Understanding the HSQC Pulse Sequence

Now that we know most of the basic building blocks of NMR pulse sequences, we should be able to use the coherence flow diagram (Fig. 11.9) to design an HSQC pulse sequence. It needs to accomplish the following steps  

Let the 13C magnetization rotate in the jd-yf plane for a period t, allowing us to indirectly measure the 13C chemical shift (evolution). 

Observe the H magnetization directly fe) so that the lH chemical shift can be determined (detection). 

Note that we do not start directly with 13 C magnetization because we want to take advantage of the larger (by a factor of 4) equilibrium population difference of lH compared to13 C, as well as the shorter T (faster relaxation) of 1H, which will permit shorter relaxation delays. We now know a lot of tricks, and the main one we need here is the heteronuclear INEPT transfer  

Evolution period simply insert a delay of t s and repeat the experiment many times with increasingly larger t delays. Increment t each time by At = 1/(2 x swl), where swl is the spectral width in the 13C dimension in hertz. 

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The HSQC experiment cannot be explained readily by using spin gymnastics arguments. To understand the HSQC and other more complex pulse sequences, we should consult other texts to learn the product operator formalism and density matrix theory. 

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