K. F. ZMBOV,0. M. UY, AND J. L. MARGRAVE
3008 This can also explain the slightly lower absorption frequencies for the amide I bands in dioxane as compared to carbon tetrachloride, where a weaker interaction between the oxygen of the dioxane and the amide hydrogen is expected. It is not certain that the carbonyl oxygen of lactams interacts with DMSO. In water, both the carbonyl oxygen and the amide hydrogen of the lactams can interact with the solvent molecules. The frequencies of the amide I bands are much lower and the frequencies of the amide I1 bands are much higher than those observed in carbon tetrachloride solution. This indicates stronger hydrogen bonding as well as an effect due to the dielectric constant of the medium. The observed solvent effect does not correlate with the order of refractive index, which
infers that hydrogen bonding is far more important than the field effect of the solvent. There is no apparent frequency shift for the amide I band in dioxane, DMSO, and water as the solution is diluted. This observation disproves any intramolecular hydrogen bonding between lactams. I n water, this can be explained by the hydrogen bonding of both the carbonyl oxygen and the amide hydrogen to the solvent molecules. The situation in dioxane and DMSO is different, as only the N-H group is bonded to solvent, leaving the carbonyl group free.
Acknowledgment. This research was supported by two grants from the Division of Molecular Biology, National Science Foundation, GB-4835and GB-2049.
Mass Spectrometric Studies at High Temperatures.
XXXI.
Stabilities of Tungsten and Molybdenum Oxyfluorides by K. F. Zmbov, 0. M. Uy, and J. L. Margrave Department of Chemietry, Rice University, Houaton, Texas 77001 (Received March 18, 1060)
WO&) and MOO&) were fluorinated in a tantalum Knudsen cell at high temperatures, and the effusing molecules were characterised mass spectrometrically. Several equilibria were observed and heats of reaction were derived from temperature-dependence studies. The heats of atomization for the oxyfluoride species are A H " ~ B S , ~ ~ ~ ~ ~ [=W651 OF~(~ 10) ]kcal mol-'; AHo298,atoms[W02F2(g)] = 572 f 10 kcal mol-l; and AH"~~S,~~~~~[MOO~F~(P;)] = 582 =!= 15 kcal mol-'.
*
Introduction This work is part of an experimental program for the study of the stabilities of transition metal oxyfluorides.'J While there have been numerous publications about the fluorides3r4 and oxides5 of the transition metals, the oxyfluorides represent an essentially unexplored class of compounds from the viewpoint of mass spectrometry.
Experimental Technique The experimental technique and procedure have been described previously.6 The mass spectrometer is a single focusing, 12-in. radius of curvature, 60" sector instrument. The Knudsen cells were heated by radiation either from a concentric molybdenum mesh or from a 10-mil tungsten three-turn spiral. Tantalum Knudsen cells were used in the experiments for both molybdenum and tungsten oxyfluorides. Other chemicals were purchased from Alfa Inorganics, Inc., Beverly, Mass. The Journal of Physical Chemistry
Results Tungsten Oxyjluorides. Oxyfluorides of tungsten CrFz mixtures in a were produced by heating WOs
+
T a Knudsen cell. At temperatures above 600" the mass spectrum of Ta-containing ions resembled that in the previously studied Ta-0-F system2and showed the presence of TaF6 and TaOF8 molecules in the vapor. (1) K. F. Zmbov and J. L. Margrave, J. Inorg. Nucl. Chem., 29, 2649 (1967). (2) K. F. Zmbov and J. L. Margrave, J . Phys. Chem., 72, 1099 (1968). (3) R. C. Feber, CT. 8. Atomic Energy Commission Report LA-3164, Nov 1964. (4) K. F. Zmbov and J. L. Margrave, J. Chem. Phys., 45, 3167 (1966),and other papers in this series. (5) J. Drowart, "Condensation and Evaporation of Solids," E. Rutner, P. Goldfinger, and J. P. Hirth, Ed., Gordon and Breach, New York, N. Y.,1964,pp 255-317. (6) K. F. Zmbov and J. L. Margrave, in "Applications of Mass Spectrometry in Inorganic Chemistry," Advances in Chemistry Series, No. 72, American Chemical Society, Washington, D. C., 1968.
3009
STABILITIES OF TUNGSTEN AND MOLYBDENUM OXYFLUORIDES The appearance potentials of W-containing ions are listed in Table I. The relatively low value of the appearance potential of W02Fz+,13.0 f 0.3 eV, as compared to that of the other W-containing species and the fact that no higher molecular species were found which could give rise to W02F2+ by fragmentation, indicate that this ion is formed from direct ionization of the WOzFz molecule. From the appearance potential of WOF3+ ion, 14.8
TaFdg)
+ WOa(s) = TaOFdg) + WOzFdg)
(31
Third-law calculations based on the equation
- TA[(G," - Hoza,,-)/T]
AHozos= -RT In K ,
(4)
were applied for reactions 2 and 3. The equilibrium constants, free-energy functions, and heats of reactions 2 and 3 are given in Tables I11 and IV, respectively.
*
Table I1 : Partial Pressures of Gaseous Species over the WOa-CrF2-Ta System Table I : Observed W-Containing Ions in the WOs-CrFz-Ta System and Their Appearance Potentials (T = 950°K)
Ion
W+ WF + WOF+
woz +
WFz + WO2F'+ W0Fz.t WO2Fz + WOFs 't WFs +
Appearance potential, eV
*
37.4 1 . 0 32.1 f 0 . 8 23,6 f 0 . 5 21.0 f0 . 5 27.6 f 0 . 5 16.3 i 0 . 5 18.4 % 0 . 5 13.0 f 0 . 3 14.6 f 0 . 5 22.8 f 1 . 0
T, OK
PT~F~, atrn
PT~oF~, atm
936 954 950 937 921 908 895
4.59 X 1.13 X lo-' 1.20 X lo-' 1.14 X 7.92 X lo+ 7.63 X 2.71 X
4.35 X 9.78 X 7.35 X 6.14 X 2.76 X lo-' 2 . 5 4 X lob7 1.34 X lo-'
PWOFI,
~
(1)
where p is the partial pressure, It+is the measured isotopic peak, at is its abundance, T the temperature of the Knudsen cell, AS the sensitivity factor, u the relative ionization cross section, y the electron multiplier efficiency, E the energy of the electron beam used in measuring the ion intensity, E m a x the energy to produce the maximum ion intensity, and A the appearance potential. The u values were calculated using the tables of relative ionization cross sections of Otvos and Stevenson8 and by applying the additivity rule. The sensitivity factor S was obtained by a silver calibration. The partial pressures of the molecules TaF5, TaOFg, WOF4, and WO2Fz are given in Table 11. The equilibria considered were 2TaFdg) and
+ WOa(s) = 2TaOFdg) + WOF4g)
~~~
+
+
*
- A)l/[Sur(-@ - A h 1
10-1 10-8 10-6 10-6 10-6 10-6
Table 111: Equilibrium Constants and Heat for the Reaction 2TaF&) WOa(s) = 2TaOFdg) WOF4g)
0.3 eV, it was assumed that this ion was formed by fragmentation of WOF4 molecules. Similarly, the appearance potentials of W02F+, 16.3 f 0.5 eV, and of WF3+, 1.0 eV, indicate these to be fragment ions from 22.8 WOzF2and WOF4 molecules, respectively. The predominant molecules in the vapor over the W03-CrF2Ta system in the temperature range 600-700" are therefore TaF5, TaOFg, WOF4, and W02Fz. Absolute partial pressures of these molecules were determined by using the relation7 = [Ir+(-@max
atm
1.50 X 2.44 X 3.67 X 5.30 X 2.99 X 4.13 X 1.64 X 2.48 X 7.88 X 1.38 x 3.63 X lop6 1.27 X 1 . 8 8 X 10-6 1.88 x
-A(QTO
p
PWOgFp
atm
(2)
T,
K,W
OK
atm
936 954 950 937 921 908 895
1.35 x 2.74 x 1.12 x 4.75 x 9.75 x 1.09 x 4.60 x
10-9 10-9 10-9 10-9 10-9 10-10 lo-"
-
H Oses)/T,
AH'ags,
Gal deg-1 mol-'
koa1 mol - 1
51 51 51 51 51 51 51
86 85 87 88 90 88 88
AHo2g3
= 87 zk 5
Table IV: Equilibrium Constants and Heat for the Reaction TaFs(g) WOds) = TaOFdd WOZFdg)
+
+
-A(G'TO
TI OK
936 954 950 937 92 1 908 895
Kes, atm
2.31 X 4.59 x 2.53 x 1 .a4 x 4.81 x 6.97 x 2.66 x
10-8 10-8
10-8 10-9 10-9
-
Hozea)/T,
AH'm,
oal deg-1 mol-]
mol-1
57 57 57 57 57 57 57
86 87 88 88 89 87 87
koa1
AH02ss = 87 f 5
(7) M. G. Inghram and J. Drowart, "Mass Spectrometry Applied to High Temperature Chemistry," in "High Temperature Technology, Stanford Research Institute," McGraw-Hill Book Co., New York, N. Y., 1960. (8) G. W. Otvos and D. P. Stevenson, J . Amer. Chem. Soo., 7 8 , 546
(1956).
Volume 79, Number 9 September 1969
K. F. ZMBOV,0. M. UY, AND J. L. MARGRAVE
3010 The only existent data for free-energy functions were those for W03(s) and WOF4(g),g while those for the molecules TaF5,TaOF3, and W02F2were estimated. The third-law calculations yielded 87 kcal mol-' for the heats of both reactions 2 and 3. The estimated uncertainty here is * 5 kcal mol-'. Because the temperature range in which these two reactions have been studied was very limited, no reliable second-law value could be obtained. Reactions 2 and 3 may also be combined to obtain the heat of reaction AHzO for the gaseous equilibrium TaFdg)
+ WO2F2(g) = TaOFs(g) + WOF4k)
(5)
By combining the heats of reactions 2 and 3 with the 10kcalmol-', heat of formation of TaOF3(g),2 -337 and of WO3(s),'o -201.46 f 0.2 kcal mol,-' the heat of atomization of TaF5(g),3719 f 2 kcal mol-', the heat of sublimation of W(s)," 200.0 kcal mol-l, and the heats of formation of O(g) and F(g)," 59.6 and 18.9 kcal mol-', respectively, one obtains A H O a t o m s [WOR(g)1 = 651 f 10 ltcal mol-'. Similar procedures yield the heat of atomization of W02F2as 572 f 10 kcal mol-'. Molybdenum Oxyfluoride. The oxyfluoride of molybdenum was produced similarly by heating Moo2 CrFz mixtures in a tantalum Knudsen cell over the temperature range 645-748". Table V lists the ions de-
0.4TaFs(g)
+ MoOz(s) = MoO2F2(g) + 0.4Ta(s)
(6)
A second-law plot of relative equilibrium constants based on pressures derived from the usual relation, Pt = Kt+T, is shown in Figure 1. From the slope of the line obtained by the least-squares method, the secondlaw heat of reaction was found to be AH"gm0x = 44.5 1.7 kcal mol-'. When this value is corrected to 298°K by using the heat content data for MOO~(S)'~ and Ta(s)" and estimated enthalpy increments for Mo02F2(g) and = 47.9 f 5 kcal mol-' TaFs(g), one calculates for the heat of reaction 6. No third-law value can be obtained for comparison because a sensitivity calibration was not performed with this experiment.
*
*
+
U
4.5
I-
-
I? N
0
2
U
.V1
Table V: Observed Ions in Moos-CrFz-Ta System and Their Appearance Potentials
(T =
973°K)
" s
4.0 -
Appearance potential,
Ion
eV
Mo + MoOF + MoOzF + MoOzFz + Ta+ TaF+ TaOF + TaFz + TaOFa+ TaF8+ TaOFs + TaF, +
... 23.0 f 0.5 15.0 f 0.5 13.0 i:0 . 3
... *..
21 f 1 28 i:1 15.0 f 0 . 5 23 f 1 12.5 f0.5 15.0 f 0 . 3
tected in the mass spectrum of the effusing vapors over the MoOz-CrFz system heated a t 700" in a T a Knudsen cell. The table lists also the appearance potentials of the ions with measurable intensities. The appearance potentials do not reveal the presence of reduced molecules and establish unambiguously only the molecules Mo02F2,TaF5, and TaOFa in the effusing vapor. The appearance potentials of the Ta-containing ions reproduced the values found previously for the ions formed by dissociative ionization from TaFs and TaOF3.2 The equilibrium considered was The JOUTnd of Physical Chemistry
33
3 9.5 10.0 10.5 11.0 1041~(OK-')
Figure 1. Plot of the ion-current analogs of the equilibrium constant us. 1/T for the reaction 0.4TaS(g) MoOz(s) = MoOZFz(g) 0.4Ta(s).
+
+
By combining the heat of reaction 6 with the heat of formation of TaFs(g),2,3437.7 kcal mol-', the heat of ,'~ =t0.2 kcal mol,-' the formation of M O O ~ ( S )-140.8 heat of sublimation of Mo(s),'Z 157.7 f 0.8 kcal mol-', the known heat contents of the elements," AHf02g8 for O(g), and F(g)," and the estimated enthalpy increments for A!loOzFz(g) and TaFS(g), one calculates AH"atoms[MoOzF2(g)] = 582 f 15 kcal mol.-' The uncertainty of *15 kcal mol-' is estimated and largely re(9) JANAF Thermochemical Tables, D. R. Stull, Ed., Clearinghouse for Federal Scientific and Technical Information, Springfield, Va.,
Aug 1965, Document No. PB-16&370. (10) E. G. King, W. W. Wellwer, and A. V. Christensen, U. S. Bureau of Mines, Report of Investigations, No. 5664, Mines Bureau, Pittsburgh, Pa., 1960. (11) D. R. Stull and G. C. Sinke, "Thermodynamic Properties of the Elements," Advances in Chemistry Series, No. 18, American Chemical Society, Washington, D. C., 1956. (12) P. A. Vorzella, A. D. Miller, and M. A. Decrescente, J . Chem. Phys., 41, 589 (1964).
STABILITIES OF TUNGSTEN AND MOLYBDENUM OXYFLUORIDES flects the possible uncertainties in the estimation, of the heat contents for MoOzFz and TaFs. The presence of molybdenum oxyfluoride, Mo02F2(g), was observed also in another experiment by heating the MoOz-MnFt mixture in a molybdenum cell a t around 900”. The observed ions and their respective appearance potentials (A.P.) are listed in Table VI. Only the
3011 100
1
Table VI : Observed Ions in the MoOz-MnFz-Mo System and Their Appearance Potentials (T = 1093’K) Appearance potentials, Ions
Mn + MnF+ MnFs + MoOF + MoOzF + MoOzFz +
0
I I
I
I
2
3
4
NUMBER OF SUBSTITUTED OXYGEN ATOMS
SV
13.0fO0.5,18.5&OO.5 13.5 f0.5 12.5 & 0 . 5 23 f 1 15 & 1 13.0 =!z 0 . 5
1
0
Figure 2. Heats of formation of gaseous Mo, W, and T a oxides, oxyfluorides, and fluorides plotted as a function of the number of substituted oxygen atoms.
TabIe VI1 : Heats of Formation of Gaseous Oxides, Oxyfluorides, and Fluorides of Mo, W, and T a AHt’zss,
two species MnFz+ (AP = 12.5 =k 0.5 eV) and MoOzF2f (A.P. = 13.0 d: 0.5 eV) appeal to come from neutral precursors. A thermochemical measurement leading to an independent value of AHatoms [MoOZFz(g)] was not feasible in this system, however, because of difficulties in characterizing the solid phases involved in the reaction. This experiment is cited here, however, as a further verification of the existence of the molecule MoOzFz and the reproducibility of its appearance potential measured earlier.
Discussion Present results on the dissociation energies of the oxyfluorides of Mo and W along with those for TaOFa from previous studies2 permit, the development of an interesting correlation among the heats of formation of the gaseous oxides, oxyfluorides, and fluorides of Mo, W, and Ta. Table VI1 lists the heats of formation of these gaseous compounds and a graphical representation of these values is shown in Figure 2. The figure is an excellent illustration of the “substitution principle”13 which predicts the existence of oxyhalides with sta-
koa1 mol-1
Compound
-
MOO^
81a -268* -372c 65a -215b -316b -4160 93d -337# -438”
MoOzFz MoF~ WOa W02F2 WOFi WFe 0.5Ta205 TaOFa
-
-
Ta8F6
a G. De Maria, R. P. Burns, J. Drowart, and M . G. Inghram, J . Chem. Phys.., 31, 1373 (1960). * This work. c Reference 3.
Estimated value.
Reference 2.
bilities intermediate between the corresponding binary oxides and halides.
Acknowledgments. This work was supported by the U. S. Atomic Energy Commission and by the Robert A. Welch Foundation. (13) 8. A. Sohukarev, et aE., Russ. J. Inorg. Chem., 4, 13 (1969).
Volume 73, Number 9 September IW3
