The NMR spin-lattice relaxation time is very short at low temperatures

Scientific Explanation

Nuclear magnetic resonance (NMR) is a measurement technique in which atomic nuclei in a strong magnetic field are excited and then return to their equilibrium state. The time this recovery takes is called the spin-lattice relaxation time T1. It measures how efficiently the nuclear spins can transfer their energy to the molecular surroundings (the “lattice”).

In water, T1 shows anomalous behavior: above 0 degrees Celsius, T1 decreases with falling temperature, which is normal — the slower-moving molecules couple more effectively to the spins. But in the supercooled regime below 0 degrees Celsius, T1 drops much more steeply than expected and reaches extraordinarily short values. This accelerated relaxation indicates that the molecular dynamics in supercooled water are qualitatively different from those of a normal liquid.

The physical mechanism involves cooperative motions in the hydrogen-bond network. At low temperatures, larger connected clusters of molecules form that move as a unit. These slow, large-scale fluctuations create strong magnetic field gradients at nuclear positions and accelerate relaxation. The phenomenon is closely linked to the second critical point hypothesis: near this point, the cooperative regions would grow ever larger and drive T1 toward zero.

Experimentally, the measurement is demanding because supercooled water tends to crystallize spontaneously. Researchers therefore use emulsions of tiny water droplets in oil or water confined in nanoporous materials to suppress crystallization and measure T1 even at minus 30 to minus 40 degrees Celsius.

Everyday Relevance

While NMR is a purely scientific technique, medical magnetic resonance imaging (MRI) is based on the same principles. The T1 relaxation time determines contrast in MRI images: different tissues appear bright or dark depending on how quickly their water molecules relax. Water’s anomalous relaxation properties are thus indirectly responsible for the excellent soft-tissue contrast that makes MRI so valuable in medical diagnostics.