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`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 poly-
`morphs” [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 Backgr0und—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 A 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
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`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
`(NaH2PO4-H20) 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” [1 1]. 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. von 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 investiga-
`tion 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
`
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`Vol. 1, N0. 2
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`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-Brandstéitter [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 charac-
`terizing polymorphs,
`there has not been a renaissance in the use of ther1norr1icroscopic
`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 masteryihas 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 develop-
`ment 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 crystalliza-
`tion 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” [5]—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
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`behavior. If sufficiently large samples could be obtained, IR 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 carried out crystal
`structure analyses. They will be described in separate publications.
`
`C00C2H5
`
`N02
`
`
`
`Benzocaine:Picric Acid, 1. 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—l63°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 chloroforrnzmethanol
`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 fonn 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 therrnomicroscopic 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
`
`O2N
`
`N02
`
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`J. BERNSTEIN and J.-O. HENCK
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`Vol. 1, No. 2
`
`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-Brandstatter’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 X 0.8 X 0.7 mm)
`appeared after 48 h.
`
`H3C
`
`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 investi-
`gations, 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. 445°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 benzocainetpicric 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, crystalli-
`zation 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
`
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`125
`
`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.
`
`
`
`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 determin-
`ing reproducible crystallization conditions for obtaining one of these room temperature
`unstable modifications (m.p. 26.5°C)_. Schaeling found that heating the melt of III in a
`sealed glass vessel up to 240°C and subsequent quenching to —79°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 modifi-
`cation 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 dif-
`fraction data the crystals of this modification transformed into the highest melting form
`(48°C) after 15 h.
`
`0
`
`/CH3
`
`N — N
`
`0 I
`
`V
`
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`J. BERNSTEIN and J.—O. HENCK
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`Vol. 1, No. 2
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`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.
`
`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 forrn(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 thermodynam-
`ically 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 recrys-
`tallization 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 X 1.5 X 2 mm)
`of the lower melting form at room temperature and solved its structure [51].
`
`
`
`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
`
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`
`for constant encouragement. The warm hospitality of the Cambridge Crystallographic Data
`Centre during a sabbatical leave of J .B. is greatly appreciated.
`
`REFERENCES
`
`
`
`G.R. Desiraju, Crystal Engineering, p. 304, Elsevier, Lausanne (1989).
`A. Gavezzotti and G.Fillipini, J. Am. Chem. Soc. 117, 12299 (1995); H.R. Karfunkel, Z.J. Wu, A.
`Burkhard, G. Rihs, D. Sinnreich, H.M. Buerger, and J. Stanek, Acta Cryst. B52, 555 (1995), and
`references therein.
`
`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).
`T. Threlfall, Analyst 120, 2435 (1995).
`J.D. Dunitz and J. Bernstein, Accts. Chem. Res. 28, 193 (1995).
`G.M.J. Schmidt, Pure Appl. Chem. 27, 647 (1971).
`M.D. Cohen and G.M.J. Schmidt, J. Chem. Soc., 1996 (1964).
`B.S. Green and G.M.J. Schmidt, Israel Chemical Society Annual Meeting Abstracts, 190, (1971);
`see also Ref. 1, pp. 186-192.
`E. Mitscherlich, Annales Chim. Phys. 19, 414 (1821).
`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.
`W.F. Ostwald, Z. Phys. Chem. 22, 289 (1897); W.F. Ostwald, Lehrbuch der Allgemein Chemie,
`Engelmann, Leipzig, 2nd ed. Part 1 (1902), pp. 448-49.
`See M. Senechal, Historical Atlas of Crystallography, ed. J . Lima-de-Faria, Chapter 3, Kluwer
`Academic Publishers, Dordrecht (1990).
`1 (1992) for an
`For instance, see B. Kahr and M. McBride, Angew. Chem. Int. Ed. Eng. 31,
`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.
`See e.g., Beilsteins Handbuch der Organischen Chemie, 4 Aufi., 1. Band, Verlag von Julius
`Springer, Berlin, 1918, p. 530: 4. Alkohole C8H18O4, 2. 2.5-Dimethyl-hexantetrol-(1.2.5.6) “. .
`.
`Schmeckt bitter. .
`.
`. ”
`
`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.
`P.H.R. von Groth, Chemische Kristallographie, 5 Volumes, Engelemann, Leipzig (1906-1919).
`L. Deffet, Repertoire des Compose Polymorphes, Desoer, Liege (1942).
`A. Findlay, The Phase Rule, 5th ed., Longmans, Green and Co., New York, (1923).
`M.J. Buerger and M.C. Bloom, Z. Krist. A96, 182 (1937).
`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).
`E.M. Chamot and C.W. Mason, Handbook of Chemical Microscopy, Volume 2, Qualitative
`Analysis, Wiley & Sons, New York (1931).
`W.C. McCrone, Acct. Chem. Res. 23, 77 (1990).
`M.C. Etter, G.M. Frankenbach, and J. Bernstein, Tetrahedron Letters 30, 3617 (1989).
`I.C. Paul and D.Y. Curtin, Accts. Chem. Res. 6, 217 (1973).
`I.C. Paul and D.Y. Curtin, Science 187, 19 (1975).
`
`.‘°9°.\‘.°‘$".“
`
`14.
`
`D1‘ 5'‘
`
`
`
`
`
`.°.‘°9°.\‘.°‘[\.)[\)D—|D—lD—lb—-1-‘
`
`22.
`23.
`24.
`25.
`
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`
`26.
`27.
`28.
`29.
`30.
`
`
`
`D.Y. Curtin, personal communication.
`F.W. McLafferty, Accts. Chem. Res. 23, 63 (1990).
`J.K. Haleblian and W.C. McCrone, J. Pharm. Sci. 58, 411 (1969).
`W.C. McCrone, Fusion Methods in Chemical Microscopy, Interscience, New York (1957).
`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