CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 5 1661-1662
Perspective ...And Another Comment on Pseudopolymorphism Joel Bernstein* Department of Chemistry, Ben-Gurion University of the Negev, Beer Sheva, Israel 84105 Received May 6, 2005
In the recent exchange of comments on the use of the term pseudopolymorph,1,2 my book3 was cited, indicating implicitly, if not explicitly, that I accept or even approve of the use of the term. Since the contrary is closer to the truth, I would like to make my position clear on the issue. The author of a book on a general subject constantly faces decisions on what material to include and what material to omit. The term pseudopolymorph fell into that category of dilemma. As both Seddon and Desiraju pointed out, the term is in use in the literature; its existence cannot be denied. In writing an opening chapter titled “Introduction and Historical Background” in a book on the general subject of polymorphism, I would have been remiss, or even irresponsible, to have ignored the term. The paragraph in question reads as follows: The literature on polymorphism and related phenomena has spawned a number of additional definitions and terms that potentially lead to confusion rather than clarification. One of these is pseudopolymorphism, and it is of interest that authors (McCrone 1965;4 Haleblian and McCrone 1969;5 Dunitz 1991;6 Threlfall 19957) who have given serious thought to the definition of polymorphism and its ramifications almost unanimously argued, strenuously in a number of cases, against the use of the term pseudopolymorphism. Even in arguing against its use, for the sake of completeness and to define some phenomena which are not to be considered as polymorphic behavior, it is impossible to ignore the term and how it has been used and perhaps misused in the past - caveat emptor! Hence, I included the definition along with the historical use. In the opening paragraph to the section titled “Pseudopolymorphs, Solvates and Hydrates”, I also indicated a lack of enthusiasm for the continued use of the term. I agree with Seddon that scientists must * E-mail:
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be precise about definitions, and I am of the opinion that pseudopolymorph is a misnomer. Solvates and hydrates are just thatsthey are not pseudo (as Seddon quoted the definition) anything, and they should be called what they are. The expanding frontiers and the blurring of distinctions between the classic disciplines of chemistry have led to many of the problems of the definition of concepts and phenomena. Some of the debates surrounding these definitions remind one of the mediaeval controversy regarding the number of angels one could put on the head of a pin. Over a quarter of a century of teaching first-year chemistry has been an enlightening experience for me in this regard. First-year students in chemistry (or beginning students in any discipline for that matter) tend to try to pigeonhole concepts and facts. It simplifies their learning process. Thus, when stating some fact or generalization in a lecture, I am often faced with an always/never question: “Is that always (or never) true?” In chemistry, we rarely deal with always/ never situations. We deal with general rules and exceptions to those rules. In fact, it is the exceptions, or outliers that ignite our curiosity and lead us to new experiments, new theories, and new developments. If everything worked as “always” or “never”, there would be nothing new to discover. Nevertheless, our definitions reflect the always/never standard. They are idealized situations. A classic example, again from introductory chemistry, is the way we teach the chemical bond. Virtually all textbooks define and describe the “pure” ionic bond, often using NaCl as the model. They define and describe the “pure” covalent bond based on the Lewis model of two shared electrons and the quantum mechanical extensions of valence bond or molecular orbital models, often using the hydrogen molecule as the prototypical example. Virtually none of the bonds that we actually deal with on a day to day basis are either purely ionic or purely covalent, so we often describe them as “covalent bonds with a certain percentage of ionic character” or “polar bonds”. The point is that the pure definitions give us a
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conceptual framework for describing and understanding the real situations. So it is with polymorphs and solvates. The discussion of definitions will almost always degenerate to a debate on how to pigeonhole a particular situation that does not quite correspond to the strict definition; attempts will then be made either to modify that definition or, as seems increasingly popular in the literature, to define some new phenomenon or new situation. Sometimes those modified definitions are indeed useful; often they are not. At the risk of being trite, however, as Shakespeare’s Juliet is often quoted, “What’s in a name? That which we call a rose By any other name would smell as sweet.” We should try to make our definitions as clear and concise as possible, knowing full well that they cannot possibly describe every (as yet unimagined) situation. Where does pseudopolymorphism come into this picture? I believe it is a misnomer that should be abandoned. I recognize that the term is in the literature, and it cannot be erased from there, but we do not need to prolong or to promulgate its use or existence. I ascribe to Seddon’s prescription: editors, referees, and authors should be encouraged to eliminate the term from current usage. I believe that will eliminate some confusion,
Perspective
especially among the non-cognoscenti and will make communicating our ideas and results more efficient. For instance, how does one today describe, say, two solvates of the same compound with identical stoichiometry but different crystal structures? Are they ”polymorphs of pseudopolymorphs”? We don’t need these linguistic gymnastics. They are polymorphs of a solvatespure and simple. If the stoichiometry or the solvent is different then they are different solvates, not polymorphs and not pseudopolymorphs. One can of course create other terms to describe these (as surely some will try), but for me at least, in most cases that would be a return to counting angels. References (1) Seddon, K. R. Cryst. Growth Des. 2004, 4, 1087. (2) Desiraju, G. R. Cryst. Growth Des. 2004, 4, 1089-1090. (3) Bernstein, J., Polymorphism in Molecular Crystals; Oxford University Press: Oxford, 2002; pp 4-8. (4) McCrone, W. C. Physics and Chemistry of the Organic Solid State; Fox, D., Labes, M. M., Weissberger, A., Eds.; WileyInterscience: New York, 1965; Vol. 2, pp 725-767. (5) Haleblian, J.; McCrone, W. C. J. Pharm. Sci. 1969, 58, 911929. (6) Dunitz, J. D. Pure Appl. Chem. 1991, 63, 177-185. (7) Threlfall, T. The Analyst 1995, 120, 2435-2460.
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