NOTES
2577
the calculated values of PT decrease as temperature increases at a greater rate than do the experimental data. I n spite of the collinear collision model and uncertainty in the expression for 2, the calculation gives reasonable values at higher temperatures. (The socalled steric factor is not introduced in the calculation.) If we used the LJ(12-6) or Morse potential, the same procedure would give PT values which are too small compared to these values. I n ref 13, we may note that complicated calculations for such potentials result in Pr almost two orders of magnitude too short. At present, a full three-dimensional analysis including the problem of rotational transitions is not possible because we do not know enough of the details of the interaction potential as a function of the various angles and distances involved in such an analysis; see ref 2a for a review of approximate studies. As scattering measurements become available and theories develop, however, 71" will ultimately be able to find complete solutions of such problems.
Mass Spectrometric Determination of the Heat of Atomization of the Molecule SiCN
by D. W. Muenow and J. L. Margrave Department of Chemistry, Rice University, Houston, Texas 77001 (Received March 6 , 197'0)
A gaseous molecule containing one atom each of silicon, carbon, and nitrogen has been mass spectrometrically observed and its heat of formation and atomization energy obtained. A molecule containing an additional oxygen atom, OSiCN, has been identified as well. The species, SiCN or SiNC, is isoelectronic with the gaseous molecules CZN and SizN reported previously. lv2 From optical spectra both the CCN and CNC isomers of CZN are known. Spectra attributable to the free radicals NCN, PCN, and AsCN have also been ob~ e r v e d ; the ~ structure AI-CN is suggested. A previous mass spectrometric study4 revealed the gaseous species NaCN, and more recently,6 similar molecules with stoichiometries AlCN, OAlCN, BCN, and OBCN. Measurements were performed with a Bendix Model 14-2068 time-of-flight mass spectrometer equipped with a high-temperature Knudsen-effusion cella6 Two reaction equilibria were studied SiC(hex)
+
+ l/zNz(g) = SiCN(g)
+
(1)
Si(g) C(graph) '/&z(g) = SiCN(g) (11) The previously unobserved ions SiCN+ and OSiCN+ were identified by their isotopic intensity distribution and shutterability. Their appearance potentials, 8.7 f 0.5 and 7.4 f 0.5 eV, respectively, suggest that both
ions are formed by direct ionization of the corresponding neutral molecules, SiCN and OSiCN, and not by fragmentation. A tantalum Knudsen cell was used for both systems. For equilibrium I several single hexagonal crystals of S i c were mixed with small chips of previously outgassed, high-purity boron nitride. The mixture was heated by radiation and electron bombardment of the cell, and the resultant molecular beam examined with 15-V ionizing electrons. Cell temperatures were measured with a Pt-Pt 10%-Rh thermocouple peened into the base of the cell. In order to obtain a third-law as well as a second-law heat for reaction I, an instrumentsensitivity constant was determined by the silver calibration t e ~ h n i q u e . ~Before temperature-dependent ion-intensity data could be collected, however, extensive outgassing of the BN chips was required to remove small amounts of boron oxide found to be present. This is necessary since the most abundant isotopic species of the BzOz+ ion has the same m/e value as S E N + . When an oxygen-rich sample was purposely examined, the ion OSiCN+ was observed as well, and was differentiated from the parent Bp03+ ion by its isotopic-intensity distribution. For the oxygen-free sample the only parent ions observed in the temperature range studied (1656-1752°K) are Nz+and SiCN+. Comparison of measured appearance potentials for the other observed ions, Si+ and Sic+, with published values8 indicate these to be ion fragments, rather than parent ions produced from the ionization of gaseous decomposition products of Sic. Loading the cell with small chips of spectroscopic grade graphite and silicon nitride permitted reaction I1 to be studied. Mass peaks for the ions Si+, N2+, SizN+, SiN+, SiO+, Sic+, SiCN+, and very small intensities for Siz+ and Si3+ were observed. Comparing measured appearance potentials with literature v a l ~ e indicates s ~ ~ ~ ~ Si+, ~ N2+,SiO+, SizN+ ions to be parents. The low value obtained for SiCN+ agrees with that measured in equilibrium I and suggests a parent ion as well. The few per cent oxygen contained within the chips of silicon nitride are undoubtedly responsible for the small amount of SiO(g). The pres-
(1) (a) A.J. Merer and D. N. Travis, Can. J . Phys., 43, 1795 (1965); (b) ibid., 44,353 (1966). (2) K. F. Zmbov and J. L. Margrave, J . Amer. Chem. Soc., 89,2492 (1967). (3) (a) N. Basco and K. K. Yee, Chem. Commun., 3, 150 (1968); (b) ibid., 3, 152 (1968); (c) ibid., 3, 153 (1968). (4) R. F. Porter, J . Chem. Phys., 35,318 (1961). (5) (a) K.A. Gingerich, Naturwiss., 54, 646 (1967); (b) J . Amer. Chem. Soc., 91,4302(1969);(0) Chem. Commun., 13,764(1969). (6) A. Kant, J . Chem. Phys., 41, 1872 (1964). (7) J. L. Mftrgrave, Ed., "The Characterization of High Temperature Vapors, John Wiley and Sons, 1967,pp 222-227. (8) J. Drowart, G. DeMaria, and M. G. Inghram, J . Chem. Phys., 29, 1015 (1958). (9) D. L.Hildenbrand and E. Murad, ibid., 51, 807 (1969). The Journal of Physical Chemistry, Vol. 74, N o . l d , 1970
COMMUNICATIONS TO THE EDITOR
2578 ence of oxygen in the system also suggests small quantities of the species OSiCN, but since the m/e values for the more abundant Si&+-ion overlap, this is not certain. (This uncertainty also explains the absence of measurements for the obvious equilibrium, SizN(g) 2C(graph) '/2Nz(g) = 2SiCN(g)). The ion intensities corresponding to Si+, Nz+, and SiCN+ were measured as a function of temperature and were used to calculate the equilibrium constants for reactions I and 11. From JANAF'O free energy functions for Si(g) and Nz(g),estimated values for SiCN(g) (71 cal deg-l mol-' at 1700"K, ref 298") and the equilibrium constants one derives the corresponding heats of reaction by the third-law method. These results are given in Table I. From a second-law treatment for
+
+
Table I : Equilibrium Data for SiCN (g) Formation
-
-A[(CTo H o dI/T,
Reaction
+
SiC(hex) 1/2Nz(g) = SiCN(g)
Temp,
-log
OK
KP
1656 1680 1701 1721 1752
7.4 7.1 7.0 6.5 6.6
cal deg-1 mol-1
(32) (32) (32) (32) (32)
AH"aB8, koa1 mol-1
109 108 109 106 109
Av 108
1780 1792 1803 1820 1853
1.2 1.0 0.9 0.9 0.8
(5.5) (5.5) (5.5)
-20 -18
-17
(5.5)
-18 -17
(5.5)
AV -18
reaction I osnealso obtains the value AH'298 3 kcal mol-,'.
=
113
Using the average of the second- and third-law heats for reaction I, AH'ZSS= 110.5 f 4 ltcal mol-' and the heat of formation of hexagonal silicon carbide, = - 18 f 4 kcal one calculates AHr' [SiCN(g)] = 92.0 f 7 kcal mol-'. Similar treatment for reaction I1 and the heat of sublimation of silicon, AH~'298 = 108.4 f 3 kcal gives aHr0[SiCN(g)] = 90.3 f 4 kcal mol-l, in good agreement with that obtained using the data for reaction I. Values for the heat of atomization of SiCN(g), as calculated from the heats of equilibria I and 11, the dissociation energy of Na(g), DO' = 226 f 2 kcal mol-',l0 the heat of sublimation of carbon, AHSo2g8= 170.9 f 0.5 kcal and the heat for the reaction, Si(g) C(graph) = SiC(hex), AH'298 = -125 f 3 kcal mol-'," are AHatoms[SiCN(g)] = 296 f 8 and 300 f 4 kcal mol-', respectively. Since fewer auxiliary thermodynamic data are required for computation using reaction 11, the latter value with smaller error limits is favored, and one finally chooses for the heat of atomization, the average [SiCN(g)] = 298 f 6 kcal weighted-value, AHostoms mol-'. This value is comparable to atomization energies given for similar molecules BC2 (294 f 6),la Sic2 (303 f 6),13RCN (301 f 5),6c and AlCN (297 A 5 ) kcal mol-l.sb Infrared matrix isolation studies are being conducted to obtain structural information on the species SiCN as well as AlCN and BCN.
+
Acknowledgment. This work was supported by the United States Atomic Energy Commission. (10) "JANAF Thermochemical Tables," Dow Chemical Co., Midland, Mich., 1967. (11) J. Drowart and G. DeMaria, Silicon Carbide; High Temp. Semicond. Proc. Conf., 16 (1960). (12) H. L. Shick, "Thermodynamics of Certain Refractory Compounds," Vol. 1, Academic Press, Inc , New York, N. Y . ,1966, p 158. (13) G. Verhaegen, F. E. Stafford, and J. Drowart, J . Chem. Phys., 40, 1622 (1964).
C O M M U N I C A T I O N S TO T H E E D I T O R
Ion Exchange between Solids Sir: Ion exchange normally occurs through the medium of aliquid phase. The exchanging ions may be dissolved in water, various nonaqueous solvents, mixtures of solvents, or they may be present in fused salts. It has now been found that certain types of ion exchangers, The Journal of Physical Chemistry, Val. 74, N o . 1% 1970
such as the various zirconium phosphates, have the ability to exchange ions directly with solids or gases. The exchanger, in the hydrogen form, is heated with an anhydrous metal salt which on exchange forms a volatile acid. The exchange reaction proceeds by continuous removal of the volatile acid. The details are given in the text which follows.
