Chemical shift spectra obtained

Estimated precision in the chemical shifts is 0.05 p.p.m. The chemical shifts are given relative to external 1,4-dioxane, which was introduced into some samples only to obtain chemical shifts. Spectra obtained at 258 for — 10% solutions. Spectra of compounds were obtained at 22.5 MHz see Ref. 20. Spectra of compounds were obtained at 22.5 MHz see Ref. 24.J Spectrum obtained at 100.6 MHz see Ref. 24. Data taken from Ref. 61. Chemical shifts for GalNAc only are given. The data given in the parentheses for compounds 51 and 32 refer to the carbon count. 

Fig. 5. (a) Two-dimensional 1SN- H dipolar/chemical shift spectrum obtained from [lSN]acetylvaline showing the dipolar and chemical shift projections. Linewidths are typically 50-150 Hz for the dipolar and 0.5-1.0 ppm for the chemical shift dimension. vR = 1.07 kHz. (b) Dipolar cross-sections taken from the 2D spectrum. Each trace runs parallel to ivh through a particular rotational sideband in u>2. (i) Experimental i5N-H spectra from [15N]acetylvaline, vR = 1.07 kHz. The two simulations (ii and iii) assume two different orientations of the dipolar and shielding tensors, (ftD = 22°, D = 0°) and (Ai = 17°, aD = 0°), respectively, and illustrate the subtle differences in orientation which can be detected in the spectra. 

The ambiguity of the interpretation of the pnmr spectrum motivated Ray et al. (1971 and eaurlier papers) to determine and analyse the cnmr spectrum of the triphenylcarbonium ion (as well as some of its derivatives). The chemical shifts they obtained are given in Table 4, and a comparison of the empirical charge densities with those calculated by the HMO and CNDO techniques... 

Just as when acquiring a chemical shift spectrum, we would set the acquisition spectral width, or sweep width (SW) (in hertz), to the field of view Av (again in hertz). (Actually, we would set the spectral width a bit larger to avoid having the spectrum or image bump into the upfteld and downfteld Nyquist spectral limits.) The spatial resolution obtained is limited by the spectral resolution of the acquisition. If we... 

A higher level of sophistication involves obtaining a full chemical shift spectrum within each image pixel. 

For both Form 1 and 4, C chemical shifts were assigned with a natural abundance C— C INADEQUATE [50] NMR spectrum, which gives the connectivity between carbons that are bonded, see Figure 7.18. Proton chemical shifts were obtained from a H— C HETCOR NMR spectrum by their connection to the carbon nuclei previously assigned (Eig. 7.18). 

Motions much faster than the static coupling result in isotopic frequency spectra, such as the C chemical shift spectrum of l,2-dilauroyl-yn-glycero-3-phosphochoIine (DLPC) obtained using MAS shown in Figure 6.87 (Leftin and Brown 2011). 

Figure 24.13a shows the ID-NMR spectra of the poly(tetrafluoroethylene-co-propylene) poly(TFE-P) the complex peak patterns were mainly attributed to couplings which made peak assignment difficult. They obtained a F J-resolved (Figure 24.14) and F COSY (Figure 24.15) 2D-NMR spectra which helped with the assignments. In the J-resolved 2D-NMR spectrum, if the data are skewed about the /i =0 axis by 45° in frequency space, a spectrum with pure F chemical shift in the/2 dimension and pure Tpp coupling in the/i dimension is obtained. A projection of this 2D-NMR spectrum onto the/2 axis produces a pure chemical shift spectrum (with homonuclear couplings removed). This broadband homonuclear decoupled spectmm is shown across the top of the 2D-NMR plot in Figure 24.14, and is reproduced in Figure 24.13b below the normal F ID-NMR spectrum. The F broadband homonuclear decoupled spectmm shows eight resolved peaks numbered from 1 to 8 (-110.3(1), -112.6(2), -113.1(3), -115.2(4), -116.0(5),...

Figure 3-7. (a) Chemical structure of aliquid crystalline compound, 4-n-pentyl-4 -cyanobiphenyl (5CB). (b) Carbon-13 chemical shift spectrum of 5CB in the nematic phase obtained via cross-polarization under static condition. Thirteen carbon sites are well resolved, (c) A 2D proton-encoded local field spectrum of 5CB in the nematic phase under static conditions. H-C dipolar coupling spectra for each carbon site are shown. Reprinted from reference 47, with permission from the Journal of Physical Chemistry... 

Figure 18.16 One-dlmenslonal NMR spectra, (a) H-NMR spectrum of ethanol. The NMR signals (chemical shifts) for all the hydrogen atoms In this small molecule are clearly separated from each other. In this spectrum the signal from the CH3 protons Is split Into three peaks and that from the CH2 protons Into four peaks close to each other, due to the experimental conditions, (b) H-NMR spectrum of a small protein, the C-terminal domain of a cellulase, comprising 36 amino acid residues. The NMR signals from many individual hydrogen atoms overlap and peaks are obtained that comprise signals from many hydrogen atoms. (Courtesy of Per Kraulis, Uppsala, from data published in Kraulis et al.. Biochemistry 28 7241-7257, 1989.)...

Some of the most important 2D experiments involve chemical shift correlations between either the same type of nuclei (e.g., H/ H homonu-clear shift correlation) or between nuclei of different types (e.g., H/ C heteronuclear shift correlation). Such experiments depend on the modulation of the nucleus under observation by the chemical shift frequency of other nuclei. Thus, if H nuclei are being observed and they are being modulated by the chemical shifts of other H nuclei in the molecule, then homonuclear shift correlation spectra are obtained. In contrast, if C nuclei are being modulated by H chemical shift frequencies, then heteronuclear shift correlation spectra result. One way to accomplish such modulation is by transfer of polarization from one nucleus to the other nucleus. Thus the magnitude and sign of the polarization of one nucleus are modulated at its chemical shift frequency, and its polarization transferred to another nucleus, before being recorded in the form of a 2D spectrum. Such polarization between nuclei can be accomplished by the simultaneous appli-... 

One-dimensional spectra obtained by projecting 2D spectra along a suitable direction often contain information that cannot be obtained directly from a conventional ID spectrum. They therefore provide chemical shift information of individual multiplets that may overlap with other multiplets in the corresponding ID spectra. The main difference between the projection spectrum and the ID spectrum in shift-correlated spectra is that the projection spectrum contains only the signals that are coupled with each other, whereas the ID H-NMR spectrum will display signals for all protons present in the molecule. 

The mechanics of obtaining a 2D spectrum have already been discussed in the previous chapter. A ID H-coupled C-NMR spectrum contains both the chemical shift and coupling information along the same axis. Let us consider what would happen if we could somehow swing each multiplet by 90° about its chemical shift so that the multiplet came to lie at right angles to the plane containing the chemical shift information. Thus, if the ID spectrum was drawn in one plane—say, that defined by the NMR chart paper—then the multiplets would rotate about their respective chemical... 

Figure 5.5 shows the heteronuclear 2Dy-resolved spectrum of camphor. The broad-band decoupled C-NMR spectrum is plotted alongside it. This allows the multiplicity of each carbon to be read without difficulty, the F dimension containing only the coupling information and the dimension only the chemical shift information. If, however, proton broad-band decoupling is applied in the evolution period tx, then the 2D spectrum obtained again contains only the coupling information in the F domain, but the F domain now contains both the chemical shift and the coupling information (Fig. 5.6). Projection of the peaks onto the Fx axis therefore gives the Id-decoupled C spectrum projection onto the F axis produces the fully proton-coupled C spectrum.

In homonuclear 2D /-resolved spectra, couplings are present during <2 in heteronuclear 2D /-resolved spectra, they are removed by broad-band decoupling. This has the multiplets in homonuclear 2D /-resolved spectra appearing on the diagonal, and not parallel with F. If the spectra are plotted with the same Hz/cm scale in both dimensions, then the multiplets will be tilted by 45° (Fig. 5.20). So if the data are presented in the absolute-value mode and projected on the chemical shift (F2) axis, the normal, fully coupled ID spectrum will be obtained. To make the spectra more readable, a tilt correction is carried out with the computer (Fig. 5.21) so that Fi contains only /information and F contains only 8 information. Projection... 

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