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Chemists re-define hydrogen bond

h bonds.jpg

The hydrogen bond is a wondrous thing. It helps give snowflakes their hexagonal symmetry; binds DNA into a double helix; shapes the three-dimensional forms of proteins; and even raises water’s boiling point high enough to make a decent cup of tea.

This ubiquitous bond is now getting a makeover, thanks to the International Union of Pure and Applied Chemistry (IUPAC).

The latest definition of the hydrogen bond that appears in IUPAC’s Gold Book – the bible of chemical nomenclature – dates from 1997, and is remarkably similar to that laid out by double-Nobel-Prize-winner Linus Pauling in his classic work of 1931, The Nature of the Chemical Bond.

Hydrogen atoms, armed with a lone electron, generally form a single bond with one other atom. For example, hydrogen can form a pretty strong covalent bond – made of a shared pair of electrons – with a carbon atom. The bond is found in every organic compound, from petrol to paraquat. In contrast, other atoms that are willing to loosely share their electrons with a hydrogen atom can sometimes form a weaker ‘hydrogen bond’ with it – as though the hydrogen atom forms a bridge between two other atoms (see picture).

But over recent years, chemists have found lots of examples of hydrogen bonding that don’t quite fit the classic Pauling definition, and the Gold Book has long been due an update, says Gautam Desiraju, a chemist at the Indian Institute of Science in Bangalore. He was on a task force established by IUPAC in 2005 to revise the current, conservative definition.

Current definition (extract) Proposed definition (extract)
A form of association between an electronegative atom and a hydrogen atom attached to a second, relatively electronegative atom. It is best considered as an electrostatic interaction, heightened by the small size of hydrogen, which permits proximity of the interacting dipoles or charges. The hydrogen bond is an attractive interaction between a hydrogen atom from a molecule or a molecular fragment X–H in which X is more electronegative than H, and an atom or a group of atoms in the same or a different molecule, in which there is evidence of bond formation.

While the previous definition stated that the hydrogen bond’s “associated energies are less than 5–6 kcal per mol”, the new definition encompasses much weaker interactions, ranging from 0.5 to 40 kcal per mol – such as the bond between a hydrogen attached to a carbon atom and a nearby oxygen atom.

“What has really happened in the last 10 years is instrumental techniques and computational techniques have reached to the point where we can measure some of these effects,” Desiraju says.

Previously, the definition suggested hydrogen bonds were ‘electrostatic interactions’, where hydrogen atoms bear a slight positive charge that is attracted by a slightly negatively-charged bonding partner. The new definition allows there to be some element of covalent bonding involved, where electrons are formally shared between the atoms.

The proposed new definition from the task force has now been released, and is out for public consultation until March. Desiraju expects it to be incorporated into the Gold Book shortly after. This is, he says, a matter of some personal satisfaction: “It’s very hard in science to say we’ve reached the end of something, but I think this definition is going to hang around for a long time.”

More

A Bond by Any Other Name – Desiraju’s new paper in Angewandte Chemie on the new definition of the hydrogen bond.

IUPAC proposal

IUPAC Gold Book

Image: hydrogen bonding in water / wikipedia

Comments

  1. Report this comment

    John Bailo said:

    Is this recognizing some of the work in hydrogen gas generation where methods “soften” the bond using RF, filtration, and so on…

  2. Report this comment

    Rob Hooft said:

    Prof. Desiraju is the world expert on weak interactions between C-H donors and X acceptors. The effort of redefining the hydrogen bond to include these weaker interactions is clearly worth it. I have tried for years to disagree with him on whether C-H…X interactions are true hydrogen bonds within the old definition.

    However, I am a bit disappointed in the phrasing “evidence of bond formation”, which sounds unscientifically vague and could include nearly anything including the mentioned groups. The distinction between a new “hydrogen bond” and general Van der Waals interactions is unclear.

  3. Report this comment

    Martin Suhm said:

    If you want to join the debate on this new definition, there is a chance to do so at a podium discussion during the 19th International Conference on “Horizons in Hydrogen Bond Research” in Göttingen in September 2011 ( see http://www.hbond.de for details)

  4. Report this comment

    Ivan Juranic said:

    As a matter of fact, the hydrogen bondigh should be considered between two electroegative atoms, having an hydrogen in between (X-H Y). Whenever the distance between two electronegative atoms is shorther than the sum od X-H bond plus vdV radii of hydrogen and of other electronegative atom, the aditional attraction is in effect, and can be rationalized as hydrogen bonding.

  5. Report this comment

    Dr.D.P.S.RATHORE said:

    In my opinion the current definition is more appropiate.It is “hydrogen bonding” and not hydrogen bond.Hydrogen bonding is a form of electrostatic interaction or association between an electronegative atom and a hydrogen atom attached to a second, relatively electronegative atom.It is because of difference in electronegativity between them, there is a displacement of electrons and polarity is developed,where hydrogen atoms bear a slight positive charge that is attracted by a slightly negatively-charged bonding partner.This electrostatic interaction between two molecule is termed as hydrogen bonding.

  6. Report this comment

    Henry Rzepa said:

    The phrase “X–H in which X is more electronegative than H” may lead to dispute. For example, consider B-H…H-N (the so-called dihydrogen bond) in which the bond occurs between electropositive and one electronegative hydrogen. Is B more electronegative than H in this context?

  7. Report this comment

    Henry Rzepa said:

    Might a clear distinction between an interaction and a hydrogen bond be useful? Thus C-H…H-C is considered an important interaction via dispersion forces. There are even some who feel that this type of “neutral” dihydrogen bond can approach the strengths of other hydrogen bonds. But at some stage, the hydrogen bond does start to become purely dispersive in character. Where to place that dividing line?

  8. Report this comment

    Henry Rzepa said:

    Might I seek what is meant by the term “an attractive interaction”? If the bond is intramolecular, attractive can only be defined in terms of another pose or conformation where the interaction does not exist. But that new pose may have other attractive forces which appear to compensate. And if it is intermolecular, where is the division between a hydrogen bond and a van der Waals term (as others have asked)? Certainly, there has been much controversy over close H…H approaches (Might I seek what is meant by the term “an attractive interaction”? If the bond is intramolecular, attractive can only be defined in terms of another pose or conformation where the interaction does not exist. But that new pose may have other attractive forces which appear to compensate. And if it is intermolecular, where is the division between a hydrogen bond and a van der Waals term (as others have asked)? Certainly, there has been much controversy over close H…H approaches (< 2A let us say, which is much closer than the sum of the vdW radii). They are close enough to be characterised by a QTAIM “bond critical point” in the electron density, but they may well be repulsive in nature. But how to produce the definitive statement of whether it truly is repulsive or attractive.

  9. Report this comment

    E. Arunan said:

    In terms of the physical forces involved, there is no difference between hydrogen bonding and van der Waals interaction. However, directionality is the hallmark of hydrogen bonding as opposed to ‘isotropic’ forces expected for van der Waals interaction. See our perspective article in PCCP last year for a dynamic criterion for a hydrogen bond.

    http://www.rsc.org/Publishing/Journals/CP/article.asp?doi=b907708a

  10. Report this comment

    R.Saravanakumar said:

    When a hydrogen atom is attached to an atom (with different electronegativity) there exists an interaction, called as hydrogen bonding.

    May this definition be more general? It can include all types of hydrogen bonding.

  11. Report this comment

    E. Arunan said:

    Saravanakumar: What you have proposed is incomplete as the simple diatomic molecule HF (hydrogen fluoride) has a H attached to an atom F with different electronegativity, but there is no hydrogen bonding in HF.

  12. Report this comment

    R. Saravanakumar said:

    Respected Prof. E.Arunan I think H-F should have hydrogenbonding, since electronegativity difference is high, so as the hydrogenbond strength. May the hydrogen bond energy is equivalent to covalent bond energy which we are unable to distinguish.

  13. Report this comment

    Paolo Manzelli said:

    Dear collegues,

    Nano technology can be a revolution in thinking about future science and tecnology becouse look at a space-time dimension where are not well applyed the traditional mechanic concepts of classical physics and also of quantum-mechamical physics. This because with the large dimension of surfice change the deeply catalytic properties of matter/ energy interactions to that this change modify the traditional expectation of chemical reactivity. For instance the traditional theorization of chemical bond ( both ionic and covalent) are not more usefull because this old conception is out of the intrepretation of entanglement effect of electonic-quantum orbitals . So that the previous very simply approaches of chemical bonds, caused only by few external electrons, now are oly too much partial or insufficient decription of the bio-chemical transformation at nano-scale level. / see BEYOND THE BOND : Ball. P.in Nature vol 469 pp 26-28 Jan- 2011. Very cordially Paolo

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