In the routine procedure of R.F. pulse optimization, distorted nutation curves can sometimes be observed, as can be seen in the calculated graph below. This is usually due to the short recycle delay time (D1) not sufficient for complete relaxation. With increasingly shorter delay time, the maximum of the nutation curve shifts to lower flip angle, which makes the curve asymmetric. Under this circumstance, the 90 degree pulse determined from the maximum of the intensity will be inaccurate. On the other hand, if the recycle delay time is not too short, say 1 to 2 times of T1, the length of 180 degree pulse can still be determined with reasonable accuracy.
Many thanks to Eric Ye of the National Ultra-high Field NMR Facility for Solids for contributing this post.