Identification of Polymorphic Forms of Crystals by
`Infrared Spectroscopy
`
`DAVID N. KENDALL
`yanamid Co., Bound Brook, .V. J.
`C
`Calco Chemical Division, American
`
`While studying the relation between the spectra and
`structure of pigments, it was found that infrared
`techniques can be conveniently and successfully
`applied to distinguish between different crystalline
`forms of substances of the same chemical composi-
`tion. Infrared spectroscopy has been applied to the
`identification and determination of polymorphic
`forms of both organic and inorganic crystals. The
`method complements and supplements the x-ray,
`microscopical, and electron diffraction techniques.
`The advantages and disadvantages of these tech-
`
`niques for study of polymorphism are discussed.
`The value of the infrared method is illustrated by
`results obtained on such pigments as the Para Reds
`and copper phthalocyanine blues. Results on the
`titanium dioxides illustrate the limitation of this
`infrared technique when applied to certain inorganic
`crystals. From application of the method knowledge
`can be obtained of how the molecular or molecular-
`ionic species are arranged and oriented in the crystal
`lattices of polymorphic substances by empirical
`interpretation of infrared absorption spectra.
`
`T scopical, and electron diffraction ( 7 ) techniques in the identi-
`
`HE use of x-ray (SO), light microscopical (16), electron micro-
`
`fication of polymorphic forms of crystals is known. It has now
`been discovered that infrared techniques can also be applied con-
`veniently and successfully to distinguish between different crys-
`talline forms of substances of the same chemical composition.
`This new infrared method can in certain cases detect smaller
`quantities of polymorphs than previously used techniques. It
`is rapid, employs small samples, and is not affected by the diffi-
`culties of crystal morphology characteristic of x-ray and micro-
`scopical methods. Permanent records in the form of infrared
`spectrograms are obtained which can be quickly referred to at any
`time in the future when the occasion demands.
`The infrared spectroscopical technique described herein was
`developed primarily for analyzing polymorphic pigments and
`dyes, but the same procedures can be applied to other organic
`and inorganic crystals. The method involves obtaining the near
`infrared spectra of the pure polymorphs and then using these
`spectra to identify qualitatively and determine quantitatively
`the polymorphic forms that are present in any given sample of
`crystals.
`Earlier workers have used infrared experimentally to show dif-
`ferences in the polymorphic forms of calcium carbonate (IO, 16,
`ZI), diamond (22, 16, 27, 29, S I ) , and quartz (28). On the theo-
`retical side papers have been published on the selection rules for
`the Raman and infrared spectra of molecular (6) and ionic (3)
`crystals, on the general theory (2,9,11) of the vibrational spectra
`of crystals, and on the polarized infrared spectra ( l a ) of inorganic
`salts.
`To date, infrared spectroscopy has been used only to show
`differences between polymorphic forms of
`inorganic crystals.
`The present work extends the infrared techniques to the identifi-
`cation and determination of polymorphic forms of both organic
`and inorganic crystals.
`The sharpest spectra of ground particles of a crystalline mate-
`rial will be obtained, as first pointed out by Pfund (18,19), when the
`crystals under study are of a size less than the wave length of the
`infrared radiation, because of scattering. The spectra reported
`on herein were obtained on particles of diameters less than 5
`microns, with the exception of the brookite spectrum.
`The absorption curves illustrated in this paper are shown for
`the region from 5000 to 650 em.-' Spectral frequencies of the
`bands unique to each polymorph of a given crystal are shown
`under Results and Discussion. Absorption spectra are reported
`for polymorphs of the para toners, copper phthalocyanine blues,
`
`and titanium dioxides. Interpretation of these spectra has been
`made by means of empirical correlations based on both published
`[ I ) and unpublished observations.
`
`EXPERIMENTAL
`All spectra reported were obtained with a Perkin-Elmer Model
`12B infrared spectrometer. The instrument was calibrated using
`known .absorption bands of water vapor, carbon dioxide,,'and
`ammonia.
`
`Figure 1. Proposed-Structural Formula of
`Para Red
`. 873
`
`I
`
`FREQ. IN CM."
`Figure 2. Polymorphism of Para Red
`A . Blueshade
`B. Yellow shade
`Samples run as Nujol mulls
`
`382
`
`Argentum EX1043
`
`Page 1
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`

`

`V O L U M E 25, NO. 3, M A R C H 1 9 5 3
`
`The crystalline powders were mulled in Nujol or perfluoro-
`kerosene on a hard opal glass plate with a glass muller. Several
`mullings were made in the preparation of each sample, the number
`depending on the ease of dispersion. Csually two grindings were
`sufficient. With t,he exception of brookite, the particle diameters
`of all samples examined were less than 5 microns before the mull-
`ing process began. The niulled sample was placed on a flat
`
`073
`
`I
`
`031
`FREQ. IN CM.-’
`
`Figure 3. Polymorphism of Para Red
`A . Blueshade
`B. Yellow shade
`Samples run as Nujol mulls
`
`rock salt plate and spread out into a uniform film by applying
`manual pressure to another flat plate atop the sample. The
`plates and sample were then held in position in a simple cell
`comprising flat metal fore- and backplates held together by three
`threaded pins, two a t one extremity, one a t the other, Threaded
`knurled knobs allowed the maintenance of the proper amount of
`pressure. Visual observation of the sample through the plates
`quickly indicated whether a satisfactory dispersed film had been
`obtained.
`
`383
`
`rocal centimeters. The energy of the source is, of course, super-
`imposed upon the radiation transmitted by the sample in each
`spectrum. While complete spectra from 5000 to 650 cm.-1
`were obtained for each substance, as will be observed in some
`plots, only segments of such spectra are shown for reasons of
`space conservation. Breaks in the spectral curves in certain
`figures indicate that less significant regions of the spectrum have
`been omitted. On the “intensity” ordinate, spectrograms have
`been arbitrarily displaced vertically in order to present compari-
`sons of polymorph spectra over the same spectral intervals.
`The dashed and full frequency verticals were for the purpose of
`oral presentation of this paper.
`In general, the full lines repre-
`sent the absorption frequencies where the differences between
`two given polymorphic forms are most readily observed. Most
`samples were run in the solid state dispersed in Nujol or perfluoro-
`kerosene. Several solution spectra are presented, however, for
`reasons given later.
`
`,1350
`
`Figure 5. Polymorphism of Para Red
`A . Blueshade
`B. Yellow shade
`Samples run ae Nujol mulls
`
`I
`
`FREQ. IN CM.-’
`’
`Figure 4. Polymorphism of Para Red
`A. Blueshade
`B. Yellow ahade
`R . Radiation
`Samples run as Nujol mulls
`
`Infrared absorption spectra were then run and the resulting
`spectra measured and interpreted. Over each of eight segments
`of the spectrum covering the range from 5000 to 650 cm.-l a con-
`stant slit width was employed, and these slit widths were the
`same for every spectrum. Philpotts et al. (go) have emphasized
`recently the importance of constant slit width to infrared spectro-
`scopical investigations.
`
`RESULTS AND DISCUSSION
`Figures 1 to 19 show the infrared absorption spectra of poly-
`morphic forms of Para Red, copper phthalocyanine blue, and
`titanium dioxide as well as the structural formulas of these sub-
`stances. The graphs show intensity versus frequency in recip-
`
`FREQ. IN CM.-’
`
`Figure 6. Polymorphism of Para Red
`A. Blueshade
`B. Yellow ahade
`Samples run as Nujol mulls
`
`Para Reds. Figure 1 shows the structural formula of Para
`Red proposed on the basis of its chemical constitution and prop-
`erties, but most particularly on the basis of the present infrared
`study, Para Red pigment is prepared (53) from diazotized
`p-nitroaniline, which yields when treated with sodium hydroxide
`a stable isodiazotate, 02NCsH,N20Na. Upon acidification with
`hydrochloric acid and coupling with 2-naphthol, Para Red results.
`
`Page 2
`
`

`

`A N A L Y T I C A L C H E M I S T R Y
`
`by Nyswander (16) in his work on calcite and aragonite. The
`infrared technique also is applicable to those crystals in which
`ionic, covalent, van der Waals, and other types of chemical bind-
`ing are all involved in the spatial arrangements of the ions and
`molecules in the unit cell and the crystal lattice.
`Figure 4 shows that no free or bonded hydroxyl absorptions are
`found in the spectrum of either the blue or yellow shade Para
`Red crystals. Figure 7 gives the absorption spectra of the blue
`shade (curve B ) and yellow shade (curve C) of Para Red in chloro-
`form solution in the high frequency -NH-
`and -OH
`region to-
`gether with the spectrum of chloroform (curve A ) in the same
`region. Here also, no free or bonded hydroxyl bands are found
`for either shade. It is, therefore, likely that extremely strong
`hydrogen bonding exists between the hydroxyl hydrogen and the
`diazo nitrogen nearest the nitro-substituted benzene ring in both
`shades of Para Red in the crystalline state as well as in solution
`in chloroform. This spectroscopical finding is compatible with
`the known virtual alkali insolubility of the Para Reds. That the
`N . . .H-0 bonding is to the diazo nitrogen nearest the nitro-
`substituted benzene ring is very certain on the grounds of ring
`strain, bond distances, and steric considerations (17).
`
`\
`
`874
`
`384
`
`Control of the appropriate variables in the process gives rise to
`the blue shade or the yellow shade of the pigment. The coupling
`in either case takes place in the I-position.
`Figures 2 through 6 show the infrared absorption spectra of
`the blue and yellow shades of Para Red as Nujol mulls. The
`spectral regions of greatest interest in the5000 to 650 em.-' range
`are presented.
`Inspection of the figures mentioned reveals that
`the blue shade and the yellow shade of Para Red have different
`solid state absorption spectra. The blue shade crystals show
`bands a t 1179, 1024, 1015, and 852 em.-' which the yellow shade
`crystals lack. The latter absorb at 1620, 1572, 1000, 946,
`844, 754, and 741 em.-' while the blue shade crystals do not.
`Figures 7 and 8 present several segments from the spectra of the
`blue and yellow shades of Para Red in 0.5% by weight chloroform
`solution together with the corresponding spectral regions of
`chloroform itself for comparison. It is evident that a sufficient
`concentration of each shade of Para Red dissolves in chloroform
`to make a valid comparison of their spectra. The presence of
`in the spectra of the blue and yellow
`the 845-cm.-' band-e.g.,
`shades (curves B and C, Figure 8)-and
`its absence in the chloro-
`form spectrum (curve A ) prove that the Para Reds have dissolved
`in chloroform in sufficient concentration to reveal their presence
`in their absorption spectra. Several other bands characteristic
`of the Para Reds will be observed in the 1075 to 1250 em.-' re-
`gion of curves B and C in Figure 8. The absence of these bands
`in curve A , the chloroform spectrum, will be noted. The com-
`plete rock salt spectrum of the blue shade in chloroform solution
`is identical to that of the yellow shade in the same solvent. This
`experimental evidence proves the two shades are identical in
`chemical composition. The fact that the infrared absorption
`spectrum of the blue shade is different from that of the yellow
`shade in the solid state shows the two shades to be polymorphic
`forms.
`
`Figure 8. Polymorphism of Para Red
`A. Chloroform
`B. Blueshade
`C. Yellow shade
`B and C i n chloroform solution
`
`The possibility of a very weak band a t 3030 em.-' was noted
`in a perfluorokerosene mull spectrum of Para Red blue shade.
`The presence or absence of such a band was ambiguous, how-
`ever, and the spectra of the Para Reds should be reinvestigated in
`-OH
`the -"-and
`region with the higher dispersion afforded
`by lithium fluoride. This 3030-em. -1 absorption, however, if
`it does exist, would appear to be of too low frequency for a bonded
`It is probably an aromatic C-H
`stretching frequency.
`-OH.
`For the Para Reds in the solid state or in solution, a bonded hy-
`droxyl absorption is entirely missing, or owing to the great
`strength of the hydrogen bonding it is shifted into and obscured
`by C-H absorptions. The absence of both free and bonded -OH
`absorptions in a substance containing an -OH grouping is unusual
`but not unknown.
`Its absence was observed by the author in
`these laboratories in 1946 in the Nujol mull spectrum of l-hy-
`droxyanthraquinone (6, 14). Here also the reason for the
`absence of any hydroxyl absorption was found to be the presence
`of extremely strong hydrogen bonding, which in this case, was an
`0 .H-0 bond.
`The possibility of a ketonic grouping with the hydroxyl hy-
`drogen shifting to the nitrogen in the molecular structure of Para.
`Red both as it exists in solution or in either of its dimorphic
`forms is ruled out by the absence of any ketonic carbonyl or
`-NH-
`absorptions in any of the Para Red spectra. The ketonic
`
`FREQ. IN CM.-'
`Figure 7. Polymorphism of Para Red
`A. Chloroform
`B. Blueshade
`C. Yellow shade
`B and C in chloroform solution
`
`Polymorphism is particularly common in the case of organic
`dyes and pigments, as first shown by extensive x-ray power dif-
`fraction studies begun about 1933 in Germany (SO). Polymor-
`phism of organic pigments has also been observed by electron
`microscopy and electron diffraction (7).
`The experimental evidence cited above for the two shades of
`Para Red shows that substances of the same chemical composi-
`tion which have their molecules or molecular species oriented
`differently spatially to form crystals of different space lattices
`can be readily identified and distinguished from each other by
`means of their different infrared spectra. This finding applies
`also to crystals made up structurally of ions, as was first reported
`
`Page 3
`
`

`

`V O L U M E 25, NO. 3, M A R C H 1 9 5 3
`
`carbonyl absence is evident from examination of Figure 9, which
`shows the C=O stretching region for the blue shade ( A ) and
`the yellow shade ( B ) . For ready comparison, the radiation curve
`for the same region is shown as curve R. The 1620-cm.-l band
`observed in the yellow shade spectrum is too strong for a para
`.uhstituted phenyl ring absorption and is believed to represent
`n type of quasi-carbonyl absorption. Khen the hydrogen of a
`6-OH
`is strongly bonded to a diazo nitrogen linked to the same
`n: omntic ring, the carbon-oyygen hond ncquire. considerable
`
`I
`
`Ih
`
`Figure 9. Polymorphism of Para Red
`A . Blueshade
`B . Yellow shade
`R. Radiation
`Samples run as Nujol mulls
`
`In the present case, the 1620-crn-’
`douhle bond character.
`absorption of this hybrid carbon-oxygen is below the normal
`frequency of about 1680 em.-’ for a true carbonyl, yet well
`nbove the normal 1235-cm.-l absorption for a true single bond
`C-0 of a +--OH. The packing of the Para Red moleculesin the
`t h e shade crystal is such as not to allow the occurrence of this
`1620-em. --1 absorption observed in the yellow shade. The -NH-
`absorption absence is evident from inspection of Figure 4.
`The structural formula proposed for Para Red given in Figure 1
`is based on the spectroscopical evidence and reasoning given
`above. Considerations of bond angles and distances ( 1 7 ) show
`that the Para Reds are trans isomers with respect to the azo
`linbagr.
`While Figures 2 through 4 point out the infrared spectral dif-
`ferences between the blue and yellow shades of Para Red, Figures
`5 and 6 emphasize the spectral similarities obsrrved when two
`polymorphs are examined in the infrared. These similarities
`are to be expected, since the two forms are but different spatial
`expressions of the same chemical composition.
`In general, poly-
`morphic forms of organic crystals with their preponderance of
`nonpolar intermolecular linkages will give rise to a greater nuin-
`her of infrared spectral differences than inorganic crystals with
`their preponderance of ionic linkages.
`Evidence that the infrared spectral differences between crys-
`tals of the blue and yellow shades of Para Red were no accident
`of chance is shown by the fact that three samples of this pigment
`of shade unknown to the author were successfully identified from
`their infrared spectra as to polymorphic form.
`From empirical correlations of the data in Figures 2 through 6
`Imed on published material ( 1 ) and studies in these laboratories,
`i t is probable that the molecules in the crystals of Para Red
`!-dlow shade are oriented in such a way as to give the 2-naph-
`thalene ring more freedom to vibrate than in the Para Red blue
`shade crystals.
`In the blue shade crystal, on the other hand,
`the benzene ring containing the hydroxyl grouping has more
`freedom to vibrate than in the yellow shade crystal. The yellow
`shade crystals were found to give more infrared absorption bands,
`
`385
`
`33, than the blue shade crystals, which gave 31 absorptions.
`This probably means that the yellow shade has a lesser degree of
`crystalline order, more types of intermolecular linkages, and hence
`more absorption bands. The blue shade is then probably the
`thermodynamically stable one, that is a t room temperature.
`Frequencies of the absorption bands for the two polymorphic
`forms of Para Red as Kujol mulls in the rock salt region are given
`in Table I.
`
`Table T. h’ujol Mull Spectra of Blue and Yellow Shade
`Para Reds in the Rock Salt Region
`(Frequency in em.->)
`
`Blue Shade
`1015 w
`1179 w
`1596 m
`98.5 u s
`1x66 w
`1590 m
`1133 m
`1313 m
`981 m
`1140 u.
`963 n.
`1330 s
`873 u.
`1104s
`1295 W
`862 m
`1094 w
`1268 m
`832 m
`1037 w
`1230 m
`1203 s
`1024 U‘
`838 s
`m , medium; s, stiong: w, weak.
`
`808
`793
`787
`768
`748
`685
`682
`
`Yellow Shade
`1620m 1 1 6 8 ~ 1000 w
`
`984 m
`1572m 1155m
`1516s
`1 1 4 0 m
`978 w
`1332s
`1111 m
`961 vw
`1 2 9 4 ~ 1107s
`946 m
`874 w
`1261 m 1 0 9 2 m
`861 s
`1228 m 1039 u.
`844 m
`1208s
`1008w
`
`836 s
`810 u-
`791 w
`766 w
`754 m
`748 m
`741 w
`688 w
`fim w
`”--
`
`A band is classified as weak if its absorbance is less than 10%
`of the absorbance of the strongest band, as medium if between
`10 and 60%, and as strong if greater than 60%.
`Copper Phthalocyanines. Figure 10 shows the structural
`formula of copper phthalocyanine. Linstead et al. ( 4 ) found the
`phthalocyanine molecule is composed of four isoindole units
`joined together by four extracyclic nitrogens to form a compound
`having a strainless 16-membered central ring with one metal atom
`in thr ctantrr in the case of the metallic derivatives of phthnlocy-
`anine.
`
`Figure 10. Structural Formula of
`Copper Phthalocyanine
`
`Robertson (23, 24) and Robertson and Xoodward (26) i n
`careful studies investigated the complete crystal structure of
`phthalocyanine and its metallic derivatives. Their results con-
`firmed the earlier work ( d ) , yielded bond lengths, and showed the
`metal atom to be located in the center of the molecule and that
`it did not alter substantially the form of the phthalocyanine por-
`tion. The structure of copper phthalocyanine must be regarded
`as one continuous conjugated system. Robertson had only one
`form of copper phthalocyanine available to him-namely,
`the
`alpha form. This was prepared by sublimation as large needle
`crystals totally unsuited for pigment use but of definite value for
`x-ray diffraction study. He reported no data on any other form.
`Susich (50) showed by x-ray diffraction that copper phthalo-
`cyanine exists in two different crystal forms. Here the Robert-
`son form is called “alpha,” because historically this crystal form
`was the first to be identified, and the first commercial pigment
`form is called “beta ”
`
`Page 4
`
`

`

`386
`
`Figures 11 through 14 show the infrared absorption spectra of
`two polymorphic forms of copper phthalocyanine. The spectra
`from 4000 to 650 cm.-' are presented for the alpha form, curve A ,
`and the beta form, curve B, as Nujol mulls.
`Inspection of these
`figures reveals that the alpha and beta forms of copper phthalo-
`cyanine yield different spectra which are readily distinguishable
`one from the other. Here again the techniques of infrared spec-
`troscopy enable one to identify and distinguish between poly-
`morphic forms of organic crystals.
`
`7 9 4
`
`. 7 3 0
`
`XB
`
`FREQ. IN CM."
`Figure 11. Polymorphism of Copper Phthalocyanines
`A. Alpha form
`B. Beta form
`Samples run as Nujol mulls
`
`I
`
`1011
`
`I ' 1003
`
`\ 899
`
`1015
`
`I
`
`998
`
`'
`
`I R E 0 IN CM -I
`
`9 4 0
`
`900
`
`83 J
`I
`I
`
`Figure 12. Polymorphism of Copper Phthalocyanines
`A. Alpha form
`B. Beta form
`Samples run as Nujol mulls
`
`The alpha form, spectrum A , has 10 bands not found in the
`spectrum of the beta form. These are the 730-, 879-, 956-,
`980-, 1003-, 1101-, 1173-, 1606-, 3152-, and 3210-cm.-' bands.
`The beta form, spectrum B , has three bands not observed in the
`spectrum of the alpha form. These are the 720-, 865-, and 3115-
`cm.-l absorptions, The most striking infrared spectral differ-
`ence between the two polymorphic forms is the strong 730-cm.-l
`band in the alpha form as opposed to the also strong 720-cm.-l
`band in the beta form (as seen in Figure 11). The other readily
`observable differences will be found upon inspection of Figures 11
`through 14.
`The difference between the crystal structure of the alpha and a
`suggested structure for the beta form (3.2) when a projection is
`taken parallel to the crystal plane of the respective polymorphic
`forms is shown in Figure 15. The two crystal forms differ only
`in the arrangement of
`the copper phthalocyanine molecules
`
`A N A L Y T I C A L C H E M I S T R Y
`
`1288
`
`1101
`
`I
`1190
`
`I
`
`FREQ. IN C M - '
`
`V
`10'91
`
`Figure 13. Polymorphism of Copper
`Phthalocyanines
`A . Alpha form
`B. Beta form
`Samples run as Nujol mulls
`
`within their respective crystal planes. The molecules in the
`alpha form present a "regular" array, those in the beta form a
`"staggered" array.
`Because the rock salt infrared spectra of the two forms are
`different, the spectral differences must arise from the different
`effects that the intermolecular forces present in one form have on
`the intramolecular forces of that form as compared with the inter-
`action of similar forces in the other form. That the effect of
`intermolecular forces on intramolecular forces ail1 be different in
`the alpha and beta forms of copper phthalocyanine is evident
`upon examination of Figure 15.
`Empirical interpretation of the spectral differences between
`the two forms reveals that probably the ortho disubstituted ben-
`zene rings and the C--T groupings are freer to vibrate in the alpha
`form of copper phthalocyanin~ than in the beta, while the C-H
`groupings are freer to vibrate in the beta form than in the alpha
`form.
`Infrared spectra in the rock salt region were run on copper
`phthalocyanines crystallized in xylene and in aniline. Each of
`these spectra showed a strong 730-~m.-~ band characteristic of
`the alpha form and also a weak 720-em. -1 band characteristic of
`the beta form. It, therefore, became of interest to work out a
`quantitative method of analysis for the determination of alpha
`and beta form copper phthalocyanines in mixtures of the two
`forms.
`
`\:587
`
`3 8 8 0
`
`A
`
`FREQ. IN CM. -1
`
`I
`Figure 14. Polymorphism of Copper Phthalocyanines
`A. Alpha form
`B. Beta form
`Samples run as Nujol mulls
`
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`

`

`V O L U M E 2.5, NO. 3, M A R C H 1 9 5 3
`
`387
`
`cause of the obvious difficulties in reproducing uniform disper-
`sions of samples in Nujol, and because of the scattering in the
`region the absorbance measurements are made.
`The qualitative and quantitative method described above for
`the identification and determination of polymorphic forms of
`copper phthalocyanine has been employed successfully on many
`preparations. The method has proved to be reliable, rapid, and
`convenient. The results have shown that not only can poly-
`morphs of organic crystals be identified and distinguished by
`means of infrared spectroscopy, but they can also be quantita-
`tively analyzed as to percentage of each polymorph present when
`samples containing more than one polymorphic form are en-
`countered.
`The polymorphic forms present in a series of six chlorinated
`copper phthalocyanines xere also identified and determined by
`the procedure given in this work. Reference to Figure 16, which
`gives the most characteristic absorption region for a typical
`sample of this series, shows bands at 735,760,909, and 1050 cm.-'
`Sone of the unchlorinated forms of copper phthalocyanine has
`an) of these four bands. ah chlorine was found in all six samples
`by means of chemical analysis and the i35- and 760-cm.-' bands
`are empirically of correct frequency for C-CI
`absorptions, it is
`certain the six samples contain chlorine.
`-4s the four absorptions
`which are unique to the chlorinated samples are also absorptions
`not found in polychlorinated copper phthalocyanine, the spec-
`trum of TT hich in the same region is shown in Figure 17, it is prob-
`able that all si.; samples are monochloro copper phthalocyanines.
`
`Beta
`Alpha
`Figure 15. -4tomic and 5lolecular Structure of Copper
`Phthalocyanine
`
`735 720
`
`1050 I
`
`FREQ. IN CM.-'
`
`v -
`
`Figure 16. Polymorphism of Copper Phthalocyanines
`A . Alpha form
`B. Beta form
`C. Monochlorinated
`Samples run as Nujol mulls
`
`A solid state technique was a necessity, as polymorphs give
`identical solution state spectra. A quantitative analysis was,
`therefore, worked out employing dispersion of the samples in
`Nujol.
`
`Samples of pure alpha form and pure beta form were used as
`standards. That these standards were pure was ascertained by
`noting the absence of the strongest beta absorption in the alpha
`form, and the absence of the strongest alpha absorption in the
`beta form. Spectra of the alpha and beta form standards and of
`known mixes of 98% alpha and 2% beta, and 95% alpha and 5%
`beta, were run as Nujol mulls in the region from 700 to 745 cm.-l
`The procedure for the preparation of each sample was to use the
`same quantity of Sujol, the same quantity of pigment, and the
`same number of mullings, 5. Mull thickness-Le., cell thickness-
`was then adjusted until the absorptipn maxima of the Nujol
`2850-em. -l doublet and the nearby N U J O ~ transmission maximum
`
`a t 2700 cm.-' were the same. The known mixes were ground
`together thoroughly in a mortar and additional mixing occurred in
`the mulling process. The well-known "base-line"
`technique
`(8) was employed for the quantitative analyses of percentage
`alpha and beta forms present in a sample. The absorbance meas-
`urements were made at 730 cm.-' for alpha and 720 cm.-l for
`beta, once adjustment of the cell length had been made to give
`equivalent thickness for known mix and unknown, as explained
`above.
`
`The copper phthalocyanine crystallized in xylene was found
`to contain 95% alpha form and 5% beta, while that Crystallized
`in aniline was found to contain 96% alpha form and 470 beta.
`No great accuracy is claimed for this quantitative method be-
`
`+
`>
`t
`cn z
`w c z
`
`I
`
`1050
`
`9[
`
`7 6 0
`I
`
`FREQ. IN CM.-'
`
`Figure 17. Polymorphism of Copper Phthalocyanines
`A. Polychlorinated
`B. rMonochlorinated
`Samples run a s Nujol mulls
`
`To illustrate the results obtained on the series of six chlorin-
`ated copper phthalocyanines, Sample 1 containing 6.4% chlorine
`was found by the infrared method described in this paper to be
`monochloro copper phthalocyanine of beta form. I t contained
`no alpha form crystals. Sample 2, which was crystallized into
`the alpha form by exposure of the beta form to boiling quino-
`line, containing 5.9% chlorine, was found to contain 62% mono-
`chloro alpha form and 38% monochloro beta form. Similarly,
`for the four remaining samples of this chlorinated copper phthalo-
`cyanine series it was possible to identify the polymorphic forms
`of the pigment present and to determine the percentage of each
`form that was present in the sample using the infrared technique
`described.
`The absorption bands observed for five of the chlorinated
`copper phthalocyanine pigments in the principal regions of in-
`terest are shown in Table 11.
`The complete rock salt spectrum at least must be considered in
`identifying the polymorphic form or forms present in organic,
`inorganic, or mineral crystals. Experience in these laboratories
`has shown that both the sodium chloride and potassium bromide
`prism infrared regions are sometimes necessary to an unamhigu-
`
`Page 6
`
`

`

`388
`
`Infrared Absorptions of Chlorinated and
`Table 11.
`Unchlorinated Copper Phthalocyanines
`3
`5
`4
`Beta0
`Alpha=
`1
`2
`. . . ..
`720 Sz
`720 Si
`. ..
`720 Si
`720 S
`720 SI
`720 SI
`726 Si
`. . . . .
`. . . .
`...
`....
`726 Si
`726 S i
`. . . .
`730 S
`...
`786'$1
`73b'&
`...
`...
`735
`735
`735
`735
`735
`760
`760
`760
`760
`760
`...
`. . .
`..,
`909
`909
`909
`909
`909
`...
`1050
`1050
`1050
`TO50
`1050
`a .41pha and beta are unchlorinated. the other samples are chlorinated.
`.4U absorptions are in om.-1 S = stroni, SI > €32. The criterion for claseifica-
`tion of a band as "strong" is the same as used in Table I.
`
`, , ,
`
`ous determination of the polymorphic modifications present in
`crvstalline materials.
`The use of infrared spectroscopical tech-
`Titanium Dioxides.
`niques to differentiate between polymorphic forms of inorganic
`ciystals is not new (IO, 16). The power of the infrared method,
`however, has not been sufficiently appreciated or applied.
`It is only in recent years that any intensive application has
`begun to be made of the techniques of infrared spectroscopv to
`the study of
`inorganic compounds and minerals. Recently
`Keller and Pickett ( I S ) reported an extensive survey of the in-
`frared spectra of clay minerals. Hunt, Wisherd, and Bonham's
`( I O ) survey of the 2- to 16-micron spectra of 64 minerals, poly-
`mineralic sediments, and inorganic chemicals is also of recent
`origin. These workers show the spectra of polymorphic forms of
`calcium carbonate.
`The identification of polymorphic forms of inorganic pigments
`is illustrated in this paper using 'titanium dioxide as an example.
`The results obtained on this pigment illustrate the limitation of
`this infrared technique when applied to certain inorganic crys-
`tals.
`Titanium dioxide exists in three polymorphic forms: anatase,
`rutile, and brookite. The first two forms are well known through
`their widespread use as commercial white pigments of unexcelled
`hiding power. The third form, brookite, is a naturally occurring
`mineral of no known commercial importance.
`Figures 18 and 19 show certain regions of the rock salt in-
`frared absorption spectra for anatase, rutile, and brookite.
`In-
`spection of these figures readily reveals the spectral differences
`among the three polymorphic forms of titania are slight. The
`infrared technique described in this work, then, would not be a
`very satisfactory m-ay to identify and determine quantitatively
`the polymorphic f o r m in titanium dioxide pigments. The well-
`known x-ray technique is a much more satisfactory method.
`The limitation of the infrared technique illustrated by the tita-
`nium dioxides as an example applies to those inorganic substances
`whose cations and anions are both highly ionic in character. That
`this limitation is not serious is realized when one considers that
`very many pigments are organic in nature and that certain in-
`organic pigments are highly covalent.
`The fundamental structural differences among anatase, rutile,
`and brookite are verv slight indeed. Each polymorphic form
`consists of Ti06 octahedra. It is only in the different manner
`that these octahedra are bound together in three-dimensional
`space that serves to distinguish the structures of the three poly-
`morphic forms. Because these Ti06 octahedra are shared in dif-
`ferent ways, however, the Ti-0
`interatomic distance varies as
`between anatase and rutile, and probably also brookite. As
`the force constants of bonds which are one factor in determining
`the vibrational frequencies of polyatomic molecules are closely
`related to interatomic distances, it follows that the force constants
`and hence vibrational frequencies will differ as between the poly-