University Of Basel - Blog Posts

The two kinds of water

Credit: University of Basel

Pre-sorted ortho-water and para-water molecules with differently oriented nuclear spins (blue or red arrows) react with diazenylium ions (centre left) at different speeds.

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Researchers from the University of Basel’s Department of Chemistry, Switzerland, has investigated how the two forms of water differ in terms of their chemical reactivity – the ability to undergo a chemical reaction. Both forms have almost identical physical properties, which makes their separation particularly challenging.

It is less well-known that water exists in two different forms (isomers) at the molecular level. The difference is in the relative orientation of the nuclear spins of the two hydrogen atoms. Depending on whether the spins are aligned in the same or opposite direction, one refers to ortho- or para-water.

The was made possible by a method based on electric fields. Using this, researchers were able to initiate controlled reactions between the pre-sorted water isomers and ultracold diazenylium ions (protonated nitrogen) held in a trap. During this process, a diazenylium ion transfers its proton to a water molecule. This reaction is also observed in the chemistry of interstellar space.

It was discovered that para-water reacts about 25% faster than ortho-water. This can be explained in terms of the nuclear spin also influencing the rotation of the water molecules. As a result, different attractive forces act between the reaction partners. Para-water is able to attract its reaction partner more strongly than the ortho-form, which leads to an increased chemical reactivity.