X-Message-Number: 13294 Date: Thu, 24 Feb 2000 22:22:30 -0500 From: Jan Coetzee <> Subject: Theory does not hold water physics : Theory does not hold water PHILIP BALL Last year, researchers claimed that they had deduced how water is held together. They said that the weak bonds linking the molecules to one another, called hydrogen bonds, are formed partly from the sharing of electrons1. This seemed to confirm a suggestion made in 1935 by the famous US chemist Linus Pauling. Now a group of chemists, from Indiana University in the USA, challenge this claim in the Journal of the American Chemical Society2. The effects seen by last year's team are if anything, they say, a sign that electron sharing inhibits the ability of two water molecules to stick together. Pauling won the 1954 Nobel Prize in chemistry for his contributions to the understanding of chemical bonding. He was the first to explain the mysterious stickiness of water molecules. By comparison with similar small molecules, H2O might be expected to be a gas under everyday conditions -- which would make the world a very different place. But water remains a liquid because hydrogen bonds give the molecules extra cohesion, preventing them from flying apart into steam. Pauling proposed in the 1930s that these bonds were primarily due to an uneven distribution of electrical charge in the H2O molecule. He pointed out that the oxygen atoms cling onto electrons, which have a negative charge, more tightly than the hydrogen atoms do. This makes the oxygens slightly negatively charged, and the hydrogens slightly positive. Since opposites attract, said Pauling, the hydrogens on one molecule will tend to stick to the oxygens on another. But Pauling refined this picture of the hydrogen bond in 1935, when he said that the hydrogen-bonding 'glue' might be supplemented by the sharing of electrons between the two linked molecules. Sharing of electrons between atoms generally results in strong, so-called 'covalent bonds', like those that hold two hydrogen atoms to an oxygen atom in individual H2O molecules. Pauling proposed that a small degree of sharing between molecules contributes to the hydrogen bond. But there had been no experimental evidence of a covalent component to the hydrogen bond -- until January 1999. Physicist Eric Isaacs of Bell Laboratories in New Jersey and his co-workers reported that they could infer it from the way that an intense X-ray beam scatters off ice, where hydrogen bonds hold water molecules in a regular, crystalline assembly. The work was widely hailed as having at last revealed the full character of the hydrogen bond. Now Ernest Davidson and colleagues at Indiana use the principles of quantum mechanics to carry out detailed calculations of the way that X-rays should interact with two water molecules positioned as they are in ice -- something that Pauling pioneered. They find that they can predict the results seen by Isaac's team -- an oscillation in the intensity of the scattered X-ray beam -- even though their calculations indicate no covalent bonding between the two molecules. The oscillations are, they say, "irrelevant to the discussion of the covalent character of the bond". In fact, these calculations show that the proximity of the electrons on the two molecules does not, in fact, cause a net sharing (which would lead to 'stickiness') but quite the opposite: a tendency for the molecules to recoil from one another, known as 'antibonding'. Apparently, the forces of electrical attraction have to overcome this effect in order to join the molecules together, implying that our seas and oceans are the result of some finely tuned push and pull. Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=13294