Nari Alireza, Szell Patrick M J, Bryce David L
Department of Chemistry and Biomolecular Sciences, Centre for Catalysis Research and Innovation, and Nexus for Quantum Technologies, University of Ottawa, Ottawa, Ontario, Canada, K1N6N5.
Department of Chemistry and Biomolecular Sciences, Centre for Catalysis Research and Innovation, and Nexus for Quantum Technologies, University of Ottawa, Ottawa, Ontario, Canada, K1N6N5.
Solid State Nucl Magn Reson. 2025 Aug;138:102014. doi: 10.1016/j.ssnmr.2025.102014. Epub 2025 Jun 3.
Quadrupolar-perturbed solid-state NMR spectroscopy is a highly useful and well-established method for studying quadrupolar nuclei. This method relies on a high ratio of the Larmor frequency to the quadrupolar frequency and is limited, therefore, by the available magnetic field strengths suitable for NMR, which are on the order of 10 T. Nuclear quadrupole resonance (NQR) provides an approach to studying strongly quadrupolar isotopes, but there are technical challenges associated with measuring high-frequency transitions, and with measuring both the quadrupolar coupling constant, C, and asymmetry parameter, η, with good precision. We describe here the technical and practical aspects of a modern implementation of Zeeman-perturbed NQR spectroscopy using an adjustable electromagnet, which overcomes the aforementioned challenges. This approach flips the quadrupolar-perturbed solid-state NMR method upside down, so that the quadrupolar interaction is dominant and the Zeeman interaction is the perturbation. Br and I Zeeman-perturbed NQR spectra are recorded for some solid bromo- and iodobenzene powders using applied magnetic fields on the order of 10 T. Various experimental considerations are discussed, including the optimal magnetic field to be used, the optimization of the coil angle, frequency stepping, the simulation of spectra using an exact diagonalization of the Zeeman-quadrupolar Hamiltonian, and how to ensure high precision in the resulting quadrupolar parameters. As an example, a C() value of 2077.25 ± 1.49 MHz (with η = 0.114 ± 0.008) is measured for sym-triiodotrifluorobenzene in less than an hour at room temperature. The approach holds promise for studying strongly quadrupolar isotopes in a range of materials and obviates the need for ultrahigh magnetic fields in many situations of interest.
四极扰动固态核磁共振光谱法是研究四极核的一种非常有用且成熟的方法。该方法依赖于拉莫尔频率与四极频率的高比值，因此受到适用于核磁共振的可用磁场强度的限制，其量级为10 T。核四极共振（NQR）提供了一种研究强四极同位素的方法，但在测量高频跃迁以及高精度测量四极耦合常数C和不对称参数η方面存在技术挑战。我们在此描述了一种使用可调电磁铁的现代塞曼扰动NQR光谱法的技术和实际方面，它克服了上述挑战。这种方法将四极扰动固态核磁共振方法颠倒过来，使得四极相互作用占主导，而塞曼相互作用是扰动。使用量级为10 T的外加磁场，记录了一些固体溴苯和碘苯粉末的溴和碘的塞曼扰动NQR光谱。讨论了各种实验考虑因素，包括要使用的最佳磁场、线圈角度的优化、频率步进、使用塞曼 - 四极哈密顿量的精确对角化对光谱进行模拟，以及如何确保所得四极参数的高精度。例如，在室温下不到一小时内就测量出对称三碘三氟苯的C()值为2077.25 ± 1.49 MHz（η = 0.114 ± 0.008）。该方法有望用于研究一系列材料中的强四极同位素，并且在许多感兴趣的情况下无需超高磁场。
