Science. 1986 Aug 1;233(4763):525-31. doi: 10.1126/science.233.4763.525.
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.
核磁共振（NMR）事件通常被视为磁场中单自旋的翻转，仅通过吸收或发射一个射频能量量子来激发。然而，通过适当设计的射频脉冲序列，也可以在耦合自旋系统中诱导出具有多于一个单位的 Zeeman 量子数的核自旋态之间的共振。以这种方式激发的自旋态对以相应的多量子跃迁频率振荡，并产生可在二维时域实验中间接监测的响应。多量子激发和响应的模式主要受耦合核群的协同相互作用的影响，在某些情况下简化了 NMR 谱，而在其他情况下则提供了重要的新信息。多量子 NMR 的应用扩展到许多不同领域的问题，从溶液中蛋白质和核酸的结构和功能研究到非晶半导体中原子排列的研究。具体的光谱技术也多种多样，包括旨在简化液体和液晶光谱分析、消除非均匀展宽、研究液态分子中原子间连接性、识别固体中原子团簇、增强固体成像实验中的空间分辨率以及探测相关分子运动的方法。
