disappearing and reappearing polymorphs—an anathema to crystal engineering?

Pergamon CRYSTAL ENGINEERING, Vol. 1, No. 2, pp. 119-128, 1998

Copyright © 1998 Elsevier Science Ltd Printed in the USA. All rights reserved

0025-5408/98 $19.00 + .00

PII S0025-5408(98)00086-5

DISAPPEARING AND REAPPEARING POLYMORPHS--AN ANATHEMA TO CRYSTAL ENGINEERING?

J. Bernstein* and J.-O. Henek Department of Chemistry, Ben-Gurion University of the Negev, Beer Sheva, Israel 84105

(Refereed) (Received November 11, 1997; Accepted January 28, 1998)

ABSTRACT "Crystal engineering" implies control over the design and preparation of desired crystal structures. That control can be compromised by the existence or appearance of polymorphs for the compound in question. Regaining control requires an understanding of the conditions for obtaining each of the crystal- line forms of the material. Chemical microscopy can be useful in determining those conditions. The historic importance of thermomicroscopic methods is reviewed and a number of examples of translating thermomicroscopic observations into single crystals suitable for X-ray study are presented. © 1998 Elsevier

Science Ltd

KEYWORDS: thermomicroscopy, chemical microscopy, metastable phases, historical perspective, crystallization conditions, polymorphism

"A number of difficult and largely unattempted problems continue to baffle and challenge the structural chemist. Polymorphism is still an imperfectly understood phenomenon which could be more of a nuisance in particular groups of structures than in others."--G.R. Desiraju, 1989

INTRODUCTION

The title of this paper echoes the title of Chapter 10 of Desiraju's landmark 1989 monograph entitled "Crystal Engineering, The Design of Organic Solids" [1], which is the subject of this new journal. That chapter is also the source of the lead quotation. Since the publication of that

*To whom correspondence should be addressed.

119

120 J. BERNSTEIN and J.-O. HENCK Vol. 1, No. 2

book, interest in polymorphism has increased significantly [2], its commercial significance has been clearly recognized and exploited [3], and the study of polymorphism has gained considerable impetus [4]. We recently reviewed examples of so-called "disappearing polymorphs" [5], materials for which more than one crystal form had been prepared and documented, but which apparently could no longer be obtained once another form appeared. At the end of that paper we stated that "we believe that once a particular polymorph has been obtained it is always possible to obtain it again; it is only a matter of finding the right experimental conditions." The purpose of this paper is to focus on this question by putting it into historical perspective and describing some recent results from our laboratories which provide further evidence for its verity.

Historical Background--Crystal Engineering. G.M.J. Schmidt's coining of the phrase "crystal engineering" [6] was intended to convey the attributes of design and control to what has always been, and what remains very much today, the art of growing crystals. The term "engineering" thus invokes thoughts of buildings, dams, bridges, all designed with a specific purpose in mind. The first principle of that design is to define the function of object--to what purpose will it be put?--before other factors such as aesthetics can be considered. As defined in Webster's New World Dictionary, engineering is "a) the science concerned with putting scientific knowledge to a particular use, divided into different branches as civil, electrical, mechanical or chemical engineering; b) planning, designing, construction or management of machinery, roads, bridges, buildings, waterways, etc."

So it is with crystal engineering. Schmidt's original desire to "engineer" crystals followed nearly a decade of success in developing and proving the topochemical principles [7], to a large extent on the basis of [2+2] solid-state photochemical reactions [6]. Trained as both an organic chemist and a crystallographer, he saw the synthetic potential of these reactions in particular, and the utilization of the chemical and physical properties of the organic solid state in general, which could result from his pioneering efforts. To further develop the field he was not satisfied being dependent on the apparently chance vagaries of crystal structures, and sought ways tO design and to control the way molecules crystallize--to engineer crystal structures.

One of the early successes of that effort was the discovery of the so-called "dichloro rule" [6]. The topochemical principles for photochemical [2+2] solid-state reactions that would lead to dimers with mirror symmetry required that the reactant crystal structure have a short 4 A translation axis, in order to bring the reactive centers into proper registry prior to the reaction. Schmidt looked for ways to generate that 4 ,~ axis--to engineer crystals with that structural property. The observation by B.S. Green, at the time one of Schmidt's postdoctoral fellows, that chlorine-substituted compounds seemed to have a tendency to crystallize with a short axis [8] led to a literature survey (before the CSD was so readily accessible or contained so much data), and the experimental determination of the cell constants of many compounds containing aromatic chlorine substituents. The results showed that such a mode of substitution led to the desired structure in more than 70% of the samples studied [8]. While such a rate of success would not suffice in the traditional engineering disciplines, it was a significant development in demonstrating that one need not necessarily rely strictly on chance when attempting to obtain crystals with desired structures.

In the more than a quarter century since that initial success, crystal engineering has developed and matured, with two significant milestones being the publication of Desiraju's

Vol. 1, No. 2 DISAPPEARING AND REAPPEARING POLYMORPHS 121

book summarizing the developments to 1989, and now the launching of this journal dedicated to the subject.

In traditional engineering, failure may be defined as when the bridge collapses or if the dam gives way. How can we classify failure in crystal engineering? Either we don't obtain the desired structure at all, or in addition to the desired structure there are others-- polymorphs--that dominate the crystal growth process.

Historical Background--Polymorphism. The phenomenon of polymorphism is not new to chemistry. It was recognized by Mitscherlich [9] in sodium dihydrogen phosphate (NaHaPOg'HaO) not long after the publication of Dalton's atomic theory [10]. Nineteenth century chemists were very much aware of the properties of solids and Ostwald defined the well-known "Rule of Steps" [ 11 ]. In the decades preceding the development of spectroscopic and X-ray crystallographic methods, the characterization of solids was a crucial aspect of the identification of materials. Chemists grew crystals carefully in order to obtain characterizable morphologies and then determined physical properties such as color, interfacial angles, indices of refraction, melting point, even taste! [12-14]. Being critically observant was essential, for there was little other information to rely on. The microscope, in particular the polarizing microscope, was an invaluable tool in these endeavors [15]. A tremendous amount of information on the properties of crystals was obtained. Much of these data were metic- ulously compiled by P.H.R. yon Groth, a professor of mineralogy at Munich, one of the giants in crystallography prior to the use of X-rays. His five-volume compendium [16], published between 1906 and 1919, of which the last three are devoted to organic compounds, contains a wealth of information on growing and characterizing the crystals of many substances. The description includes details on the preparation of polymorphs when they were known to exist. A subsequent compilation of polymorphic organic substances [17] by Deffet, entitled "Repertoire des Composes Polymorphes" and strangely published in Liege in 1942, contains references to many of the polymorphic substances described by Groth as well as subsequent ones. Both of these compilations have been somewhat "lost" because of the limited distribution during the World Wars and the fact they are written in German and French, respectively. Nevertheless, they testify to the widespread recognition and investigation of polymorphic materials during the 19th century and well into the 20th. Alexander Findlay, in the fifth edition (1923) of his classic "The Phase Rule and Its Applications" [18] (first published in 1905), noted that " . . . polymorphism is now recognized as of very frequent occurrence indeed." Nevertheless, M.J. Buerger, one of the pioneers of modern X-ray crystallography, was prompted to write in 1937 that " . . . to most chemists [polymorphism] is still a strange and unusual phenomenon" [19].

With the development of X-ray crystallography, and the increasing emphasis on the microscopic structure of crystals (i.e., elucidation of the details of molecular structure and intermolecular interactions), the optical microscope in the hands of most organic crystal chemists and chemical crystallographers, at least, has been relegated from a principal research tool [20] to merely an aide in choosing and mounting crystals for diffraction experiments. This has not always been the case. The art of doing chemistry under the polarizing microscope reached a high degree of sophistication, and an entire textbook of microscopic qualitative analysis was published [21]. Recently, these techniques, employing nanogram size samples, were used by W.C. McCrone in the ongoing controversy over the origin and authenticity of the "Shroud of Turin" [22], and to investigate and verify the mechanism of a solid state reaction [23]. The early thermal and gas/solid reactions studied by


Curtin and Paul at Illinois [24,25] in the 1960s and 1970s were also investigated by similar techniques on a vintage 1911 Bausch & Lomb polarizing microscope resuscitated from the laboratory debris in the basement of Noyes Laboratories [26]. This demise of the polarizing microscope as a research tool was also lamented by Fred McLafferty in an editorial in Accounts of Chemical Research [27].

In the latter half of this century the practice of chemical microscopy in organic chemical crystallography has been maintained by essentially two groups--that of McCrone himself [28-30] and the Innsbruck school (Institute of Pharmacognosy at the University of Inns- bruck) founded by L. Kofler and A. Kofler [31] and their succeeding generations, first Marie Kuhnert-Brandstatter [32] and more recently Artur Burger. The latter group has concentrated mainly on pharmaceutical substances, and has been particularly successful in identifying many polymorphic substances [33,34], only some of which have been further characterized by infrared, X-ray diffraction, and DSC techniques.

In spite of the success of the McCrone and Innsbruck groups in identifying and characterizing polymorphs, there has not been a renaissance in the use of thermomicroscopic methods. In fact, while thermomicroscopy is one of the most rapid and most sensitive methods for detecting the presence of polymorphism, for instance in pharmaceuticals, there is not a single U.S.P. standard that is based on microscopic examination [35]. The reason appears to be quite simple. The examination is still often a subjective one, easy to show to someone with a picture, but more difficult to explain in words, and virtually impossible to quantify. The real practitioners of chemical microscopy are very skilled masters of their

science, but that mastery has become one acquired and passed on much more by appren- ticeship than through textbooks (which do exist).

Nevertheless, if the identification and characterization of polymorphs is so facile by thermomicroscopic methods, why haven't chemical crystallographers become devotees of the technique? The answer, we think, lies in the fact that very little has been done to translate microscopic observations into macroscopic crystals which are suitable for spectroscopic investigation or X-ray structure determination. That gap is closing because of the development of instruments capable of spectroscopic and X-ray measurements on increasingly smaller samples. The IR microscope [36] and the CCD detector for X-rays [37] are but two examples of this instrumental revolution.

These are very promising developments indeed, but we have taken a different approach. We believe that the thermomicroscopic observations can provide guidelines for crystallization experiments that can lead to macroscopic crystals suitable, for instance, for X-ray structure determination. To that end we have recently undertaken the thermomicroscopic study of a number of the "disappearing polymorphs" cited earlier [5], with the eye to preparing crystals for subsequent X-ray structure determination.

EXPERIMENTAL

We concentrated our choice of compounds for study mainly, but not exclusively, on those we earlier described as "disappearing polymorphs" [5J--that is, they had once been shown to exist, but had become elusive with the appearance of an additional polymorph. The goal was to prepare crystals of as many as possible of the previously reported polymorphs, and additional ones, if we encountered them in the course of our preliminary studies. The strategy was to study the material first on the hot stage microscope to characterize the thermodynamic


behavior. If sufficiently large samples could be obtained, 1R spectra, X-ray diffraction data, and DSC were measured, but this was not necessary or always possible; the microscopic investigation is sufficient to give a good measure of the relative stabilities of the various phases; in many cases an energy/temperature-diagram [38] and/or phase diagrams may be prepared, summarizing the (thermodynamic) relationships among the various phases. Fol- lowing the growth of crystals, to be described here, we subsequently carded out crystal structure analyses. They will be described in separate publications.

C COOC~5 NO2

:O2N.~ NH2 OH

Nt>z

Benzocaine:Picric Acid, I. This binary system was studied at least three separate times [39-41]. At the start of our study the material was known to exhibit a low melting (132°C) form (used as a pharmacopoeial standard) and a high melting (162-163°C) form. The latter may be obtained from the former by "excessive drying of the isolated substance at 105°C '' [39] for at least 1 h or by vacuum drying/sublimation. Togashi and Matsunaga [41] (without referring to the earlier work) had apparently also observed an additional complex of com- position (2:1) [42]. No single crystal experiments or structure determinations had been reported.

Since the higher melting form is the thermodynamically preferred one, an "equilibrium" crystallization is preferred over a "kinetic" one. The drastic conditions described are not conducive to an "equilibrium" situation. Also, the presence of water is clearly problematic in this process. Hence we resorted to a non-aqueous gel-diffusion crystallization [43,44] using Sephadex as the gel medium. Benzocaine was dissolved in a 3:1 chloroform:methanol mixture in the gel. Picric acid was dissolved in the same solvent mixture. Large (1 x 1 X 2 mm) single crystals were obtained after 3 days at 20°C. The lower melting form is less stable, so a high temperature (80°C) crystallization was attempted, with water as the solvent yielding single crystals (1.2 X 1.3 X 0.5 mm). Seeds of the stable form must be excluded. Hence, we attempted and succeeded in obtaining the less stable form prior to attempting experiments to obtain the more stable one.

The thermomicroscopic evidence clearly showed two eutectics, strongly suggesting the presence of an additional complex. Since the second eutectic appeared in the benzocaine regions of the microscopic preparation, this was suspected to be the complex with 2:1 stoichiometry [40]. Crystals of the complex were obtained by slow evaporation (ca. 4 weeks) of a 1:1 mixture of the components in isopropanol at 4°C. Admittedly, this was not an experiment designed to obtain the 2:1 complex, but there was already ample evidence to suggest its existence, which increased our care in examining all the crystals obtained. All


complexes were yellow, but were distinguished by their morphology, and clearly, their melting points.

The rather drastic drying procedure described by Nielsen and Borka to obtain form I, Borka and Kuhnert-Brandst~itter's report, and our observations on the hot stage microscope indicated the existence of a hydrate of the complex. Hence we attempted a crystallization from a saturated water solution in a sealed virgin flask (to prevent the unintentional incursion of seeds of any of the other forms) at 20°C; crystals (approximately 4 × 0.8 × 0.7 mm) appeared after 48 h.

o

II

p'-Methylchalcone, II. This substance has been shown to exist in 13 different polymorphic forms. Weygand and coworkers investigated this substance by thermomicroscopy over a period of more than 10 years and summarized their results in a review in 1929 [45]. To the best of our knowledge, p'-methylchalcone is the "world record holder" in the polymorphic behavior of an organic compound. However, to date no structural information on this material has been published. From Weygand's precise description of his thermomicroscopic investigations, seven of these modifications (he called them "main forms") are monotropically related with a very high probability. That means these crystal forms do not have an intersection point of their G-isobars between 0 K and the melting point of the lowest melting modification (m.p. 44.5°C) at ambient pressure [46]. On the other hand, the experiences described in [5] showed that the crystallization of a room temperature thermodynamically unstable crystal form can take place only if seeds of the stable, in this case the highest melting form, are excluded. Because of the very small sample size and the fact that investigations are carried out between glass slides, thermomicroscopic investigations more nearly approach seed-free conditions than most other crystallization experiments. However, in the case of II the material has to be synthesized (and crystallized) in larger quantities in advance just to begin these experiments. This differs from benzocaine:picric acid, where the preparation of the complex can be performed by means of thermomicroscopy. Since chemists are trained to maximize reaction yields, in the case of II such a strategy will lead to the highest melting form (m.p. 75°C) from the crystallization process after the reaction. To then take this material and try to grow single crystals of a thermodynamically unstable form from solution by recrystallization experiments is well nigh impossible, because once one has the stable form then there are seeds of it around the laboratory, which will tend to induce the crystallization of the stable form. The sensitivity of a particular system to unintentional seeding from a particular polymorph is variable from compound to compound. In some instances it may be possible to "decontaminate" a laboratory from the unwanted seeds [39]. In our experience, in the case of II, such decontamination is considerably more difficult. Therefore, crystallization of a thermodynamically unstable form of II must be carried out with the reaction solution in virgin glassware. Since our aim was to obtain at least one single crystal of each


of a number of the previously reported unstable modifications (also observed by us on the hot stage microscope) suitable for X-ray diffraction experiments, we attempted to carry out the reaction and the crystallization at three temperatures (20°C, 4°C, and -13°C) and with three different solvents (methanol, ethanol, and 2-propanol). These experiments led to single crystals of five thermodynamically unstable modifications. To date, three of these structures have been solved, the other two modifications were so unstable that they transformed at room temperature (20°C) into another form during data collection, but we are confident that the X-ray diffraction technology is sufficiently advanced that these problems will soon be overcome.

0

III

Benzophenone, III . Five different modifications of benzophenone are described in the "old literature," as summarized by Groth [47]. A 1910 Ph.D. thesis from Marburg (Germany) submitted by K. Schaeling in 1910 [48] reports extensive work on determining reproducible crystallization conditions for obtaining one of these room temperature unstable modifications (m.p. 26.5°C): Schaeling found that heating the melt of I I I in a sealed glass vessel up to 240°C and subsequent quenching to - 7 9 ° C (dry ice in acetone) will always lead to the crystal form with a melting point of 26.5°C. He also warned that care must be exercised to exclude seeds of the room temperature stable modification. We had no problem crystallizing this modification and confirming Schaeling's results by means of thermomicroscopy. Subsequently, we obtained single crystals of this modification at - 13°C by growing them from the melt and have verified its existence by other analytical methods. Unfortunately, during our attempts to collect low temperature diffraction data the crystals of this modification transformed into the highest melting form (48°C) after 15 h.

O jcH3 IV


N-(N'-methyl-anilino)phthalimide, IV. Knowledge of the thermodynamic properties of the crystal modifications of a substance is essential to design an experiment to obtain the desired crystal form(s). There are even cases where difficulty is encountered in crystallizing the room temperature thermodynamically-stable modification in an enantiotropically related system. Enantiotropism is defined as the situation in which the G-isobars of two modifications have an intersection point below the melting point of the lowest melting crystal form at ambient pressure. This intersection point is the thermodynamic transition point. If this point in a given system is above room temperature then the lower melting modification is the thermodynamically stable crystal form (from 0 K up to the transition point) at this temperature and the higher melting form is stable between the transition point and its melting point.

In the case of IV, Chattaway and Lambert reported in 1915 [49] that the thermodynamic transition point of the two enantiotropically related modifications is 55.25°C. In 1979 Barlow et al. [50] published the crystal structure of the higher melting modification (without citing the earlier Chattaway and Lambert work). Our own experiments showed that "usual recrystallization procedures" using different solvents consistently led to the higher melting form. The reason is that seeds of this form do appear at a higher temperature and have a higher crystal growth velocity than the lower melting form. Crystal growth velocity and transfor- mation velocity (from one modification into another) are related to kinetic effects. In the case of IV, this can easily by shown by means of thermomicroscopy. We confirmed by this method that the two modifications of IV are indeed enantiotropically related and under thermodynamically controlled crystallization conditions we grew crystals (1 × 1.5 x 2 mm) of the lower melting form at room temperature and solved its structure [51 ].

SUMMARY AND CONCLUSIONS

Our lack of understanding of and control over polymorphism may be impediments to crystal engineering, but by no means warrant abandoning what is an increasingly fruitful approach to chemistry by design. None of the methods described above is foolproof, none of the recipes is guaranteed to yield large single crystals. But the determination of the crystallization conditions for various polymorphic forms need not be a completely random process. Hot stage microscopy combined with keen, thoughtful observation can provide extremely useful guidelines, if not for Success, then at the very least for further experiments. Crystallization is almost never a surefire procedure, especially When one is trying to selectively produce a particular polymorph. When at least one crystal structure is known, considerably more sophisticated techniques may be used to produce a particular polymorph preferentially, for instance by the addition of "tailor-made" additives [52], conformational mimicry [53], or solvent-mediated transformations [54]. However, there are rather simple techniques for increasing the odds at obtaining a desired product when none of the crystal structures are known (the "engineered crystal") and, after all, increasing the odds is a lot of what preparative chemistry is all about.

ACKNOWLEDGMENT

J.-O.H. wishes to thank the Alexander von Humboldt Foundation for a Feodor Lynen Postdoctoral Fellowship. This work was supported in part by the U.S.-Israel Binational Science Foundation (Jerusalem) under Grant 94-00394-2. We wish to thank Prof. J.D. Dunitz


for constant encouragement. The warm hospitality of the Cambridge Crystallographic Data Centre during a sabbatical leave of J.B. is greatly appreciated.

REFERENCES

1. G.R. Desiraju, Crystal Engineering, p. 304, Elsevier, Lausanne (1989). 2. A. Gavezzotti and G.Fillipini, J. Am. Chem. Soc. 117, 12299 (1995); H.R. Karfunkel, Z.J. Wu, A.

Burkhard, G. Ribs, D. Sinnreich, H.M. Buerger, and J. Stanek, Acta Cryst. B52, 555 (1995), and references therein.

3. C. Leadbeater, Financial Times, April 9, 1991, p.1; Wall Street Journal, Sept. 20, 1993, p. A5B; U.S. District Court, Eastern District of North Carolina, No. 91-759-CIV-5-BO Glaxo v. Novo- pharm; J.-O. Henck, U.J. Griesser, and A. Burger, Pharm. Ind. 58, 165 (1997).

4. T. Threlfall, Analyst 120, 2435 (1995). 5. J.D. Dunitz and J. Bernstein, Accts. Chem. Res. 28, 193 (1995). 6. G.M.J. Schmidt, Pure Appl. Chem. 27, 647 (1971). 7. M.D. Cohen and G.M.J. Schmidt, J. Chem. Soc., 1996 (1964). 8. B.S. Green and G.M.J. Schmidt, Israel Chemical Society Annual Meeting Abstracts, 190, (1971);

see also Ref. 1, pp. 186-192. 9. E. Mitscherlich, Annales Chim. Phys. 19, 414 (1821).

10. Dalton actually inscribed the principles of the atomic structure of matter in his notebook on 6 Sept. 1803, and lectured on them at the Royal Institution on 22 December of the same year. The formal publication did not come until five years later. See J.R. Partington, A History of Chemistry," Vol. 3, Martini Publishing, New York, 1961.

11. W.F. Ostwald, Z. Phys. Chem. 22, 289 (1897); W.F. Ostwald, Lehrbuch derAllgemein Chemic, Engelmann, Leipzig, 2nd ed. Part 1 (1902), pp. 448-49.

12. See M. Senechal, Historical Atlas of Crystallography, ed. J. Lima-de-Faria, Chapter 3, Kluwer Academic Publishers, Dordrecht (1990).

13. For instance, see B. Kahr and M. McBride, Angew. Chem. Int. Ed. Eng. 31, 1 (1992) for an account of the historical development of the phenomenon of optical anomalies, a field which went dormant for nearly half a century for reasons similar to those involving activity in the field of polymorphism.

14. See e.g., Beilsteins Handbuch der Organischen Chemie, 4 Aufl., 1. Band, Verlag von Julius Springer, Berlin, 1918, p. 530: 4. Alkohole C8H1804, 2. 2.5-Dimethyl-hexantetrol-(1.2.5.6) " . . . Schmeckt bitter . . . . "

15. The polarizing microscope had developed to its modern form essentially by 1879; see Historical Atlas of Crystallography, ed. J. Lima-de-Faria, Chapter 3, (1990), pp. 68-69, Kluwer Academic Publishers, Dordrecht.

16. P.H.R. von Groth, Chemische Kristallographie, 5 Volumes, Engelemann, Leipzig (1906-1919). 17. L. Deffet, Repertoire des Compose Polymorphes, Desoer, Liege (1942). 18. A. Findlay, The Phase Rule, 5th ed., Longmans, Green and Co., New York, (1923). 19. M.J. Buerger and M.C. Bloom, Z. Krist. A96, 182 (1937). 20. E.M. Chamot and C.W. Mason, Handbook of Chemical Microscopy, Volume 1, Principles and use

of Microscopes and Accesories; Physical Methods for the Study of Chemical Problems, 2nd ed., Wiley & Sons, New York (1938).

21. E.M. Chamot and C.W. Mason, Handbook of Chemical Microscopy, Volume 2, Qualitative Analysis, Wiley & Sons, New York (1931).

22. W.C. McCrone, Acct. Chem. Res. 23, 77 (1990). 23. M.C. Etter, G.M. Frankenbach, and J. Bernstein, Tetrahedron Letters 30, 3617 (1989). 24. I.C. Paul and D.Y. Curtin, Accts. Chem. Res. 6, 217 (1973). 25. I.C. Paul and D.Y. Curtin, Science 187, 19 (1975).


26. D.Y. Curtin, personal communication. 27. F.W. McLafferty, Accts. Chem. Res. 23, 63 (1990). 28. J.K. Haleblian and W.C. McCrone, J. Pharm. Sci. 58, 411 (1969). 29. W.C. McCrone, Fusion Methods in Chemical Microscopy, Interscience, New York (1957). 30. In 1956 McCrone founded McCrone Associates, a private analytical laboratory in which the

principal analytical technique employed was polarized light spectroscopy. Over the years he and his staff learned to visually identify over 30,000 particles [27]. McCrone Associates specialized in the identification of asbestos samples, airborne impurities, and the identification and classifi- cation polymorphism, among others. McCrone recently partially endowed a chair of chemical microscopy to Cornell University.

31. L. Kofler and A. Kofler, Thermo-Mikro-Methoden zur Kennzeichnung organischer Stoffe und Stoffgemische, Wagner, Innsbrnck (1954).

32. M. Kuhnert-Brandstatter, Thermomicroscopy in the Analysis of Pharmaceuticals, Pergamon Press, Oxford (1974).

33. A.Burger and R Ramberger, Mikrochim. Acta [Wien], 259 (1979 II). 34. A. Burger and R. Ramberger, Mikrochim. Acta [Wien], p. 273 (1979 II). 35. S.R. Byrn, personal communication. 36. U.J. Griel3er and A. Burger, Sci. Pharm. 61, 133 (1993). 37. E. Hovestreydt, J. Phillips, J. Chambers, J. Fait, M. Schuster, and R. Sparks, J. de Physique IV 6,

23 (1996). 38. A. Grunenberg, J.-O. Henck, and H.W. Siesler, Int. J. Pharm. 129, 147 (1996). 39. T.K. Nielsen and L. Borka, Acta Pharm. Suecica 9, 503 (1972). 40. L. Borka and M. Kuhnert-Brandst~itter, Arch. Pharm. 307, 377 (1974). 41. A. Togashi and Y. Matsunaga, Bull. Chem. Soc. Jpn. 60, 1171 (1987). 42. Although we carried out a rather thorough literature search prior to beginning our crystallization

experiments, we did not locate reference 41. It did not come up in a citation search, since it did not cite the original Nielsen/Borka paper. Another reason is that other searches were based on "benzocaine" while reference 41 refers to the molecule as ethyl aminobenzoate. One of us (J.B.) came across reference 41 in his own reprint collection subsequent to our determination of the structure of the 2:1 complex. This incident simply demonstrates the reproducibility of these phenomena, and the necessity for keen observation to characterize a system.

43. H.K. Henisch, Crystallization in Gels, Pennsylvania State University Press, University Park, PA, (1970).

44. G.R. Desiraju, D.Y. Curtin, and I.C. Paul, J. Am. Chem. Soc. 99, 6148 (1977). 45. C. Weygand and H. Baumgartel, Lieb. Ann. 469, 251 (1929). 46. W.C. McCrone, Fusion Methods in Chemical Microscopy, pp. 134-138, Interscience, New York

(1957). 47. P.H.R. von Groth, Chemische Kristallographie, Vol. 5, p. 89, Engelmann, Leipzig (1919). 48. K. Schaeling, Ph.D. thesis, University of Marburg (1910). 49. F.D. Chattaway and D.F.S. Wiinsch, J. Chem. Soc. 99, 2253 (1911). 50. J.H. Barlow, R.S. Davidson, A. Lewis, and D.N. Russell, J. Chem. Soc., Perkins H, 1103 (1979). 51. M. Botoshansky, A. Ellern, N. Gasper, J.-O. Henck, and F.H. Herbstein, Acta Cryst. B57, 277

(1998). 52. E. Staab, L. Addadi, L. Leiserowtiz, and M. Lahav, Adv. Mater. 2, 240 (1980). 53. R.J. Davey, N. Blagden, G.D. Potts, and R. Docherty, J. Am. Chem. Soc. 119, 1767 (1997). 54. R.J. Davey and J. Richards, J. Cryst. Growth 71, 597 (1985).

disappearing and reappearing polymorphs—an anathema to crystal engineering?

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