Sample: 5% 2,3-dibromopropionic acid in [D.]benzene.

Record a standard ¹H NMR spectrum and optimize the spectral width. Change to the 2D mode of the spectrometer software and load the pulse program for phase-sensitive E.COSY with TPPI mode. Note that the pulse sequence shown here can only be performed on spectrometers capable of phase shifts less than 90°. You have to set:

td2: 2 k data points in F, td1: 256 data points in F, sw2: 1.5 ppm swl: 1.5 ppm o1: middle of ¹H NMR spectrum p1, p2, p3: 90° ¹H transmitter pulse dl: 2s d2: 4 μs initial value for ¢, evolution: 3 μs increment for ¢, evolution: 1/[2-sw1] ds: 2 ns: 12

5. Processing

Apply zero-filling in F, to 1 k real data points to obtain a symmetrical matrix of 1024x1024 real points. Use exponential windows in both dimensions. Apply real Fou- rier transformation corresponding to the TPPI mode of data acquisition in F;. Phase correction in F; is usually not necessary.

6. Result

The figure shows an expansion of the phase-sensitive E.COSY spectrum obtained on an AMX-500 spectrometer, displaying the diagonal peak for H-3a (upper left corner) and three cross-peaks connecting H-3a with H-2 (lower left corer), H-3b with H-3a (upper right corner) and H-3b with H-2 (lower right corner). The dotted contour lines represent negative signals, the solid contour lines positive signals. In this type of COSY spectrum the active coupling (the one that causes the cross-peak) is in anti- phase, whereas the passive one disappears. From the appropriate cross-sections the three-spin coupling constants J(H-3a,H-2) = 11.0 Hz, J(H-3b,H-3a) = –10.1 Hz, and J(H-3b,H-2) = 4.6 Hz can be measured. As in Experiment 10.6, the relative sign of the coupling constants can be taken from the slope of the cross-peaks.

7. Comments

A disadvantage of the E.COSY sequence is that it is less sensitive than the COSY-45 procedure. Furthermore, if there are four- and three-spin systems in the same molecule it is better to perform the sequence twice, adapted to the spin system in question. There exists a complementary E.COSY sequence with a different phase cycle, yielding in