in Fig. 83. In contrast, the t /2 values of the protonated aromatic carbons reflect the ring flips. The fraction of phenyl rings undergo-... 

Mao, J. D., and Schmidt-Rohr, K. (2004). Separation of aromatic-carbon BC NMR signals from di-oxygenated alkyl bands by a chemical-shift-anisotropy filter. Solid State Nucl. Magn. Reson. 26, 36—45. 

Finally, the aromatic carbon chemical shift anisotropy (R j spin-lattice relaxation rates are determined from Eq. (4.4-10). Equation (4.4-11) is then used to calculate the chemical shift anisotropy (Au) for an axially symmetric chemical-shift tensor. 

The chemical shift anisotropy arises from the nonspherical electron density around the nuclei, and is particularly prominent for aromatic and carbonyl (C=0) carbon types. These carbon types experience different shieldings of the magnetic field depending on whether the bond axes are parallel or perpendicular to the external magnetic field. For polycrystalline or amorphous materials all orientations are possible, including these two extremes. 

The effect of magic-angle spinning on the C NMR spectra of oil shales is shown in Fig. 1. The improvement in resolution is clearly evident, particularly for shales that have a sizable aromatic component. This is because aromatic carbons have a larger chemical shift anisotropy than the aliphatic carbons. In the absence of spinning, the aromatic carbons yield broad anisotropic line shapes. Note also that for the spinning spectra, the maximum intensity of the aromatic band shifts to the isotropic value, which lies between the perpendicular and parallel components of the chemical shift tensor. 

The chemical shift anisotropies for the carbonyl and aromatic carbons of Hytrel were reconstructed from a Herzfeld-Beiger analysis (24) of the intensities of the sidebands from NMR magic angle spinning experiments. The results in Table III indicate that the carbonyl carbon chemical shift anisotropy is axially symmetric for each terephthalate ester. We attribute this axial symmetry to a general property of terephthalate esters, rather than as a consequence of molecular motion, as the highly crystalline dimethyl terephthalate also has an axially symmetric carbonyl carbon chemical shift tensor. 

Chemical shift anisotropies for aliphatic carbons are 15-50 ppm, but as much as 120-200 ppm for aromatic and carbonyl sp -type carbons (since directional variation in electron density is larger within multiple bonds). 

The chemical shift anisotropies (CSA) of nuclei in different structural environments [8] vary from about 30 ppm for CH2 carbons to about 200 ppm for aromatic carbons. Unlike the dipolar interactions between and spins, the strength of the CSA depends on the strength of the applied magnetic field Bq, because the strength of the local magnetic field Bq experienced by a nucleus depends on the strength of the electronic currents in the vicinity... 

A typical example of a partial motional averaging of the chemical shift anisotropy is that of an unprotonated aromatic carbon belonging to a para-substituted phenyl ring rotating about its local symmetry axis. For such a... 

---