In 2D NMR measurements involving 19F, unlike those for 1H and 13C only, wide chemical shifts of 19F make it difficult to detect correlation peaks. RF pulses should manipulate magnetizations as designed, and otherwise signal intensities become considerably low. In multiple region selective (mrs) HMQC,1) 19F pulses are replaced by a selective excitation E-BURP1 pulse and a selective refocusing RE- BURP pulse, and several frequency regions are simultaneously excited/refocused using shifted laminar pulses, permitting all the correlation peaks to be detected.
Fig. 1 Pulse diagram of mrs-HMQC
Fig. 2 mrs-HMQC spectra of CF3CHFCF2OCH2CH3
Conventional HMQC method, where the offset of 19F rectangular pulses is fixed at -80ppm, does not give a correlation peak at -210ppm, but all the correlation peaks are detected in the mrs-HMQC spectrum.