Infrared Spectra of Evaporated Films JOHN E. TYLER' AND SHIRLEY A. EHRHARDT Znterchemical Corp. Research Laboratories, New York, IV. Y.
M
The infrared spectra of films made by vacuum evaporation were compared with the spectra of these same materials as mulls. I n the process, i t is possible t h a t the material may condense in one of four ways: as t h e original compound, as a polymorph, as an isomer, or as a decomposition product. Examples of t h e first three cases are included together with their x-ray powder patterns and photomicrographs. It is conceivable t h a t some compounds may decompose in a rational manner on evaporation and t h a t
useful information may be obtained, but to date the only compounds which have decomposed have done so completely. This technique has been useful i n obtaining spectra having a high background transmission, a reduction of scattered light, and an increase in sharpness and intensity of absorption. I t should be of value in the investigation of isomers and polymorphs, in obtaining spectra of various fractions of a homogeneous mixture of materials, and as a method of purification.
AXY organic as well as inorganic compounds will evaporate or sublime when heated in vacuum. If the vapor from such
both before and after evaporation. This case is illustrated by Figure 1 LThich compares the infrared spectra of mineral oil and hexachlorobutadiene mulls and an evaporated film of anthracene. Basically these two spectra are the same, but the process of evaporation has resulted in a spectrum which shows higher background transmittance, together TT-ith stronger and sharper absorption bands. The x-ray powder patterns of the original and evaporated material are shown in Figure 2. Here again, the patterns are basically the same but the one for the evaporated material shows, if anything, less detail. One can see that the evaporated film is made up of crystals, but their size is probably suboptimum for powder patterns. This conclusion is confirmed by a comparison of the photomicrographs reproduced in Figure 3. Anthraquinone is another example of this class of material. The infrared spectra of anthraquinone prepared as a mull and by evaporation are shown in Figure 4. The respective x-ray pon-der patterns are shown in Figure 5 and the photomicrographs are reproduced in Figure 6. A number of compounds, such as tetrahydrosyanthraquinone, 1-aminoanthraquinone, and p-hydroq-azobenzene, have been
materials is allowed to condense on a suitable plate, the resulting film can be used as a sample for absorption spectroscopy. Although this technique is Fell known to electron microscopists and others, it does not seem to have been widely used by spectroscopists. Blout and Fields (f), Scott, Sinsheimer, and Loofbouronr (4,5 ) , Wagner and Hornig (6, 7 ) , and others ( 2 , 3 ) have used this technique in the preparation of films of materials such as biologicals and ammonium halides. Other than this there seems to be very little discussion of the method in the literature that would interest the infrared spectroscopist. Khen a pure organic compound is evaporated in vacuum the condensed vapor may be the original compound, an isomeric form of the compound, a polymorphic form of the compound, or a decomposition product-or a new compound. ORIGINAL COMPOUND
If the original compound is obtained, one would expect to get the same infrared spectra and the same x-ray powder patterns
Present address, University of California, Scripps Institute of Oceanography, La Jolla, Calif.
WAVELENGTH
p
Figure 1. Infrared Spectra of Anthracene as Nujol and Hexachlorobutadiene Mulls and as a n Evaporated Film
390
VOLUME
25,
NO. 3, M A R C H
391
1953
found which exhibit this same hasic activity. In all cases of this type, the evaporated film can he considered a representative sample of the compound under examination. The obvious advantages t o this method of sample preparation are: 1. The elimination of mineral oil bands (or the need for two mulling media). 2. The reduetian of scattered light in certain regions of the spectrum with an accompanying increase of hackground transmittance. 3. An increase in intensity and sharpness of absorption. 4. The ease of control of film thickness. -"S.T...i
>
The obvious disadvantage, if it can be called so, is the fact that one cannot work with completely unknown materials. ISOMERS OR POLYMORPHS
If an isomeric or a polymorphic form of the original material is produced by evaporation without decomposition, one would expect to find differences in infrared spectra, x-ray powder patterns, and appearance under the microscope. A case of this type is shown in the next series of platetes. Figure 7 presents the infrared spectra. of a mull of benzidine yellow
.*.,.,.%
... ..
.
Figure 2. X-Ray Powder Patterns of Anthracene 0.
Figure 3.
Before evsporntion
m.
b. After evaooration
b.
WAVELENGTH
Figure 4.
Photomicrographs at Equal Magnification of Anthracene Before evaporation After euewration
p
Infrared Speetra of Anthraquinone as Nujol and Heraehlomhutadiene mulls and as an Evaporated Film
ANALYTICAL CHEMISTRY
392 made from o-acetaacetotoluidide and of the same material evaporated. The new bands a t 4.42 and 12.75 microns and the changes in intensity indicate that this material has undergone some sort of c h n g e on evaporation. Both x-ray patterns and photomicrographs of the two materials reveal a marked lack of structure in the case of the evaporated sample (see Figures 8 and 9). It can he shown that this new material is not a decomposition product by heating the evaporated sample a t 165' C. for about 100 hours. When subjected to this treatment, the material slowly returns to its original state, as shown by its infrared
Figure 5. X-Ray Powder Patterns of Anthraquinone
spectrum and by its x-ray powder pattern. Whether this new material is rn isomer or a polymorph has not been critically answered, although it seems likely that it is an isomer. Several other compounds such as copper phthalocyanine (Figure 10)and beneidine yellow made from aeetoacetanilide also apparently yield isomers or polymorphs on evaporation. I n cases of this type, the evaporated film can still be used for identification purposes if evaporation consistently yields the same product. Perhaps more important, however, are the opportuni-
Figure 6. Photomicrographs at Equal Magnification of Anthraquinone a. Bsiors evaporation b. After sssporstion
a. Before erawrnfion
b.
After evaporation
I
I
I
MULL
WAVELENGTH
p
Figure 7. Infrared Spectra of Benzidine Yellow from o-Acetoaeetotoluidide .as Nujol and Hexaohlombutadiene Mulls and a s an Evaporated Film
V O L U M E 25, N O , 3, M A R C H 1 9 5 3
393
Finally, a decomposition product may be obtained on evaporation. It is conceivable that such decomposition might be rational and that useful information could therefore be obtained. To date, hmx-ever, such a compound has not been found. ThoRe
chemicals which have decomposed have yielded nothing but gases and carbon. Another possible use of this method to the infrared spectroscopist would be for fractionation of compounds. When a homogeneous mixture of materials is evaporated in high vacuum, the condensate will, in general, represent the composition of the vapor phase for the mixture under the particular conditions of temperature and pressure used in the experiment.
Figure 8. X-Ray Powder Patterns of Benzidine Yellow
Figure 9. Photomicrographs of Benzidine Yelloiv from
ties far producing isomers, far augmenting the study of structure, and for altering the chemical activity of certain materials. DECOMPOSITION PRODUCTS
from "-xcrl"acTjL"I"I"I"Lluli .~--.-~.-.>....!>:J. ~
1.
b.
Before evaporation After evaporation
rOI
I
a.
b.
I
.....I .
I
MULL
r
EVAPORAT
WAVELENGTH
Figure 10. Infrared Spectra of Copper Phthalocyanine as Nujc Evaporated Film
Before evaporation (SOOX) After wreporation (20,OOOX)
I
I
394
ANALYTICAL CHEMISTRY
boiling fraction may be obtained in the case of certain resins, or an impure material may be purified by the process. INSTRUMENTATION
’
For the benefit of those who have had limited experience with vacuum systems, the authors include a short description of the system currently used. The most important feature is the filament-heating circuit which consists of a Type V-10 Variac whose secondary taps are connected to the primary of a 1-kv.-amp welding transformer (Eider Engineering, 760 South 13th St., Newark, N. J.). The secondary coil of this transformer has 10 turns of 6/8 X 3/g inch copper busbar with a connecting tap on every coil. With 110 volts on the primary, secondary voltages from 0.7 to 7 volts in 0.7-volt steps can be selected a t will. Storage battery cables connect the binding posts of the evaporator with the welding transformer. This circuit permits one to bring to white heat a tungsten or molybdenum strip 2 X 0.5 X 0.005 inch thick for the purpose of cleaning it. It also permits the use of a boatrtype filament with enough holding capacity for a lar e charge of material. The bell jar is small (5.5-inch opening 8 inches tall) and is evacuated by means of an oil diffusion pump of small size, together with a fore pump. Vacuums of about mm. of mercury can be obtained. This is a very small and simple system. It is, nevertheless, adequate for the job, inexpensive, and easy to clean.
6,
ACKNOWLEDGMENT
The authors would like to express their thanks to the microscopy department under John J. Kelsch, and especially to Ralph Bainbridge and Vincent Salines, who supplied the photographs for this paper, and to Frank JV, Dunne, who built the sniall evaporator described here. They are also grateful to Dan Hurley and Anthony Sonnessa, who did the x-ray work. LITERATURE CITED
Blout, E. R., and Fields, RI., Science, 107, 252 (1948). Brown, J. IT., Trans. Roy. Soc., Can., 111, 26, 173-5 (1932). Reinkober, O., 2. Physik, 5, 192-7 (1921). Scott, J. F., Sinsheimer, R. L., and Loofbourow, J. R., Science, 107,302 (1948).
(5) Sinsheimer, R. L., Scott, J. F., and Loofbourow, J. R., iYatur~, 164, 796 (1949). (6) Wagner, E. L., and Hornig, D. F., J . Chem. Phys., 18, 296-301 (1950). (7) Ihid., pp. 305-12.
RECEIVED for review April 15, 1952. Accepted Sovember 18, 1952. Prerented before the Pittsburgh Conference on Analytical Chemistry and Spectroscopy, March 1962.
Identification of Some Organic Acids by Paper Chromatography A. R . JONES, E. J. DOWLING, A N D W . J. SKRABA Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn.
In the course of investigations of the acidic products formed by the action of gamma-rays from cobalt-60 on aqueous acetic acid solution, the paper chromatographic R values for a number of common acids were determined in several solvent combinations. A pseudo-two-dimensional chromatograph was devised for presenting and using R value data for characterization. .4new acidic solvent combination with great resolving power for nonvolatile acids, and a new basic solvent combination w-hich increases the sensitivity of the paper chromatographic method as applied to volatile acids, were developed.
D
U R I S G preliminary investigations of the identity of substances by paper chromatography, it is advantageous to be able to compare the R values for the unknowns with R values reported in the literature for knoiin substances. In the course of research on the acidic products formed by the irradiation of dilute aqueous solutions of acetic acid with gamma-rais from cobalt-60, the paper chromatographic R values for a number of common acids nere determined in several solventq and are reported in Tables I and 11. Although R values in any one solvent combination m a r not characterize an unknown material because the solvent conibination lacks sufficient resolving polqer, the use of several solvent Combinations minimizes the possibility of error. This is particularly true vhen the unkrionn substance is chromatographed in different chemical forms in different solvent combinations-e.g., as a free acid and as the acid anion. MATERIALS AND REAGENTS
The chromatographic developments were carried out in cylindrical jars 18 inches high by 12 inches in diameter, having ground lips and covered a i t h desiccator lids. A stainless steel scaffold in each jar supported the paper sheets and, in the case of descending chromatograms, the solvent trays. Whatman No. 1 filter paper was used in all experiments. The test acids were commercial products purified before use. Paper chromatograms of the volatile acids were sprayed with
a solution of 50 mg. of bromophenol blue in 100 ml. of water, thr solution being made acidic with 200 mg. of citric acid. Indicator solutions (0.04%) of bromophenol blue, chlorophenol red, or bromocresol green in alcohol, the solutions being made definitely basic with sodium hydroxide solution, were used for spraying paper chromatograms of nonvolatile acids. Chromatograph solvents were prepared as follows: Solvent Combination A, 100 volumes of n-butyl alcohol. 15 volumes of water, and 1 volume of diethylamine. Solvent Combination B, 15 volumes of ether, 3 volumps of acetic acid, and 1 volume of water (4). Solvent Combination D, equal volumes of 1-pentanol and 5 .If aqueous formic acid (3). Solvent Combination E. 2 volumes of 2-ethyl-I-butanol and 3 aqueous formic acid. volumes of 5 Solvent Combination F, 8 volumes of 95% ethyl alcohol. 1 volume of water, and 1 volume of concentrated ammonium hydroxide (1). The dried papers were sprayed with indicator solution until clearly defined spots were obtained. Care was taken to avoid making the paper wet with the indicator solution. The paper? were resteamed and resprayed if solvent removal was not complete. GENERAL PROCEDURE
Ether or ether-acetone solutions (2 mg. per ml.) of the test acids were spotted by means of micropipets on paper sheets 18 em. wide and sufficiently long to provide a chromatograph path of 30 em. They were allowed to dry in air. Before the solutions of the volatile test acids were placed on the paper, the spot posi-
