J. Phys. Chem. 1981, 85,1624-1626
1624
2oc
7 100 s.
0 0
10.0
[02]
20.0
30.0
(mtorr)
Flgure 2. Observed values of kas a function of the O2partial pressure. These values are for 75 f 1 O C , as reported in Table I.
to eq VII, if k is plotted against the O2 concentration, a straight line should result with slope kl and intercept k-l k,. The values of k in Table I have been plotted in this manner in Figure 2. Although there is considerable scatter in the data, the points are consistent with a linear relationship. A weighted linear least-squares treatment gives kl = (4.38 f 0.88) mt0rr-ls-l and k-l = (26.0 f 9.6) s-l, where error limits are again 90% confidence limits. The ratio of these two values gives another value for the equilibrium constant, K,, = kl/k-, = (128 f 54) X lo3
+
atm-l, which agrees well with the average of values determined by using eq IX. The more precise value, (132 f 14) X lo3 atm-’, will be used as the best estimate of the equilibrium constant at an average temperature of 348.4 K. This equilibrium constant for reaction 1 can be used to estimate the bond energy for the allyl peroxy radical. The AGO for reaction 1is calculated to be -8160 f 130 cal/mol. Reasonable estimates for the entropies of R and ROz have been taken from B e n ~ o n ; by ~ J correcting for the temperature change from 300 to 348 K, the for reaction 1is estimated to be -26.4 eu. From these two values one can calculate AHo= = -17.4 kcal/mol, with an uncertainty of approximately f l . O kcal/mol due mainly to the uncertainty in the estimated ASo. Using Benson’s estimated C, values to correct to 300 K gives AH0300= -17.2 kcal/ mol. This calculated bond energy for the allylperoxy radical is only sightly larger than the estimate of 15 kcal/mol made by B e n ~ o n . ~ By measuring the equilibrium constant for reaction 1 over a range of temperatures, it should be possible to calculate ASo and AHo directly, thus reducing the uncertainty in the bond energy. Such experiments are currently in progress. Acknowledgment. Support for this work was provided by the National Science Foundation under Grant CHE7823867 and by a NATO Research Grant, No. 1849. (7) S. W. Benson, “Thermochemical Kinetics”, 2nd ed, Wiley, New York, 1976.
Proton Affinity of the Gaseous Azide Ion. The N-H Bond Dissociation Energy in HN3 Mark J. Pellerite, Robert L. Jackson,+ and John I. Brauman” Department of Chemistry, Stanford University, Stanford, California 94305 (Received: March 27, 198 1)
Three different sources of azide ion have been used in conjunction with ion cyclotron resonance spectrometry to measure the gas-phase proton affinity of azide ion, PA(N3-). The result, PA(N3-) = 344 f 2 kcal/mol, is in significant disagreement with that calculated by use of literature data obtained via indirect methods. Reliability of our value is confirmed by the consistency of results obtained from the different azide sources. Our experiments also allow other thermochemical parameters to be determined: iVI,0(N3-(g))= 48 f 2 kcal/mol, AHfo(N3(g)) = 112 f 5 kcal/mol, and Do(H-N3) = 92 f 5 kcal/mol.
Introduction Organic and inorganic azides have been the subject of extensive investigation for many years from the standpoint of both chemical reactivity and thermodynamic properties.l The simplest azide, hydrazoic acid (”,), has also been well studied and characterized.2 However, owing to the thermal instability of HN3, direct study of its gas-phase thermochemistry is difficult. In the past, indirect metho d ~such ~ ~as ,Born-Haber ~ cycles have been used in determinations of such quantities as AHfo(N3-(g)) and Do(H-N3). The value of AHHfO(N3-(g))is currently the subject of some c o n t r o v e r ~ yand , ~ ~widely ~ different values of Do(H-N3) can be found in the literature.lG5 We present here results of some experiments which allow direct determination of these quantities, and thus more accurate and complete characterization of the thermochemical t Corporate Research Center, Allied Chemical Corporation, Morristown, N J 07960.
0022-3654/81/2085-1624$01.25/0
properties of gaseous HN,. Our initial work in this area6 was a measurement of the electron affinity of the azide radical, EA(NJ. Here we report the proton affinity of (1)(a) Patai, S., Ed. “The Chemistry of the Azido Group”; Interscience: London, 1971. (b) Fair, H. D.; Walker, R. F., Ed. “Energetic Materials”; Plenum: New York, 1977; Vol I. (c) Evans, B. L.; Yoffe, A. D.; Gray, P. Chem. Reu. 1959,59,515. (d) Gray, P. Q.Rev. Chem. SOC. 1963, 17, 441. (2) (a) Mason, K. G. In “Mellor’s Comprehensive Treatise on Inorganic and Theoretical Chemistry”; Wiley: New York, 1967; Vol. 8, Suppl. 11, pp 1-15. (b) Jones, K. In “Comprehensive Inorganic Chemistry”;Bailar, J., et al., Ed.; Pergamon Press: Oxford, 1973; Vol. 2, pp 276-293. (3) (a) Gray, P.; Waddington, T. C. Proc. R. SOC.London, Ser. A 1956, 235, 106. (b) Gray, P.; Waddington, T. C. Ibid. 1956, 235, 481. (c) Jenkins, H. D.; Pratt, K. F. J. Phys. Chem. Solids 1977,38, 573. (4) Franklin, J. L.; Dibeler, V. H.; Reese, R. M.; Krauss, M. J. Am. Chem. SOC. 1958,80, 298. (5) (a) Clark, T. C.; Clyne, M. A. A. Trans. Faraday SOC.1970,66,877. (b) Okabe, H. “Photochemistry of Small Molecules:” Wiley-Interscience: New York, 1978; p 287. (6) Jackson, R. L.; Pellerite, M. J.; Brauman, J. I. J. Am. Chem. SOC. 1981,103,1802.
0 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85, No. 12, 1981 1625
Letters
azide ion, PA(N
