2365
Communications to the Editor loosely attached to the Na+ ions since they can be almost entirely removed by outgassing the zeolite at room temperature (the U.C. variations are nearly reversible). Therefore, the unsaturated molecules may well be associated with the Na+ ions through ions to induced-dipole forces as previously ~ u g g e s t e d This . ~ also corroborates the findings of Y a t e ~ . ~ However, -g in contrast with ethylene, the acetylene molecules cannot be entirely removed at room temperature. This again shows that the cation to electrons sorbed molecule bond is stronger when more i~ are present. This study proves that the interaction between unsaturated molecules adsorbed on Y zeolites and Na+ ions is the main cause for the observed U.C. decrease. The variation observed must be taken into consideration before applying relations such as that given in a previous paper.2 References aind Notes J. V. Smith, Advan. Chem. Ser., No. IO?, 171 (1971,). P. Gallezot and B. Imelik, J. Phys. Chem., 77, 652 (1973). P. Pichat, J. Vedrine, P. Gallezot, and B. Imelik, J. Catal., in press. P. Gallezot and B. Imelik, J. Chim. Phys., 68, 34 (1971). P. Gallezot, Y . Ben Taarit, and B. Imelik, J. Catal., 26, 295 (1972). G. V. Tsitsishvili, G. D. Bagratishvili, and N. i. Onashvili, Russ. J. Phys. Chem., 43, 524 (1969). (7) A. A. Amaro and K. Seff, J. Chem. SOC.,22,1201 (1972). (8) J. L. Carter, D. J. C. Yates, P. J. Lucchesi, J. J. Elliott, and V. Kevorkian, J. Phys. Chem., 70, 1126 (1966). (9) D. J. C. Yates, J. Phys. Chem., 70, 3693 (1966).
(1) (2) (3) (4) (5) (6)
lnstitut de Recherches sur la Catalyse, C. N. R.S. 69100 Villeurbanne, France
P. Gallezot B. Imelik*
Received June 29, 1973
Selective Hydrogen Atom Abstraction by Hydrogen Atoms in Neopentane-Alkane Mixtures at 77 K Publication costs assisted by Faculty of Engineering, Nagoya University
Sir: Recently quite interesting phenomena have been reported on the hydrogen atom abstraction reaction by radicals in the sollid phase at 77 K.Is2 Here we report that H atoms produced by the photolysis of hydrogen iodide in neopentane containing a small amount of alkane react selectively with the solute alkane at 77 K. Experimental procedures were identical with those described in the previous ~ t u d i e s . When ~ , ~ the photolysis of hydrogen iodide (0.05 mol %) is performed in neopentane with 2537-A radiation at 77 K, an esr spectrum of the neopentyl radical is obtained. H atoms produced by the photolysis of hydrogen iodide abstract hydrogen atoms from neopentane to form the neopentyl radical. When the photolysis of hydrogen iodide is performsd in neopentane containing a small amount of an alkane such as ethane, propane, or isobutane, a quite different esr spectrum of the solute radical, such as ethyl, propyl, or the t-butyl radical, is obtained. The H atoms produced by the photolysis react selectively with the solute alkane to form the solute radical, even if the solute concentration is very low. The yields of solute radicals in the photolysis of neo-CsHlz-iC4Hlo(l%)-HI(O.O5%)are 35 times as high as those in the
photolysis of neo-CeHlz -HI(0.05%). Therefore, most of the H atoms produced by the photolysis cannot react with neopentane in the pure neopentane matrix, while they can react with the solute alkane in the neopentane-alkane mixture. One possible explanation for the selective formation of solute radicals is that hydrogen iodide and the alkane form a complex and dissolve in juxtaposition in the neopentane matrix. Though the possibility cannot be neglected at present, it may be small for the following reasons. First, there is no evidence to support the idea that the hydrogen iodide forms a complex with the alkane except neopentane. Since the yields of solute radicals in the photolysis of neo-C5H12-i-C4Hlo(l%)-HI(O.l%) are about 6 times as high as those in the photolysis of neo-C~Hl2C3H,(l%)-HI(O.l%), it is expected from the complex hypothesis that isobutane is more favorable for the formation of the complex than propane. Propyl radical, however, is selectively formed in the photolysis of i-CIHIOC3Hs(l%)-HI(O.l%) at 77 K. Therefore, the results cannot be explained by the hypothesis of the complex formation. Secondly, it was found that solute radicals were selectively formed in the radiolysis of neopentane containing a small amount of alkanes.3 The results show that the selective C-H bond scission does occur in the absence of hydrogen iodide and that it is a phenomenon characteristic of the neopentane-alkane mixtures at 77 K. Thirdly, it has been observed that a large fraction of 3-methylpentyl radicals produced by the photolysis of HI in 3-methylpentane glass at 77 K decays within a few minutes, apparently by combination with iodine atoms formed near the same locations in the matrix when the HI is dissociated.* The ~ - C ~ H radicals Q produced by the photolysis of HI in a neopentane-isobutane(l%) mixture, however, do not decay at all at 77 K, even if the sample is stored for 5 hr after the photolysis. Therefore, it seems that the H atoms produced by the photolysis of hydrogen iodide migrate through the neopentane matrix and react selectively with the solute alkane. The yield of t-C4H9 radicals in the photolysis of ~ ~ O - C ~ H ~ ~ - ~ - C ~ H ~ Oat-77 H IK(was O . studO~%) ied as a function of concentration of isobutane. Since the yields become a plateau over 0.2 mol % of isobutane, most of the H atoms react selectively with isobutane at 0.2%. The selective hydrogen atom abstraction by H atoms was also observed in the isobutane-propane mixture. The relative probabilities for hydrogen atom abstraction by H atoms in a variety of alkane mixtures at 77 K are summarized in Table I. It is surprising that the relative probabilities for hydrogen atom abstraction from the solute alkanes are much larger than that from neopentane. The relative probabilities for deuterium atom abstraction in the solid phase are shown in the third column of Table I.5 The relative rate constants of hydrogen atom abstraction by hot D atoms in the gas phase are also given in the fourth column of Table 1.6 The reported relative probabilities for hydrogen atom abstraction from alkanes are of the same order of magnitude as that from neopentane even in the solid phase. The extremely large values for the alkanes in the present work may be caused by the specific physical property of the neopentane-alkane mixture in the solid phase. Classical activation energies of hydrogen atom abstraction by thermal H atoms are shown in the last column in Table I. It should be noted that the activation energy for neopentane is smaller than that for ethane. The present results obtained for the solid alkane mixture are quite mysterious for us. It is conceivable, howThe Journalof Physical Chemistry, Vol. ??, No. 19, 1973
Communications to the Editor
2366 TABLE I: Relative Probability for Hydrogen Atom Abstraction in Alkane Mixtures at 77 K
__
Alkane
Relative probability for H-atom abstraction in alkane mixturesa
Relative probability for D-atom abstractionb
Relative rate constant for H-atom abstractionc
Classical activation energy for H-atom abstraction by thermal H atom, kcal/mol
neo-CsH12-RH(1%)-HI (0.05%) System
1
200 620
0.73
1.oo 0.73
1.1
9.3d 9.5d 8.9
500
3500 1400 2400 1300 270 1300 1000 800
1.6
9.9e 8.4e 8.4e 7.4e
4.6
i-c4Hl0-C3H8(1 %)-HI(O.l %) System i-C4H10 (pure) C3Hs
1 40
1.o
0.69
8.5d
a.4e
a The relative probability, measured at 77 K by uv illumination for 3 min, is equal to (yield of solute radical per 3 mini X (concentration of neo-C5H12)/ (concentration of RH). The yields of solute radical were measured by doubie integration of the first-derivative esr signals of the solute radical. Since the formation of the solute radical is quite selective, serious overlapping of the spectra of the solute radical and the neopentyl radical does not occur. The microwave power level used did not result in saturation of any of the signals. The relative probability for D-atom abstraction by CH3 radical was measured by the photolysis of perdeuterated alkane containing methyl iodide (2 %) in the solid phase at 20 K.5 It was reported that the relative probabilities for Datom abstraction from alkanes in the solid phase are the same as those in the gas phase and that there is no isotope effect between deuterium and hydrogen atom abstraction. The rate constant of H-atom abstraction by hot deuterium atom was measured by the photolysis of alkane containing deuterium iodide in the gas phase at 298 K.6 T. Kagiya, Y . Sumida, I. Inoue, and F. S. Dyachkovskii, Bull. Chem. Soc. Jap., 42, 1812 (1969). e B. A. Thrush, Progr. React. Kinet., 3, 89 (1965).
*
ever, that the physical condition of the matrix, such as the local crystalline structure near the additive and the crystalline structure of the matrix, would affect the selective hydrogen atom abstraction reaction.
References and Notes (1) (a) E. D. Sprague and F, Williams, J: Amer. Chem. SOC., 93, 787 (1971); (b) J , T. Wang and F. Williams, ibid., 94, 2930 (1972); (C) A. Campion and F. Williams, ibid., 94, 7633 (1972). (2) T. Wakayama, T. Miyazaki. K. Fueki, and Z. Kuri, Bull, Chem. SOC. Jap.. 44, 2619 (1971). (3) (a) T. Miyazaki, T. Wakayama, M. Fukaya, Y. Saitake, and Z. Kuri,
The Journal of Physical Chemistry, Vol. 77, No. 79, 1973
Bull. Chem. SOC. Jap., 46, 1030 (1973); (b) M. Kato, Y. Saitake, T. Miyazaki, and Z. Kuri, ibid., 46, 2004 (1973). (4) W. G. French and J. E. Willard, J. Phys. Chem., 72, 4604 (1968), (5) R. E. Rebbert and P. Ausloos, J. Chem. Phys., 48, 306 (1968); cf. footnote b in Table I . (6) R. J. Carter, W. H. Hamill, and R. R. Williams, J. Amer. Chem. Soc., 77, 6457 (1955); cf. footnote c in Table I.
Department of Synthetic Chemistry Faculty of Engineering Nagoya University Chikusa-ku, Nagoya, Japan Received March 14, 1973
Terunobu Wakayama Tetsuo Miyazaki" Kenji Fueki Zen-ichiro Kuri
