The NMR shift reaches a maximum at low temperatures

Scientific Explanation

The chemical shift in the proton NMR spectrum of water describes how strongly the local magnetic field at hydrogen nuclei is shielded by the electronic environment. In water, this shielding depends critically on the strength and number of hydrogen bonds: the more strongly a proton is engaged in a bond, the further its NMR signal shifts.

At room temperature, the chemical shift increases with falling temperature because more hydrogen bonds remain intact at lower temperatures and the average bond strength grows. This behavior is normal for a hydrogen-bonding liquid. What is surprising is that the shift reaches a maximum at supercooled temperatures — around minus 30 to minus 35 degrees Celsius — and then appears to decrease at even lower temperatures.

This maximum means that below a certain temperature, the shielding no longer increases even though the bond network becomes ever more complete. The explanation likely lies in a structural transformation: in the deeply supercooled regime, the molecular arrangement approaches that of ice, and the geometry of the bonds changes in a way that reduces proton shielding. This behavior is another indirect sign of a transition between two distinct liquid phases.

Experimentally, such measurements require special techniques: emulsified water in fine droplets, confinement in nanoporous materials, or ultrafast pulsed NMR sequences that capture the signal before crystallization can set in.

Everyday Relevance

Although this phenomenon appears purely scientific, it has practical implications for cryobiology — the science of biological behavior at low temperatures. The structural changes reflected by the NMR shift maximum affect how proteins and cells interact in supercooled water. In cryo-electron microscopy, which was awarded the Nobel Prize in Chemistry in 2017, water is cooled so rapidly that it vitrifies — understanding the water structure in this transition region is essential.