四极核的零场J谱
Zero-Field J-spectroscopy of Quadrupolar Nuclei
AI总结：
本研究首次报道含四极核（I=1）分子的零场J谱，通过原子磁力计直接测量铵阳离子同位素异构体，提取精确J耦合值并发现初级同位素效应，分析了能级结构对谱线位置的影响。
摘要：
零至超低场核磁共振（ZULF NMR）是核磁共振的一种强大形式，它允许在由内禀自旋-自旋相互作用主导的体系中研究分子及其转化。虽然零磁场下的自旋动力学可以通过间接方式探测——即将在零场下演化的样品穿梭至高场核磁共振谱仪进行检测——但J谱也可以使用非感应传感器（例如原子磁力计）在零场下直接测量。迄今为止，尚未有关于含四极核耦合的分子的零场J谱的报道。在此，我们展示了可以从含有自旋I=1的四极核的分子中收集零场J谱，并针对含有不同铵阳离子同位素异构体的溶液进行了演示，即14NH4+和15NDxH(4-x)+（其中x=0、1、2或3）。与质子相比，含有更多氘核的分子观察到较低的ZULF NMR信号；这归因于总磁化强度较低，而非第二类标量弛豫。从ZULF NMR谱中提取了15N-1H和14N-1H的J耦合值，分别为-73.416(3) Hz和52.395(2) Hz。对J(15NH)/J(14NH)比值的精确测量得到的结果在1.4009-1.4013范围内；这在统计上与文献报道的γ_{15N}/γ_{14N}=1.4027不同，表明存在初级同位素效应。我们详细分析了所研究分子阳离子的能级结构，并证明谱线位置既显著依赖于每个J耦合的符号，也显著依赖于磁脉冲长度。
英文摘要：
Zero- to ultralow-field nuclear magnetic resonance (ZULF NMR) is a powerful version of NMR that allows studying molecules and their transformations in the regime dominated by intrinsic spin-spin interactions. While spin dynamics at zero magnetic field can be probed indirectly - via shuttling a sample that underwent evolution at zero field to a high-field NMR spectrometer for detection - J-spectra can also be measured directly at zero field by using non-inductive sensors, for example, atomic magnetometers. To date, no zero-field J-spectra of molecules featuring the coupling to quadrupolar nuclei were reported. Here we show that zero-field J-spectra can be collected from molecules containing quadrupolar nuclei with I = 1 and demonstrate this for solutions containing various isotopologues of ammonium cations, namely, 14NH4+ and 15NDxH(4-x)+ (where x = 0, 1, 2, or 3). Lower ZULF NMR signals are observed for molecules containing larger numbers of deuterons compared to protons; this is attributed to less overall magnetization and not to the scalar relaxation of the second kind. Values for the 15N-1H and 14N-1H J-couplings of -73.416(3) Hz, 52.395(2) Hz, respectively, are extracted from the ZULF NMR spectra. Precision measurement of the J(15NH)/J(14NH) ratio resulted in the value within a range of 1.4009-1.4013; this is statistically different from {γ_{15N}}/{γ_{14N}}=1.4027 reported in the literature indicating the presence of the primary isotope effect. We analyze the energy structure for the studied molecular cations in detail and demonstrate that spectral line positions depend dramatically both on the sign of each J-coupling and on the magnetic pulse length.
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