Abstract
Two-dimensional double-quantum spectroscopy can be of great assistance for the assignment of complicated high-resolution spectra and for the characterization of networks of scalar-coupled spins ( 1-9). Double-quantum spectroscopy is an attractive alternative to two-dimensional correlation spectroscopy (COSY), for the spectra are not obscured by a dominant diagonal ridge, and the fine structure of the multiplets is less complicated. In a double-quantum spectrum, pairs of multiplets that appear symmetrically disposed with respect to the skew diagonal have distinct structures, so that unfortunate signal cancellations are unlikely to occur on both sides of the spectrum ( 9). Furthermore, double-quantum spectra actually provide a great deal more information than COSY spectra: the so-called remote connectivity peaks allow one to identify three-spin subunits, in a manner that is reminiscent of the information derived from two-dimensional spectra obtained by relayed coherence transfer. Unfortunately, the most widely used pulse sequences for double-quantum spectroscopy suffer from a major drawback: the efficiency of the excitation of the various double-quantum coherences is not uniform, since it depends on the magnitude of the coupling constants and on the complexity of the coupling network. The following experiment, henceforth called the “conventional” sequence, contains a constant preparation interval rP:
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