Chapter 7. NMR Spectroscopy Using Several Parallel Receivers

Abstract

Recent instrumental innovations in NMR spectrometers now make it possible to operate up to four receivers independently and in parallel. In a single measurement, spectra from different nuclear species (e.g., 1H, 13C, 15N, 19F or 31P) can be recorded simultaneously. More importantly, several standard pulse sequences can be combined into a single entity, offering complementary types of structural information in a single pass. For small molecules, direct and long-range correlation measurements can combine to reveal the entire molecular structure, starting with the basic carbon framework, using samples with the natural 13C isotopic abundance. As a demonstration of this concept, the molecular structures of two “unknown” small molecules (actually melatonin and quinine) were derived “from scratch.” Where necessary, these techniques can be extended into three dimensions for the purpose of measuring an extended range of multiple-bond couplings. New methods are described to speed up these experiments. One of these, Hadamard-encoded multiple irradiation, combines with two other standard pulse sequences to derive the molecular structure of menthol in an experiment lasting only 56 s. The same general concept can be extended to treat moderately sized biomolecules with global isotopic enrichment; nuclease A inhibitor, a 143-residue protein, is presented as in illustrative example.