Magnitude mode

Figure 3.10 Effect of different window functions (apodization functions) on the appearance of COSY plot (magnitude mode), (a) Sine-bell squared and (b) sine-bell. The spectrum is a portion of an unsymmetrized matrix of a H-COSY I.R experiment (400 MHz in CDCl, at 303 K) of vasicinone. (c) Shifted sine-bell squared with r/4. (d) Shifted sine-bell squared with w/8. (a) and (b) are virtually identical in the case of delayed COSY, whereas sine-bell squared multiplication gives noticeably better suppression of the stronger dispersion-mode components observed when no delay is used. A difference in the effective resolution in the two axes is apparent, with Fi having better resolution than F. The spectrum in (c) has a significant amount of dispersion-mode line shape.

If phase-sensitive spectra are not required, then magnitude-mode Pico) (or absolute-mode ) spectra may be recorded by combining the real and imaginary data points. These produce only positive signals and do not require phase correction. Since this procedure gives the best signal-to-noise ratio, it has found wide use. In heteronuclear experiments, in which the dynamic range tends to be low, the power-mode spectrum maybe preferred, since the S/N ratio is squared and a better line shape is obtained so that wider window functions can be applied. 

The frequency-domain spectrum is computed by Fourier transformation of the FIDs. Real and imaginary components v(co) and ifi ct>) of the NMR spectrum are obtained as a result. Magnitude-mode or powermode spectra P o)) can be computed from the real and imaginary parts of the spectrum through application of the following equation ... 

So far, we have talked about phasing 1-D spectra but this is also valid for some 2-D experiments. Phase-sensitive 2-D experiments also require phasing in one or both dimensions. Similar approaches are used as described here. Note that this is not necessarily the case for all 2-D experiments as some of them are collected in magnitude mode where we look at only the intensity of the signals, not their sign. 

Figure 27 Edited broadband HMBC spectrum of cyclosporine using the pulse sequences shown in Figure 26 in an interleaved manner. The two subspectra, CH + CH3 (left) and C + CH2 (right), exemplify the editing properties. The spectrum in the bottom displays the two subspectra, CH + CH3 (black) and C + CH2 (grey) in the same frame. The number of scans was 32 for each of the 128fi increments, the relaxation delay was 1 s, and the range for the third-order low-pass. /-filter was 115 Hz < Vch < 165 Hz. The spectra were processed to maintain the absorptive profiles in F, while a magnitude mode was done in F2.

X/Y coherence transfer steps as in the original HNCA experiment.41 As in the previous case, X nuclei are selectively irradiated by low power rectangular or shaped pulses, and coherence selection is accomplished by the matched pulsed field gradients Gb G2 and further assisted by a spoil gradient Gs. Owing to the need to avoid -pulses in the X-channel, the spectra are processed in magnitude mode in the fft dimension. 

In its basic (magnitude mode) version the length of PO is adjusted either to maximize the sensitivity (PO = 90° ) or to yield structured cross peaks (PO = 45° ). In the latter case information regarding the relative signs of coupling constants may be deduced. 

The final 2D spectrum has a projection onto the F2 axis which is the normal H-decoupled "C spectrum with the quaternary carbon.s missing, and a projection onto the Fl axi,s which is the normal H spectrum, including "J, coupling. Thi.s experiment is not phase-sensitive, and must be displayed in magnitude mode. 

Fig. 3.29a 2D spectrum of the C-detected, magnitude mode C/H-COSY experiment... 

The cross peaks in the 2D spectrum are a combination of absorption and dispersion lineshapes and consequently spectra are displayed in magnitude mode. 

Load the 2D COSY (magnitude mode) spectrum of peracetylated glucose D NMRDATA GLUCOSE 2D HH GHHCO 001999.RR. The 2D spectrum appears in the default contour display mode and shows only positive cross peaks. Use the mouse to select a suitable expansion. 

Now load the basic 2D magnitude mode H/ H COSY spectrum... 

Use again the 2D magnitude mode H/ H COSY spectrum D NMRDATA GLUCOSE 2D HH GHHCO 001999.RR and switch to the Stacked display mode. Select a region of interest. Use the Grid button in the button panel and the mouse to move and tilt the 3D spectrum according to your ideas. Set up your output device and plot the expanded COSY spectrum in stacked display mode. 

Check it in 2D WIN-NMR C/H-long range-COSY (HMBC, magnitude mode)... 

Try out the effect of several modes for DC correction available with 2D data. Load the raw data of the magnitude mode 2D COSY spectrum D NMRDATA GLUCOSE 2D HH GHHCO 001001.SER and choose no, quad and qpol as BC mod in F2. Note that usually for Fl no baseline correction is applied in the time-domain, i.e. BC mod (F1) = no. Fourier transform the data and store the individual spectra using ascending processing numbers. Compare corresponding rows to inspect the effect of different baseline corrections. 

Load the raw data of the 2D magnitude mode COSY experiment of... 

Fig. 8.16. Aromatic ring part of the COSY spectra of the complex shown in the inset. (A) Magnitude mode spectrum (B) phase-sensitive spectrum, absorption mode (C) phase-sensitive spectrum, dispersion mode (D) ISECR [20] spectrum (sequence in Fig. 8.2D). Sin2 weighting functions have been used for spectra (A)-(C) and a cos2 weighting function for spectrum (D). Peaks in (A) and (D) are in phase and positive. The positive components of the 6-5 peak in (B) and (C) are shown in the enlargements.

---