Selective excitation in spin systems with homogeneous broadening

We have found that the application of a weak radio frequency (rf) pulse to a spin system with indirect spin–spin (J) couplings can produce a narrow inverted peak in spectral regions where there are many overlapping peaks. Examples of three compounds with 1H–1H J couplings and one with 1H–13C J couplings are given. The dependences of the signals on the frequency, duration, and amplitude of the weak rf irradiation have been studied. For an rf power of γB1/2π=1.0 Hz, pulse widths longer than 0.25 s consistently produce inverted narrow peaks in spectral regions with unresolved peaks. An interpretation of the origin of this unusual result of selective excitation in spin systems with homogeneous broadening is given: the inverted signal observed can be considered as a cumulative effect of the weak rf irradiation acting on many uncorrelated transitions that are slightly off resonance on both sides of the irradiating frequency; the only role of the J couplings (or dipolar couplings in liquid crystals) is to produc...