A Discussion of the Term "Polymorphism" Helmut Reinke, Heinz Dehne, and Martin Hans University of Rostock, Department of Chemistry, Giistrow Division, Goidberger Str. 12, 0-2600 Giistrow, Germany The attempt of Sharma (1)to redefine allotropes and ~ o l v m o r ~and h s our results in the field of analytical charHctErizaiion of sulfonamide derivatives ( 2 ) ciused us to have a closer look into problems of definition. Although we quite agree with the general approach to define allo60pes on the basis of different chemical bonding and polymorphs by the help of different crystal packing (I), some problems remain unsolved, especially when the liquid state is taken into consideration. Authors dealine with thermotro~icsubstances do not hesitate to consider different liquid ihases of a mesogen to be ~olvmorphs(3).In fact there is no reason to confine oneseli'tdthe solid state because of the general definition of a phase by means of thermodynamics. A second problem that is related to the first one is that of elassv ~roducts.' Bureer holds the view that elasses ihouid Lot be regardei as polymorphs (5).~ow&er,we feel that one should speak of polymorphs as long as a certain degree of supermolecular order can be realized, irrespective of the actual state of the system. This degree has to be fixed, of course, for each single problem, otherwise a mesogen would be defined as polymorph above the melting point (e.g., nematic phase), but the same nematic could not be regarded as a polymorph in the supercooled melt. However, if the liquid state is generally accepted for the range of existence of polymorphs, there should be no difficulties to speak of helium I and helium I1 as polymorphs rather than as allotropes (I).In addition to this, the classification of those two helium forms as allotropes is in contradiction to the basic mle of different chemical bonding being responsible for allotropes. Finally the liquid state is considered to be of meat importance for the recomition of polymorphs. It is common to-searchfor polymorp6s by par'The oroblem of alassv, .wlvmers which mav show different s t ~ c , tures oie lo lneir t&rmodynamic histories, shHh be ment one0 here forcompleteness only ( 4 ) .
allel investigations of a substance in the soli d state and in solutions (6).This approach will succeed in many cases but we would like to point out the following. The parallel investigation of a suhstance must be done with diluted systems in order to prevent the solute particles from aggregating into lyotropic solutions. The causes for polymorphism may he due to associative or steric (van der Waals) interrelations of the molecules, but they are never due to different primary valence honds. Examples for associative polymorphism can be found in oreanic solids with diff'erentcwstal onchine due to different LYdrogen bonding. These blnds &e gen&ally referred to as a special type of valency (7), possibly resulting in different species of polymorphs. As for the second case of steric interrelations, the fmdings of Bernstein et al. who introduced the term "conformational polymorphism", must be mentioned (8).As far as the interrelation between isomerism and polymorphism is concerned, we agree with Burger (5, who approaches this problem pramnatricall~Because of the argumentation of plastic Erysta~s,o r d e h s o r d e r transitions, and the phenomenon of different conformers in a unit cell, there always will be obstacles to differentiating between isomerism and polymorphism. For all these reasons we propose to use an extended and modified definition, at least for polymorphism in low molecular weieht is the ~ h e .. comwunds. Here wlvmomhism . nomenon where supermolecular structures with different, well defined ohvsical properties can be fbrmed bv chemically u n i f ~ ~ s ~ e cdoth'in i e s the liquid and the soiid state.
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Literature C i d Shama.B.D.J Chrm Bdvc 1S87, €4,404407.
1.
H.;Hans, M.; Dphne,H.M i k m h i m Ada, in press. 3. Kofler, I*:Kofler, & Brandstat&& M. T k - M i k m m e l h d n lur Xenrurrchnll* ogonischerStfle und Sfoffgomi&:; Verlag Chemie: Weinheim, 1954. 4. M & n w JE.;DoIdrtein,M J Roa.Net. Bur Stand. SoctA 1811.78331453. 5. Bulper,AAetoPharm. Technol lssP.28.1-20. 6. Rao, B.; Rao,G.;Avdhanulu,A B. J S d ind Roa. 11881,46,46%463 7. Ratajaak. H.;Orville-Thomu, W. J. Mdeculorintemetfo~:Wiley: New Ymk. 1980: 2. Reinke.
"7
3 yr "* &,
3-9" *-""
8. Bemstein. J.:Hag1er.A.T. . J A W % Chom. S a . 1818, lW. 673-681.
Volume 70 Number 2 February 1993
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