J. Phys. Chem. 1981, 85,971-976
971
Thermodynamic Stabilities of Gaseous Carbides of Iridium and Platinum Satish K. Gupta, Bianca M. Nappi, and Karl A. Glngerlch" Department of Chemlstty, Texas A &
M University, College Station, Texas 77843 (Received: September 4, 1980)
The Knudsen-effusion technique in conjuction with mass spectrometry was employed to study the new gaseous molecules IrC3,IrC4,PtCI, PtC4,and PtC5, and the previously known molecules IrC, IrC2,PtC, and PtC2. The species were observed in the vapors above the Ti-Ir-C, and Ir-C, and Pt-Y-C systems at high temperatures. The third-law analysis was employed to determine the enthalpies of the M(g) + nC(graphite) = MC,(g) type reactions from the measured equilibrium partial pressures of the various metal-containing species. The reaction enthalpies, when combined with the thermodynamic data taken from the literature, yielded the following atomization energies, AHa:,, (kJ mol-', or kcal mol-'): IrCz(g),1144 f 29, 273.4 f 6.9; IrC3(g),1769 f 29,422.8 f 7.0; IrC4(g),2393 f 45, 571.9 f 10.6; PtC2(g),1146 f 28, 273.9 f 6.7; PtC3(g),1822 f 29,435.5 f 6.9; PtC4(g), 2477 f 43,592.0 f 10.3; and PtC,(g), 3176 f 43,759.1 f 10.4. In addition, the dissociation energy of the previously known molecule IrC(g) was redetermined as D O 0 = 627 f 5 kJ mol-' or 149.9 f 1.2 kcal mol-'.
Introduction Iridium and platinum are metals of considerable technological importance. The knowledge of the thermodynamic stabilities and bonding properties of gaseous carbides of these metals would be of relevance to such applications as catalysis, high-temperature corrosion, and refractory materials. As part of our ongoing program concerned with the investigation of the molecular carbide species evaporating from metal-carbon systems, we have studied the gas phase above the titanium-iridium-carbon, iridium-carbon, and platinum-yttrium-carbon systems by Kudsen-effusion mass spectrometry. We report here the identification and the atomization energies of several polyatomic carbide species of iridium and platinum observed in the vapors above these systems a t high temperatures. Nearly all of the transition metals are known to form stable gaseous carbides which exist in the vapors above metal-carbon systems at high temperatures.'*2 The technique of Knudsen effusion combined with mass spectrometry has been almost exclusively the experimental method which has been utilized to identify these carbide molecules and to determine their atomization energies. The predominant species found in the equilibrium vapors above the early transition metal-carbon systems are the dicarbides, MC2, and the tetracarbides, MC4. Recent studies have revealed the existence of even higher gaseous carbide molecules, such as MC5 and MC6 in ~ e r i u myt,~ t r i ~ mand , ~ thorium and uranium5 systems. In general, gaseous carbide species of the type MC,, where n = 1-6, can be expected to exist for the early transition metals. On the other hand, only the gaseous monocarbides6-s and dicarbides8-10are known for the platinum metals, with the former being the predominant species in the equilibrium (1) K. A. Gingerich, Curr. Top. Mater. Sci., 6, 345 (1980).
(2) G. DeMaria and G. Balducci, Int. Reu. Sci.: Pys. Chem., Ser. One, 10, 209 (1972). (3) K. A. Gingerich, D. L. Cocke, and J. E. Kingcade, Inorg. Chim. Acta, 17, L1 (1976). (4) K. A. Gingerich and R. Haque, J. Chem. Soc., Faraday Trans. 2, 76, 101 (1980). (5) S . K. Gupta and K. A. Gingerich, J. Chem. Phys., 71,3072 (1979); ibid., 72, 2795 (1980). ( 6 ) A. Vander Auwera-Mahieu and J. Drowart, Chem. Phys. Lett., 1, 311 (1967). (7) N. S. McIntyre, A. Vander Auwera-Mahieu, and J. Drowart, Trans. Faraday Soc., 6 4 , 3006 (1968). (8) K. A. Gingerich and D. L. Cocke, Inorg. Chim.Acta, 28, L171 (1978). (9) D. L. Cocke and K. A. Gingerich, J . Chem. Phys., 57, 3654 (1972). (10) K. A. Gingerich, J. Chem. SOC.,Chem. Commun., 199 (1974).
vapor. The occurrence of high polyatomic gaseous carbides of the early transition metals can be attributed to the fact that the M-C, ( n > 1)bond energies for these metals are quite high5 whereby thermodynamically stable structures of the type Cz-M-C, C2-M-C2, C2-M-C3, Cz-M-C4, etc., and M-C, are possible. In contrast, in the case of the platinum metals, the M-C2 bond is considerably weaker than the M-C bond and than the M-C, ( n > 2 ) bonds in the early transition-metal carbides. Also, the electronic configurations of the platinum metal atoms are not favorable to forming bonds of reasonable strength with more than one atom. Hence, if they at all occur, the higher platinum metal carbides, MC, (n > 2), would be expected to be found in rather small concentrations in the vapors in equilibrium with the metal-carbon systems. It is not surprising, therefore, that the polyatomic carbides of the platinum metals containing more than two carbon atoms have never been observed previously. Establishing the existence of these higher gaseous carbides of the platinum metals would lend credence to the viability of the M-C, linear chain structures for the polyatomic carbides of transition metals.
Experimental Section Three experiments were run. The tri- and tetracarbides of iridium were first observed during an investigation of the titanium-iridium-carbon system whose primary objective was to study the formation of gaseous titaniumiridium intermetallic species. However, because of the overlap of IrC4+ion mass with that of the TiIr+ ion in the mass spectrum, a definite identification or measurements could not be accomplished for iridium tetracarbide. Therefore, a second run was carried out with an iridiumcarbon system. The carbides of platinum were studied in a third run in connection with the investigation of the molecules Ptz and PtY l1 employing a platinum-yttriumcarbon system. A 90' sector, 12-in. radius magnetic focusing type mass spectrometer (Nuclide, 12-90-HT) equipped with a Knudsen cell assembly was employed in these investigations. The Knudsen-effusion cells were heated by a spiral resistance heater made from 0.125 -in. diameter rod of thoriated tungsten.lZ The cells were constructed from solid tantalum rod and fitted with graphite inner liners. Both the cell and the liner were provided with concentric (11) S. K. Gupta, B. M. Nappi, and K. A. Gingerich, Inorg. Chem., in press. (12) K. A. Gingerich, J. Chem. Phys., 49, 14 (1968).
0022-3654/81/2085-0971$01.25/00 1981 American Chemical Society
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knife-edge orifices in the lids a t the top. The orifice diameter of the Knudsen cell employed for the iridiumcarbon system run was 0.020 in., and for the other two runs it was 0.040 in. The cell temperature was measured by sighting a Leeds and Northrup optical pyrometer a t a blackbody hole provided at the bottom of the tantalum cells. The pyrometer had been calibrated in situ a t the melting point of an NBS mp reference gold sample. In a previous experiment the temperature measured at the bottom of the cell had been compared that measured by sighting the pyrometer directly at the orifice (top of the cell), and no measurable temperature gradiant could be detected under the conditions of the experiment. The Knudsen cell charge consisted of 300 mg of a titanium-iridium alloy (5 at. % iridium) for the first run, iridium (100 mg, powder) plus graphite (excess, powder) for the second run, and platinum (200 mg, wire) plus yttrium (10 mg, chips) plus graphite (excess, powder) for the third run. Small amounts of pure silver (run 1) or gold (runs 2 and 3) were also included with the cell charges for instrument sensitivity determination purposes. The ion source was operated at an electron beam energy of 22 eV, and the emission current was regulated at 0.3 mA, which corresponded to a trap current of -50 FA. The ions were accelerated through a potential of 4.5 kV and were detected with a 20-stage electron multiplier whose entrance shield was maintained a t ca. -2-kV potential. Before reaching the electron multiplier, the ions passed through a 50% transmission grid collector so that the multiplier current gain could be determined easily when desired. The ionic species were identified by their m / e ratio and isotopic abundance. Wherever possible, ionization efficiency curves for the ions were recorded and ion appearance potentials determined by the linear extrapolation method using AP(Au+) = 9.22 eV13 as the reference point for the electron energy scale. The absolute sensitivities of the instrument in each run, defined by the pressure calibration constant ki = Pili+T (atm A-l K-I), weie determined14 from the sets of Au+/ Au2+or Ag+/Ag2+ion currents measured in each case and the thermodynamic data for the equilibrium reactions AuJg) = 2Au(g)15and Ag2(g) = 2Ag(g).16 The pressure calibration constants (in atm A-l K-l thus calculated are k, = 1.02, kAu= 4.64, and kAu = 4.12, respectively, for runs 1-3.
Results and Thermochemical Evaluations The measured currents (multiplier anode currents) of the iridium- and platinum-containing species studied in the three experiments can be found listed in Tables 111-V as a function of temperature. The ion currents correspond to the Ig3Ir isotope of iridium and the lg5Ptisotope of platinum. In the iridium systems, the new species observed were IrC3+ and IrC4+, in addition to the previously known species IrC+ and IrC2+. The higher carbides, IrC5+,IrC6+, etc., could not be observed, since, by the time the Knudsen-cell temperature in the iridium-carbon run was high enough where these molecules would be expected to occur in detectable concentrations, the activity of carbon had dropped. The nonunit activity of carbon in the condensed system was ascertained from the decrease in the C3+ion (13) C. E. Moore, Natl. Bur. Stand. (U.S.),Circ. No. 467, 186 (1958). (14) R. T. Grimley in “Characterization of High Temperature Vapors”, J. L. Margrave, Ed., Wiley, New York, 1967, pp 195-243. (15) J. Kordis, K. A. Gingerich, and R. J. Seyse, J. Chern. Phys., 61, 5114 (1974). (16) K. P. Huber and G. Herzberg, “Constants of Diatomic Molecules”, Van Nostrand, New York, 1979.
current relative to that of C1+ or Czt and the overall drop in the ion currents of the carbon species. In the Ti-Ir-C system experiment, the identification of I C 4 + was not conclusive because of t h overlap of its mass with that of the TiIr+ ion which was positively identified and measured by its isotopic distribution. Thus the IrC4+ion currents listed in Table I11 are presumed to contain major contributions from TiIr+ ion. The new platinum carbide species observed in run 3 were PtC,+, PtC4+,and PtC,+. The ion currents of all of the carbide species, except IrC+ and PtC+, were rather small. Therefore, ionization efficiency curves could not be obtained for IrC3+,IrC4+,PtC4+, and PtC,+. The ionization efficiency curves of IrC+, IrC2+, PtC+, PtC2+, and PtC3+ did not show any evidence of fragmentation losses and yielded an appearance potential of -9.5 f 1.0 eV in each case. This indicated that these ionic species originated from the corresponding neutral molecules. The same was assumed to be true for the rest of the species by analogy. Any possible fragmentation of ean MC, species according to the reactions MC, MC,-l+ + C + 2e- would not be expected to amount to more than a few percent of the parent species, and the resulting errors in the MC,+ and MC,-l+ intensities would be well within the error limits of the measurement of MC,+ ( n 1 2 ) ion current. The ion currents were converted to the partial pressures of the neutral species through the relation Pi = kili+T.The pressure calibration constant for each species was calculated relative to that of gold or silver determined earlier, lz,, through the expression
+
ki = k,
+
acycEi ~
UiYiEcni where u is the maximum ionization cross section, y is the electron multiplier gain, E is the correction factor for converting the ion current measured a t 22-eV electron beam energy to that corresponding to the maximum in the ionization efficiency curve, n is the fractional isotopic abundance of the ion measured, and the subscript c refers to the respective calibrant used, gold or silver. The maximum ionization cross sections for the atomic species were taken from the literature,17 and those for the molecular species were taken to be 0.75 times the sum of the respective atomic cross sections.18 The only species for which the multiplier gain could be experimentally measured were Ir+, IrC+, and Pt+. For the other species, the approximations that y(IrC,+) = y(IrC+) and y(PtC,+)/y(Pt’) = y(IrC+)/y(Ir+)were made by assuming cancellation of the mass and molecular effects. Similarly, it was assumed that E(IrC,+) = E(IrC+) and E(PtC,+)/E(Pt+) = E(IrC+)/E(Ir+). The various experimental and estimated quantities and the resulting pressure constants are listed in Table I. The equilibrium partial-pressure data were treated by the third-law and, where appropriate, second-law methods to calculate the enthalpies of the following reactions M(g) + nC(graphite) = MC,(g) which are independent of the absolute pressure calibration of the instrument. The third-law reaction enthalpies are given by the expression AHoo = -RT In K,(T) - TA[(GoT - H o ~ ) / T l where K,(T) is the equilibrium constant, A [ ( G o ~- Hoo)/Tl (17) J. B. Mann, Recent Deu. Mass Spectrosc., Proc. Int. Conf.Mass Spectrosc., 1969, 814-9 (1970). (18) J. Drowart and P. Goldfinger, Angew. Chern., 79, 589 (1967); Angew. Chern., Int. Ed. Engl., 6, 581 (1967).
Stabilities of Gaseous Carbides of Ir and
The Journal of Physical Chemistry, Vol. 85, No. 8, 198 1 973
F’t
TABLE I: Experimental or Estimated Parameters and the Calculated Pressure Constants for the Metabcontaining Species over the Ti-Ir-C, Ir-C, and Pt-Y-C Systems intenionizaity pressure tion multi- correc- concross plier tion stant, section, gain, factor, h i , atm cri, A * 10Sri Ei A-’ K-’
isotopic abundance, ni
Titanium-Iridium-Carbon System Ap’(107) 5.05 0.972 1.00 1.02 Ir’(i93)’ 7.71 0.945 1.08 1.21 1.11 1.15 1.17 IrC+ 1205) 7.27 IrC2+‘(21?) 8.75 1.11 1.15 0.986 1.11 1.15 0.85 IrC3+(229) 10.24 1.11 1.15 0.75 IrC4+(241) 11.72
0.5187 0.615 0.6082 0.6014 0.5947 0.5881
species
(m/z)
Au’ (197) Ir+ (193) IrC’(205) IrC,+ (217) IrC,+ (229) IrC4’ (241)
Iridium-Carbon System 5.85 0.977 1.06 1.08 1.10 7.71 7.21 1.27 1.17 8.75 1.27 1.17 10.24 1.27 1.17 11.72 1.27 1.17
4.64 5.35 5.21 4.38 3.78 3.34
Platinum-Yttrium-Carbon System Au+ 5.85 0.583 1.18 4.12 Pt’ (195) 6.66 0.639 1.25 10.44 PtCt (207) 6.435 ‘0.751 1.33 9.81 F’tC,+(219) 7.92 0.751 1.33 8.07 PtC,’ (231) 9.405 0.751 1.33 6.86 PtC,+ (243) 10.89 0.751 1.33 6.00 PtC,’ (255) 12.375 0.751 1.33 5.33
1.000 0.615 0.6082 0.6014 0.5947 0.5881 1.000
0.338 0.334 0.330 0.327 0.323 0.320
is the free-energy function change for the reaction, and 0 = 0 ox 298.15 K. The second-law enthalpies are obtained from the slope of log K (7‘)vs. 1/T plot. The thermodynamic functions needed in these evaluations were taken from the compilation by Hultgren et for Ir(g) and Pt(g). The same for the gaseous carbide molecules were calculated by the standard statistical thermodynamic expressions from estimated molecular parameters. For the monocarbides, IrC and PtC, the spectroscopically determined constants listed by Huber and HerzberglGwere utilized. Linear structures M X , were assumed for all of the polyatomic carbide molecules for the reasons discussed earlier. The bond distance and the force constant for the M-C, bonds were obtained from the corresponding M-C parameters by making adjustments of an elongation of the bond and a decrease in the stretching force constant because of the relatively smaller M-C, bond energies antici ated. The bond-distance (8) and force-constant (mdyn/ ) values used, respectively, are as follows: Ir-C, 1.683 and 7.35; Ir-Cn, 1.75 and 5.88; Pt-C, 1.6767 and 6.39; and Pt-C,, 1.84 and 6.27. All C-C bond lengths and force constants were taken to be the same as those for the C2 molecule, 1.31 8 and 9.25 mdyn/A, respectively.20 The bending force constants, k6 (mdyn A/ rad) were approximated as numerically equal to one-tenth of the average of the two adjacent stretching force constants. The vibrational frequencies were calculated by the F-G matrix method of Wilson.21 The electronic contributions to the thermodynamic functions of the polyatomic carbides were computed from the spectroscopic levels16for
p:
(19) R. Hultgren, P. D. Desai, D. T. Hawkins, M. Gleiser, K. K. Kelley, and D. D. Wagman, “Selected Values of the Thermodynamic Properties of the Elements”, American Society for Metals, Metals Park, OH, 1973. (20) G. Herzberg, “Molecular Spectra and Molecular Structure”, Vol. 1, 2nd ed., Van Nostrand, New York, 1950, Appendix. (21) J. H. Schactschneider and R. F. Snyder, Spectrochim. Acta, 19, 117 (1963); E. B. Wilson, Jr., J. C. Decius, and P. C. Cross, “Molecular Vibrations”, McGraw-Hill, New York, 1955.
the corresponding monocarbide molecules. The calculated free-energy functions, (GoT- Hoo)/T,and the heatrcontent functions, HOT- Hootfor the gaseous iridium and platinum carbides in the 2200-3200 K temperature range are listed in Table 11. The estimated uncertainty in the free-energy functions for the MC2 and MC3 molecules is 1 1 0 J mol-’ K-l, and that for the MC4 and MCSmolecules f15 J mol-’ K-1. Details of the third-law evaluations of the reaction enthalpies, AHo,,, for the thermochemical equilibria studies in the three experiments are presented in Tables 111-V. The uncertainties quoted with the average moo values are the standard deviations from the mean. The reaction enthalpies were combined with the l i t e r a t ~ r e values ’~ for the heats of vaporization of iridium (AHv0298 = 69.4 f 6.3 kJ mol-l), platinum (AHv0298= 564.8 f 1.3 kJ mol-’), and carbon (AHvoo= 711.2 f 2.1 k J mol-l) to derive the standard enthalpies of formation and atomization energies of the iridium and platinum carbides which have been summarized in Table VI. Here, the error limits given correspond to the overall uncertainties in the reaction enthalpies which are also shown in this table. The overall uncertainties in the third-law reaction enthalpies have been calculated from the uncertainties of f20 K in temperature, the error quoted above in the free-energy functions of the carbides, and the uncertainties in the K (T) values of f20% for the MC species, 30% for the &IC2 and MC3 species, and f40% for the MC4 and MC5 species. The free-energy functions calculated for IrC and PtC were assumed to be exact since they were derived from experimental spectroscopic constants. IrC(g) was the only species which could be measured over a wide enough temperature range (2465-2935 K) to allow a second-law treatment of the equilibrium data. The second-law enthalpy of the reaction Ir(g) + C(graphite) = IrC(g) was determined to be AHoo = 79.5 f 2.9 k J mol-l, in good agreement with the third-law value of 86.6 f 5 k J mol-’. The Moo and the other quantities for IrC given in Table VI correspond to the meean of the second- and third-law values of run 2 and the third-law value of run 1. For IrC2 and IrC3 the Moo values and other quantities were based on the average of the third-law value. No reliable data could be measured for IiC4+in run 1because of the overlap with IrTi’, which was more abundant.
Discussion The dissociation energies of IrC(g) and PtC(g) have been reported previously by Drowart et al.6p7and by Gingerich.22 The present value for the dissociation energy of IrC(g), Doo = 627 f 5 kJ mol-l, is in very good agreement with their values of 621 f 137and 624 f 10 kJ mol-’,22respectively. We take ours as the selected value for the dissociation energy of IrC(g) by virtue of the large temperature range (2465-2935 K) of measurements and the relatively smaller second- and third-law standard deviations f2.9 and f0.6 kJ mol-l, respectively. The present value for the dissociation energy of PtC(g), 593 f 6 k J mol-’, is somewhat lower than either of the two literature values, 608 f 6 kJ mol-l by Drowart et a1.6 and 605 f 10 k J mol-’ by Gingerich.22 The discrepancy cannot be traced to any systematic error in the experimental measurements. However, there are reasons to suspect that the activity of carbon was nonunity when PtC,(g) species were measured in the present investigation of the Pt-Y-C system. A comparison of the equilibrium data measured in this in(22) K. A. Gingerich, Chem. Phys. Lett., 23, 270 (1973). (23) H. R. Leider, 0. H. Krikorian, and D. A. Young, Carbon, 11,555 (1973).
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TABLE 11: Estimated Free-Energy Functions, (GOT - H",)/T, and the Heat-Content Functions, Carbides of Iridium and Platinum
HOT
- H",,for Gaseous
- H",)/T, J mol-' K-'
- ( G O T
(H"T- H",),kJ mol-' molecule Ir C
temp, K
IrC, IrC, IrC, PtC PtC, PtC, PtC, PtC,
298.15 204.03 8.755 218.52 10.220 232.34 12.617 246.34 15.002 204.14 8.756 219.18 10.164 231.34 12.014 243.20 13.929 254.96 15.877
2200 268.92 80.984 309.01 123.67 351.03 167.07 393.11 210.47 268.06 76.804 308.59 119.60 345.93 161.47 383.21 203.45 420.49 245.46
2400
2600
2800
3000
3200
272.13 89.156 313.94 136.68 357.68 184.91 401.50 233.14 271.11 84.294 313.34 131.88 352.36 178.58 391.32 225.39 430.28 272.22
275.12 97.341 318.52 149.73 363.89 202.81 409.33 255.90 273.93 91.834 317.76 144.20 358.35 195.74 398.89 247.40 439.42 299.09
277.90 105.55 322.81 162.81 369.70 220.77 416.66 278.73 276.55 99.432 321.89 156.54 363.96 212.96 405.98 269.48 448.00 326.04
280.51 113.79 326.84 175.94 375.17 238.79 423.57 301.63 279.01 107.10 325.76 168.92 369.23 230.21 412.65 291.62 456.07 353.06
282.97 122.07 330.64 189.19 380.32 256.88 430.09 324.62 281.32 114.84 329.41 181.32 374.20 247.51 418.96 313.82 463.71 380.15
TABLE 111: Thermochemical Equilibria between Graphite and the Iridium-Containing Species above the Titanium-Iridium-Carbon System (Run 1 ) and Their Evaluation by the Third-Law Method
TABLE IV: Thermochemical Equilibria between Graphite and the Iridium-Containing Species above the Iridium-Carbon System (Run 2 ) and Their Evaluation by the Third-Law Method
temp,
K
2503 2551 2647 2674 2698 2732 2753 2796
2647 2674 2698 2732 2753 2796
IIr+ra
A
IIr~nA + , a A H " , , b kJ mol-'
Ir(g) t C(graphite) = IrC(g) 3.17 X lo-'' 1.62 X lo-'' 85.5 6.70 X lo-'' 3.30 X lo-'' 86.6 1.27 X 10"O 7.35 X lo-'' 85.6 1.86 X lo-'' 1.06 X lo-'' 86.5 2.44 x lo-'' 1.39 x lo-', 87.2 3.00 X 1.81 X lo-'' 86.6 3.70 x lo-'' 2.31 x 86.4 4.05 X lo-'' 2.78 X lo-'' 85.2 av 86.1 t 0.8 Ir(g) + 2C(graphite) = IrC,(g) 1.27 X lo-'' 1.05 X 275.8 1.86 X lo-'' 1.80 X lo-', 274.8 2.44 X lo-'' 2.25 X 278.2 3.00 X lo-'' 2.91 X lo-', 280.3 3.70 X lo-'' 3.15 X 285.3 5.70 X 277.7 4.05 X 10"O av 278.7 * 3.5
Ir(g) t 3C(graphite) = IrC3(g) 2.44 X lo-'' 4.56 X 361.3 1.05 X 363.1 3.70 X lo-'' av 362.2 a The ion currents listed correspond to the lg31r isotope of iridium. The error limits quoted with the average AH", values are the standard deviations from the mean. For overall uncertainties see Table VI and the text. 2698 2753
vestigation with those of the two previous studies reveals measured in the present that, while the ratios IptC+/IPt+ work are similar to that reported by Gingerich,22they are smaller than those measured by Drowart et al. by a factor of -2. This suggests that the activities of carbon in the condensed systems in the earlier work by Gingerich and in the present work were similar and less than unity. However, Gingerich derived his value for DDo(PtC)from the gaseous equilibrium reaction RhC + P t = PtC + Rh, which is independent of the activity of carbon in the condensed phase. Drowart et al., on the other hand, determined the dissociation energy of PtC(g) from the enthalpy of the reaction Pt(g) + C(graphite) = PtC(g), as in the present investigation; but since they employed a pure graphite Knudsen cell, a unit activity of carbon was assured in their experiments. Also, a third-law evaluation
Ir(g) + C(graphite)= IrC(g) 2.85 X lo-'' 1.30 X lo-'' 85.7 1.16 X lo-'' 87.1 2.68 X lo-'' 0''' 3.90 x 86.6 7.89 x 1 1.36 X lo-'' 6.78 X lo-'' 86.5 5.35 x 86.4 1.05 x lo-'' 1.80 x lo-" 1.03 x lo-'' 85.4 1.20 x lo-" 86.8 2.20 x lo-" 87.0 2.35 x 10-l' 1.28 x lo-'' 1.65 X lo-'' 86.2 2.85 X lo-'' 1.29 X lo-" 86.3 2.23 X lo-'' 2.50 X lo-'' 86.0 4.12 X lo-'' 3.90 X lo-'' 86.2 6.45 X lo-'' 3.70 x lo-'' 2.20 x lo-'' 87.4 5.33 x lo-'' 87.2 8.58 x lo-" 86.8 1.21 x 10-1° 8.10 x lo-'' 1.68 x lo-'' 1.14 x lO-'O 87.8 2.25 X lo-'' 87.6 3.20 X lo-'' av 86.6 * 0.6 Ir(g) t 2C(graphite) = IrC,(g) 2719 6.45 X lo-'' 6.00 X 279.9 2790 8.58 x 1 0 - l ~ 1.20 x 10-13 277.2 2849 1.21 x io-io 1.95 x 10-13 279.2 2906 1.68 x 10-10 3.54 x 10-13 277.9 2935 3.20 X lo-'' 7.80 X lo-'' 276.9 av 278.2* 1.2 Ir(g) t 3C(graphite) = IrC,(g) 2906 1.68 x 10-10 9.00 x 1 0 4 4 366.8 2935 3.20 x io-1o 1.89 x 10-13 367.9 av 367.4 Ir(g) + 4C(graphite) = IrC4(g) 2935 3,20 x lo-'' 6.00 x 451.7 a The ion currents listed correspond to the Ig3Irisotope of iridium. The error limits quoted with the average AH", values are the standard deviations from the mean. For overall uncertainties see Table VI and the text. 2465 2476 2559 2563 2577 2636 2650 2657 2679 2684 2714 2719 27 54 2790 2849 2906 2935
of the heat of vaporization of graphite, C(graphite) = C(g), from the C1+ ion currents measured in the present work showed that carbon had been at a reduced activity when platinum carbide species data were obtained. When the present value of 593 kJ mol-l for the dissociation energy of PtC(g) is compared with the mean of the literature
The Journal of Physical Chemistry, Vol. 85, No. 8, 1981 975
Stabilities of Gaseous Carbides of Ir and Pt
TABLE V: Thermochemical Equilibria between Graphite and the Platinum-Containing Species above the Platinum-Yttrium-Carbon System (Run 3 ) and Their Evaluation by the Third-Law Method temp,
K 2649 2684 2704 2707 2736
2649 2684 2704 2707 2736
2684 2704 2707 2736
Ipt+,aA
IPtCn+ra A
kJ mol-'
Pt(g) + C(traphite) = PtC(g) 1.52 X lo-' 1.57 X lo-'' 116.1 118.6 1.35 X lo-' 1.32 X lo-'' 8.55 X lo-'' 8.25 X lo-'' 119.6 8.76 X lo-'' 8.85 X lo-'' 118.7 6.90 X lo-'' 6.90 X lo-'' 120.0 av 118.6 i 1.4 Pt(g) + 2C(graphite) = PtC2(g) 1.52 X 2.25 X 281.0 1.35 X lo-' 2.25 X 281.8 8.55 X 10"' 2.40 X 276.7 8.76 X lo-'' 2.19 X 274.9 6.90 X lo-'' 1.65 X 278.6 av 276.6 c 2.6 Pt(g) + 3C(graphite) = PtC,(g) 1.35 X lo-' 1.86 X lo-" 310.9 1.60 X 310.8 1.06 X lo-' 8.55 X lo-'' 1.05 X 315.9 6.90 X lo-'' 1.35 X 10'13 308.5 av 311.5 i 2.7
2736
Pt(g) + 4C(graphite) = PtC,(g) 6.90 X lo-'' 3.00 X 367.5
2736
Pt(g) + 5C(graphite) = PtC,(g) 6.90 X lo-'' 5.00 X lo-', 380.1
a The ion currents listed correspond to the '"Pt isotope of platinum. The equilibrium constants were based on the estimated activity of carbon a , = 0.54 for all molecules except PtC (see Discussion section), and the error limits quoted with the average AH", values are the standard deviations from the mean. For overall uncertainties see Table VI and the text.
values given above, 606.5 kJ mol-l, the activity of carbon in our Pt-Y-C system is estimated as 0.54 f 0.05 from the third-law analysis of the reaction Pt(g) + C(graphite) = PtC(g). This value of the carbon activity has been incorporated in the equilibrium constants, Kp, for the reactions Pt(g) nC(graphite) = PtC,(g), n = 2-5, listed in Table V. The gaseous dicarbide molecules IrCz and PtC, have been previously observed by Gingerich,'O who obtained the tentative values of 11088 f 42 and 1084 f 38 k J mol-l, respectively, for their atomization energies, AHato0.The present values of AHat',, (IrC2) = 1144 f 29 kJ mol-' and AHatoo(PtC2)= 1146 28 kJ mol-' are consistent, within
+
*
the combined range of experimental error, with the earlier values. However, Gingerich derived his values from the equilibrium reaction MC2(g) + M(g) = 2MC(g), M = Ir or Pt, by the third-law method using the approximation that the free-energy function change for this reaction was the same as that for the corresponding reaction with M = RhSe When the free-energy functions for the respective species calculated in this work are employed, the atomization energies of IrC2(g) and PtC2(g) are recalculated from Gingerich's equilibrium data as 1159 and 1141 k J mol-l, respectively, in much better agreement with the results of the present investigation. The gaseous molecules IC3, IrC4, PtC3, PtC4, and PtC, have not been observed previously. As was indicated in the Introduction, these species occur in very low equilibrium concentrations in the vapor above the respective condensed systems. For both iridium and platinum, the concentrations of the various polyatomic carbide molecules are all within the range of 1 order of magnitude and nearly 3 orders of magnitude lower than the respective monocarbide concentrations. Thus, the gas phase above these metal-carbon systems is composed primarily of the free metal atom and the monocarbide. The concentration of the polyatomic carbide molecules would, of course, be expected to increase relative to that of the monocarbide with increasing temperatures. Therefore, under optimum experimental conditions it should be possible to observe the molecule IrC6(g) and possibly the higher carbides of both iridium and platinum. Some additional thermochemical properties of the gaseous carbide species studied have been listed in Table VI. The bond dissociation energies, Doo(M-C,), given were derived from the experimental atomization energies by subtraction of the atomization energies of the respective C,(g) molecules. For the sake of internal consistency of the tabulated bond energies, the C, atomization energies were taken from a single source (Leider et al.23)notwithstanding several discordant data found in the literature for C2 and higher species. In consideration of the experimental uncertainties associated with the atomization-energy values one may infer that the polyatomic gaseous iridium and platinum carbides have equivalent thermodynamic stabilities. Nevertheless, because of the consistently positive deviations of the PtC,(g) atomization energies from the corresponding IrC,(g) energies, excluding the case of the monocarbides, the platinum carbides are indicated to be somewhat more stable than the iridium carbides. In contrast, platinum monocarbide is definitely less stable than iridium monocarbide. This may be interpreted in terms of an empirical valence bond approach.
TABLE VI: Reaction Enthalpies,u Standard Heats of Formation, Atomization Energies, Bond Dissociation Energies, and Other Thermochemical Data of the Gaseous Carbides of Iridium and Platinumb
reaction Ir(g) + C(graphite) = IrC(g) Ir(g) + 2C(graphite) = IrC,(g) Ir(g) + 3C(graphite) = IrC,(g) Ir(g) t 4C(graphite) = IrC,(g) Pt(g) + C(graphite) = PtC(g) Pt(g) + 2C(graphite) = PtC,(g) Pt(g) + 3C(graphite) = PtC,(g) Pt(g) + 4C(graphite) = PtC,(g) Pt(g) + 5C(graphite) = PtC,(g)
APO,
AH"29S3
A H f " 298 9'
A Hat" o
kJ mol-' 84.1 i: 4.5 278.5 f 28.7 364.8 i 28.4 451.7 f 43.7
k J mol-'
k J mol-
kJ mol'
85.6 e 280.5 c 368.7 f 456.3 c
4.5 28.7 28.4 43.7
753 c 8 950 i. 29 1037 2 29 1123 i: 44
>'
A Ha too (MGl+l), - A Hat' 0 -
IMC,)
D",(M-C.)d
molecule
627i 5 627c 5 627 5 5 IrC 1144 c 29 517 * 29 549 * 30 IrC, 17692 29 6 2 5 c 41 474 IrC, 2393 f 45 624 i 54 565 IrC, 606 * 5 606i 5 606 c 5 PtC 276.6 c 27.9 278.1 f 27.9 843 e 28 1 1 4 6 i 28 540 c 28 551 c 29 PtC, 311.5 i 28.0 1822 c 29 313.8 e 28.0 879 c 28 6 7 6 t 40 527 PtC, 367.6 i 42.1 935 i 42 2477 c 43 370.6 c 42.1 655 e 52 649 PtC, 949 c 42 380.1 e 42.1 384.1 c 42.1 6 9 9 c 61 659 PtC , 3176 c 43 a All values are from the third-law analysis, except that for IrC corresponds to the mean of the second- and third-law values of run 2 and of the third-law value of run 1. The error limits quoted are the overall uncertainties (see text). Data derived from the present w o r k for all molecules except PtC(g) for which the literature value was used (see text, Discussion). No error limits are cited for D",(M-C,), n > 2, values because of the uncertainties in the C, data.
J. Phys. Chem. 1981, 85,976-981
976
While the platinum atom with a valence-state electronic configuration of des1can form only two valence bonds with a C atom, a d8s1valence state for the iridium atom allows it to form more than two valence bonds. However, some of the bond energy gained in IrC must be expended in promoting the ground state of Ir atom, d7s2,to the valence-state configuration. Alternatively, in an MO description there would be one extra electron in the antibonding orbitals of PtC over the total in IrC molecular orbitals. That the polyatomic platinum carbides are more stable
than the iridium carbides is again indicated by the quantities AHatoo(MC,+l)- AHatoo(MC,)given in Table VI. For each metal, the difference, corresponding to the gain in atomization energy per additional carbon atom, becomes roughly constant for species higher than MC2 and is larger for the platinum carbides than for the iridium carbides.
Acknowledgment. We appreciate the financial support given this work by the Robert A. Welch Foundation (Grant No. A-387) and the National Science Foundation (Grants No. CHE-08711 and CHE-8007549).
Inversion Barriers in Methyl-Substituted Amines Robert A. Eades, David A. Well, David A. DIxon,*+ Chemistry Department, University of Minnesota, Minneapolis, Minnesota 55455
and Charles
H. Douglass, Jr.
Chemistry Department, Drake University, Des Moines, Iowa 503 I I (Received: September 25, 1980)
The barriers to inversion for the simple methyl-substituted amines NH,, CH3NH2,(CH3)2NH,and (CH&N have been calculated from molecular orbital theory. Calculations have been done by using the approximate PRDDO method and at the ab initio level with a variety of basis sets. The largest basis set, double { plus polarization (DZP), gives barriers of 4.7,5.1,5.4, and 9.6 kcal/mol for NH3, CH3NH2, (CH,),”, and (CH&N, respectively. These calculated values are in good agreement with experiment except for the barrier for (CH&N, which is more accurate than the experimental estimate. The effects of the various basis sets on the barrier height are discussed. The rotation barrier in CH3NH2has been calculated with the same basis sets. The value with the DZP basis set is 2.46 kcaljmol. Rotational conformers of planar (CH3)2NH2and (CHJ3N were examined at the PRDDO level. The rotational barriers are small and are higher in the former in comparison with the latter compound. These conformational changes in the planar forms are employed in a discussion of the inversion process. Dipole moments and ionization potentials from Koopmans’ theorem are also presented.
Introduction Molecular inversion barriers have long fascinated chemists, and studies of such phenomena are extremely active t0day.l Indeed recent work on barriers and molecular conformations has introduced novel stereochemical concepts.2 The inversion barriers for simple hydrides of the form AH3, AH
