Redefining hydrogen bonds.

[Jim__]

An excerpt from New Scientist:

...

These bonds govern the way the molecules fold up, like DNA's double helix. They also change their bulk properties: without H bonding, it would take less energy to separate water molecules from each other so water would boil at a lower temperature. But an exact definition has proved elusive. Scientists argue about exactly which atoms must be involved for an interaction to be classified as an H bond, as well as the bond's spectroscopic signatures and the forces that hold it together.

A previous definition, from the 1997 version of the International Union of Pure and Applied Chemistry (IUPAC)'s Gold Book, lists nitrogen, oxygen or fluorine - some of the most electronegative atoms in the periodic table - as most likely to tug on hydrogen's electron cloud and lead to an H bond. Recent experiments have found this definition wanting. It's now clear that a hydrogen bonded to a carbon, which is slightly more electronegative than hydrogen, but much less than nitrogen, oxygen or fluorine, can still form an H bond in some circumstances.

So, under the new definition, the brainchild of an IUPAC committee, an atom bonded to hydrogen need only be more electronegative than the hydrogen itself to be a candidate for an H bond. Spectroscopic observations, or even computer simulations, could then show that the particular interaction meets the criteria to be an H bond, such as having the characteristic bond length, angle or energy density (Pure and Applied Chemistry, DOI: 10.1351/pac-rec-10-01-02).

The new definition could improve computer models used to predict molecular structure, says Steven Scheiner of Utah State University in Logan who co-led the IUPAC group. Current versions don't allow for the possibility of hydrogen bonds forming between a hydrogen atom covalently bonded to a carbon and sitting near an oxygen, even though it is now clear this is possible.


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[benld74]

1. For a sec, I thought this was some NEW financial crap the banksters created,,,,

[RC]

2. Except here the verbiage is more easily understood.

[Jim__]

5. All I can say is that for those types of bonds, I would have put it in the Economy Forum. - n/t

[struggle4progress]

3. wish the popular article had given some example of how the new definition helps.

not a good enough chemist myself to know: does this provide improved calculations on keto-enol tautomerism, for example?

[Jim__]

4. The only advantage suggested by the popular article is in predicting molecular structure.

I can't answer your question, but my reading of the article is that the new definition broadens the criteria so that hydrogen bonding would be considered in additional situations. Based on the article, it sounds like some of these additional situations are already recognized by the field, for example: (c)urrent versions don't allow for the possibility of hydrogen bonds forming between a hydrogen atom covalently bonded to a carbon and sitting near an oxygen, even though it is now clear this is possible.

Does that sounds like the keto-enol tautomerism (I thought it might based on a quick look under wikipedia)? If so, the field may already recognize this; but the current definition of a hydrogen bond doesn't allow for the possibility. Looking at the IUPAC publication, my guess is that this reaction may fit under the C2 characteristic: (C2) Hydrogen bonds are involved in proton-transfer reactions (X–HY → XH–Y) and may be considered the partially activated precursors to such reactions.

[Salviati]

6. Just like those librul scientists--- trying to redefine hydrogen bonding!11!!!

I'm in favor of traditional hydrogen bonding. It's H and O, not H and Steve!