180º pulses placed into heteronuclear pulse sequences can be used to refocus a desired chemical-shift, homo- or heteronuclear J-coupling information, depending of the position into the pulse sequence and on the nuclei that they are applied.

If refocusing 180º pulses are applied on both ¹H and ¹³C nuclei, both ¹H and ¹³C chemical shifts are refocused at the end of the echo period while ¹H-¹H and  ¹H-¹³C coupling constants are not affected.
 

Some typicals examples are i) the variable evolution period in the standard 2D SPIN-FLIP experiment which can be used to measure heteronuclear coupling constants and ii) the evolution periods in INEPT and refocused INEPT pulse trains.

Easy implementation on AVANCE spectrometers.

There are 32 duration parameters, d0-d31,  to be specified in seconds. They are executed as delays without any further actions by the pulse program commands d0. They can be incremented or decremented using the pulse program commands id0-id31 or dd0-dd31, respectively. The changes are given by the parameters IN0-IN31. The pulse program commands rd0-rd31 reset a respective delay to its original value. The delays may also be changed by means of arithmetic expressions during pulse program execution.

The most standard way to implement a variable heteronuclear spin-echo t₁ period should be:

... d0 (p2 ph1) (p4 ph2):f2 d0 ...

in which:

• d0 is the incremented delay (in seconds).
• p2 is the 180º X  pulse (in microseconds) applied at a power level pl1 from the f1 channel.
• p4 is the 180º ¹H pulse (in microseconds) applied at a power level pl2 from the f2 channel.
• All phases may be specified at the end of pulse program.