Water Suddenly Becomes Mysterious

An anonymous reader writes "Logical to assume that scientists know the structure of water. But wrong. A study in April by Anders Nilsson from the Stanford Linear Accelerator Center concluded the molecular bonds are looser than thought. Now a new study by Richard Saykally's group at UC Berkeley appears to debunk the April results. So a new debate is born. Both scientists agree on one thing: They don't fully understand how water molecules interact."

Comments

Hmm...

[Canar]

Considering that even fluid dynamics has a hard time making water behave in a way that looks realistic, why is this surprising?

Re:Hmm...

[Mark of THE CITY]

Different scale, for one thing...

Spectroscopists use ultrafast experiments for the same reason photographers use fast flashlamps: to capture the action, unblurred. But the motion of water, one of the most labile molecules around, is too fast, and there is argument (at least there was in the mid-90s, when I was going to seminars) about how meaningful the ball-and-stick models of water clusters were. Was it a real geometry, or an average over the time of the laser pulse? Apparently, arguments continue over this.

There are other systems where the molecules in a cluster can't move much, due to a higher degree of mutual interference than is the case with water. So the cluster models are more believable.

I'm skeptic

[hankwang]

A claim that seems to disagree with everything else that is known should have solid evidence. It is of course possible that these guys are right and that the bookshelves full of other experimental data all are flawed. However, it might also be that the way they measured and modelled ignored some important effect.

From my background, I can mention that the infrared absorptions of the two OH bonds are at a wavenumber of 3650 and 3750 cm-1 (around 2.7 micrometers wavelength). In liquid water, these absorptions shift to a band around 3400 cm-1 (2.95 micron). It is a widely accepted fact that this shift of the OH vibration frequency occurs as a result of the hydrogen bond forming between the H of the OH and the O of the next water molecule:

H-OH ... OH2

There is hardly any absorption in liquid water at 3650 and 3750 cm-1, which would strongly suggest that nearly all water molecules have both of their hydrogen atoms bonded to other water molecules (that means four H-bonds per molecule). If the claim in the article were true, half of the OH groups would be free and absorb at a higher wavenumber.

The idea that the OH absorption wavelength depends on whether it has a hydrogen bond is in agreement with a large number of studies in which for example clusters of two, three, four water molecules embedded in another material or in vacuum have different infrared absorptions. They are also in agreement with calculations on what happens with an OH bond when you let it form a hydrogen bond, and with fully quantum-mechanical so-called ab initio calculations of how water molecules should behave, although with the latter, one might object that computers still are not powerful enough to do these calculations with more than a couple of ten molecules at a time.