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The Weak Hydrogen Bond: In Structural Chemistry and Biology (International Union of Crystallography Monographs on Crystallography)

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Chemistry is all about interactions, and chemists have traditionally classified interactions into various categories such as covalent and ionic, hydrogen bonding and Van der Waals. But this classification is primarily for convenience, and there are many borderline cases which any chemist should be aware of, if he wants to notice interesting phenomena.

One such borderline interaction that is very important in maintaining the structure of crystals is the weak hydrogen bond. Crystallographers are in a unique position to observe and catalog such an interaction, because they are constantly looking at structures frozen in time in the solid state. These are also structures that represent the dazzling chemical diversity inherent in nature. In this book, the authors, both of whom are leading authorities in the field, provide a comprehensive and extremely readable overview of this unique interaction, which should challenge the traditional wisdom of any chemist, and should allow him or her to greatly expand his or her horizons in the world of molecular interactions.

The book starts with a lucid and excellent introduction to what are usually described as 'normal' and 'strong' hydrogen bonds. The authors then gracefully demonstrate in the rest of the book by virtue of countless examples of organic, organometallic, and biological structures, how the strong and all important traditional picture of a hydrogen bond smoothly transitions to the domain of the weak hydrogen bond. Many of the rules that chemists usually apply to the notion of the hydrogen bond need to be modified and challenged, and excursions into weak hydrogen bonds actually exemplify the whole paradigm of weak intermolecular interactions. The authors explore all the evidence for such weak interactions including statistical, energetic, and spectroscopic. The crystal structures included reinforce the astonishing variety of molecular structures around us, both artificial as well as natural. There is also great simplicity in some of these structures, which makes them and the interactions in them truly beautiful to comprehend, in terms of their stability and symmetry. The discussion in every chapter is lucid, to the point, and shows the authors' own appreciation of their subject and its ramifications.

Their discussions drive home the point that chemists always need to think in terms of a continuum of interactions, if they truly want to understand the nature of molecules. In today's specialized compartments, with rigid definitions and rules, chemistry is often perceived as a science with rigid boundaries. This is far from being the case, and the weak hydrogen bond is a superb vehicle for proving the continuous nature of the science. It also demonstrates the much more general paradigm of always thinking in terms of all kinds of interactions, 'strong' and 'weak', which any chemist, no matter what his specialty, has to appreciate. More than anything else, the study of such weak interactions proves that chemistry is still very much an art with many thin boundaries between concepts, and not just a science. It is not an exact science like physics, but it is precisely this ambiguity in it which nonetheless can be classified, that makes it a unique discipline. This book is a striking example of this fact.

"The Weak Hydrogen Bond in Structural Chemistry and Biology" is an excellent reference and collection of research that emphasizes the presence and importance of weak hydrogen bonds. I heartily recommend it. My own work starting in 1967 and continuing to the present (Hansen Solubility Parameters: A User's Handbook, 2nd Edition, CRC Press, Boca Raton FL, 2007, and Hansen Solubility Parameters in Practice - eBook/software at www.hansen-solubility.com) has also quantified the weak hydrogen bonds in terms of the cohesive energy they create. There is mutual agreement in the present book as well as my work as to their presence and importance. The so-called Hansen solubility parameters (HSP) quantify not only the weak and "strong" (molecular) hydrogen bonding cohesive energy, but also quantify the simultaneously acting permanent dipole-permanent dipole (molecular) bonding cohesive energy and the cohesive energy deriving from (atomic) dispersion interactions, estimated from the cohesion energy of the hydrocarbon homomorph using a corresponding states procedure. The presence and importance of the weak hydrogen bonds has been confirmed in both approaches to describe and to understand them. In my own work it has been shown many times that one can predictably mix two non-solvents (having the same Hildebrand or total solubility parameters) to dissolve a solute. The HSP of the mixture, found by a simple volume fraction additivity rule, need only be close enough to the HSP of the solute to allow solution. The HSP, including the (weak plus strong) hydrogen bonding parameter, have been applied successfully to predictions on the behaviour of gasess, liquids, solids, organic salts, inorganic salts and ionic liquids, polymers, DNA, pharmaceuticals, skin, and a host of other materials. There is no doubt that the weak hydrogen bonds play an important part in solubility in all of these situations. About 10,000 chemicals have been assigned HSP, and with knowledge of SMILES, for example, additional chemicals can be added to the list.

I only wish that I had been aware of this book earlier.

Charles M. Hansen