593
C O M M U N I C A T I O N S T O THE E D I T O R
The C-H Bond Dissociation Energy in Fluoroform
Sir: Current estimates of the bond dissociation energy D(CF3-H) do not agree. Whittle and co-workers1B2 studied reactions 1, -1, 2, and 3 using competitive methods.
+ CF3H HBr + CF3 Br + CH4 +HBr + CH3 CF3 + +CF3I + I
Br
(1,
- 1) (2) (3)
12
They obtained an activation energy difference of El - EZ = 3.76 f 0.14 kcal mole-' using C2F5H to bridge the gap between CF3H and CHI which differ too greatly in reactivity to compete directly for bromine atoms. They also obtained E-1 - E3 = 2.98 f 0.12 kcal mole-' which equals E-' if we assume that E3 = 0. These results lead to D(CF3-H) - D(CHrH) = 2.2 f 0.5 kea1 mole-' at 298°K Trotman-Dickenson and co-workers3 have recommended that D(CH3-H) = 103.8 kcal mole-'. This is a weighted mean of determinations which include D(CH3-H) = 103.9 kcal mole-' from bromination studies. However, the latter value should be corrected4 to 102.9 kcal mole-'. Also, since Trotman-Dickenson's recommendation, Benson and co-workers5 have obtained D(CHrH) = 104.1 kcal mole-' from a study of the equilibrium I2 CH4 HI CHJ. Their equilibrium data are confirmed by Goy and Pritchard.6 The combined result of these two amendments (giving the iodination work slightly more weight) endorses Trotman-Dickenson's recommendation of D(CH3-H) =, 103.8 kcal mole-' at 298°K; this is probably accurate to k0.5 kcal mole-'. If this value is combined with the difference D(CF3-H) - D(CH3-H) given above, we obtain
+
+
+
D(CFrH) = 106.0 f 0.7 kcal mole-' at 298°K However, reaction 4 CD3
+ CF3H
CD3H
+ CF3
(4,-4)
was studied by Pritchard, et al.,' and more recently by Pritchard and Thommarson.8 Their value of E4 together with independent data on E-4 leads to D(CF3-H) - D(CHrH) = -0.6 f 1.9 kcal mole-' so that D(CFrH) = 103.2 kcal mole-'. Since this dis-
agrees with our value, we have checked our results as follows. First, the difference E1 - Ez was redetermined using another bridge compound, n-C3F7H, the result being E1 - E2 = 3.71 f 0.11 kcal mole-'. Secondly, an absolute value of E-1 has been measured by studying the reactions
+ HC1+ CF3H + C1 CFs + CFs +CzFe CF3 + Brz CF3Br + Br
CF,
(5) (6)
(7)
--f
By photolyzing hexafluoroacetone, HFA, with HC1 in the vapor phase, it was found that Eb - 1/zE6= 5.1 f 0.1 kcal mole-' so that, if Es = 0,9E5 is known. Next, HFA was photolyzed with a mixture of HC1 Br2 from which E5 - ET = 4.43 i 0.08 kcal mole-'. Tucker and Whittle2 found that E-1 - E7 = 2.17 i 0.16 kcal mole-', so that combining all these results, we have E-1 = 2.84 f 0.30 kcal mole-'. The new determinations of E1 - E2 and E-' agree well with our previous ones and the need to assume that E3 = 0 has been eliminated. A further determination of D(CFrH) has been made by studying the reactions
+
C1
+ CF3H +HC1 + CFj
C1+ CH4 +HC1+ CH3
(-5) (8)
From a study of the competitive chlorination of mixtures of CF3H C2F5H and CH4 CzF5H, we find that E-6 - EB = 4.53 f 0.08 kea1 mole-'. Knox'O gives E8 = 3.85 kcal mole-' so that E-5 = 8.4 kcal mole-'. Thus AH--6 = E-5 - E5 = 3.3 kcal mole-' at an average temperature of 360°K. At this temperature, D(H-C1) = 103.3 kcal mole-' and, since
+
+
(1) A. M. Tarr, J. W. Coomber, and E. Whittle, Trans. Faraday Soc., 61,1182 (1965). (2) B.G. Tucker and E. Whittle, ibid., 61,866 (1965). (3) J. Greechowiak, J. A. Kerr, and A. F. Trotman-Dickenson, Chem. Commun., 109 (1965). (4) P. Corbett, A. M. Tarr, and E. Whittle, Trans. Faraday SOC.,5 9 , 1609 (1963). (5) D.M.Golden, R. Walsh, and S. W. Benson, J . Am. Chem. Soc., 87,4053 (1965). (6) C.A. Goy and H. 0. Pritchard, J.Phys. Chem., 69, 3040 (1965). (7) G.0. Pritchard, H. 0. Pritchard, H. I. Schiff, and A. F. TrotmanDickenson, Trans. Faraday Soe., 52,849 (1956). (8) G. 0.Pritchard and R. L. Thommarson, J . Phys. Chem., 68, 568 (1964). (9) R.D.Giles and E. Whittle, Trans. Faraday SOC.,61, 1425 (1965). (10) J. H.Knox, ibid., 58, 275 (1962).
Volume 70, Number 2 February 1966
COMMUNICATIONB TO THE EDITOR
594
A H , = D(CFrH) - D(H-CI), we have D(CFaH) = 106.3 kcal mole-’ after correction to 298°K. This agrees well with D(CF3-H) = 106.0 kcal mole-’
1070, 1030, 1025, 1008, 988, 969, 958, 933, 923, 912, 900, 891, 882, 877, 871, 867, 861. It should also be noted that evidence of the 2060-cm-‘ band obfrom our bromination work and we believe that the served and reported early2 was found in the studies average value of D(CFrH) = 106.2 kcal mole-’ is of the nickel films reported above, and the background accurate to *0.5 kea1 mole-’. data in this region were also free of structure. Further studies of carbon monoxide on evaporated metal CHEMISTRY DEPARTMENT J. C. AMPHLETT UNIVERSITY COLLEGE J. W. COOMBER films are in progress. CATHAYS PARK CARDIFF,GREATBRITAIN
E. WHITTLE
RECEIVED OCTOBER 29, 1965
Acknowledgment, The infrared studies of chemisorbed molecules at the University of Kentucky are supported in part by the United States Atomic Energy Commission Contract No. AT-(40-1)-2948.
Infrared Studies of Carbon Monoxide
(1) C. W. Garland, R. C. Lord, and P. F. Troiano, J . Phys. Chem., 69, 1188, 1195 (1965). Also, for the purposes of this Communica-
Chemisorbed on Metallic Surfaces
tion these papers should be referred to for literature references and review of studies in this field. (2) H.L. Pickering and H. C. Eckstrom, ibid., 63, 512 (1959). (3) Water spectra appears in their calculations because of very small changes in water vapor content in the “comparison cell” after the taking of the “background data,” upon which all calculations are based. It should be emphasized that no water vapor is present in the cell used for the chemisorption studies and any contamination is very unlikely because of outgassing procedures and vacuums maintained during film preparation.
Sir: Relatively recent papers by Garland, Lord, and Troiano’ presented a new method of forming metal films evaporated in carbon monoxide onto the windows of the infrared cell. They report a new band at 1620 cm-’ which was not reported previously for supported nickel surfaces and was not reported by Pickering and Eckstrom2 for bulk evaporated nickel films. Our work on infrared studies of carbon monoxide chemisorbed on nickel and on rhodium evaporated films using the multiple-reflection technique2 reveals the possibility of a very weak band at approximately 1620 cm-l for carbon monoxide on nickel and several very weak absorption bands in the region from 1350 to 750 cm-l; many of these bands are coincident for both nickel and rhodium. The experimental method used by Pickering and Eckstrom makes the unequivocal assignment of absorption bands in the water region difficulta and, in addition, the band at 1620 cm-l is very weak, appearing as a shoulder on a water line; in the region from 1350 to 750 cm-I several bands may be assigned, and even though they are very weak they are nevertheless “real.” The assignment of many bands to carbon monoxide chemisorbed on nickel and on rhodium confirms the complicated nature of “surface complexes” and complicates the theoretical interpretation. The number of assignments for hydrogen chemisorbed on evaporated rhodium films2 was large and our later studies have indicated there are more bands. Also, our studies of deuterium chemisorbed on rhodium films are showing approximately as many bands as for hydrogen. The “strongest” of the bands we have observed in this region for carbon monoxide are as follows: on nickel (cm-l): 1195, 1069, 1056, 1036, 1028, 1017, 1011, 1004, 999, 981, 964, 948, 933, 927, 920, 906, 896, 891, 883, 875, 864, 856, 775; on rhodium (cm-I): The Journal
of
Physical Chemistry
DBPARTMENT OF CHEMISTRY UNIVERSITY OF KENTUCKY LEXINGTON, KENTUCKY
HARTLEYC. ECKSTROM
RECEIVED NOVEMBER 5, 1965
The Change of the Rate-Determining Step of the Ammonia Decomposition over an Ammonia Synthetic Iron Catalyst Sir: Two mechanisms have been proposed on the ammonia decomposition over the doubly promoted iron catalysts. Thus, on the one hand, Temkin and Pyzhev’ proposed the rate equation on the basis of the desorption of adsorbed nitrogen as the rate-determining step which could explain many of the experimental resu1tsa2 On the other hand, the dehydrogenation of NH3(a), NH2(a), or “(a) was proposed as the ratedetermining step.3 Here the (a)’s signify the adsorbed state. This disagreement has frequently been considered as due to differences in the catalysts used. _ _ (1) M. I. Temkin and V. Pyahev, Acta Physicochim. U.R.S.S., 12, 327 (1940). (2) W. G. Frankenburg, “Catalysis,” Vol. 111, P. H. Emmett, Ed., Reinhold Publishing Corp., New York, N. I-.,1955, p 171; C. Bokhoven, C. van Heerden, R. Westrik, and P. Zwietering, ibid., p 265; A. Nielsen, Advan. Catalysis, 5, 1 (1953). (3) S. Enomoto and J. Horiuti, Proc. Japan Acad., 28, 493, 499 (1952); J . Res. Inst. Catalysis, Hokkaido Univ., 2 , 87 (1952); J. Horiuti and I. Toyoshima, ibid., 5, 120 (1957); 6, 68 (1958); J. Horiuti and N. Takezawa, Chid., 8, 170 (1961). ~
