J. Phys. Chem. 1985,89, 2901-2905
2901
Reaction of O(3P) with Trimethylsilanet H. Hoffmeyer,l 0. Horie, P. Potzinger, and B. Reimann* Max-Planck-Institut fur Strahlenchemie. 0 - 4 3 3 0 Mulheim a.d. Ruhr, FRG (Received: November 27, 1984)
The title reaction has been investigated at room temperature in a discharge flow system as well as by stationary Hg(3P1) sensitized N 2 0 photolysis experiments. O(3P) atoms abstract hydrogen from the silicon center with a rate constant of k( 1) = (2.6 0.3) X cm3 s-l. Hydroxyl radicals formed in the primary step react in a second abstraction reaction to yield H 2 0 with a rate constant larger than k( 1) by a factor of about 20. The fate of trimethylsilyl radicals, the other primary product, depends on the experimental conditions. In the flow system they combine with 0 and OH; the combination products decompose unimolecularly, yielding CH3and CH4, respectively,and the common product (CH3),Si0. In stationary photolyses they mainly abstract 0 from NzO to ultimately form (CH3)3SiOSi(CH3)3.In a separate static experiment a ratio of k(14)/k1/z(7) = 2.5 X cm3l2s-’/’ has been determined for oxygen abstraction of trimethylsilyl radicals from N 2 0 vs. combination of two trimethylsilyl radicals. The rate constant of N(4S) with trimethylsilane has been measured in the flow system to be cm3 s-I. k(2) = (2.6 f 0.8) X
Introduction The reactions of 0 atoms with silanes have so far attracted little attention. Only two investigations are known to us. Worsdorfer’ measured the title reaction with a predecessor of the apparatus cm3 used in the present study and obtained k(1) = 1.4 X s-l. H e did not, however, perform product analysis. Atkinson and PittsZ studied the reaction of 0 atoms with SiH4 by a flash photolysis chemiluminescence technique. Arrhenius parameters were given for the primary process, but its chemical identity was not established. From the discussion it can be surmised that the authors thought of a simple abstraction reaction. Because of the high affinity of silicon to oxygen other primary steps such as substitution or addition cannot be excluded a priori. We have therefore investigated the reaction of 0 atoms with trimethylsilane, which is less prone to unwanted wall effects than SiH4,3-5by the discharge flow technique as well as by stationary photolysis experiments, with special emphasis on product analysis. Experimental Section Flow Experiments. The measurements were carried out at room temperature (297 f 3 K). The apparatus is shown schematically in Figure 1. Three different reactor configurations were used in this work. Reactor A is a quartz tube with an inner diameter of 27 mm. Reactor B differs from A only in that Teflon rings have been inserted at regular distances (see Figure 1). The effective cross section of the flow tube is reduced to the inner diameter of the teflon rings (18.8 mm); this separates the flowing gas from the wall. An appreciable decrease in the influence of the wall has been observed for the reaction of H atoms with silanes when reactor B is used.7 A detailed account of this system will be given elsewhere.s Reactor C, consisting of a Pyrex tube of the same inner diameter as reactor A, is coated with Halocarbon wax. The atoms are generated in a side arm by a microwave discharge and enter the actual reactor via a perforated Teflon stopper whose holes ensure a uniform distribution of the atoms over the reactor cross section. The substrate is added by a movable injector ( 5 mm 0.d.) concentric with the flow tube. The injector has a Teflon tip which is radially perforated to provide uniform injection of the reactant gas. In reactor C the movable injector has a larger diameter (10 mm 0.d.) and also larger holes at its tip compared to that used with reactors A and B, allowing the introduction of atoms through the injector for measuring wall constants. For this purpose a second microwave discharge has been attached to the rear part of the movable injector. The reaction mixture flows through the reactor with linear flow velocities ranging from 3 to 40 m/s. The pressure in the reactor can be varied but is usually ‘Dedicated to Prof. Dr. G. Wilke on the occasion of his 60th birthday. *Presentaddress: Deutsche L’Air Liquide Edelgas GmbH, Karlstr. 104, D-4000 Diisseldorf. FRG.
0022-3654/85/2089-2901$01.50/0
about 300 Pa. A small portion of the reaction mixture is extracted by a stainless steel skimmer and expanded into the first vacuum chamber, the pressure of which is kept below lo-, Pa by an oil diffusion pump (Leybojet 3000/7, Leybold Heraeus). The molecular beam formed in this way is chopped by a tuning fork (Bulova L 40 C), allowing phase-sensitive detection. The inner part of the beam passes through a second skimmer constructed from Delrin, then enters the ion source of a quadrupole mass spectrometer (Extranuclear 4-324-9) and is eventually pumped off by a turbomolecular pump (Turbovac 450, Leybold Heraeus). The background pressure in the spectrometer chamber is less than Pa. Special care is taken to keep the air leakage into the reactor and into the gas supply tubes as low as possible because of the high reactivity of silanes with oxygen. Flow rates of the carrier gases, helium or argon, are monitored by calibrated flow meters (Datametrics, Model 800 L) or by using a calibrated sonic orifice whereas all other gases are measured by the dp/dt method (pressure drop). 0 atoms are generated by two methods: (i) microwave discharge of a mixture of O2 and Ar or He, where the 0 atom concentration is calculated from the decrease in the mass spectrometric O2 signal and (ii) microwave discharge of N2 and subsequent titration of N atoms with NO. There is excellent agreement between the visually determined titration point and the mass spectrometric appearance of NO and disappearance of N, respectively. The detection limit of 0 atoms with 17-eV ionizing electrons is about 1 X lo1, cm-3 and for (CH3)3SiHat m / e 59 and 70 eV I 1 X 1Olo ~ m - ~It .is found that the sensitivity of 0 atoms is a factor of 4smaller than for N atoms despite similar ionization cross sections9 and negligible wall constants in both cases. One reason for this could be the loss of 0 atoms at the second skimmer. All rate constant determinations were carried out with 0 atoms in excess (4 X l O I 3 cm-3 I[ O ] I3 X IOl4 ~ m - [(CH,),SiH] ~, smaller by a factor 30-120), thereby avoiding (i) the introduction of stoichiometric correction factors made necessary by secondary (1) Worsdorfer, K. Ph.D. Thesis, University of Essen, 1978. (2) Atkinson, R.;Pitts, Jr., J. N. Int. J . Chem. Kinet. 1978, 10, 1151. (3) Mihelcic, D.; Potzinger, P.; Schindler, R. N. Ber. Bunsenges. Phys. Chem. 1974. 78. 82. (4) Miheicic,’Dy; Schubert, V.; Schindler, R.N.; Potzinger, P. J . Phys. Chem. 1977,81, 1543. ( 5 ) Cowfer, J. A.; Lynch, K. P.; Michael, J. V. J . Phys. Chem. 1975, 79, 1139.
(6) Homann, K. H.; Solomon, W. C.; Warnatz, J.; Wagner, H. G.; Zetzsch, C. Ber. Bunsenges. Phys. Chem. 1970, 74, 585. (7) Worsdorfer, K.; Reimann, B.;Potzinger, P. Z. Naturforsch. A 1983, 38, 896. ( 8 ) Worsdorfer, K.; Horie, 0.;Hoffmeyer, H.; Potzinger, P.; Reimann, B., to be published. (9) Kieffer, L. J. JILA Inf. Cent. Rep. 1969, 6, 95.
0 1985 American Chemical Society
2902
Hoffmeyer et al.
The Journal of Physical Chemistry, Vol. 89, No. 13, 1985 Reactant I A r , He
Movable injector
02 or
Microwave discharge cavity
O-ring sliding seal
/'I1 1
100
N2 IAr, He
c [ICH313 S i H l
I
c
Manometer
10
=
Perforated Microwave discharge cavity
Teflon rings
b I/ d I
I I1 I
Pump
d
Figure 2. Typical first-order plot for reaction 1 with 0 atoms in excess. Z and I, are the intensities at m / e 59 with discharge on and discharge off, respectively. z is the distance between the tip of the injector and the
first skimmer.
1
1
I
600
-
500
-
LOO
-
300 Turbomolecular pump
Figure 1. Schematic drawing of the flow apparatus. The differential pumping stage is basically of the same design as that of Homann et
reactions, in particular by reaction 4 and (ii) deposition of partially oxidized silanes at the wall, which would lead to changes in the wall constant. Experiments with substrate in excess were not feasible for a number of reasons, e.g., wall effects, mass interference between 0 and CH4 formed in secondary reactions, and the low sensitivity for 0 atoms. Static Experiments. Stationary photolyses were performed in cylindrical quartz cells of 5 cm diameter and 10 cm length. The oxygen atoms were generated by Hg(3P,) sensitized photolysis of N20. The reaction mixture, typically consisting of 0.2 Pa of Hg, 130 Pa of trimethylsilane, and N 2 0 in at least hundredfold excess, was irradiated by the partially collimated beam of a low-pressure mercury resonance lamp (Grantzel, Type 5). The largest diameter of the light beam used was about 4 cm. Without this precaution inconsistent results were obtained. If the light beam is allowed to reach the walls of the photolysis cell, (CH3)3SiOH especially appears as a prominent product. With a spherical reaction vessel of 4 L volume, of which only 0.8 L are illuminated, it was possible to suppress the silanol formation completely. The noncondensable products were analyzed by mass spectrometry (MAT 3 1 1 A) and their pressure measured by a membrane manometer (Datametrics 51 1-10); all other products were measured by gas chromatography (Carlo Erba 2900, 50-mglass capillary column OV 1). The purities of gases as stated by the manufacturers were as follows: He 99.996%, Ar 99.997%, N2 399.99%, N O >99.8%, O2 99.995%, N 2 0 399.0%, and (CH3)+3iH 399.5%. Argon was passed over P205to remove traces of moisture. N2 was used as delivered. NO was passed over Ascarite (Flub) to remove higher oxides. N20,whose main impurities were N2and 02,was degassed before use by several freeze-pumpthaw cycles. Trimethylsilane contained traces of di- and tetramethylsilane as impurities (
