R. A. KENT,J. D. MCDONALD, AND J. L. MARGRAVE
874
The molecules need not have over-all planar symmetry but planarity would probably be retained in the individual boroxine rings. We can visualize the formation of H4B607 as elimination of a H2 molecule in a reaction of H3B303 a t a B-H bond with HzB303(0H) at the 0-H bond. Similarly, H4B404 could be formed from H3B303 and HzB(0H) (hydroxyborane).* Ion currents of HBO+ and HBOH+ are always observable in these experiments but, because these ions are formed by fragmentation from a number of precursors, it is difficult to assess the importance of the molecular contribution of HBO and HzBOH. These species may be
important in the kinetics of reaction as precursors to boroxine. The pressure dependences of reactions 1 and 3 show that the equilibrium concentrations of H4B404and H4B60 7 should increase relative to H3B303 as the pressure of H3B303 increases. Relatively high concentrations of H4B404 and H4Be07should be possible in a static system where the partial pressure of Hz over B-Bz03 mixtures is of the order of 1 atm at temperatures near 1400'K. (8) R.F. Porter and S. K. Gupta, J . Phys. Chem., 68, 2732 (1964).
Mass Spectrometric Studies at High Temperatures. IX.
The Sublimation
Pressure of Copper(I1) Fluoride
by R. A. Kent, J. D. McDonald, and J. L. Margrave Department of Chemistry, Rice University, Houston, Texas 77001 (Received October 8, 1965)
Mass spectrometric studies of CuFz sublimation from a Knudsen cell have established CuFz(g) as the major vapor species and AH"~~~[sublimation] = 63.9 f 1.0 kcal mole-'. The dissociation energy of CuF(g) is 87 f 9 kcal mole-' (3.8 f 0.4 ev).
I. Introduction have reported mass Previous papers in this spectrometric and microbalance measurements of Knudsen and/or Langmuir vaporization and/or sublimation rates for various transition metal fluorides. I n this investigation, the sublimation rate of CuFz has been measured, and the vapor species have been identified by the Knudsen technique employing a mass spectrometer.
11. Experimental Section The mass spectrometer has been described previ0us1y.~ Temperatures were measured with an infrared pyrometer as described previously. The CuFz sample, 99.8% purity, was generously provided by Professor J. W. Stout of the University of Chicago. The JOUTnd of Physical Chemistry
Choice of the Knudsen cell material presented a slight problem inasmuch as CuFz has one of the weakest metal-fluorine bonds of any of the first-row transition metal fluorides, and, thus, CuFz is an effective hightemperature fluorinating agent. Preliminary experiments showed that CuFz reacted with tantalum a t high temperatures to produce Cu(s) and TaFs(g). Finally, the CuFz was contained by using an MgO (1) R. A. Kent, T. C. Ehlert, and J. L. Margrave, J . Am. Chem. SOC.,86, 5090 (1964). (2) T. C. Ehlert, R. A. Kent, and J. L. Ahfargrave, ibid., 86, 5093 (1964). (3) R.A. Kent and J. L. Margrave, ibid., 87,3582 (1965). (4) R. A. Kent and J. L. Margrave, ibid., 87, 4754 (1965). (5) G. D.Blue, J. W. Green, R. G. Bautista, and J. L. Margrave, J . Phys. Chem., 67,877 (1963).
SUBLIMATION PRESSURE O F COPPER(I1)
FLUORIDE
Iinudsen cell to line the tantalum cell. The MgO had previously been heated to 800" in an atmosphere of fluorine gas. I n the temperature range covered in this investigation, the sublimation pressure of MgFz is several orders of magnitude lower than that of C U F ~ , ~and . ' the expectation was that a protective layer of MgF2 would be formed which would prevent reaction between the MgO cell and the CuFz sample. This expectation was justified when the only ions formed by electron bombardment of the eff usate from the cell were CuF2+,CuF+, and Cu+. 111. Results and Discussion The Sublimation of CuF2. The rates of sublimation were measured, and the vapor species were identified between 897 and 1026°K by means of the mass spectrometric technique. The ionic species which were formed by electron bombardment of the effusate from the Knudsen cell were CuF2+, CuF+, and Cu+. S o dimers or higher polymeric species were observed although the spectrum was scanned to mass 400. Using as a standard the known value of the ionization potential of mercury, 10.4 ev,8 the appearance potentials of the ions CuF2+,CuF+, and Cu+ were observed to be 11.3, 12.4, and 16.5 ev, respectively, with estimated uncertainties of f 0 . 3 ev. These values indicate that the ions CuF+ and Cu+ result from the dissociative ionization of CuF2(g) rather than from the simple ionization of the species CuF(g) and Cu(g). The ion CuF2+ is attributed to the simple ionization of CuFz(g). The relative abundances of ions at masses 63 and 65, 82 and 84, and 101 and 103 were checked and found to correlate with the known isotopic abundances of 63Cuand 66Cu. Two independent methods were employed to determine the heat of sublimation of CuFz from the experimentally observed ion currents of CuF2+. The intensity of the lo3CuF2+peak using 75-v electrons was followed as a function of temperature. By making use of the ion current-pressure relationship P = k I T 9 and the integrated form of the Clausius-Clapeyron equation, a value of AHT which is independent of the proportionality constant k may be found from the slope of the curve obtained by plotting log (I+T) us. 1/T. The slope of this plot yielded a heat of sublimation of AH0970 = 59.5 f 0.6 kcal mole-', where the uncertainty given is the standard deviation of the leastsquares treatment. The true uncertainty may be two or three times this figure owing to such factors as temperature gradients in the crucible. Because no heat capacity data for CuF2(s) were available, the thermodynamic functions for CuF2(s) were estimated from a combination of available data for hlnFz(s),lo
875
XiF,(s)," and Mn(s), Si(s), and Cu(s)I2. Therniodynamic functions for CuFz(g)were calculated from the parameters presented by Brewer and co-urorkers.13 When corrected to 298°K the second-law heat of sub= 62.8 f 0.6 kcal mole-'. limation becomes An alternative approach is to calculate AH"298 for each temperature from the absolute pressure and the free energy function change, Afef, for the reaction. I n order to determine the instrument constant k a weighed sample of previously degassed CuFz was vaporized from the crucible at a constant temperature, and the intensity of the lo3CuF2+peak was followed as g effused a function of time. At 950"K, 3.86 X in 150 min through an orifice whose area mas 8.02 X cm2 (Clausing factor, 0.479). By use of the Knudsen equation, the pressure was calculated to atm. The value of k determined was be 7.7 X combined with the least-squares equation for log ( I + T ) as a function of reciprocal temperature to yield the vapor pressure equation log Patm= - (1.300
f
0.013) X 104/T
+ (8.58)
f
0.14)
The value of k was also used to calculate a value of log P for each observation. The values of log P were then combined with the free energy functions for CuFz (s) and CuFz(g)to obtain the third-law heat of subliniation. The results of the mass spectrometric runs are presented in Table I and plotted in Figure 1. While the small standard deviation in the thirdlaw heat reflects the reproducibility of the measurements, the true uncertainty must reflect errors in the estimated thermodynamic functions; hence, the thirdlaw heat is taken to be 63.9 f 1.0 kcal mole-'. The major inherent errors in the second-law result are those in temperature measurement. In practice, uncertainties of f 3 kcal mole-' are usual in the teniperature range covered in this investigation, and the second-law heat of sublimation is taken to be 62.8 f 3.0 kcal mole-'. The heat of formation of (6) J. 1%'. Green, G. D. Blue, T. C. Ehlert, and J. L. Margrave, J . Chem. Phys., 41, 2245 (1964). (7) M. A. Greenbaum, H. C. KO,M. Wong, and AI. Farber, J . Phys. Chem., 68, 965 (1964). (8) C. E. Moore, National Bureau of Standards Circular 467, U. S. Government Printing Office, Washington, D. C., 1949. (9) W. A. Chupka and M. G. Inghram, J . Chem. Phys., 21, 371 (1953). (10) A. D. Mah, U. S. Department of Interior, Bureau of Mines, Report of Investigation 5600, Mines Bureau, Pittsburgh, Pa., 1960. (11) A. Glassner, Argonne National Laborators Report ANL 5750, Jan 1958. (12) D. R. Stull and G. C. Sinke, Advances in Chemistry Series, No. 18, American Chemical Society, Washington, D. C., 1956.
Volume 7 0 . S u m b e r S
March 1966
R. A. KENT,J. D. MCDONALD, AND J. L. MARGRAVE
876
- 4.0 5i w cc
2g
-4.5
5
I-
5 w
cc
2
v)
W -50 a a W
0
J
A ABSOLUTE PRESSURE
MEASUREMENT
-5 5
9.6
10.0
10.8
10.4
11.2
I04/ T,OK
Figure 1. Vapor pressure data for CuF2.
Table I: Mass Spectrometric Data for CuFz Sublimation Temp,
OK
983 991 1004 1015 1026 1021 1008 1004 995 988 981 985 978 974 971 963 959 950 948 93: 933 929 922 912 897 950“
I(CuF2+), arbitrary units
1197 1677 2352 3750 4350 4250 3470 2900 2502 211s 1752 2046 1704 1413 1248 1098 933 703 633 434 377 319 257 184 118 680
-Log
-A / d ,
AH020s,
cal deg -1
kcal mole - 1
Pstm
mole - 1
4.679 4.531 4.378 4.219 4.132 4.162 4.256 4.336 4.514 4.590 4.674 4.605 4.687 4.770 4.827 4.885 4.958 5.085 5.132 5.300 5.364 5.439 5.535 5.685 5.885 5.113
43.76 43.76 43.75 43.77 43.78 43.78 43.76 43.75 43.75 43.76 43.77 43.76 43.77 43.77 43.7s 43.78 43.78 43.79 43.80 43.81 43.81 43.81 43.82 43.83 43.85 43.79
64.1 63.9 64.0 64.0 64.3 64.2 63.8 63.8 64.0 64.0 63.9 63.9 63.8 63.9 64.0 63.7 63.7 63.7 63.8 63.8 63.8 63.8 63.8 63.7 63.5 63.8 Av 63.9 f 0.2
Absolute pressure measurement, corrected for change in multiplier gain. ~~
The Journal of Physical Chemistry
CuFz(g) from the elements in their reference states at 298°K is calculated to be -63.0 f 3.0 kea1 mole-’, based on the available heat of formation of CuFz(s).13 The Dissociation Energy of CuF. When the heat of sublimation of CuFz is combined with the heat of formation of CuF2(s),13 the heat of sublimation of copper,’2 and the atomization energy of fluorine16 one calculates a dissociation energy of CuFz(g) into atoms of 181.8 f 4.0 kcal mole-’ and an average Cu-F bond strength of 90.9 kcal mole-’ (3.94 ev). From the appearance potential data one calculates AHa 5 202?& kcal mole-’, with the large uncertainty caused by the unknown thermodynamic state of the fluorine produced on coniplete dissociation. If the ratio D(JIF)/AHa(AIF2) = 0.46 f 0.02 as found for the alkaline earth fluoridess and other transition metal fluoride~,~ then, the value of D(CuF) becomes 83.6 f 4.0 kcal mole-’ (3.63 f 0.2 ev). Calculations of dissociation energies, based on an ionic model like that used by Rittneri4 for the alkali halides, appear to be useful. One may calculate the polarizability for the Cuf ion from the prescription CY = 32/21(M), given by Kauzmannls with the value for r‘i calculated by Hirshfelder, et aE.,16 and the second ionization potential for Cu from Moore.s The polarizability of F- was calculated by Pauling,” and the electron affinity of F atoms was taken from the JAh’AF tables.18 The bond Iength in CuF, re, was estimated as 1.70 A while Herzberglg gives we = 623 em-’. From these parameters one calculates D(CuF) = 3.66 f 0.4 ev, in good agreement with the prediction based on the heat of atomization. Manganese metal powder was added to CuF2 in the A4gO cell in an attempt to produce CuF(g) and determine its dissociation energy. However, the h1n reduced the CuFz to Cu(s), and the major species which effused from the crucible were B!tnF2(g) and UnF(g). The dissociation energy of \lnF(g) has been established as 4.39 f 0.15 ev,’ and this value can be taken as an upper limit for the dissociation energy of CuF(g). (13) L. Brewer, G.R. Somayajulu, and E. Brackett, Chem. Rea., 63, 111 (1963). (14) E.S. Rittner, J. Chem. Phys., 19, 1030 (1951). (15) W. Kauzmann, “Quantum Mechanics,” Academic Press Inc., New York, N. Y.,1957,p 514. (16) J. 0.Hirshfelder, C. F. Curtiss, and R. B. Bird, “Molecular Theory of Gases and Liquids,” John Wiley and Sons, Inc., New York, N. Y., 1954,p 955. (17) L. Pauling, Proc. Eoy. Soc. (London), A114, 181 (1927) (18) D. R. Stull, Ed., “JANAF Thermochemical Data,” Dow Chemical Co., Midland, hfich. (19) G. Heraberg, “Molecular Spectra and Molecular Structure. I. Spectra of Diatomic Molecules,” D. Van Nostrand Co., Inc., New York, N.Y., 1953.
VAPORPHASE REACTION OF METHYLRADICALS WITH TOLUENE
CuFz was reduced when heated in a tantalum Knudsen cell. The appearance potential of the ion CuF+ produced in this reducing system, 8.6 f 0.3 ev, when combined with the value obtained in the nonreducing system 12.4 f 0.3 ev, yields D(FCu-F) = 4.2 f 0.5 ev and D(CuF) = 3.7 h 0.5 ev, with the uncertainty mainly to allow for the unknown thermodynamic state of the fluorine after electron impact; i e . , is it F, F-, Fz,or Fz-? Simple subtraction (12.4 - 8.6 = 3.8 ev) for D(FCu-F) would seem to indicate D (CuF)/AH,(CulF2) > 0.5 in disagreement with available data for similar fluorides, but would give D(CuF)
877
= 4.1 + 0.5 ev. Until further data are available, D(CuF) = 3.8 f 0.5 ev is a reasonable choice. Acknowledgment. The authors wish to thank Professor J. w. Stout of the University of Chicago for the CuFz sample. Financial support for high-temperature research at Rice University is provided by the United States Atomic Energy Commission, by the National Aeronautics and Space Administration, by the Advanced Research Projects Agency through the Army Research Office, and by the Robert A. Welch Foundation.
The Vapor Phase Reaction of Methyl Radicals with Toluene at 100-300"
by Mark Cher, C. S. Holliigsworth, and F. Sicilio' hlorth American Aviation Science Center, Thousand Oaks, California (Received October 15, 1965)
The rates of reaction of methyl radical, produced by photolysis of acetone between 100 and 300" or by pyrolysis of asomethane a t 300", with the aliphatic and aromatic C-H bonds in toluene, toluene-ds, toluene-d3, and toluene-& were determined from measurements of the rates of production of methane and ethane and from the isotopic composition of the methane. The dominant reaction is the abstraction from the side chain, for which the activation energies are 9.5 kcal/mole for C,&CH3 and 11.3 kcal/mole for CeHsCD3 and the A factor is 1011.6cc mole-' sec-' independent of deuteration. The ring reaction is complex with a t least two mechanisms having different temperature dependences operating simultaneously. At high temperatures direct abstraction from the ring takes place, the rate constant is given by ?'h = exp(-10 kcal/RT), and the deuterium isotope effect is rh/rd 10-15. At low temperatures methyl radicals appear to add to the aromatic ring, and the approximate values of the Arrhenius parameters are log A 7 and E 4 kcal/mole. These values are little affected by deuterium substitution in the ring.
-
Introduction I n a previous publication2 we have shown that at 60" methyl radicals abstract hydrogen from both the ring and the side-chain positions of toluene and that the ratio between these two rates of reaction is sensitive to the deuterium content of the toluene. In ordinary toluene the ring abstraction is about one-tenth of the total abstraction; in toluene-d3 (CsHsCD3) the ring
-
-
abstraction is about equal in magnitude to the sidechain abstraction; and in toluene-& (C6DsCHa) the ring abstraction is essentially negligible. Since the C6H5-H bond strength is approximately 2&25 kcal/ mole higher than the CeH&H,-H bond strength, (1) Visiting Scientist, summer 1963. (2) M. Cher, J. Phys. Chem., 68, 1316 (1964).
Volume 70, Number S
March 1966
