1H-X refocused INEPT followed by a X-S INEPT

Multidimensional NMR experiments must be specifically designed in order to reduce the number of pulses and the length of total experiment time to avoid sensitivity losses due to rf field inhomogeneities and T₂ relaxation effects. A widely used building block in a three-spin system is one consisting of two following sequential steps:

• X evolution period of heteronuclear ¹J_HX coupling constant.
• X evolution period of heteronuclear ¹J_XS coupling constant.

This approach allows i) to refocuse antiphase magnetization (2I_zX_x ) to in-phase S magnetization (X_y) and ii) to prepare anti-phase S magnetization wirth respect to the thirs spin (-2S_zX_x) by a subsequent transfer step.The same pathway can be used in the reverse sense.We can design a concatenated approach in which one of the two 180º X pulses is avoided and the overall duration of the sequence is minimized.

An alternative approach can also be designed by incorporating ¹H-decoupling.

An example is the ¹⁵N chemical shift evolution period in a 3D HNCO (and 3D HNCA experiment) in which ¹J_NH is refocused at the end of the 2*d5 delay and antiphase ¹⁵N magnetization with respect to ¹³CO is ready for heteronuclear polarization transfer.

Easy implementation on AVANCE spectrometers.

The general way to implement such an approach is:
 

Original	Concatenated
...  d4  (p2 ph2) (p4 ph1):f2  d4  d5  (p4 ph3):f2 (p22 ph4):f3  d5  ...	...  d4  (p2 ph2)  d20  (p4 ph1):f2 (p22 ph4):f3  d4  ...

in which:

• d4 and d5 are the evolution delays optimized to 1/(4*J_XH) and 1/(4*J_XS), respectively (in seconds), and d20=d5-d4
• p2 is the 180º ¹H  pulse (in microseconds) applied at a power level pl1 from the f1 channel.
• p4 is the 180º X pulse (in microseconds) applied at a power level pl2 from the f2 channel.
• p22 is the 180º S pulse (in microseconds) applied at a power level pl3 from the f3 channel.
• All phases may be specified at the end of pulse program.