KOTES
924
Vol. 67
protons of CH, if this group is not rotatinglo-further broadening may be caused by an unresolved hyperfine interaction with the nitrogen atom. The chloride behaves very similarly to the bromide except that the methyl radical signal is only just observable and slowly anneals at 77°K. The spectra of the irradiated iodide and bromide did not change significantly after several weeks a t 77°K. The difference between the behavior of the iodide and that of the bromide and chloride is interesting. Since the crystal structures are the same and the dimensions of the unit cell are very similar,ll the difference may be related directly to the chemical properties of the halides. The methyl radical observed in all three irradiated salts probably is formed by decomposition of an excited tetramethylammonium species.
DPPH
C
The species tentatively identified as (CH&CCW2. could be formed as a result of hydrogen abstraction from the tetramethylammonium cation by bromine or chlorine atoms. Photochemical evidence12 shows that the iodine aton? cannot undergo such a reaction. KO e.s.r. signal was observed from halogen atoms, and it is assumed that matrix interaction broadens the peaks beyond detection.6 Acknowledgment.-'The author is indebted to Dr. N. Sutin for helpful discussion and comment. (10) E. L. Cochran, F. J. Adrian, a n d V. A. Bowels, zbad., 34, 1161 (1'261). (11) R. W. G. Wyckoff, Z. Krzst., 67, 91 (1928). (12) 'E. W. R. Steacie, "Atomic and Free Radical Reactions," Vol. 11, Reinhold Publ. Gorp., New York, N. Y., 1964, p. 701.
THE DENSITY OF LIQUID ARSENIC AND T H E DENSITY OF ITS SATURATED VAPOR1 BY P. J. XCGONIUAL~ ASD A. V. GROSSE Research Institute of Temple Cnidersity, Philadelphia 44, P a .
Receieed September 26, 196B
60 gOUSS MAGNETIC F I E L D STRENGTH,
Fig. 1.-(A): The e.s.r. spectrum of RIe4S"I- ?-irradiated and observed a t 77°K.; ( B ) the e.5.r. spectrum of MedK+Br?-irradiated and observed a t 77°K; (C) the e m . spectrum of Me4N+Br- ?-irradiated a t 77°K. then thermally annealed a t 148°K. and the spectrum observed a t 77°K. -411 the e.s.r. spectra are first derivative curves.
tet, and this would indicate that a species such +
as (CH3) ,NCH2. is observed and not CH2.+. Spectra taken using a single crystal of the chloride did not give an increased resolution. The microwave power was reduced to the lowest possible levels but no further fine structure could be observed in the t r i ~ l e t . The ~ variation of signal intensity with microwave power showed that saturation was not occurring at the low power levels used. The lines may be broadened by an unequal coupling of the unpaired electron with the two (9) J. A. Simmons, J. Chem. P h y s , 36, 469 (1962).
The density of liquid arsenic is difficult to measure in spite of its comparatively low melting point (1090°K.) due to the fact that the vapor pressure of arsenic at its melting point is already 35.8 atm.3 I n the present m7ork the density of liquid arsenic and the density of its saturated vapor mere measured over a range extending from the m.p. to 1323°K. The density of liquid arsenic was determined by cathetometric measurement of the liquid level in calibrated, sealed Vycor tubes of known volume which contained a known mass of arsenic. The mass of the liquid was obtained by subtracting from the total the mass present as vapor. A total of 49 points was obtained from eight series of experiments. Least squares treatment of the data yielded the equation
D g . / ~ m= . ~5.80 - 5.35 X lO-*T
(OK.)
The probable error is d0.02 g . / ~ m . ~ . Vapor densities were determined by observing the temperature at which a known mass of arsenic vaporized completely in a sealed Vycor tube of known volume. Determinat,ions were made at seven different tempera(1) This work was supported in part b y the Kational Science Foundation under Grant 18829. (2) -4 report of this work will constitute a portion of a dissertation t o be submitted by P. J. McGonigal to the Graduate Board of Temple University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. (3) S.Horiba, 2. physik. Chem., 105, 295 (1922).
SOTES
April, 1963 tures in the range of interest. Th.e experimental vapor densities fall on a smooth curve which is best represented by the equation obtained by the method of least squares D g . / ~ m= . ~0.2072 - 6.182 X 10--4Tt 5.043 X 10-’T2
(OK.)
The probable error is &0.0007 g . / ~ m . ~ . A summary of t:he smoothed results, together with several derived quantities, is shown in Table I. The vapor pressures were calculated from Horiba’s experimental data,3extrapolated as necessary. TABLE I DENSITY,AT’OMIC VOLUME, AND COEFFICIENT O F CUBICAL EXPANSION OF LIQUIDARSENIC; VAPORDEKSITYAKD VAPOR PRESSURE
--D.
Liquid---
T, OK.
g./lcm.n
At. vol., cm.a
1090 m.p. 1150 1200 1250 1300 1320
5.22 5.19 5.16 5.13 5.10 5.09
14.35 14.44 14.52 14.59 14.67 14.70
Coeff. of cubical exp., T-1 X 100
Vapor density. g./cm.s
Vapor pressure, atm.
102.5 103.1 103.7 104.2 104.8 105.0
0.1326 .1632 .1916 .2224 ,2558 .269g
35.8 46.8 57.6 69.2 83.2 87.1
The experiments were carried out in a specially constructed furnace equipped with observation ports and three separate resistance windings. Temperature measurement was accomplished by three Chromel-Alumel thermocouples. The arsenic used was supplied by Penn Rare Metals, Inc., and had a purity of 99.99%. The major impurities were O . O O l ~ o Cu, O.OOl~oFe, 0.004% Sb, and 0.002% Si. No detectable reaction occurred between the arsenic and the Vycor glass. However, several tubes burst due to the high vapor pressures (about 90 atm. a t 1323’K.) and extension of the measurements to higher temperatures was not considered feasible with Vycor tubes. The design of the furnace incorporated several features to minimize the danger from bursting tubes. Arsenic is a very interesting element from the standpoint of allotropy. It exists in several solid modifications of which the gray, or metallic, form is stable at ordinary temperatures. On the basis of its physical properties Klemm4 has termed it a “Halbmetall.” Liquid arsenic is opaque but the vapor in equilibrium with it is yellow. Brewer and Kane5 examined arsenic vapor from a Langmuir type experiment with a mass spectrometer and found it to be predominantly Asd. Stull and Sinke6 state that according to the data of Brewer and Kane5 there is no appreciable concentration of the As2 species below 1000°K. The vapor densities obtained in this work are higher than those calculated from the ideal gas law assuming 100% As4 molecules. This corroborates the predominance of the As4 species up to 1323OK. JoliboisJ7who determined that liquid arsenic is opaque a t llOO°K., has suggested that liquid arsenic may change into a yellow transparent form (4) W . Kleinm, Angeu. Chem., 62, 133 (1950). ( 5 ) L. Brewer and J. 5.Kane, J . Phys. Chem., 59, 105 (1955). (6) D R. Stull and G. C. Sinke, “Thermodynamic Properties of the Elements,” -4dvances in Chemistry Series, No. 18,American Chemical Society, Washington, D. C., 1956. (7) P. Johbois, Compt. rend., 162, 1767 (1911).
925
AS^) a t some higher temperature. Rassow* has determined by direct observation that the critical temperature of arsenic is greater than 1400’ and presumably the liquid is still opaque a t this temperature since no information to the contrary is reported. At temperatures above 1700OK. a t atmospheric pressure, the As2 species predominates6 in the vapor but the pressure effect on the equilibrium is unknown. Dissociation to the monatomic species appears to be unfavorable even at 30OO0K. I n view of the fact that arsenic is a “Halbmetall,” it is not excluded that it may become nonmetallic a t higher temperatures. The critical temperature of arsenic is obviously higher than that of phosphorus, 993.8°K.,9 and probably less than that of antimony. The density of liquid antimony recently has been measured over a wide temperature rangelo and, in contrast to typical metals (whose density us. temperature behavior is in general linear up to the normal boiling point) shows significant curvature downward with increasing temperature. It is most likely that this deviation from linearity is due to structure changes in the liquid. I n view of their positions in the periodic table, it is reasonable to assume that arsenic will behave in a manner similar to that of antimony. That is, at higher temperatures the usual rectilinear diameter line mill begin to curve downward and significant structure changes may be expected. XOTEADDEDIN PROOF.--W~ have only recently become aware of previous measurements of the density of liquid arsenic. W. Klemm, H. Spitzer, and H. Niermann, Angew. Chem., 72, 985 (1960), report five points between 830 and 850”. These points show considerable scatter and it is not possible to establish a slope from them. H. Niermann, Dissertation, Miinster, 1961, reports six points between 771” (subcooled) and 960” which fall approximately on a straight line. Both sets of measurements are about 4% lower than our values. (8) €1. Rassow, Z. anorg. allgem. Chem., 114, 117 (1920). (9) W. Nlarckwald and K. Helmholtz, ibid. 124, 81 (1922). (10) &4.n. Kirshenbaum and J. $. Cahill, A m . SOC.Metals, Trans. Quart., 66, 849 (1962).
GASEOUS FLUORIDES OF XESO1C” BY M.4RTIN H.
~ T U D I E R4 N D
Chemistry Dzeision, Argonne .\-atzonal
ERICN. SLOTH
Laboratory, Argonna, Illinozs
Received September 88, 1962
The existence of xenon tetrafluoride, first reported by Claassen, Selig, and Malm of this Laboratory,2has been verified further by observation of gaseous XeF4 in the Bendix Time-of-Flight mass spectrometer. Their suggestion of the existence of a lower fluoride of xenon has been confirmed by the observation of the difluoride of xenon as an independent species. I n addition, a number of oxyfluorides of xenon were seen. The masses of the observed ions correspond to the formulas Xe +
Xe++
XeO +
XeF +
XeF + +
XeOF +
XeF2+
XeFz++
XeOF2+
(1) Based on work performed under the auspices of the U. S. Atomic Energy Commission. (2) H. €1. Claassen, H. selig, and J. G. Malm, J . Am. Chem. SOC.,84, 3593 (1962). (3) D. B. Harrington, “Encyclopedia of Spectroscopy,” Reinhold Publ. Corp., New York, N. Y.. 1960, pp. 628-617.
