2D spectrum

P correlation of two inorganic hydrated phosphates, (a) bnishite and (b) a bone, are shown [48]. In both cases two phosphorus sites that completely overlap in the 1D MAS spectrum are clearly visible in the 2D spectrum. 

Each cross peak has x and y coordinates One coordinate corresponds to the chem real shift of a proton the other to the chemical shift to a proton to which it is coupled Because the diagonal splits the 2D spectrum m half each cross peak is duplicated on the other side of the other diagonal with the same coordinates except m reverse order This redundancy means that we really need to examine only half of the cross peaks To illustrate start with the lowest field signal (8 2 4) of 2 hexanone We assign fhis signal a friplef fo fhe protons af C 3 on fhe basis of ifs chemical shifl and fhe spin fmg evidenf m fhe ID speefrum... 

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-... 

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.

Figure 5.6 If proton broad-band decoupling is applied in the evolution period, t, then the resulting 2D spectrum contains only chemical shift information in the F, domain, while both chemical shift and coupling information is present in the F domain. Projection onto the /-j-axis therefore gives the H-decoupled C spectrum, whereas projection along F. gives the fully coupled C spectrum.

Figure 7.1 Selective excitation of only one multiplet by a selective pulse transforms a 2D experiment into a ID technique. A selective pulse generates the transverse magnetization. The result is a trace of the corresponding 2D spectrum. (Reprinted from Mag. Reson. Chem. 29, H. Kessler ei al., 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)...

Diagonal peaks Cross-sections of peaks that appear on or near a diagonal line in a 2D spectrum. The projection produces the ID spectrum. They give no shift[Pg.413]

Dispersion mode A Lorentzian line shape that arises from a phase-sensitive detector (which is 90 out of phase with one that gives a pure-absorption-mode line). Dispersion-mode signals are dipolar in shape and produce long tails. They are not readily integrable, and we need to avoid them in a 2D spectrum. 

Fx axis The axis of a 2D spectrum resulting from the Fourier transformation of the tx domain signal. 

Projection The one-dimensional spectrum produced when one of the two axes of a 2D spectrum is collapsed and the resulting projection spectrum recorded. 

Zero-filling A procedure used to improve the digital resolution of the transformed spectrum (e.g., in the tj domain of a 2D spectrum) by adding zeros to the FID so that the size of the data set is adjusted to a power of 2. Zero-quantum coherence The coherence between states with the same quantum number. It is not observable directly. 

The H,H COSY spectrum of model compound 1 is shown in Fig. 24. In fact you can see a total of three spectra the central square which is the actual 2D spectrum and two proton ID spectra at the top and on the left. The computer software generates this combination of spectra automatically using a previously recorded ID proton spectrum. 

Since phosphorus and protons are both abundant spin-1/ nuclei, it is simple to design an experiment in which we correlate protons and phosphorus rather than protons with themselves. The result of this experiment, a P,H correlation, is shown in Fig. 26. Again we have the 2D spectrum in the form of a central rectangle and two (previously recorded) ID spectra parallel to the axes. One is the proton spectrum, the other the phosphorus spectrum. The latter of course consists of a single line, and in the 2D spectrum we do not need to look for a diagonal as there cannot be one. 

By now we are used to the appearance of such spectra, and again the central rectangle contains the actual 2D spectrum, while the carbon spectrum (decoupled) is shown on the left and the proton spectrum at the top. 

Figure 29 shows the P,C correlation for compound 1 carried out by selecting a J value of 15 Hz. The decoupled phosphorus signal is shown at the top, the proton decoupled carbon-13 spectrum on the left. The actual 2D spectrum... 

The first thing that we can see is that the 2D spectrum is not decoupled with respect to the phosphorus the methine carbon C-P doublet in the 13C spectrum is associated with a doublet along the phosphorus axis, from which can of course be extracted. 

Now, as in the H,H correlation, the actual 2D spectrum forms the central square, and the ID phosphorus spectra are depicted above and to one side of the square. There is a diagonal and there are cross peaks. 

Figure 3 2D 27Al-29Si RAPT-CP-CPMG HETCOR data for zeolite ZSM-4 recorded at 14.1 T with 10 kHz MAS. The 2D spectrum was acquired in 30 rows of hypercomplex data, with 64K scans per row, h increments of 100 ps, and acquisition time of 38 hours. In the right, ID slices taken through the centers of Ah and Al2 resonances are compared with ID 29Si MAS spectrum.

Fig. 11 (a) 2D NCO experiment with optimal control element inserted for 15N — 13C transfer. Transfer efficiencies for the ocNCO experiment optimized for 12 kHz spinning speed as function of (b) resonance offsets for 13C and 1SN and (c) rf inhomogeneity/adjustment in terms of scaling factors on the nominal rf field strengths for 13C and 15N. (d) Experimental ocNCO 2D spectrum of uniformly 13C,15N-labeled ubiquitin with the projections to the left comparing ocNCO experiment most intense) and DCP (less intense) based NCO experiments [reproduced with permission from [161] (a, d) and [164] (c)]... 

The signals S+ and S are now amplitude-modulated as a function of tp, therefore, a double hypercomplex Fourier transformation of these data, following for instance the States-Haberkom-Ruben procedure, yields a pure-absorption 2D spectrum with sign discrimination in the 12 j dimension [169]. 

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