A new nuclear magnetic resonance (NMR) spectroscopy technique developed at the University of Ottawa allows physical chemists to distinguish between certain types of magnetically equivalent atoms in molecules, something that was previously thought to be almost impossible.

Certain atomic nuclei — those with non-zero values of the quantum property known as spin — act like tiny magnets. If placed in a strong enough magnetic field, they can induce an electrical current in the receiving coil at specific frequencies that can be measured; this is the basis of all NMR spectroscopy. Of course, these tiny magnets also interact with each other and with nearby electrons, which gives rise to slight shifts and coupled peaks in NMR spectra. These variations can be interpreted to provide information about the structure of a molecule. However, not all NMR-active nuclei are created equal: those with a spin of ½ (like 1H) are called dipolar and give relatively clear signals, while those with larger spins (like 14N) are called quadrupolar, and give signals that are much harder to resolve.

In a paper recently published in the Journal of Chemical Physics, David Bryce and Frédéric Perras analysed samples containing quadrupole-quadrupole bonds, using a technique known as double-rotation NMR to cancel out as much broadening as possible from the resulting spectra. Though the signals were still complicated, a careful NMR theoretical analysis showed that certain peaks could indeed be assigned to specific interactions — called dipolar coupling and J-coupling — that were previously thought to be intractable for quadrupolar nuclei.

An interesting outcome had to do with nuclei in identical electronic environments, a phenomenon known as magnetic equivalence. In dipolar nuclei, these show up as a single peak in the spectrum: a good example is the three identical hydrogens in a methyl group. Bryce and Perras showed that if magnetically-equivalent atoms are quadrupolar, previously unexpected spectral splittings can be used to distinguish one from another. This finding could greatly improve spectral analysis of problematic molecules, from proteins to metal-organic frameworks. “It gives us direct insight into electronic structure that we didn’t have before,” says Bryce.