NOTES
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April, 1957 system immersed in a constant temperature bath. The lines to the bourdon gage as well as the gage itself were kept a t a temperature above that in the bath. The 1000 p.s.i.g. bourdon gage used in these measurements had a stated accuracy of of 1%. Critical temperature was measured on samples sealed in 2 mm. i.d. capillary tubing. The tube was placed directly into a constant temperature bath and observations made on the disappearance and reappearance of the meniscus. Temperature was measured during the density and precise vapor pressure determinations either by means of a platinum resistance thermometer or by means of thermocouples which have been compared with the thermometer. The thermometer had been compared with the National Bureau of Standards temperature scale. During the equilibrium periods of 15-20 min. during which measurements were taken, the temperature drift in the metal system or the dilatometer was no more than 0.002'/min. and 0.004'/min., respectively. For the critical temperature and high vapor pressure determinations a mercury in glass thermometer calibrated by the National Bureau of Standards was used. Material.-The perchloryl fluoride for these measurements was prepared and purified by A. Engelbrecht at this Laboratory. Purification was accomplished by fractional distillations. Purity was assayed by infrared analysis and by mass spectrometer analysis kindly furnished by the staff of the National Bureau of Standards. The mass spectrometer analysis showed a purity of 99.9+ mole %.
Results and Discussion.-The data obtained for the liquid density are given in Table I, and are represented by the equation d = 2.266
- 1.603 X
10-8T
- 4.080 X 10-8T'
(1)
where d is the density in g./cm.3 a t the absolute temperature T. Deviations between the experimental and values calculated using equation 1 are given in column 3 of Table I. TABLE I THEDENSITY OF LIQUIDPERCHLORYL FLUORIDE BETWEEN 131 AND 234°K. (OOC. = 273.16"K.) T,OK.
Density, gJcrn.8
131.31 144.46 149.58 154.79 159.84 164.66 169.62 174.50 179.52 184.54
1.989 1.950 1.935 1.920 1.905 1.891 1.877 1,862 1.846 1.831
Deviation obsd. oalcd.
-
0.004 .001 ,000
.ooo ,000 ,000
.OOl ,000 ,000 ,000
T,OK.
Density, g./cm.*
189.50 194.44 199.38 204.34 209.21 214.21 219.15 224.11 229.23 234.22
1.816 1.801 1.786 1.770 1.751 1.734 1.718 1.701 1.684 1.667
Deviation obsd. calod.
-
0,001 .001
.002 ,002 - ,001 ,001 ,000 ,001 ,000 .003
-
499
TABLE I1 THEVAPORPRESSURE OF PERCHLORYL FLUORIDE (O'C. = 273.16"K.) Temp., OK.
Deviation, mm. Pressure, obsd. mrn. calcd.
-
152.81 3 . 0 8 -0.15 157.67 5 . 5 3 - .03 162.58 9 . 3 1 - .05 167.50 14.97 .02 172.33 23.19 - .10 177.40 35.73 - .03 182.33 53.05 - .01 187.31 77.13 .02 192.25 109.48 .12 197.19 151.77 .23 202.11 207.50 .38 207.19 279.66 - .35 212.20 370.73 .08 217.13 481.49 - .20 222.13 619.33 - .10 227.09 785.44 1.00 232.04 980.28 -0.97 237.14 1220.89 -0.80 242.09 1496.31 1.20
-
Temp., OK.
227.09 232.04 237.14 242.09 251.99 262.07 272.04 282.10 292.13 292.24 302.17 312.21 322.33 332.41 342.50 352.66 362.92
Pressure, atm.
Deviation, atm. obsd. cnlcd.
-
1.033 -0.008 1.290 - ,006 1.606 - .001 ,004 1.969 2.891 ,025 4.048 - ,041 5.748 .086 .lo8 7.789 10.170 - ,027 10.177 ,050 ,074 13.361 17.034 .012 21.388 - .122 26.490 - .285 32.782 - .127 39.959 - ,066 48.259 .024
-
2 is 226.40"K. The data to the critical region are represented by the equation logloP(atm.) = 4.46862
1010.81 -T
Deviations between the experimental and calculated values for these two equations are given in column three of Table 11. Temperature was known in these measurements to *0.05° for the data to 2 atmospheres and to f0.10" for the data to the critical region. The heat of vaporization a t the normal boiling point calculated using equation 2 and a gas imperfection correction of -3.21%, derived from the Bethelot equation, is 4609 cal./mole. The critical temperature was determined to be 368.33 f 0.10"K. From this value the critical pressure was calculated using equation 3 to be 53.0 atm. absolute. Acknowledgments.-The author wishes to express his appreciation to Dr. J. J. Fritz of the Pennsylvania State University and Mr. H. C. Miller of the Pennsylvania Salt Manufacturing Company for their advice during the course of this work. He also wishes to thank Dr. A. Engelbrecht, now at the University of Innsbruck, for purifying the sample of perchloryl fluoride, and Mr. Gordon E. Webb of the Pennsylvania Salt Manufacturing Co. for his excellent assistance.
The uncertainty of the measurements is estimated to be f O . l % . Temperature was known to f0.05O . The volume values for the dilatometer were corrected using the expansion coefficient data of Buffington and Latimer.g Corrections were applied for the quantity of material in the dead space POLYMORPHISM I N MONOCHLOROACETIC above the liquid. ACID The vapor pressure data are given in Table 11. The data to two atmospheres have been fitted by BY R. E. KACARISE the equation Naval Research Laboratory, Washington 86, D. C . log,oP(mm.) = 18.90112
1443 467 - 4.09566 logloT -A T
(2)
The normal boiling point calculated using equation (9) R. 19. Buffington and W. M. L,atimer, J . A m . Chem. SOC.,4 8 , 2305 (1926).
Received October 86, 1966
It is well known that monochloroacetic acid exists in at least three distinct crystalline varieties-l Re(1) "Beilsteins Handbuch der Organischen Chemie," J u l i u s Springer, Berlin, Fourth Edition, H2, 194, 1920; E.12, 87 (1929); E112, 187 (1942).
NOTES
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500
cent thermodynamic studies2r3 have shown that these three varieties form a monotropic system consisting of a stable a-form and metastable p- and y-forms. The melting points of the three forms are 62.0, 56.5 and 50.5", respectively. Moreover, these investigations have shown that the varieties are capable of isolation provided the proper cooling techniques are employed. The problem of determining the crystal structure of these three species does not appear to have been studied previously, except for several investigations of the pure quadrupole spectra. Allen4 observed two C136 resonance frequencies in the spectrum of the ordinary acid (a-form) which he attributed to the acid dimer. More recently, Negita5 has observed the pure quadrupole spectrum of all three crystalline species and found only single resonance frequencies in the metastable p- and y-forms. Moreover, the frequency in the p-form is near the higher frequency of the a-modification, while that of the y-form is very close to the lower one of the aform. These results have been interpreted in terms of two forms of the dimer resulting from the hindered rotation of the -CH2C1 groups about the C-C bond. Thus the y-modification is regarded as the gauche-form, the p-modification as the transform, and the a-modification as an equilibrium mixture of the two.
LL-'v . I
b
,
YICiONI.
,
I
I
Fig. 1.-Infrared spectra of liquid and the a-, p- and y crystalline forms of monochloroacetic acid. Thickness of samples is approximately 0.01 mm.
In view of the successful application of infrared absorption spectroscopy to similar studies on symtetrabromoethane6it was considered worthwhile to examine the infrared spectra of the various crystalline species of monochloroacetic acid. Experimental The sample of chloroacetic acid used in this investigation was an Eastman Kodnk White label product, which was recrystallized five times from benzene. The product. was stored over PzOb in a vacuum desiccator to remove final traces of water and benzene. (2) M. Aumeras and R. Minangay, Bull. sac. chim. France, 1100 (1948). (3) L. M. Katayeva and Z. S. Smutkina, Zhur. Pg. Khim., 29, 428 (1955). (4) H. C. Allen, P h y e . Rev., 87, 227 (1952); J . A m . Chem. Sac., 74, 0074 (1952); THISJOURNAL, 67, 501 (1953). ( 5 ) H. Negita. J . Chem. Phys.. 23, 214 (1955). (1;) R . E. Kagsrise, ibid.,24, 300 (195G).
Vol. 61
All absorption spectra were obtained with a PerkinElmer Model 21 spectrophotometer equi ped with NaCl optics. The absorption cell consisted o f a conventional liquid cell which was heated by means of an electrical heater wound around the periphery of the cell. Since CHZClCOOH is a solid a t room temperature, it was necessary to fill the absorption cell in the following manner< Two sodium chloride windows were heated to a temperature slightly above the melting point of the acid. An appropriate amount of sample was placed on one window and allowed to melt. The second window and a Teflon spacer were placed on top of the first and the cell assembled in the usual manner. The temperature of the sample was measured with an ironconstantan thermocouple. To prepare specimens of the various crystalline forms the following cooling procedures were employed. The a-form was obtained readily by slowly cooling the liquid while the 7-form usually resulted when the liquid was rapidly cooled. The p-form was very difficult to prepare in situ since most of the procedures advocated by earlier workers213,s are not applicable to closed systems. It was observed, however, that under certain conditions (which were not definable) the y form was converted to the p-form with aging. The various forms were identified by observing their melting points.
Discussion of Results The observed infrared absorption spectra of liquid and the a-,p- and y-modifications of crystalline monochloroacetic acid are shown in Fig. 1, while the wave lengths and wave numbers of the absorption maxima are listed in Table I. An examination of Fig. 1 shows that the four spectra are quite different and the individual spectra cannot be represented by a combination of the others. For example, in the region between 10.75 and 12.25 1 none of the three crystalline forms have bands in common. The spectra are also appreciably different in the neighborhood of 7 p . These observations are in contrast to those previously reported for symtetrabromoethane.6 In this case the observed spectrum of the liquid was very similar to the synthetic spectrum obtained by a superposition of the spectra of the two crystalline forms. According to the interpretation of Negita,6 one would have predicted that the spectrum of the or-form would be similar to the sum of those of the 0- and y-forms. An examinination of the region beyond 12 1 in Fig. 1, shows that apparently only a single band is present which can be attributed to a C-CI stretching mode. One must conclude, therefore, that if several isomeric species are present, the stretching frequency of the carbon-chlorine bond is the same in each. This is also in opposition to observations on other halogenated compounds which are known to possess rotational isomer^.^ Moreover, t,he observed conversion of the y-form to the p-form appears to contradict the interpretation of Negita. If the stable a-form is an equilibrium mixture of /3 and y such a conversion would be highly improbable. It seems certain, therefore, that the various crystalline species of monochloroacetic acid cannot be attributed simply to different species of rotational isomers. Before discussing the differences between the spectra of the various crystalline species it is appropriate to consider briefly the possible modes of vibration of the dimeric molecule. This question has been treated quite extensively by Hadzi and (7) 8. Mizushima, "Structure of Molecules and Internal Rotation," Academic Press Inc., New York, N. Y . , 1954.
TABLE I INFRARED ABSORPTION SPECTRA OF LIQUID AND
x
(PI
Liquid
3.18s
... 3.36~ 3.69~ 3.85~
(cm.-x)
3144
...
2970 2710 2597
...
...
2360
...
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Y
4.22
...
... ...
...
...
5.74vs 7.05s
1742 1418
...
...
7.75sh 7.86m 8.23s
1290 1272 1215
...
10.74m
... ...
50 1
NOTES
-4pri1, 1957
...
93 1
...
... ... ...
12.58s
795
a-Crystal (P)
3.25vs
... ...
Y
&Crystal
(cm.-')
3076
... ....
2717 3.68~ 2597 3.85~ 2506 3.99w 4 . 1 4 ~ ~ 2415 4 . 5 2 ~ ~ 2212 4 . 7 0 ~ ~ 2127 5.49w 1821 5.75vs 1739 7.04s 1420 7.16m 1396 7.68m 1302 7.75m 1290 8.26~s 1210 8.48m 1179 10.73m 932 908 1l.Olvw
...
...
11.88s 12.65s
842 790
a-,p- AND
(P)
Y
3.18m 3.31s 3.36sh 3 . 6 5 ~ 3 . 8 2 ~ 3.96~ 4 . 1 1 ~ 4 . 2 8 ~ 4 . 7 1 ~ 5.46~ 5.75vs 6.96, 7.13m 7.65m 7.70sh 8,14vs 8 . 4 8 ~ 10.66s
... 11.45s
(cm.-1)
3144 3021 2976 ~ 2730 ~ 2617 2525 ~ 2433 ~ 2336 ~ 2123 1831 1739 1436 1402 1307 1298 1228 ~ 1179 938
...
873
...
...
12.51s
799
Sheppard.8 They conclude that in the region of the spectrum between 1500 and 700 cm.-l, one might expect to observe absorptions due to the C-0 stretching, C-0-H angle bending, and the motion corresponding principally to the motion of the hydrogen atom of the OH group perpendicular to the plane of the dimer. These conclusions are based on the assumption that the dimers have either C2= or Ci symmetry. In either case, the rule of mutual exclusion applies, so that there is a corresponding set of frequencies which are active only in the Raman effect. In addition to the above mentioned modes, one would also expect to observe the O-H and C=O stretching frequencies in the NaCl region of the spectrum. The assignment of the latter two modes is firmly established, and Hadzi and Sheppard have given strong evidence that the out-ofplane bending mode absorbs in the region from about 870 to 940 cm.-l. The two remaining modes, namely, the C-0 stretching and C-O-H angle bending are attributed to the absorption frequen. cies around 1400 and 1300 cm.-l. However, the degree of interaction between these two vibrations is so great, that one cannot make an unequivocal assignment. Therefore, the absorption bands in the 1200-1400 cm.-l are referred to as COOH-I, I1 and III. An examination of the data shows that those absorption bands due to the -CH&I end groups, e.g., v(C-H), 6(C-H), v(C-C) and v(C-Cl), remain relatively constant in both intensity and frequency in all of the spectra. Those bands attributable to the -COOH portion of the molecule, on the other hand, vary considerabIy both in intensity and frequency. Consider, for example, the band at -3150 cm.-l due to the O-H stretching mode. In the a-form, the absorption is quite strong and completely over(8) D. Hadzi and N. Bheppard, Proc. R o y . Soc. (London), 216, 247 (1953).
?-FORMS
OF
CH2ClCOOH
y-Crystat
x (P) 3,20sh 3.28s 3.35s 3 , G5m 3.80~ 3.94w 4.25sh 4.47vw 4.72~ 5.44w 5.78~s 6.98m 7.15m 7.67m 7.8Gm 8.10m 8.45m 10.75m 11.15m.
...
Y
Interpretation
(om.-'>
3125 3048
v
2985
I
v
(O-H) (O-H) (C-H)
2730 2631 2538 2350 2237 2118 1838 1730 1-132 1398 1303 1252 1234 1183 930 897
s (OH)
...
6 (OH)
...
...
12.52m
599
(C=O) COOH-I 6 (C-H) COOH-I1 v
-
C00H III v
(C-C)
s (OH) v
(C-CI)
laps the nearby C-H stretching band a t -3025 cm.-l, while in the P-modification the OH band is relatively weak. The same situation prevails in the case of the COOH-I band near 1420 cm.-l. It is also apparent that considerable frequency shifts take place since the out-of-plane OH deformation band falls at 842, 873 and 897 crn.-I in the a-,Pand y-species, respectively. As previously mentioned, one would predict two frequencies in the 1200-1400 em.-' region for dimers having C2a or C; symmetry. Since three frequencies due to COOH group vibrations are observed in this region one must conclude that either the dimers are of lower symmetry, or other molecular species (e.g., trimers or higher polymers) are present. Indeed, recent studies (see Chapman9 have shown that the molecules of solid formic acid are linked by hydrogen bonds a t both ends, forming extended chains and not' dimeric units. However, since little is known about the crystal structure of these compounds, one cannot differentiate between these two possibilities. Conclusions The infrared absorption spectra of liquid and the a-,P- and y-crystalline forms of monochloroacetic acid are significantly different. These differences cannot be explained logically in terms of rotational isomers, since the more pronounced changes occur in absorption bands characteristic of the bondedCOOH groups. This being the case, it seems logical to conclude that the kind and degree of hydrogen bonding is appreciably different in the various crystalline forms. A more detailed study of this problem can perhaps be carried out when crystallographic data become available. These data together with infrared polarization studies of oriented crystals are (9) D. Chapman, J . Chem, Soc., 225 (1956).
502
NOTES
needed before one can make any conclusions with regard to the structure or symmetry of the crystalline species of monochloroacetic acid. COMPARISON OF THE INFRARED AND RAMAN SPECTRA OF SOME CRYSTALLINE HYDROXIDES BYBETTYA. PHILLIPS AND WILLIAMR. BUSIN@
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Contribution No. 1404 from the Sterling Chemistry Laboratory, Yale University, New Haven, Connecticut Received October 6 , 1866
The crystal structures of LiOH,3 LiOH.H20,4 NaOH5 and Ca(OH)2a-9 are known from X-ray or neutron diffraction studies, and, while the atomic arrangements of these substances are all different, their structures have one feature in common: in each primitive unit cell there are two hydroxide ions and these are related to each other by a center of symmetry. This means that in the vibrational spectrum of each material there will be two fundamental OH stretching modes, one of which will be symmetric with respect to the symmetry center and Raman active, while the other will be antisymmetric with respect to this center and infrared active. The frequency difference between these vibrational modes is a measure of the coupling between the vibrations of the two hydroxide ions.lo The infrared spectra of these compounds have been observed,l1-l3 and the hydroxide stretching frequencies are listed in Table I. The probable errors of these frequencies are all about 1 cm.-l. The purpose of this note is to report some careful measurements of the corresponding Raman frequencies, since the published values of these frequenciesl4,16 are given to the nearest 10 cm.-l only. The samples used were white crystalline powders of LiOH and LiOH.HZ0, reagent grade pellets of NaOH (assay 97.0% minimum), .and coarsely crystalline Ca(OH)2 obtained by slow precipitation from interdiffusing solutions.10 (1) This research was supported in part by the U. 8. Air Force under contract monitored by the Office of Scientific Research, Air Research and Development Command. The material is taken from a thesis to be submitted t o Yale University b y Betty A. Phillips in partial fulfillment of the requirements for the degree of doctor of philosophy. (2) Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee. (3) T. Ernst, 2. physik. Chem., 2OB,65 (1933). (4) R. Pepinsky, 2. Krist., A102, 119 (1939). (5) T. Ernst, Nachr. Akad. Wiss. Gdttingen, Math.-physik. K l . , Math.-physik. chem. Abt., 7 6 (1946). ( 6 ) J. D. Bernal and H. D. Megaw, Proc. Roy. Sac. (London), AlBi, 384 (1935). (7) H. E. Petch and H. D. Megaw, J. Opt. S O CAm., . 44, 744 (1954). (8) H. E. Petch, Phys. Rev., 99, 1635 (1955). (9) W. R. Busing and H. A. Levy, J. Chem. Phya., in press. (10) D. F. Hornig, Disc. Faraday Sac., 9, 115 (1950). (11) L. H. Jones, J . Chem. Phys., 22, 217 (1954); LiOH and LiOH.HzO. (12) W. R. Busing, ibid., 28, 933 (1955). This is a detailed discussion of the spectrum of NaOH which refers t o the Raman work presented here. (13) H. W. Morgan and W. R. Busing, t o be published. The infrared spectrum of Ca(0H)a is complicated by the appearance of a number of bands believed to be due to the combination of vibrational modes with the stretching fundamental, but the latter is easily recognized b y its strong polarization in the c-direction. (14) P. Krishnamurti, Znd. J . Phys., 6 , 651 (1930). (15) G. S. Landsberg and E'. S. Baryshanskaya, Izvest. Akad. Nauk S S S R , Ser. Fiz., 10, 509 (1946). (16) F. W. Ashton and R. Wilson, A m . J . Sci., 18,209 (1927).
Vol. 61
Suitable spectra of these inhomogeneous samples were obtained by using a Wood's tube in which the exit window was replaced by a Pyrex cone extending about 4 cm. into the sample. Six AH-2 mercury lamps supplied the exciting light which passed through a filter jacket surrounding the sample. A filter solution of gentian violet and p-nitrootoluene in isopropyl alcohol was used to isolate the 4358 A. H line, and a few measurements were made with the 4047 Hg line isolated by a filter of iodine in CCla. The spectra were photographed with an f/12 Schmidt and Haensch spectrograph using two glass prisms. Exposure times ranged from 20 to 48 hours. Only one Raman line was observed for each sample and in tach case it was a fairly sharp line in the OH stretching region. The fine structure found in the infrared spectrum of Ca(OH)27Jawas not observed in these Raman spectra. Any low frequency bands which may have been present were obscured by the halation from the exciting line, and even the water bands of the LiOH.HZ0 were too weak to be seen against the background.
.f .
TABLE I A COMPARISON OF THE INFRARED AND RAMAN OH STRETCHING FREQUENCIES IN SOMECRYSTALLINE HYDROXIDES Compound
LiOH LiOH.H20 NaOH Ca(OHh
Frequency (om. -9 Infrared"-13 Raman
3678 3574 3637 3644
3664 3563 3633 3618
Difference
14 11 4 26
The spectrum of a neon discharge was superimposed on each Raman spectrum for calibration. The Raman frequencies were determined in the usual way, correcting to vacuum, and the final values which are the averages of from three to eight measurements are listed in Table I. The probable errors as determined from the deviations of the observations are about 1 cm.-1, and test measurements of the 4916 8. H g line and of the Raman spectrum of benzene indicate that this estimate of the errors is realistic. With the exception of the LiOH frequency, these results are all within 8 cm. -' of the previously reported Raman value^.^^^'^ The literature value of 3630 cm.-I for LiOH appears t o be in error. The observed spectra are consistent with the symmetries of these crystals in that one infrared active fundamental and one Raman active one have been observed in each case. Comparing the frequency differences, it is seen that the coupling of the hydroxide ion vibrations is greatest in Ca(OH)2 and least in NaOH, and that the sign of the interaction force constant is the same for all of these compounds
.
THE SYSTEM %,4,6-TRINITROTOLUENE%,4,6-TRINITRO-m-XYLENE BYLOHRA. BURKARDT Chemistry Division, U. S. Naval OTdnance Test Station, China Lake, California Received October 88, lg66
I n the course of investigations of the viscosities of liquid mixtures of 2,4,6-trinitrotoluene and 2,4,6trinitro-m-xylene it became evident that the freezing point data reported by Efremov and Tikhomi-
