The relaxation delay is the first step in any NMR experiment in which the initial equilibrium state is prepared to be modified by the effect of pulses and other delays. This period also defines the repetition rate of the experiment and must be taking in account when quantitative NMR work is required.

Delays are executed by the pulse program commands d0-d31, defined in seconds. In particular, d1 is always used as the relaxation period.

d1 is usually set to 1-5*T₁ of the nuclei to be perturbated. Thus, in experiments starting from a 90º ¹H pulse, d1 is set to 1-5*T₁(¹H). Usually d1 is set to 1-1.5 seconds for routine applications. On the other hand, in experiments starting from a 90º ¹³C pulse, d1 is set to 1-5*T₁(¹³C) which can be longer when quaternary carbons are involved. The duration of d1 is very important to define the total experimental time of the NMR experiment to be recorded.

The standard way to implement the relaxation delay in a pulse program is very simple:

... d1 ...

However, during the relaxation delay a series of specific settings are applied in order to achieve an specific goal. For instance:

• Specific power levels, transmitter or decoupler offsets, lists definitions, ... to be used later can be defined during the relaxation period. Such settings only require minimum delays (often defined by the d12 command, 20 us).
• Specific perturbations as, for instance, Solvent presaturation.
• Broadband or selective ¹H decoupling before a 90º ¹³C (or X) pulse, to improve sensitivity by means of heteronuclear NOE effects in heteronuclear experiments.

In all these cases, the overall pre-scan duration will be defined by the addition of all delays existing between the end of the FID acquisition (see the go command in any pulse program) and the first pulse creating transverse magnetixation to be applied.

Some examples: