How to record ID NMR spectra of paramagnetic molecules

When the spectral width is of hundreds of parts per million, i.e. more than 105 Hz on high field instruments, a very short excitation pulse is needed. Of course, high power is needed to reach the r.f. energy corresponding to a 90° pulse in a short time. To best exploit the short relaxation times, it is often convenient to use a full 90° excitation pulse and to recycle fast, because magnetization equilibrium is reached quickly. With suitable power supplies and purpose-built probes, short H 90° pulses can be achieved (as short as 2 p,s at 800 MHz) [1]. 

When very large spectral widths are needed, pulses of less than 90° can always be used. 

The finite duration of the pulse in a way causes the same kind of problems, 

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