Herzfeld-Berger analysis

Fig, 16. 50.1-MHz 13C MAS spectra of benzaldehyde-a-13C and benzene reacting on zeolite HY. The spectrum acquired at 120 K after the sample was heated at 448 K clearly shows an isotropic chemical shift at 207 ppm, consistent with the chemical shift of the trityl cation. Furthermore, the Herzfeld-Berger analysis of the sideband intensities reveals an axially symmetric tensor, thus providing unambiguous evidence for the trityl cation 16. 

It has to be stressed that the Herzfeld-Berger analysis is restricted to relatively simple molecules, since it is necessary to be able to resolve, at a low spinning rate, the sidebands of different resonances. As the number of distinct resonances increases, the ID spectrum becomes increasingly more crowded (Figure 8). 

The spinning-side band (SSB) analysis has been applied [26-28], i.e., the Herzfeld-Berger analysis, to determine the exact principal values of the carbonyl carbons. The tensor components of the other samples are determined in the same manner. It has been reported that 5n is in the amide sp plane and lies along the direction normal to the C=0 bond, the 622 component lies almost along the amide C=0 bond, and the S33 component is aligned perpendicular to the amide sp plane [29]. 

Reconstruction of Chemical Shift Anisotrc s. The principal values of the chemical shift tensors for the carbonyl, protonated aromatic, and non protonated aromatic carbons were determined by a Herzfeld-Berger analysis (24) of the intensities of the sidebands from magic angle spinning (14). Herzfeld and Berger have defined two parameters, p and p, which are related to the chemical shift tensor elements en, 0-22, and <733 by ... 

RF pulse and spinning speed effects/ (iv) When the second-order quadrupolar interaction is much smaller than the chemical shift anisotropy, the quadrupolar parameters can be estimated by monitoring the frequency shift of the centre band as a function of the Bo field and the chemical shift anisotropy can be determined by Herzfeld—Berger sideband analysis/ We shall discuss the applicabihty of these methods to Co systems in turn. 

Experimentally, the isopropyl cation was prepared by the low temperature reaction of 2-bromopropane-2-13C with frozen SbF5. The 13C spectrum was measured at 83 K using slow speed magic angle spinning. Analysis of the spectrum using the method of Herzfeld and Berger yielded tensor values of 8n = 497 ppm, 822 = 385 ppm and 833 = 77 ppm for the central carbon, which results in an isotropic chemical shift of 320 ppm. 

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