Z-Filtered double-quantum NMR spectra and automated analysis by pattern recognition

Abstract

A simple variation of double-quantum spectroscopy is described where the transfer of double- into single-quantum coherence is carried out by means of two pulses with smallflip angles via eigenstate populations and zero-quantum coherences. For systems with four or more spins, the sensitivity of the new experiment is comparable to or better than that of conventional double-quantum spectroscopy with a single 90° monitoring pulse. The resulting spectra feature simple 2D multiplets that are composed of juxtapositions of antiphase doublets. The inclusion of pathways involving zero-quantum coherences greatly simplifies experimental procedures but leads to small deviations from pure 2D absorption peakshapes in systems with three or more spins. A simple variation of the technique yields a complementary 2D spectrum which is useful for automated analysis by pattern recognition techniques. Algorithms are described that allow one to determine the topology of the coupling network and the values of the shifts and coupling constants. The likelihood that a postulated topology is correct may be determined from the degree of consistency between expected and experimental remote multiplets in double-quantum spectra.