Practical aspects of Zeeman-perturbed NQR spectroscopy using an adjustable electromagnet

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 10¹ 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_Q, 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. ⁷⁹Br and ¹²⁷I Zeeman-perturbed NQR spectra are recorded for some solid bromo- and iodobenzene powders using applied magnetic fields on the order of 10⁻² 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_Q(¹²⁷) 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.