RF pulses are the most important elements into NMR pulse sequences. A RF pulse applied from a specific axis (x or y) rotates the magnetization around its perpendicular plane in accordance to the following counter-clockwise convention:   Using the PO formalism, the following general expressions can be applied:

 
Thus, using the classical vectorial description, a RF pulse applied from the x axis rotates the magnetization aroung the yz plane as shown:

In particular, the effect of a 90 degree angle pulse applied from the x axis (specified as 90_x) rotates the z magnetization (I_z) to the -y axis (-I_y):

On the other hand, the effect of a RF pulse on each single component of a product operator may be treated separately. Thus, a 90 degree pulse from the x axis converts I_1yI_2z to  -I_1zI_2y .