2450 We attempted to study the 170splitting in the esr spectrum of labeled NO$ in the gas phase14 and also in NOz trapped in uv-irradiated Nz04 and Nz04-ice.16 Only in the case of NO2 in Nz04were the lines narrow enough for possible observation of oxygen satellite lines, but these could not be assigned unambiguously. We note, however, that the 1 7 0 isotropic hyperfine interaction for N170160has been measured from the microwave spectrum:’B a0 = -63.76 MHz, i.e., -23 G. The analogy between NO2 and iminoxy radicals has already been pointed out ;17 it is of interest that the 170 splittings are of the same magnitude and suggests that the oxygen coupling constant in the iminoxy radical is also negative.
Acknowledgment. The authors gratefully acknowledge support of this research from the National Science Foundation through Grant GP-7783 and through Grant GP-1687 for partial support for purchase of the esr spectrometer. gen spin density of -50% which, assuming negligible delocalization into the remainder of the radical, leaves ~ 5 0 %on oxygen. The observation of a larger value of a0 in the iminoxy radical than in the nitroxide may be an indication of the importance of spin transmission to oxygen through the c system of the former. (14) T. J. Schaafsma, Chem. Phys. Lett., 1, 16 (1967). (15) T. J. Schaafsma, G. A. v.d. Velde, and J. Kommandeur, Mol. Phys., 14, 501 (1968). (16) P. D. Foster, J. A. Hodgeson, and R. F. Curl, J . Chem. Phys., 45, 3760 (1966). (17) P. W. Atkins and .M, C. R. Symons, “The Structure of In-
organic Radicals,” Elsevier Publishing Co., New York, N. Y., 1967, p 140.
Dissociation Energy of Fez
by Sin-Shong Lin and Arthur Kant A r m y Materials and Mechanics Research Center, Watertown, Massachusetts 08172 (Received November 1.4, 1968)
A combination of Knudsen effusion and mass spectrometer techniques has been used to identify the Fez molecule and establish its dissociation energy. Samples of spectroscopically pure iron powder or chips were vaporized at 1900-2100°K from a tantalum crucible containing either a thoria or zirconia liner, and the eff usates were analyzed mass spectrometrically by means of a 12-in. radius, 90” magnetic sector mass spectrometer using a 20-eV electron beam as the ionizing source. The crucible assembly, electron bombardment furnace, and temperature measuring system were similar to those previously described.l A movable shutter with a l-mm slit placed between the crucible and the mass spectrometer ionizing electron beam ensured that only species having their sources in the crucible contributed to the measured intensities. This was done by taking the difference of intensities between The Journal of Physical Chemistry
NOTES the shutter at the “on” and the “off” positions relative to the molecular beam. The ion intensity of dimer was measured by a 14-stage electron multiplier and that of monomer by a Faraday cup. The gain of the multiplier a t mass 56 (Fe) is found to be 4.0 X lo5. Ions were identified from both their mass positions and isotopic abundances. I n the zirconia liner runs, a correction was made for the ZrO+ at the mass 112 of Fez+. No correction was required in the thoria liner experiments. The dissociation energies as calculated from the observed dimer to monomer ion intensity ratios corrected for isotopic abundances I(Fez+)/I(Fe+), and the temperature, by using the third-law method1g2 are given in Table I for both the thoria and zirconia runs. The atomic parameters required in the third-law computation are taken from Edwards, et aL3 (vapor pressure), and Moore4 (electronic partition function). The molecular constants employed in the calculation are interatomic distance, 2.33 8, and vibrational frequency, 365 cm-l. The latter is 1.25 times the Debye frequency of the metal (O(Fe) = 420)5 and is based on a previous comparison1 of the vibrational properties of elemental crystals and the corresponding diatomic molecule. The probable error resulting from this choice of the interatomic distance and the frequency to the calculated dissociation energy is estimated to be 3 kcal a t 2000°K. The electron partition function of the diatomic molecule, for reasons given previously,’ is taken as the geometric mean of the minimum multiplicity, 1 (‘2 molecular state), and the maximum estimated multiplicity 30 (15hmolecular state). The latter multiplicity can be obtained from a combination of two excited iron atoms in the configurations 3d54s4p2. The activity of iron, as calculated from the extrapolation of I(Fe+) at low temperatures, is found to be approximately unity so that the resulting error from the use of the vapor pressure data3 is about 2 kcal. Based on the rule of addivitity of atomic ionization cross sectionss and an earlier method’ of estimating electron multiplier efficiencies, the ratio of partial pressures of dimer to monomer is taken as 1.7 times the corresponding ion intensity ratio with an estimated uncertainty of a factor of 2 (equivalent to 2.8 kcal a t 2000°K). Thus, from the above-estimated uncertainties and using the geometrical mean of the multiplicities ds,the third-law dissociation energy of Fez is 30 f 5 kcal. (1) A. Kant, J . Chem. Phys., 41, 1872 (1964). (2) J. Drowart and R. E. Honig, J . Phys. Chem., 61, 980 (1957). (3) J. W. Edwards, H. L. Johnston, and W. E. Ditmars, J. Amer. Chem. Soc., 75, 4729 (1951). (4) C. E. Moore, Natl. Bur. Std. (U.9.) Circ., No. 467, 2, 49 (1959). (5) F. Seitz and D. Turnbull, “Solid State Physics,” Vol. 2, Academic Press, Inc., New York, N. Y., 1956, P 233. (6) J. W. Otvos and D. P. Stevenson, J . Amer. Chem. soc., 78, 564 (1956). (7) A. Kant, J . Chem. Phys., 44, 2450 (1966).
NOTES
2451
Table I : Dissociation Energy of Fen
Temp,
Run
Fe-ZrOz-Ta
O K
2073 2041 2004 2062 2049 2070 2106 2158 2176
I(Fez +)/ I(Fe+) x 108
0.925 0.676 0.510 0.800 0.715 0.871 1.17 1.83 2.01
Doo from the third-law method AHTO from baaed on the the aecondmean multiplicity law method
29.3 29.3 29.0 29.3 29.4 30.0 29.9 30.4 30.4
Fe-ThOz-Ta
1963 2050 2108 2188 2112 2085 2102 1988 1931 2028 2096 2168 2098 2045
0.487 0.703 1.87 2.75 2.10 1.12 1.14 0.55 0.386 0.691 1.09 2.77 1.73 0.748
29.8 29.3 31.8 31.4 32.2 30.3 29.9 30.0 30.2 29.7 29.8 32.0 31.7 29.7 __ 3 0 . 6 =!= 1 . 6
+
+
29.7 rt 0.5
the first period and the atomic valence state energies of the corresponding atoms. The appearance potential of Fez+was measured to be 2 f 0.2 eV less that of Fe+ [I.P.(Fe) = 7.90 f 0.01].4 The reaction Fe Fe+ = Fez+as a spurious source of dimer is improbable because the ratio I(Fez)/I(Fe) was found to be independent of repeller voltage, and it requires a three-body collision. The charge-exchange reaction Fe2+ AB = Fe+ A + B (AB is a neutral molecule), as discussed in a previous paper,” which could affect the dimer ion intensity, was eliminated by using ionizing electrons (20 eV) of energy less than the Fez+ appearance potential.
l8
*
+ +
(8) R. Hultgren, R. L. Orr, P. D. Anderson, and K. K. Kelley, “Selected Values of Thermodynamics Properties of Metals and Alloys,” John Wiley & Sons, Inc., New York, N. Y.,1963. (9) The maximum value of the heat function may be obtained by maximizing the quantity (AHT - AHo), = Zg,E,e-fldRT/(l Fgie-EdRT) with respect to E , assuming E = E”,g = Zgi and using g = 180 which is obtained from the sum of states, IKA, 1*A, . , , LA, 1 6 2 , 1 3 2 , , , , 12. (10) A. Kant and B. Straws, J. Chem. Phys., 41, 3806 (1964). (11) A. Kant, S.-S. Lin, and B. Straws, ibid., 49, 1983 (1968).
+
The Molecular Hydrogen Yield in Irradiated 2-Propanol Vapor1 18 f 3
by M. G. Bailey and R. S. Dixon2 Atomic Energy of Canada Limited, The dissociation energy can also be obtained from the Whiteshell Nuclear Research Establishment, plot of log [I(Fez+)/I(Fe+)1 vs. reciprocal temperature, Pinawa, Manitoba, Canada (Received November 16, 1968) known as the second-law method. A least-squares treatment of the data of Table I, using the heat of vaporization of iron from Hultgren, et uZ.,* results in The effect of propene on G(H2)3from irradiated 2AHo2~00= 18 f 3 kcal for the reaction Fez(g) = Fe (g) propanol vapor has indicated the presence of an unfor both the thoria and zirconia liners. Reduction of scavengable or molecdar yield of hydrogen, g(Hz) = this value to absolute zero using the tabulated heat 1.7 =!= 0.2.4 The effect of propene is to capture thermal function of iron and the previously mentioned vibraH atoms 3) tional frequency of Fez gives DOo(Fez) = (13 H C3He --t C3H7 (1) (AHT - AHo),kcal where the last quantity, the molecular electronic heat function, is unknown and must be thus preventing them from forming hydrogen by abapproximated. The minimum and estimated maxistraction mum contributions of the molecular electronic heat H CsH70H --t Hz C3HsOH (2) function are 0 and 12 kcal19 respectively. Using the mean value of 6 f 6 kcal for the heat function, 19 f 7 I n this study, the possible abstraction reaction from kcal is obtained for the second-law value of Doo(Fez). propene It should be noted that the third-law computations, H C3Ha -+ Hz C3H5 (3) based on 16A and $A molecular states, give values of Doo(Fez) of 23 f 5 and 25 f 5 kcal which are not in was considered to be negligible. Clearly this molecular much disagreement with the second-law result. Apparhydrogen yield which persists at very high propene ently both a high degeneracy and large number of lowlying energy levels are required to bring the second- and (1) Issued as AECL No. 3277. (2) T o whom correspondence should be addressed. third-law values into agreement. (3) Primary yields are written as g(X) and experimentally measured The relative low dissociation energy of Fez is consisyields as G(X). tent with a previous correlationlo between the dissocia(4) R. S. Dixon and M. G. Bailey, Advances in Chemistry Series, tion energies of transition element diatomic molecules of No. 82, American Chemical Society, Washington, D. C., 1968, p 247.
* +
+
+
+
+
+
Volume 73, Number 7 July 1969
