Broadband multiple-quantum nuclear magnetic resonance with frequency-modulated ‘‘chirp’’ pulses: Applications to pairs of scalar-coupled spin I=1/2 nuclei

Abstract

It is shown by simulation and experiment that frequency‐modulated ‘‘chirp’’ pulses are very effective for exciting multiple‐quantum coherences over large bandwidths. Chirp pulses can be considered as an alternative to composite pulses to combat the effects of large offsets and tilted effective fields. By way of example, we discuss the excitation of double‐quantum coherences for pairs of scalar‐coupled spin I=1/2 nuclei over large ranges of chemical shifts. Refocusing of the phase dispersion of double‐quantum coherence can be combined with suitable detection sequences to yield pure absorption two‐dimensional double‐quantum spectra. The idea of symmetrical excitation and detection by time reversal may be applied to obtain t₁‐modulated longitudinal magnetization, which may then be converted into observable single‐quantum coherence by a chirp echo sequence. Similar approaches can be used for many other NMR experiments involving coherence transfer.