Spin me right round: Ortho-Para water

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  • Published: Sep 1, 2015
  • Author: David Bradley
  • Channels: NMR Knowledge Base
thumbnail image: Spin me right round: Ortho-Para water

Nuclear spin isomers

The research team confined single water molecules in C60 carbon cages or ‘buckyballs’ to produce supramolecular endofullerene H2O@C60. Credit: Image courtesy of University of Southampton

Research by an interdisciplinary team including chemists, physicists and engineers from the University of Southampton has found that water molecules react differently to electric fields, which could provide a new way to study spin isomers at the single-molecule level and offer new insights to the phenomenon of magnetic resonance.

Chemists Benno Meier, Salvatore Mamone, Maria Concistrè, Javier Alonso-Valdesueiro, Andrea Krachmalnicoff, Richard Whitby, and Malcolm Levitt of the University of Southampton describe in Nature Communications how they have used fullerene molecules to isolate single water molecules so that they could examine their spin behaviour at the single molecular level when the endofullerenes are bathed by an electric field. Water molecules are known to exist as two spin "isomers" - an ortho and a para form - that have distinct nuclear spin states. In ortho water, the nuclear spins are parallel to each other, and in para water, the spins are antiparallel.

The switch to and from ortho and para forms of water is relevant to nuclear magnetic resonance (NMR) and studies of chemistry in space. The spin isomers of water can be separated but it is difficult to study their properties in bulk water because rapid proton exchange between molecules and hindered molecular rotation essentially blur the differences. What is needed, therefore, is a way to observe just a single water molecule and to somehow control its isomerisation between the ortho and para forms.

On the inside

By confining a single water molecule in the "hollow" interior of the spherical, all-carbon cage molecule, [60]fullerene, to make an H2O@C60 supramolecular structure, the Southampton research team was able to observe a watery bulk without the possibility of individual water molecules being able to interact with each other and confuse the signals they wished to see. The carbon cages preclude freezing of water so that they continue to rotate freely even at the very low temperatures needed to make observations of the ortho and para isomers. The approach is a lot simpler and more versatile than earlier attempts at studying ortho and para water that relied on molecular beams or an inert gas matrix.

Probe potential

The team measured the dielectric constant of H2O@C60 at cryogenic temperatures and found that it decreases as water converts from ortho to para, which corroborates the quantum theory associated with spin isomers as well as previous NMR spectroscopic studies of the conversion kinetics. Meier, lead author on the work says: "The bulk dielectric constant of H2O@C60 depends on the spin isomer composition of the encapsulated water molecules. The observed time-dependent change in the bulk dielectric constant at 5 Kelvin, as encapsulated water converts from the ortho to the para isomer, is due to a change in molecular polarisibility on spin conversion." He points out that the work is a result of a long-standing collaboration between the teams of Levitt and Whitby.

"The different response of ortho and para water to electric fields provides a sensitive alternative means to study the spin isomers. The phenomenon opens up the prospect of using Kelvin probe force microscopy to study water spin isomers on a single-molecule level," the team says.

Levitt told SpectroscopyNOW that the next steps in this research would include their attempting single-molecule detection and studying other systems displaying spin-isomerism in the hope of providing the scientific community with new experimental tools for studying spin-isomerism. In the long-term they hope to achieve better resolution, contrast and brightness in magnetic resonance and magnetic resonance imaging (MRI) and to find new methods for dense information storage where each molecule carries a single bit.

Related Links

Nature Commun, 2015, 6, online: "Electrical detection of orthopara conversion in fullerene-encapsulated water"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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