Weak hydrogen bonds – strong effects?

“That which we call a rose – by any other name would smell as sweet”. Shakespeare's relativization of names and definitions is particularly true in the realm of weak intermolecular interactions. They are always present when molecules approach each other. They are subtle, multifaceted, more than the sum of their parts, and undoubtedly important. Theoretical models not taking them into account can fail dramatically in describing structure, physical properties, and even reactivity. In organic matter and beyond, it is likely that hydrogen atoms are involved in the interaction. That has to do with the abundance of this element and its preferred terminal position in chemical bonds. It does not imply that the contacts are attractive, if the binding partners have only a modest electronegativity. Instead, they can be repulsive or even neutral. Because of their abundance and importance, there is a natural desire to better understand hydrogen contacts between molecules and to quantify their bonding character.

Due to the subtle nature of such bonding situations, this requires concerted efforts by the best available experimental techniques and the most advanced computational approaches. The identification of systems with particularly pronounced effects and of role models with a minimum of complexity is most helpful in this context. This themed issue provides a collection of such intermolecular investigations which help to bring experiment and theory together and assist in quantifying the bonding character of the participating hydrogen atoms. The issue coincides with the publication of the new IUPAC definition of the hydrogen bond^1,2 and with the 19th meeting of the traditional conference on “Horizons in Hydrogen Bond Research”.³ There appears to be a growing consensus that hydrogen bonding is not a binary on/off phenomenon which is unconditionally linked to the sign of a frequency shift⁴ or to the existence of a bond path in the electron density.⁵ Neither is it a “contact-like interaction” that is “zero as soon as the contact is broken”.⁶ It is—as most interesting things are—not easily cast into a rigid frame.

An impressive breadth of topics and methods is covered in the nearly 40 contributions to this issue. They range from solids and ionic liquids to layered nanodroplets to isolated clusters and from quantum chemistry to spectroscopy to diffraction. Low temperature techniques such as computed potential energy minima, matrix isolation, liquid rare gases and supersonic jets prevail. Statistically, CH bonds have the most prominent abundance in organic matter. Therefore, the role of weak CH hydrogen bonds in small model systems, in conformational selection, and in proteins is summarized in three Perspectives. However, these Perspectives also cover more traditional and more exotic donors. The latter are a rich source of anomalies and have consequently been dealt with in several contributions. Another class of compounds which is covered extensively is fluoro-, chloro-, and chlorofluorocarbons, exploiting the polarizing action of the electronegative substituents on neighbouring CH bonds. Similar effects may be found for hydrogen atoms directly attached to a carbonyl group and to other multiple bonds. A rich diversity of phenomena arises from the interplay between stronger and weaker hydrogen bonds during solvation, which may be studied at the level of clusters, in nanodroplets, on surfaces, or in the bulk. When ions are involved, the effects usually intensify. Interactions with π-systems are among the best characterized weak hydrogen bond phenomena, ranging from acetylene to benzene to heterocycles. Hydrogen bond bifurcation is a recurring binding motif. Chirality recognition profits from multiple contacts, which may be of weakly interacting nature and still tip over or narrow down conformational preferences. Weak and strong remain relative categories, and sometimes their role is interchanged, e.g. due to steric constraints. In their theoretical description, dispersion interactions may cease to be the most critical challenge in the future. Interchange and modulation of weak hydrogen bonds via tunnelling is regularly observed in the gas phase. Cooperativity is confirmed to be a key characteristic of hydrogen bonding, but anti-cooperative effects may also occur. Predicted and observed frequency shifts need to be carefully separated into direct and indirect effects. Overall, the diversity of the weak hydrogen bond phenomenon and the challenges in its rigorous categorization are confirmed. In conjunction with the size of some of the reported effects, one is occasionally tempted to turn the title question around: “strong effects – weak hydrogen bonds?”

The concept of a weak hydrogen-mediated union between valence-saturated molecules is now about 100 years old,⁷ with a typical ‘error’ bar for such concepts of ±10 years, bridging the early work of Werner and the later work by Huggins, Latimer and Rodebush. Its key role in shaping molecular recognition phenomena will be appreciated for many years to come, while its definition should probably always remain a bit fuzzy.

We are grateful to the members of the Cambridge Phys. Chem. Chem. Phys. team, most importantly E. Wise, for putting together this themed issue in a very professional manner, and we thank T. Wassermann for the cartoon map in the table of contents. We thank all contributors and referees for further sharpening the picture we have of hydrogen atoms located somewhere in between two molecules or even between parts of a single molecule—tightly bound to one side, but not without interest in the other side.

References

1. G. R. Desiraju, Angew. Chem., Int. Ed., 2011, 50, 52–59.
2. E. Arunan, G. R. Desiraju, R. A. Klein, J. Sadlej, S. Scheiner, I. Alkorta, D. C. Clary, R. H. Crabtree, J. J. Dannenberg, P. Hobza, H. G. Kjaergaard, A. C. Legon, B. Mennucci and D. J. Nesbitt, Pure Appl. Chem., 2011, 83, 1619–1636,  DOI:10.1351/PAC-REP-10-01-01; E. Arunan, G. R. Desiraju, R. A. Klein, J. Sadlej, S. Scheiner, I. Alkorta, D. C. Clary, R. H. Crabtree, J. J. Dannenberg, P. Hobza, H. G. Kjaergaard, A. C. Legon, B. Mennucci and D. J. Nesbitt, Pure Appl. Chem., 2011, 83, 1637–1641,  DOI:10.1351/PAC-REC-10-01-02.
3. http://www.hbond.de .
4. B. Michielsen, J. J. J. Dom, B. J. van der Veken, S. Hesse, Z. Xue, M. A. Suhm and W. A. Herrebout, Phys. Chem. Chem. Phys., 2010, 12, 14034–1044.
5. S. Grimme, C. Mück-Lichtenfeld, G. Erker, G. Kehr, H. Wang, H. Beckers and H. Willner, Angew. Chem., Int. Ed., 2009, 48, 2592–2595.
6. P. Atkins and J. de Paula, Atkins' Physical Chemistry, Oxford University Press, 9th edn, 2009.
7. T. S. Moore and T. F. Winmill, J. Chem. Soc. Trans., 1912, 101, 1635–1676.

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