A hard 90º ¹³C pulse can be the starting point in any heteronuclear multidimensional NMR experiment requiring transverse ¹³C magnetization. A 90º_x(¹³C) pulse converts the pre-equilibrium longitudinal (S_z) magnetization to transverse magnetization (-S_y) that usually evolves under the effects of chemical shifts and/or coupling constants during the following predefined delay.

However, the major drawbacks of such an approach are the sensitivity and relaxation effects. If ¹³C excitation is desired, it is usually performed by means of different options:

• Using a preparation NOE-buildup period in which broadband ¹H irradiation (WALTZ-16) is applied before the 90º ¹³C pulse. A simple routine ¹H-decoupled ¹³C spectrum is a clear example. In this case, the relaxation period is defined as 1-5*T₁(¹³C).
• Using a sensitivity-improved polarization-transfer pulse scheme as, for instance, the INEPT or DEPT pulse trains. In this case, the relaxation period is defined as 1-5*T₁(¹H). However, the final result can be carbon magnetization in anti-phase or in-phase states depending if J refocusing delays are included.
• Using a heteronuclear cross-polarization transfer. This is widely used in solid-state NMR techniques and several applications in liquid NMR have also been reported.

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 most often used as a 90º and 180º ¹³C pulses at power level pl1, respectively, when the f1 channel is defined for ¹³C (¹³C-detected experiments). However, in inverse experiments, all carbon pulses are delivered from the decoupler or f2 channel and, in this case, p3 and p4 are most often used as a 90º and 180º ¹³C pulses, respectively, at power level pl2.

The excitation bandwith of a 90º ¹³C 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.

Accurate ¹³C pulse calibration via transmitter and/or decoupler is required. For further details about pulse calibrations in AVANCE spectrometers see Practical Tutorials: Pulse Calibrations.

The most standard way to implement an initial 90º ¹³C pulse in a pulse program is:

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

in which d1 is the relaxation delay (in seconds), p1 is the 90º ¹³C pulse (in microseconds) applied at a power level pl1 from the f1 channel, and ph1 is the phase specified at the end of pulse program. 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:

... d1 p1 ph1 ...

When ¹³C pulses are delivered from the decoupler, the syntax must specify from which channel the pulses amd power level are applied:

...    d12 pl2:f2  (p3 ph1):f2 ...

in which the p3 is the 90º ¹³C pulse (in microseconds) applied at a power level pl2 from the channel f2 or decoupler. In this case, the 90º ¹³C pulse length can be slightly different compared to the analogous observe ¹³C pulse. See high-power 90º ¹³C decouple pulse calibration for more details.

 See some examples: