Diagonal peak

Suppression of the tme diagonal peaks by double-quantum filtering (DQF-COSY) may resolve such problems. Finally, quantitative measurements of the magnitude of the coupling constants is possible using the Z-COSY modification, These experiments ate restricted to systems of abundant spins such as H, and which have reasonably narrow linewidths. 

Figure 18.17 Two-dimensional NMR spectnim of the C-terminal domain of a cellulase. The peaks along the diagonal correspond to the spectrum shown in Figure 18.16b. The off-diagonal peaks in this NOE spectrum represent interactions between hydrogen atoms that are closer than 5 A to each other in space. From such a spectrum one can obtain information on both the secondary and tertiary structures of the protein. (Courtesy of Per Kraulis, Uppsala.)...

Figure 5.27 Schematic diagram of a COSY spectrum illustrating how intrinsically different cross-peaks are from diagonal peaks. The circles represent the cross-peaks in an AB coupled system. The diagonal peaks have dispersion shape, and the crosspeaks have an absorption shape though they alternate in sense. The diagonal peaks are centered at d 8,i) and (8 , 8 ) the cross-peaks appear at (8., 8 ) and (8 , 8 ).

One problem associated with COSY spectra is the dispersive character of the diagonal peaks, which can obliterate the cross-peaks lying near the diagonal. Moreover, if the multiplets are resolved incompletely in the crosspeaks, then because of their alternating phases an overlap can weaken their intensity or even cause them to disappear. In double-quantum filtered COSY spectra, both the diagonal and the cross-peaks possess antiphase character, so they can be phased simultaneously to produce pure 2D absorption line... 

Homonuclear shift-correlation spectroscopy (COSY) is a standard method for establishing proton coupling networks. Diagonal and off-diagonal peaks appear with respect to the two frequency dimensions. 

Fi and F. The off-diagonal peaks (cross-peaks) represent the direct coupling interactions between protons. Working through cross-peaks, one can easily correlate protons that are coupled to each other. Several versions of the COSY experiment have been designed to get optimum performance in a variety of situations (such as DQF COSY, COSY-45°, and COSY-60°). 

The SECSY spectrum of buxapentalactone displays a number of off-diagonal peaks. Pairs of interconnected cross-peaks can be identified... 

Cross-diagonal peaks (o>i = Cross-diagonal peaks (oj. = w,) Back-transfer peaks Diagonal peaks... 

Figure 6.4 Three-dimensional spectrum of a three-spin system showing peak types appearing in a three-dimensional space. Three diagonal peaks, six (wi = Wj) and six (wj = w,) cross-signal peaks, six back-transfer peaks, and six cross-peaks are present in the cube, (a) The cubes (b-d) represent three planes in which crossdiagonal peaks and the back-transfer peaks appear on their respective (atj = 0)2), u>2 = cof), and ( >i = Wj) planes. (Reprinted from J. Mag. Reson. 84, C. Griesinger, et al., 14, copyright (1989), with permission from Academic Press, Inc.)...

A 3D spectrum of a three-spin subsystem in which all the nuclei are coupled to one another, such as C(H/i)(Hb)-C(Hc), will lead to 27 peaks, comprising six cross-peaks, 12 cross-diagonal peaks (six at o) — o>2 and the other six at 0)2 = (O3), six back-transfer peaks, and three diagonal peaks. However, in the case of a linear three-spin network (e.g., CH -CHg-CHc), the number of peaks will depend on whether two equal (e.g., COSY-COSY or NOESY-NOESY) or unequal (e.g., COSY-NOESY) mixing processes are... 

So a simplified 3D spectrum is obtained having eight peaks, i.e., one crosspeak, four cross-diagonal peaks (two at V] = V2 and two at V2 = vj), three diagonal peaks, and no back-transfer peaks (Fig. 6.5b). 

Cross-peaks The off-diagonal peaks in a 2D experiment that appear at the coordinates of the correlated nuclei. 

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]

Fig. 9.12 (A) Cross-relaxation rates with respect to the correlation time and the spectrometer frequency cOq. (B) While only positive NOE (resulting in cross-peaks with inverted sign compared to the diagonal peaks, left) are present in a mixture of free ligands, the addition of the receptor leads to the...

Now let us look at the NOESY spectrum (b) just as in COSY, we can identify a diagonal and a series of associated off-diagonal cross peaks. Thus the interpretation of the results is analogous to the method we have already learned for COSY. However, the cross peaks are not due to spin-spin coupling but to NOE effects between the protons concerned. However, if we look more closely we can see one big difference between the diagonal peaks, which look like irregular circles, and the cross peaks, which look just like all the peaks in the COSY spectrum. 

Schematic COSY spectrum of a two coupled spins, denoted A and X. For convenience, the normal one-dimensional spectrum is plotted alongside the F and F2 axes and the diagonal (F t = F2) is indicated by a dashed line. This spectrum shows two types of multiplets those centred at the same F t and F2 frequencies, called diagonal-peak multiplets, and those centred at different frequencies in the two dimensions, called cross-peak multiplets. Each multiplet has four component peaks. The appearance of a cross-peaked multiplet centred at I = A, F2 = 8x indicates that the proton with shift A is coupled to the proton with shift A. This observation is all that is required to interpret a COSY spectrum.

As with the COSY experiment, the sequence starts with a pulse followed by an evolution period, but now the mechanism that couples the two spins (which must be in close proximity, typically <6 A) is the Nuclear Overhauser Effect (NOE). The second pulse converts magnetization into population disturbances, and cross-relaxation is allowed during the mixing time. Finally, the third pulse transfers the spins back to the x-y-plane, where detection takes place. The spectrum will resemble a COSY spectrum, but the off-diagonal peaks now indicate through-space rather than through-bond interactions. 

This molecule exists of isomers of the complex shown in Figure 11.7 that inter-convert rapidly, even at temperatures below 200 K, as shown by the off-diagonal peaks between the double doublets of one structure and the broad doublet of the other structure [23]. The isomeric structures are caused by the relative positions of the two ligands with respect to each other (i.e., parallel or anti-parallel orientation of the NMe2 groups). 

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