Caging water to study spin

A pioneering method of caging and cooling water molecules to study its nuclear spin conversion could enhance the diagnostic power of MRI scans. By trapping water molecules in spheres of fullerene, researchers from the Universities of Southampton, Nottingham and Columbia University in New York were able to study the change in orientation of the magnetic nuclei of hydrogen atoms. Water exists as two isomers depending on the spin of their two hydrogen atoms: if the nuclear spins are parallel, the molecule is said to be ortho; if anti-parallel, the molecule is para. It is thought that any given molecule can transform between the two spin isomers, a process known as nuclear spin conversion. “Currently, mechanisms for this conversion are not completely understood, nor how long it takes the molecules to transform from one spin isomer to the other,” said Salvatore Mamone, a post-doctoral physicist from Southampton. “To study this, we have to figure out how to reduce the strong intermolecular interactions that are responsible for grouping of molecules and lowering the rotational mobility of the water molecules.” The researchers caged them in fullerene spheres – C₆₀ – and used chemical reactions to open a hole in the sphere. They injected water molecules inside before closing them, forming a complex referred to as H₂O@C₆₀. “At the end of this synthetic preparation nicknamed ‘molecular surgery’ we find that 70 to 90% of the cages are filled, giving us a significant quantity of water molecules to examine,” said Mamone. “Because the molecules are kept separate by the cages, there is a large rotational freedom that makes the observation of ortho and para isomers possible.” Individual H₂O@C₆₀ samples were quickly cooled from 50K to 5K and monitored for their nuclear magnetic resonance (NMR) signal every few minutes over several days. “As the observed NMR signal is proportional to the amount of ortho-water in the sample, we can track the percentages of ortho and para isomers at any time and any temperature,” Mamone said. “At 50K, we find 75% of the water molecules are ortho, while at 5K they become almost 100% para. Therefore, we know that after the quick temperature jump, equilibrium is restored by conversion from ortho to para – and we see that conversion in real time.” The work was published in Journal of Chemical Physics. Nuclear spin conversion of water inside fullerene cages detected by low-temperature nuclear magnetic resonance