Multiple-quantum nuclear magnetic resonance spectroscopy.

A nuclear magnetic resonance (NMR) event is popularly viewed as the flip of a single spin in a magnetc field, stimulated by the absorption or emission of only one quantum of radio-frequency energy. Nevertheless, resonances between nuclear spin states that differ by more than one unit in the Zeeman quantum number also can be induced in systems of coupled spins by suitably designed sequences of radio-frequency pulses. Pairs of states excited in this way oscillate coherently at the frequencies of the corresponding multiple-quantum transitions and produce a response that may be monitored indirectly in a two-dimensional time-domain experiment. The pattern of multiple-quantum excitation and response, influenced largely by the concerted interactions of groups of coupled nuclei, simplifies the NMR spectrum in some instances and provides significant new information in others. Applications of multiple-quantum NMR extend to problems in many different areas, ranging from studies of the structure and function of proteins and nucleic acids in solution to investigations of the arrangements of atoms in amorphous semiconductors. The specific spectroscopic techniques are varied as well and include methods designed, for example, to simplify spectral analysis for liquids and liquid crystals, eliminate inhomogeneous broadening, study interatomic connectivity in liquid-state molecules, identify clusters of atoms in solids, enhance the spatial resolution in solid-state imaging experiments, and probe correlated molecular motions.