4-band center

Figure 2. Cs MAS NMR spectra of Cs-exchanged hectorite sample dehydrated at 500°C before analysis at temperatures from 80 to -80°C. The labeled peaks near -120 and +30 ppm are true center bands, and the other peaks are spinning sidebands. After reference 22.

FIGURE 39. MAS 13C NMR spectra of (a) 13C-11, (b) 13C-12 and (c) 13C-13 rhodopsin. Center-bands and rotational sidebands of the retinal resonances are marked with asterisks. Reprinted with permission from Reference 55. Copyright (1990) American Chemical Society... 

Like the mycoplasma, the E. coli cell is bounded by an 8-nm membrane which consists of -50% protein and 50% lipid. When "stained" (e.g., with permanganate) for electron microscopy, this single membrane appears as two very thin (2.0 nm) dark lines separated by an unstained center band (-3.5 nm) (Fig. 1-4 see also Fig. 8-4). Single membranes of approximately the same thickness and staining behavior occur in all cells, both of bacteria and of eukaryotes. 

Fig. 19. Proton. NMR spectrum at 220 He of ferricytochrome c. Different scales were used for the different spectral regions. The high field line at +23.2 ppm was observed as an inversed resonance of the center-band of the spectrum (the HR-220 operates with a 104 cps field modulation, and one usually observes the first upfield side-band. With the occurrence of large hyperfine shifts the center band and the side bands sometimes overlap). Heme c and the axial ligands of the heme iron are shown at the bottom. (Reproduced from ref. (777))...

Figure 1. 199Hg MAS NMR spectra recorded in 4.7T field (35.79MHz). The center bands (denoted by ) of the full spectra are given in the right panels together with the Gaussian fits.

Fig. 2 shows that relative intensities of the center band lines of the sample 1 fit well to the random scenario, whereas those of the sample 3 indicate deviation from the random distribution. Here the large intensity of the line B suggests that the oxygen atoms tend to be separated by at least one lattice period. 

C2 units are also found in solid-state compounds with C-C separations that depend on formal electron count. These are viewed as deprotonated ethyne, ethylene or ethane using a popular solid-state idea the Zintl-Klemm concept. This concept is based on the simple idea that the metals transfer their valence electrons to the non-metal atoms thereby generating filled anion-centered bands at low energy, well separated from empty cation-based bands. Of course, this concept fails when the electronegativities of the metal and non-metal are not very different,... 

These resonances can be detected as very low intensity satellites on either side of the center band that is derived from molecules containing only isolated atoms. For bonded pairs of y is about 30-50 Hz, and the satellites are separated from the center band by half that amount. Coupling also may be present over two or three bonds ( 7, J) in the range of about 0-15 Hz. Not only are these satellites low in intensity and possibly obscured by the center band, but also, spinning sidebands, impurities, and other resonances may get in the way. 

BIRD-HMQC. The most difficult aspect of implementing the HMQC experiment is the suppression of signals from protons attached to C (the center-band or single quantum coherences) in favor of the protons attached to C (the satellites or double quantum coherences). The use of pulse field gradients (PFG, Section 6-6) is the most effective technique, but relatively few spectrometers are equipped with the hardware required for their generation. Fortunately, there is an effective alternative for the suppression of center bands by means of the BIRD Bilinear Rotation Decoupling) sequence, which is outlined by the vector... 

Finally, let us reexamine the 2D INADEQUATE experiment (Section 6-4) in terms of its coherence-level diagram (Figure A6-9). This experiment utilizes pulses only at the frequency of the insensitive nucleus, such as C. The initial 90° pulse generates single quantum coherence, and the periods t are selected to produce an antiphase disposition of " C nuclei with C coupling partners. The center-band signal from C nuclei lacking adjacent... 

Figure 15. Al NMR spectra (taken at Vo = 104.2 MHz, without MAS) of ciyohte polycrystalline powder, with vertical exaggeration to emphasize the +(3/2,1/2) and +(5/2,3/2) satelhte transitions. Decrease of spectral width with increasing temperature corresponds to decrease of Cq (Eqn. 3). In die P-phase all the transitions occur in the narrow center band, indicating Cq = 0 and average cubic symmetry at the Al-position. [Used by permission of the editor of Physics and Chemistry of Minerals, from Spearing et al. (1994), Fig. 6, p. 378, Springer-Verlag 1994.]...

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