Zero-Field J-spectroscopy of Quadrupolar Nuclei
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.