Quantitative analysis of Earth's field NMR spectra of strongly-coupled heteronuclear systems

M.E. Halse, P.T. Callaghan, B.C. Feland, R.E. Wasylishen

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In the Earth's magnetic field, it is possible to observe spin systems consisting of unlike spins that exhibit strongly coupled second-order NMR spectra. Such spectra result when the J-coupling between two unlike spins is of the same order of magnitude as the difference in their Larmor precession frequencies. Although the analysis of second-order spectra involving only spin-1/2 nuclei has been discussed since the early days of NMR spectroscopy, NMR spectra involving spin-1/2 nuclei and quadrupolar (I > 1/2) nuclei have rarely been treated. Two examples are presented here, the tetrahydroborate anion, BH , and the ammonium cation, NH . For the tetrahydroborate anion, J( B, H) = 80.9 Hz, and in an Earth's field of 53.3 μT, ν( H) = 2269 Hz and ν( B) = 728 Hz. The H NMR spectra exhibit features that both first- and second-order perturbation theory are unable to reproduce. On the other hand, second-order perturbation theory adequately describes H NMR spectra of the ammonium anion, NH , where J( N, H) = 52.75 Hz when ν( H) = 2269 Hz and ν( N) = 164 Hz. Contrary to an early report, we find that the H NMR spectra are independent of the sign of J( N, H). Exact analysis of two-spin systems consisting of quadrupolar nuclei and spin-1/2 nuclei are also discussed.
Original languageEnglish
Pages (from-to)88-94
Number of pages7
JournalJournal of Magnetic Resonance
Issue number1
Publication statusPublished - 1 Sept 2009

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