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Reflections on the Hydrogen Bond in Crystal Engineering Published as part of the Crystal Growth & Design 10th Anniversary Perspective Gautam R. Desiraju Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India ABSTRACT: Comments on aspects of the new definition of the hydrogen bond specific to crystal engineering are given.
important is that “the hydrogen bond is an (attractive) interaction, in which there is evidence of bond formation.” These interactions actually arise from attractions, and we call them attractive interactions. One could have just as well dropped the word attractive from the preamble, with no loss in scientific meaning. However, the phrase “attractive interaction” is used often in the crystal engineering literature—it suggests that the interaction is one that arises from an attraction between certain atoms and that the mutual arrangement of these atoms in the equilibrium geometry leads to a net stabilization. Such matters become more important when one considers contact distances between nonbonded atoms that are shorter than the equilibrium distance. At these shorter distances, there is a net repulsion between the atoms. This may be seen in some CH 3 3 3 O contacts formed by particularly unactivated CH groups, say a methyl group at the end of an alkyl chain. In certain cases, the contact distances are so short that the geometries are destabilizing, in other words the potential energy is greater than zero. Many of these rather short CH 3 3 3 O contacts are also characterized by small hydrogen bond angles, and one must be particularly watchful and not call them significant hydrogen bonding interactions just because the C 3 3 3 O distance is short.4 Again, there are two aspects in the new definition that are of particular interest to crystal engineers. The first pertains to the geometrical criteria employed to characterize an XH 3 3 3 Y interaction as a hydrogen bond. The definition states “Historically, the X to Y distance was found to be less than the sum of the van der Waals radii of X and Y, and this shortening of the distance was taken as an infallible indicator of hydrogen bonding. However, this empirical observation is true only for strong hydrogen bonds. This criterion is not recommended.” Unfortunately, the van der Waals
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he recent recommendations on the definition of the hydrogen bond, submitted to the International Union of Pure and Applied Chemistry, are worthy of attention.1 Hydrogen bonding has been extensively studied, and yet, 90 years after the term entered the literature, there still seems to be no decline in the interest it elicits. A new paper on the subject is added to SciFinder every hour. A number of sites, such as Chemical and Engineering News, Chemistry World, and Nature Blogs, have reported this new recommendation, which supersedes the earlier 1997 IUPAC definition. It is likely that the new definition will soon find its way into the Gold Book. I, among others, have commented on the IUPAC recommendation in generality.2,3 The editor-in-chief of Crystal Growth and Design subsequently asked for my opinion on the new definition, and I agreed because I know that there are sufficient aspects specific to crystal engineering to warrant this additional commentary. The definition itself consists of a preamble, lists of criteria, characteristics, and footnotes. The complete definition is quite long, running into a few pages. The short preamble begins with the statement that “the hydrogen bond is an attractive interaction between a hydrogen atom from a molecule or a molecular fragment XH 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”. All crucial features of the interaction are touched upon in the preamble. Noteworthy among these is that H must be more electropositive than X (ruling out the BHB bond from the domain of hydrogen bonding). Curiously, the preamble does not specify what evidence is good evidence and, more importantly, does not define the word “bond”. I would like to say a few words about the phrase “attractive interaction”. We all know that it is forces that are attractive or repulsive and that interactions are either stabilizing or destabilizing depending on whether the potential energy associated with them is lesser or greater than zero. So, how can an interaction be attractive? The crucial phrase in the preamble, however, is the phrase, “in which there is evidence of bond formation”. What is r 2011 American Chemical Society
Received: January 23, 2011 Revised: February 28, 2011 Published: March 17, 2011 896
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Crystal Growth & Design distance criterion still seems to be applied to the heavy atom distance X 3 3 3 Y in the assessment of a contact as a potential hydrogen bond. This does not cause serious problems for strong hydrogen bonds, but it may result in certain weak hydrogen bonds being overlooked. The use of the van der Waals criterion is always problematic for the weakest of hydrogen bonds, such as the CH 3 3 3 π interactions. More relevant is how good the hydrogen bond positions are. When the van der Waals criterion is applied to the H 3 3 3 Y separation (with say, a hydrogen van der Waals radius of 1.2 Å), there are no known problems.5 For example, there are hardly any viable CH 3 3 3 π hydrogen bonds in which the H 3 3 3 π distance is greater than 2.9 Å. As the hydrogen bond becomes weaker, it may become necessary, while assessing it, to consider both the length H 3 3 3 Y and the hydrogen bond angle XH 3 3 3 Y. As mentioned earlier, short contacts with small hydrogen bond angles need to be examined especially carefully. Some longer contacts may be viable, provided the angle is close to 180°. The second aspect of the definition that is of relevance to crystal engineering pertains to the directionality of hydrogen bonds and the ways in which they influence crystal structures. The definition states that “Hydrogen bonds are directional and influence crystal packing modes in chemically understandable ways. The crystal packing of a non-hydrogen-bonded solid (say, naphthalene) is often determined by the principle of close-packing, and each molecule is surrounded by a maximum number of other molecules. In hydrogen bonded solids, there are deviations from this principle to a greater or lesser extent depending upon the strengths of the hydrogen bonds that are involved. Correspondingly, the hydrogen-bond geometries are conserved with fidelities that depend on their strengths.” It has always been well-known that hydrogen bonds are directional, and the present definition seems to reiterate the obvious, but this aspect of hydrogen bonding, which is so important in crystal engineering, has generally not found a mention in previous formal definitions of the interaction. As far back as 1953, J. M. Robertson wrote that “the hydrogen bond is something much more specific than merely a stronger type of attraction between molecules. It is effective only in certain definite directions, and this directive power is sometimes capable of maintaining an unusually open structure, where ordinary packing considerations would indicate the possibility of alternative structures of higher density”.6 Robertson also stated that “another generalization derived from a study of these various crystal structures is what may be termed the principle of maximum hydrogen bonding. All the available hydrogen atoms attached to the electronegative groups are generally employed in hydrogen bond formation. Some of the bonds formed may be weaker than others, but the molecular packing is generally capable of adjustment in such a way as to fulfill this condition. Sometimes the resulting structures may not be the most compact that might be devised, but this condition and the steric requirements are nevertheless generally obeyed.” Surely, few will miss the point that, during the 60 years since Robertson wrote these words, many of us have been proving his generalizations again and yet again. As for the distinction between hydrogen bonded and closepacked structures, Robertson referred mostly to ice and resorcinol, calling them open structures. In general, however, the conflict between packing and interactions cannot be observed very well in molecular solids, unlike in extended inorganic solids, wherein the contrast is marked. For example, a ccp or hcp metal is close packed while bonding interactions dominate in the more open diamond and ZnS structures. Kitaigorodskii’s close packing principle7 seems to muffle any kind of directionality in the overall
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packing of organic molecular solids that hydrogen bonding might be likely to confer. Of course, we have compounds such as adamantane-1,3,5,7-tetracarboxylic acid8 and trimesic acid,9 in which hydrogen bonding leads to open framework structures that are interpenetrated. It is only in recent times when studies of polymorphic structures have reached a certain standard of detail and precision that we are actually beginning to see the packing versus interaction dichotomy illustrated experimentally.10 We have open structures that are characterized by numerous directed hydrogen bonds on the one hand and then thermodynamic structures which are characterized by lower energies and very often by higher densities and higher packing coefficients. The latter structures are generally poorer in terms of their interaction metrics. An early example is furnished by the dimorphs of pentachlorophenol.11 The ordered form contains OH 3 3 3 O hydrogen bonds. The second high temperature form is disordered and is isomorphous to hexachlorobenzene. Here, hydrogen bonding seems to have disappeared. A major area of crystal engineering where there is still much work to be done is in understanding and eventually controlling the balance between directional interactions and the drive toward close packing. In this connection, there is a link between these themes and the presence of multiple molecules in the crystal asymmetric unit, Z0 . Generally, though not always, higher Z0 structures are associated with better intermolecular interactions but have a looser packing.12 These structures are kinetically favored. Analogously, and I have mentioned this elsewhere,13 every compound must have a low energy structure with a Z0 value of unity, even if the establishment of such a structure leads to suboptimal hydrogen bonds or other directional interactions—or even their absence. Such a crystal structure could well be disordered. Perhaps such thermodynamic crystal structures, which defy Robertson’s generalization, will be observed experimentally for compounds with melting points that are sufficiently high so that the compounds are still solids at the temperatures that are required to obtain them. Alternatively, and in the absence of experimental isolation, I suspect that such Z0 = 1 structures could be obtained computationally. A drawback in the presently available software is that we are unable to simulate disordered structures. There is a definite need to be able to handle this lacuna. A reviewer suggested that I include here a definition of the hydrogen bond that is more specific to crystal engineering than the general definition submitted to IUPAC. I am not in philosophical agreement with this suggestion. The hydrogen bond is a complex phenomenon, and in the description of all complex phenomena there is a conflict between accuracy and generality. I would always prefer a definition that is more general, that each scientist can evaluate and then find specifically in it whatever he or she wants to behold. The present definition is of very broad scope and should stand the test of time in a general chemical sense. Let me conclude then by saying that there is still a great deal of research on hydrogen bonding that can be undertaken by crystal engineers. This is a limitless field where new discoveries will only be constrained by the imagination of the researcher.
’ ACKNOWLEDGMENT I thank the DST (New Delhi) for the award of a J. C. Bose fellowship, and E. Arunan (Indian Institute of Science) for several valuable discussions. 897
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’ REFERENCES (1) http://www.iupac.org/web/nt/2010-10-25_hydrogen_bond. (2) Desiraju, G. R. Angew. Chem., Int. Ed. 2011, 50, 52–59. (3) Arunan, E. Curr. Sci. 2010, 99, 1493. (4) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond in Structural Chemistry and Biology; OUP: Oxford, 1999; pp 108113. (5) Raghavendra, B.; Mandal, P. K.; Arunan, E. Phys. Chem. Chem. Phys. 2006, 8, 5276–5286. (6) Robertson, J. M. Organic Crystals and Molecules; Cornell University Press: Ithaca, 1953; p 224, 239. (7) Kitaigorodskii, A. I. Molecular Crystals and Molecules; Academic Press: New York, 1973. (8) Ermer, O. J. Am. Chem. Soc. 1988, 110, 3747–3754. (9) Herbstein, F. H. Top. Curr. Chem. 1987, 140, 107–139. (10) Desiraju, G. R. Angew. Chem., Int. Ed. 2007, 46, 8342–8356. (11) Masood, M. A.; Desiraju, G. R. Chem. Phys. Lett. 1986, 130, 199–202. (12) Nangia, A. Acc. Chem. Res. 2008, 41, 594–601. (13) Desiraju, G. R. CrystEngComm 2007, 9, 91–92.
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