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
                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.
                was the first to explain the mysterious stickiness of
                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
                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
                these bonds were primarily due to an uneven distribution
                electrical charge in the H2O molecule.

                He pointed out that the oxygen atoms cling onto
                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
                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'
                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',
                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
                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
                the way that an intense X-ray beam scatters off ice,
                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

                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
                water molecules positioned as they are in ice --
                that Pauling pioneered. They find that they can predict
                results seen by Isaac's team -- an oscillation in the
                of the scattered X-ray beam -- even though their
                calculations indicate no covalent bonding between the
                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
                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
                another, known as 'antibonding'. Apparently, the forces
                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

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