Off-diagonal peaks

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.)...

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

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). 

The NOESY spectrum has a diagonal set of peaks (open circles) as well as peaks which are off the diagonal (filled circles). The off-diagonal peaks occur at positions where a proton on the Fi axis is close in space to a one on the F2 axis. In the schematic spectrum on the right, the off-diagonal signals show that Ha must be located near Hd and Hb must be located near He. 

Fig. 4. Locations of the diagonal and off-diagonal peaks in the 2D-IR spectra in the rephasing (upper) and nonrephasing (lower) quadrant of (to, to,)- (A) single-frequency (B) two pulses one centered at coi and the other at ton- D and d are the diagonal and off-diagonal anharmonicities, respectively. The cross peaks near (I, II) and the (II, II-D) peaks are omitted for clarity.

Fig. 10.4 An example of NOE connectivities in a DNA molecule (top). These connectivities may be seen in the spectrum as off-diagonal peaks or crosspeaks . A section of the spectrum is shown at the bottom. The stronger the peak in the spectrum, the closer the two atoms are in the space.

Figure 6 shows the calculated absolute value of the 2D PE signal S( 22, 2i). The resonances representing various correlations between one-exciton states (—ea, ea) (diagonal peaks) and (sa, sb) a / b and a, b = 1,..., 5 (off-diagonal peaks) as well as between one- and two-exciton states (—ea, b — a ), a, b = 1,..., 5 (cross-peaks) are well resolved in model A. Model B has the same resonant energies, but since the homogeneous broadening is comparable with the anharmonicity and coupling energies, many resonances are unresolved. Five slices of the 2D spectra along the 0.2 direction at 2i = — ea, a = 1,..., 5, for models A and B are displayed in Fig. 7. The well-resolved resonances in model A are marked in the plot and identified in Table 1 the broader curves represent model B.

---