The amide ¹⁵N nucleus N-9 (dy = – 233) couples with one of the protons 11, whereas the other one, presumably because of the small coupling constant (Karplus relation- ship), does not give a correlation signal. The tertiary amine nitrogen (dy = – 345) does show correlation signals to all protons that are separated by two bonds (16, 18 and 20) and furthermore a correlation signal over three bonds to one of the protons 15.

7. Comments

The experiment uses nearly the same parameters as explained in Experiment 12.4. ¹⁵N GARP decoupling was not applied for the same reasons as outlined in Experiment 12.5. Note that the gradient ratios are quite different, since the ratio of the gyromagnetic ratios of ¹⁵N and ¹H is about | : 10. It is advantageous to use the exact gradient ratios as extracted from the carrier frequencies in both channels of the instrument. Double-quantum magnetization, which in the coherence pathway diagram is labeled as Ii IX, is first dephased with the relative gradient strength g1 = 55, corresponding to ti + © 11. The 180° pulse in the proton channel transforms the coherence into Iqly. At this stage, the relative sum of ~values is -9. During acquisition, only 1); is present, with a relative +value of –10. Thus, with the relative gradient strength used and applying the exact ratios, Equation (1) yields 20.14 for the last gradient g3.

g1 (i+ wn) + 82(-m1 + wm) +23 (-m) = 0 (1) Gradients should not only select the desired coherences but also most efficiently dese- lect the undesired ones, and there are computer programs which perform this task. An- other often-used gradient ratio for the ¹⁵N HMQC experiment is 70 : 30 : 50.

8. Own Observations