Rewriting the textbooks: Hydrogen bonds in a bind

There is a reason why ice floats on water, and it is called the hydrogen bond. Whatever that is.

Nobel laureate Linus Pauling thought he knew. In fact, the International Union of Pure and Applied Chemistry (IUPAC), which concerns itself with such things, still bases its official definition on the one that appears in Pauling's classic 1939 book The Nature of the Chemical Bond.

A hydrogen bond, in this picture, is what forms when a hydrogen atom that is already stably bound into one molecule finds itself attracted to a highly electronegative atom - one like oxygen, nitrogen or fluorine that likes to suck in electrons and turn into a negatively charged ion - elsewhere in the same molecule or in a nearby molecule.

Take good old H₂O. The two hydrogen atoms of a water molecule are bound covalently, through shared electrons, to its central oxygen atom. But should a second water molecule come near, the electron orbiting one of the hydrogen atoms can be drawn towards the second molecule's electron-hungry oxygen.

Ice is less dense than liquid water because, when water molecules are cold and still, weak hydrogen bonds between them keep them consistently at arm's length. In free-flowing water, however, the bonds are continually breaking and reforming, allowing the molecules to jostle closer together.

That is all fine and dandy. But this traditional picture also implies a strict range of admissible hydrogen-bond strengths. Over the past 40 years, though, reams of evidence about much weaker bonds, including ones between hydrogen and elements like carbon, which are not very electronegative, have come to light.

Six years ago, IUPAC set up a committee to clear up the confusion. Its conclusion, set out in a seven-page draft redefinition published last year, is that the hydrogen bond is a far fuzzier entity than we thought. "It is not an interaction with sharp boundaries," says Gautam Desiraju from the Indian Institute of Science in Bangalore, a member of the IUPAC committee.

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