An IUPAC commission has delivered a technical report on the definition of the hydrogen bond. Unfortunately, it does not as yet seem to be available through Pure and Applied Chemistry, but one of its lead authors, Gautum Desiraju, has written a personal perspective in the first issue of this year’s Angewandte Chemie.1
Hydrogen bonding may be to some extent within the eye of the beholder. If the “hydrogen bond” is worth less than a single kcal mol-1, how does that really differ from van der Waals interactions or London dispersion? If the interaction is upwards to 40 kcal mol-1, do we benefit from not simply calling that a bond? Further complexity comes in the nature of the hydrogen bond: is it simply strong dipole-dipole attraction? Does it possess some covalent character? What is its dispersion component? And can it have some charge transfer character? Is it perhaps some or all of these? Or does the particular environment dictate the nature?
Desiraju argues really for as broad a swath as possible, and the new definition borrows from Pauling’s original definition:
Under certain conditions an atom of hydrogen is attracted by rather strong forces to two atoms, instead of only one, so that it may be considered to be acting as a bond between them
ads in a dash of the Pimentel and McClellan definition:
(1) There is evidence of a bonds and (2) there is evidence that this bond specifically involves a hydrogen atom already bonded to another atom
to come up with
The hydrogen bond is an attractive interaction between a hydrogen atom from a molecule or molecular fragment X-H in which X is more electronegative than H, and an atom or a group of atoms in the same or different molecule, in which there is evidence of bond formation. A typical hydrogen bond may be depicted as X-H…Y-Z, where the three dots denote the bond. X-H represents the hydrogen bond donor. The acceptor may be an atom or an anion Y or a fragment or molecule Y-Z, where Y is bonded to Z. In specific cases X and Y can be the same with both X-H and Y-H bonds being equal. In any event, the acceptor is an electron-rich region such as, but not limited to, a lone pair in Y or a π-bonded pair in Y-Z.
Broad enough to cover just about everything! But it demands “evidence of bond formation” and the commission spells out a series of experiments/computations that might provide this evidence. One might wonder if this list is acceptable and complete.
While I think this is an interesting and necessary step forward, debate on hydrogen bonding is sure to rage on!
References
(1) Desiraju, G. R., "A Bond by Any Other Name," Angew. Chem. Int. Ed. 2011, 50, 52-59, DOI: 10.1002/anie.201002960
Henry Rzepa responded on 07 Jan 2011 at 1:37 am #
This indeed sounds like the kind of argument which might in some circumstances be likened to pondering how many angels can dance on the point of a needle. The concept of a bond introduced the idea of functional groups and transferability between molecules. This concept has allowed generations of researchers to eg analyse spectroscopic data etc, or develop synthetic strategies for molecules, and has obviously proved immensely useful. But the idea that bonds cluster into kinds (single, double,aromatic, etc) may I fear not hold for hydrogen bonds. For example, one might introduce two real life metaphors. Firstly, there is the fixed ratio gearbox in the transmission of a car. It has ~4-6 speeds only. Then there are the continuously variable gearboxes. I suspect the hydrogen bond may have some attributes of the latter. Thus here I speculate whether the dihydrogen bond might be continuously variable. Depending on substituents, it may be partially covalent, through electrostatic up to dispersion and beyond. If a passenger in a car using a continuously variable transmission asks what gear are we in, how do you answer them?
Perhaps ultimately, the only way to characterise a weak bond is to develop good techniques for probing the Laplacian of the electron density. This relatively recent review (DOI: 10.1002/anie.200501734 outlines how this quantity may be experimentally measured (and indicates how very difficult it actually is to do so). I would suggest that the instrumentation for such measurements (which, as I understand is available in very few laboratories) become more widely used. It is also of interest that the Laplacian may be one of those properties where calculating it reliably may be cheaper than measuring it! Like cosmologists, we may be moving into an era in chemistry where calculations are the only alternative to difficult if not impossible experiments?