2D spectroscopy finds fleeting interactions between water molecules

A new spectroscopy technique reveals that pairs of gaseous water molecules form short-lived complexes (Proc. Natl. Acad. Sci. U.S.A. 2021, DOI: 10.1073/pnas.2020941118).

“You might think, come on, it’s the most studied molecule in the universe. What more could be learned?” says researcher Keith A. Nelson of the Massachusetts Institute of Technology. He and Robert W. Field, also of MIT, have now collected the first experimental evidence for these metastable complexes which had previously only existed in theory, using 2D terahertz spectroscopy, a method developed in the last decade that had never before been applied to gaseous water. They say their measurements and methods could reveal new details about water vapor and associated chemistry.

Their team used terahertz laser pulses to induce and then halt rotational motions in gas-phase water molecules. Looking at pairs of rotational spectra measured during these experiments, they spotted instances where a water molecule’s rotational energy differed slightly from what would be expected of a lone water molecule. That indicated the molecule had been interacting with another water molecule, the researchers say. Nelson pointed out that these interactions are much weaker than those in a hydrogen-bonded water molecule dimer, and shorter-lived, lasting about 100 ps. He says in an email that these complexes are nonetheless common: “In the room you are in, there are certain to be some molecules in the water vapor around you that are in these transient complexes.”

Field says this new kind of spectroscopy will help scientists create a more complete picture of water behavior in the gas phase. Future work is “surely going to contradict simple ideas of what water is doing,” he adds.

“This work will open the door to consideration of the chemistry of metastable water clusters that might occur in the atmosphere,” spectroscopy and water expert Richard J. Saykally of the University of California, Berkeley, says. Nelson says that chemistry might include related interactions between water and other molecules that play important roles in atmospheric chemistry, like nitrogen dioxide or sulfur dioxide.