1249
RECOIL TRITIUM REACTIONS WITH HEXAMETHYLDISILANE studied as spread-films in the range of high surface pressures. Since the subphase is usually not saturated with respect to both lipid components, one cannot establish unequivocally whether the mixed film is homogeneous. However, in at least one system-dipalmitoy1 lecithin plus cholesterol-we have demonstrated that the components are immiscible. Thus, the spreadfilm for which a condensing effect has been reported5 must contain two surface phases. Therefore, the assumption of film homogeneity has been violated, and the reported condensing effect cannot be interpreted rneaningf~lly.~~-2~ A condensing effect has also been reported for mixtures of cholesterol with oleic acida&and, on the basis of less direct evidence, with octadecyl sulfate.27 In these cases, while film miscibility is indicated by our adsorption studies, it is again important to recognize that this does not necessarily indicate complete miscibility; the second phase, if it exists, will be excluded from the surface. I n summary, we have demonstrated that the behavior of mixed-lipid films at high surface pressures is
consistent with regular solution theory, where one would predict either large positive deviations from Raoult’s law or phase separation. For lipid mixtures at high surface pressures, where condensing effects have been reported, phase separation has been shown to occur. While the reported condensing effect may be shown ultimately to involve a decrease in molecular areas for surface mixtures of lipids (much as the mixing of hydrocarbons in bulk results in a decrease in volume), at present the significance of this effect must remain in doubt until a more rigorous method for studying spread films in the high surface pressure region can be devised. (24) The immiscibility of dipalmitoyl lecithin and cholesterol has also been verified in the low surface pressure region: R. E. Pagano and N. L. Gershfeld, unpublished results. (25) Surface viscosity studies have recently been used to test for cholesterol-lecithin interactions. The significance of these studies must also be questioned on the basis that similar results were obtained with both immiscible and miscible systems.16 (26) P. Joos, Chem. Phys. Lipids, 4, 162 (1970). (27) M. Muramatsu and N. L. Gershfeld, J . Phys. Chem., 73, 1157 (1969).
Recoil Tritium Reactions with Hexamethyldisilane in the Gas Phase
by S. H. Daniel, G. P. Gennaro, K. M. Ranck, and Y.-N. Tang* Department of Chemistry, Texas A&M University, College Statwn, Texas Y784.9 (Received November 19,1971) Publication costs assisted bu the U.8. Atomic Energy Commission
A series of recoil tritium experiments is described with hexamethyldisilane in which hydrogen abstraction and substitution, heavy group displacement, and reaction a t the Si-Si bond are observed. Effects of additives, total system pressure, and radiolysis were evaluated. Reactivities on a “per bond” basis were determined for each bond in hexamethyldisilane relative to those in neopentane. The relative order was found to be: Si-Si > C-H > C-Si > C-C. I n fact, the Si-Si bond was estimated to be the most reactive single bond toward high-energy tritium yet encountered. This high reactivity is thought, on the basis of 0 2 and CZH4 additive effects, to be due t o both a low threshold energy and a high reaction cross section. Results are interpreted in terms of possible chemical effects on these high-energy reactions, such as a low bond dissociation energy.
Introduction The importance of various chemical parameters as controlling factors in high energy atom reactions has been indicated by several recent investigations. 1-12 Bond dissociation energies have been shown to be a major deterjminant in both recoil tritium abstraction and substitution reactions. For the abstraction reaction, the excellent correlation between D(C-H) and HT yields of various organic compounds has been established by Rowland and c o ~ o r k e r s . ~ -For ~ the substitution reaction, a correlation between D(C-X)
and product yields from the T*-for-X substitution in bot,h CH,X and the substituted benzoic acids has been rep~rted.l~~O~l~ (1) Y.-N. Tang, E. K. C. Lee, E. Tachikawa, and F. S. Rowland, J . Phys. Chem., 75, 1290 (1971). (2) S. H. Daniel and Y.-N. Tang, ibid., 75, 301 (1971). (3) F. S. Rowland, E. K. C . Lee, and Y.-N. Tang, ibid., 73, 4024
(1969). (4) J. W. Root, ibid., 73, 3174 (1969). (5) W. Breckenridge, J. W. Root, and F. S. Rowland, J. Chem. Phys., 39, 2374 (1963).
The Journal of Physical Chemistry, VoE. 76, No. 9 , 1972
S. H. DANIEL,G. P. GENNARO, K. M. RANCK,AND Y.-N. TANG
1250
As for other chemical factors, a most noteworthy result is that an electron density effect was demonstrated by the correlation between NMR proton chemical shifts and the T*-for-H substitution yields in a number of alkanes and halomethanes. l 9 3 All of these observations, although not denying the possible effect of physical-geometrical parameters (e.g., the rotational inertia hypothesis used for the explanation of heavy-group substitution by tritium atoms),14-17 have positively identified the active role of certain chemical factors in hot atom reactions. The present series of studies on recoil tritium reactions with silicon-containing compounds has been designed to search for further evidence of chemical effects in hot atom reactions by increasing or extending our working parameter^.'^^'^ Our previous work on trimethylfluorosilane has convincingly demonstrated the existence of a bond strength effect in the T*-for-X substitution in highly substituted silanes.18 I n the present case, we wish to examine in detail how the bond strength effect works in recoil tritium reactions with hexamethyldisilane, the major feature of this molecule being the presence of a weak Si-Si bondaZ0 Furthermore, we wish to identify any possible clues for the presence of additional chemical factors, such as the use of 3d atomic orbitals.21 The primary products expected from reaction of recoil tritium with hexamethyldisilane are as follows
+ (CH3)3Si-Si(CH3)3+ H T + (CH3)3Si-Si(CH3)zCHz (1) T* + (CH3)3Si-Si(CH3)3+ (CH3)3Si-Si(CHa)2CH2T+ H (2) T* + (CH3)3Si-Si(CH3)3+ CH3T + (CH3)3Si-Si(CH3)z (3) T * + (CH3)3Si-Si(CH3)3+ (CH3)3Si-SiT(CH3)z+ CH3 (4) T * + (CH3)3Si-Si(CH3)3+ T*
(CHd3SiT
+ (CH&Si
(5)
All of the expected tritiated products, with the exception of pentametliyldisilane, for which identification is tentative, are observed.
Experimental Section General Procedure. The standard procedures used in recoil tritium reactions were f01lowed.l~ Samples containing 3He, hexamethyldisilane vapor, and additives were filled into Pyrex 1720 bulbs and sealed with standard high-vacuum techniques. The nuclear reaction, 3He (n,p) 3 H , was used for the tritium production. Irradiations were carried out at the Texas A&WI University Nuclear Science Center reactor with a thermal neutron flux of 1 X neutrons/(cm2 sec) for The Journal of Physical Chemistry, VoZ, 76, N o . 9,1978
5 or 8 min. The tritium-labeled products were analyzed by radiogas chromatography.22 Mass peaks were detected by a thermal conductivity detector and measured by disc integration. Measurement of radioactivity in the eluent was accomplished by gas proportional counting of the helium stream after it had been mixed with propane. There was no indication of quenching by the hexamethyldisilane. Chemicals. Hexamethyldisilane, with a purity level of >98%, was obtained from Peninsular Chemical Co. A gas chromatographic purity check showed no detectable impurities, (CHa)3SiH included. 3He was obtained from Mound Laboratory, Monsanto Research Corp., and has a tritium content of less than 2 X 10-11%* Oxygen (Airco), COz (Matheson, >99.5% purity), CzH4 (Matheson, >99% purity), and neo-CaH12 (Matheson, >99% purity) were all used without further purification. Gas Chromatographic Columns. For most of the samples, two columns were sufficient for a complete analysis, a 50-ft TTP (tri-o-tolyl phosphate) column a t 50” for the evaluation of labeled parent and (CH3)3SiT yields and a combination set of columns (glass beads, activated alumina, and molecular sieves) for the H T and CHaT separation. For ethylene-containing samples, a SO-ft T I B (triisobutylene) column at 0” was also used. This permitted separation of (CH3)3SiT without interference from tritiated hydrocarbons. A small radioactive peak which appears somewhat before the parent on the TTP column and whose activity is generally 145% of that found in the tritiated (6) J. W. Root, 43, 3694 (1965). (7) E.
W. Breckenridge, F. 9. Rowland, J . Chem. Phys.,
Tachikawa, Y.-N. Tang, and F. S.Rowland, J . Amer. Chem.
SOC.,90, 3584 (1968).
(8) E. Tachikawa and F. 5. Rowland, ibid., 90, 4767 (1968). (9) E. Tachikawa and F. S. Rowland, ibid., 91, 559 (1969). (10) Y.-N. Tang, Ph.D. Thesis, University of Kansas, 1964.
(11) F. Schmidt-Bleek and F. S. Rowland, Angew. Chem., 76, 901 (1964). (12) F. S.
Rowland, “Proceedings of the International School of Physics, “Enrico Fermi” Course XLIV-Molecular Beam and Reaction Kinetics,” Ch. Schlier, Ed., Academic Press, New York, N. Y., 1970. (13) R. M. White and F. 5. Rowland, J . Amer. Chem. Soc., 82, 5345 (1960). (14) R. Wolfgang, Progr. React. Kinet., 3, 97 (1965). (15) R. Wolfgang, Annu. Rev. Phys. Chem., 16, 15 (1965). (16) R. A. Odum and R. Wolfgang, J . Amer. Chem. SOC.,83, 4668 (1961). (17) R. A. Odum and R. Wolfgang, ibid., 85, 1050 (1963). (18) S.H. Daniel and Y.-N. Tang, J . Phys. Chem., 73, 4378 (1969). (19) S. H. Daniel, Ph.D. Thesis, Texas A&M University, 1971. (20) I. M. T. Davidson and I. L. Stephenson, J . Chem. SOC.A , 282 (1968). (21) E. A. V. Ebsworth, “Volatile Silicon Compounds,” MacMillan, New York, N. Y., 1963. (22) J. K. Lee, E. K. C. Lee, B. Musgrave, Y.-B. Tang, J. W. Root, and F. S.Rowland, Anal. Chem., 34, 741 (1962).
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RECOILTRITIUM REACTIONS WITII. HEXAMETHYLDISILANE
Table I: Recoil Tritium Reactions with Hexamethyldisilane in Systems Scavenged by
Gas pressure, Tori
7
(CH&Si-Si (CHa)a *He 02 OZ/(CH3)sSiz.ratio
20 20 26 1.3
20 20 28 1.4
25 26 66 2.6
a
100 16 f 1 30 f 1 72 =IC 1
7
25 25 100 4.0
25 25 80 3.2
27 50 167 6.2
--.
Product yieldsa-
c
HT CHaT (CHa)aSiT (CHa)aSi-Si((>Hg)zCHzT
0 2
100 16 f 1 35 =k 1 67 =IC 1
100 19 =IC 1 16 f 1 57 f 1
100 19 f 1 18i. 1 58 f 1
100 18f 1 20 f 1 56 f 1
100 19 f 1 13f 1 54 f 1
Relative to €IT as 100.
parent is probably (CH3)6SizT. The identification is only tentative, since no authentic sample was available for column calibration.
Results Variation of Product Yields with Scavenger Concentration. Recoil tritium reactions with hexamethyldisilane in the presence of various amounts of oxygen as a scavenger were carried out, and the results are shown in Table I. The O2/parent ratio ranged from 1 to 6. I n this table, HT, which is normally used as a pressure-independent reference, was chosen as a comparison standard because, for well-scavenged systems, the H T yield from the abstraction of hydrogen from a reasonably strong C-H bond should not be very sensitive to the scavenger concentration. Error limits in this and subsequent tables are computed from counting statistics only, unless otherwise specified. The data in Table I reveal that as the Oz/parent ratio goes from 1 to 6, the relative yields of (CH3)3SiT decrease by a factor of approximately 3. However, the relative yields of all the other primary products show much less variation. Radiolysis of Hexamethyldisilane. A small amount of trimethylsilane (
