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E. R. Austin and F. W. Lampe
Rate Constants for the Reactions of Hydrogen Atoms with Methylgermanes' E. R. Austin and F. W. Lampe" Davey Laboratory, Department of Chemistty, The Pennsylvanla State University, University Park, Pennsylvania 16802 (Received Recember 30, 1976) Publication costs assisted by the U.S. Energy Research and Development Administration
Absolute rate constants for the reaction of H atoms, formed by mercury photosensitizationof H2-substrate mixtures at 32 O C , with (CH3),GeH4-, for n = 1, 2, 3 have been determined in experiments involving the competitive reaction of H atoms with Si2HG.Arrhenius parameters and bond dissociation energies have been estimated for this series of methylgermanes.
Introduction While considerable kinetic information pertaining to the gas-phase reactions of radicals with various silanes and alkylsilanes has become available during the last 10 very little kinetic data are available for radical reactions with germanes31@and no such information exists for methylgermanes containing Ge-H bonds. Recently we have developed a mass-spectrometrictechnique to measure rate constants for the reaction of hydrogen atoms with various silanes and mono- and digermane,33and it seemed worthwhile to apply this same technique to the methylgermanes. We have conducted mass-spectrometric studies of the uced decomposition of the rates of hydrogen-atom methylgermanes relative the rate of hydrogen-atom reaction with disilane a t 32 O C . From these studies we have determined the specific reaction rates for hydrogen-atom attack on the methylgermanes, and this paper is a report of our results. Experimental Section Hydrogen atoms were generated by Hg(3P1) photosensitization of hydrogen-substrate mixtures which contained about 98% hydrogen. Under these conditions more than 95% of Hg(3P1)atoms that are collisionally quenched react with hydrogen to yield hydrogen atoms. At the total pressures used in these experiments, namely 30-60 Torr, gas-phase recombination of hydrogen atoms and diffusion of hydrogen atoms to the walls are negligible when compared with the rates of reaction with substrate molecule^^^^^^ (cf. Table I). The reactions were carried out in a photolysis cell containing a pin-hole leak leading into the ionization region of a Bendix Model 14-101time-of-flight mass spectrometer. The photolysis cell is connected via 1/4-in,stainless steel and 6-mm Pyrex tubing to large reservoirs (5-12 L) containing the reactants. Reactant gases thus flow continuously through the pin-hole leak into the mass spectrometer. The flow rates are such that during the course of a single experiment (typically 2-3 min) the pressure decrease in the reservoir is of the order of 0.5% or less. The apparatus has been previously described in The source of 2537-A radiation for generation of Hg(3P1) atoms was a General Electric 4-W G4T4/1 germicidal lamp, Measurements of the initial rate of formation of n-C4Hlo in Hz-CzH4 mixtures indicate that in our experiments the lower limit for the rate of formation of hydrogen atoms was 6.7 X 1013atom/cm3 s.33 The temperature of the photolysis cell was measured using a chromel-alumel thermocouple and was found to be 32 f 2 oc. The Journal of Physical Chemlstry, Vol. 81, No. 16, 1977
TABLE I: Rate Constants for B-Atom Induced Decomposition at 3 2 ,'C
No. Of
Substrate CH,GeH, (CH,),GeH, (CH,),GeH (CH,),Si (CH,),Ge (CH,),Sn
mle runs k , ( k , + k , ) 74 11 0.84 ~t0.08 90 1 1 0.90 i 0.06 105 1 1 1.22 i: 0.07 88 1 165
2
k,(cm3/s)x lo', 3.1 i: 0.6 3.3 i: 0.6 4.6 i: 0.7
