Chemical shift anisotropy, carbon

From the NMR data of the polymers and low-molecular models, it was inferred that the central C—H carbons in the aliphatic chain in polymer A undergo motions which do not involve the OCH2 carbons to a great extent. At ambiet temperatures, the chemical shift anisotropy of the 0(CH2)4 carbons of polymer A are partially averaged by molecular motion and move between lattice positions at a rate which is fast compared to the methylene chemical shift interaction. 

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. 

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... 

The carbon-proton dipolar interaction and the chemical shift anisotropies broaden the lines in solid state 13C NMR spectra. The major effect arises from the dipolar coupling of the carbon nuclei with neighboring protons homonuclear dipolar couplings between two adjacent 13C nuclei are neglegible because of their low natural abundance. The large magnitude of dipolar 13C— H coupling (up to 40 kHz) results in broad and structureless proton-coupled 13C NMR absorptions. 

As is seen from Eq. (4), chemical shift (unlike dipolar coupling) is field dependent. This has important consequences for nuclei with large chemical shift anisotropies, such as13C. While chemical shift anisotropy (CSA) for sp3 carbons is 15-50 ppm, it is 100-200 ppm for sp and sp2 carbons. In an applied field of 1.41 T, 200 ppm translates as 3.02 kHz, but in the very large field of 11.74 T (in a 500 MHz magnet), it translates as 25.2 kHz. Since the sample must be spun at a rate comparable with the size of the CSA (in hertz) it is... 

The resulting principal values of the 13C chemical shift tensors of the C60 carbons are 8n = 228 ppm, 822 = 178 ppm, and 833 = -3 ppm. Tycko et al reportet the experimental values 8n = 213 ppm, S22 = 182 ppm, and 833 = 33 ppm obtained from low temperature measurements of a powder pattern spectrum (18). However, the spectra have a low signal to noise ratio and a wide slope so that a larger error for the experimental value can be assumed. The chemical shift anisotropy of 217 ppm corresponds quite well with the spectral range of about 200 ppm reported by Kerkoud et al for low temperature single crystal measurements (19). 

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-... 

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