A hard 90º ¹H pulse is the most typical starting point in any homo- and heteronuclear multidimensional NMR experiment basically due to the sensitivity and relaxation effects. A 90º_x(¹H) pulse converts the pre-equilibrium longitudinal (I_z) magnetization to transverse magnetization (-I_y) that usually evolves under the effects of chemical shifts and/or coupling constants during the following predefined delay. In some cases, other pulse flip angles can be applied.

The excitation bandwith of a 90º ¹H pulse depends of its duration and, therefore, of the applied power level. In AVANCE spectrometers, usually short pulses (microseconds) are achieved at maximum power level. On the other hand, multiplet-selective excitation would require more longer pulses (milliseconds) at lower power levels.

Easy implementation on AVANCE spectrometers. Pulses are executed by the pulse program commands p0-p31, which execute a pulse on the specified channel during the respective duration parameter. p1 and p2 are the 90º and 180º pulses from the observer f1 channel, respectively. Otherwhise, p3 and p4 are the 90º and 180º pulses from the decoupler f2 channel, respectively.

Accurate 90º ¹H pulse calibration via transmitter and/or decoupler is required. For further details about pulse calibrations in AVANCE spectrometers see Practical Tutorials: Pulse Calibrations.

The standard way to implement an initial 90º ¹H pulse in a pulse program is (see mlevph):

... d1 d12 pl1 (p1 ph1):f1 ...

in which:

• d1 is the relaxation delay (in seconds)
• p1 is the 90º ¹H pulse (in microseconds) applied at a power level pl1 from the f1 channel
• ph1 is the phase specified at the end of pulse program, and depends of each experiment.
• A short delay d12 (20 usec) is usually needed to set the pl1 value.

However, in some cases, both power level and channel syntax can be omitted because they are applied by default from the f1 channel at pl1 power level (see zg):

... d1 p1 ph1 ...

When ¹H pulses are delivered from the decoupler, the syntax can be different:

...  d1   d12 pl2:f2  (p3 ph1):f2 ...

in which the p3 is the 90º ¹H pulse (in microseconds) applied at a power level pl2 from the channel f2 or decoupler. In this case, the 90º pulse length can be slightly different compared to the analogous observe pulse. See high-power 90º ¹H decouple pulse calibration for more details.

See some examples: