4. Acquisition

Time requirement: 4.5 h Sample: 3% strychnine in CDCl₃; for the best results the sample should be degassed.

Run a normal ¹H NMR spectrum of the sample and optimize the spectral width. Change to the 2D mode of the spectrometer software and load the NOESY pulse program. The length of the mixing time d2 is an adjustable parameter. For small organic molecules a trial value of | to 2 s is reasonable. You have to set:

td2: | k data points in F» tdl: 256 data points in F, sw2: 10 ppm swl: 10 ppm o1: middle of ¹H NMR spectrum d1:2s d2:2s p1, p2, p3: 90° ¹H transmitter pulse initial value for 7; evolution: 3 μs increment for ¢, evolution: 1/[2-sw1] ds: 2

5. Processing

Apply zero-filling in F,; to 512 real data points to obtain a symmetrical matrix of 512x512 real data points. Use exponential windows in F2 and F,. Apply real Fourier transformation corresponding to the TPPI mode of data acquisition in F\. Phase correction is usually only necessary in F>. Adjust the phase of the diagonal signals so that they are negative. The NOESY correlation signals will then be positive if the compound has a molar mass below 1000. Correlation signals caused by chemical exchange will have the same phase as the diagonal signals.

6. Result

The figure shows the result obtained on an AMX-500 spectrometer. Note that the phase of the diagonal signals is opposite to that of the cross-peaks as can be seen from the dotted contours. There is a wealth of information to be taken from the spectrum, which can best be studied using a molecular model. Notice, for instance, that only one of the H-20 protons has an NOE contact with one of the H-15 protons, from which a relative assignment of the protons in these methylene groups can be derived.