PROPERTIES OF TRAPPED H AND D ATOMS I n our system, we are unable to detect acetylene.1° Since the initial pressure of ethane is >lo0 mm, it is very likely that if excited ethylene is formed, it is quickly deexcited by collision to normal ethylene. Experimental results on the CHaCDa photolysis (Table 11) indicate that the molecular detachment of hydrogen (-75%) takes place from the same carbon ~ from CH3CDa atom. The formation of ~ 2 5 of7 HD may occur by a four-center reaction mechanism involving hydrogen atoms from both the carbon atoms. It is noted from the data of the low intensity photolysis that elimination of molecular hydrogen from the terminal carbon atom was less favorable with a decrease of wave-
1207 length. However, in flash photolysis this appears to be invariant with a change in rare gas continuum. In brief it may be pointed out that the mechanism of the photodecomposition of ethane, both by continuous and flash irradiation, appears to be generally similar, with, however, a few minor differences. These differences we believe are due to different secondary reactions of products in the continuous and flash photolysis. (10) I n order t o be sure that small amounts of acetylene in the reaction products did not get absorbed on the “0”rings, an artificial mixture of small amount of acetylene and large excess of ethane was introduced in the reaction cell: about 1 hr later it wa8 condensed out of the reaction cell and analyzed. It was noted that within the limit of experimental uncertainty, acetylene was quantitatively recovered.
Properties of Trapped H and D Atoms Produced by the Photolysis of HI in 3MP-d,, Glass1 by Mervyn A. Long and John E. Willard Department of Chemistry, University of Wisconsin,Madison, Wisconsin 63706 (Received September 18, 1969)
Photolysis of H I dissolved in perdeuterated 3-methylpentane (3MP-d~)glass a t any temperature from 4 to 50°K produces trapped H atoms and trapped D atoms observable by esr, as well as C6D13 radicals formed by abstraction of D from the 3MP-dl4 by hot H. CeHl3 radicals, but no trapped H atoms, are produced by photolysis of HI in 3MP-h14 under identical conditions, indicating a major effect of isotopic substitution on the trapping capability of the matrix. The initial quantum yield of trapped H in 3MP-dl4 (ca. 0.03) is independent of temperature over the range of a t least 20-40’K but decreases with time of photolysis until a steady-state concentration is reached, while the concentrations of C6Dl3 and D continue to grow linearly. The fractional rate of decay of trapped H atoms following short illuminations decreases rapidly with time, but the decay curves for samples with different initial concentrations are superimposable after normalization for dose. The properties of trapped H produced in 3MP-d14in Kel-F tubes are the same as when produced in quartz tubes. Trapped H atoms may be produced by photolysis of empty quartz tubes after certain conditions of aging and y radiolysis. Trapped D (or H )atoms are not produced by the radiolysis of 3MP-dl4 (or 3MP-hl4) with or without dissolved H I present, although photolysis of H I in radiolyzed 3MP& produces them. The implications of the data with respect to the mechanism of H atom trapping and decay are discussed.
Introduction Recent investigations have shown that (a) hydrogen and deuterium atoms produced by photolysis of H I in perdeuterated 3-methylpentane (3MP-du) a t 20-50°K can be trapped in the matrix and observed by their ear spectra;2 (b) photolysis of H I in perprotiated 3-methylpentane (3MP-hl4) under identical conditions does not produce trapped H atoms;2 (c) radiolysis of 3 M p - d ~under conditions demonstrated to trap D atoms produced by photolysis of H I does not produce trapped D atoms.* The present paper reports further investigations of
the photolysis of HI in 3MP-dl4 and 3MP-h14designed to answer the following questions. (1) What are the kinetics of growth of trapped H and D atoms and of free radicals during photolysis of HI in 3MP-d14? ( 2 ) What are the kinetics of decay of trapped H and D atoms? (3) What do the kinetics indicate about the mechanisms of trapping and decay? (4) Does D (1) T F s work has been supported in part by U. S. Atomic Energy Commission Contract AT(11-1)-1715 and by the W. F. Vilas Trust of the University of Wisconsin. (2) D. Timm and J. E. Willard, J. Amer. Chem. SOC., 91,3406 (1969). (3) D. Timm and J. E. Willard, J.Phys. Chem., 73,2403 (1969). Volume 74, Number 6 March 19, 1970
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atom production occur by a displacement by hot H atoms or by photolysis of DI formed by disproportionation (CeD13 I -+ COD12 DI)? (5) Are there conditions under which 3 h I P - h ~will trap H atoms as they are trapped in 3MP-d14? (6) What is the nature of the inhibiting action of isobutene on reactions initiated by photolysis of HI in hydrocarbon matrices? (7) Are H atoms ever photolytically produced in the walls of the quartz sample container? I n addition to investigation of these questions, the fact that radiolysis of 3MP-dl4 does not produce D atoms has been further confirmed.
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Experimental Methods The methods used for preparing purified degassed samples of H I in hydrocarbons and for examining them with a Varian 4500 X-band esr spectrometer from 10 to 77°K during and after photolysis with a medium pressure mercury lamp have been d e ~ c r i b e d . ~ -In ~ the present work photolysis was also carried out in the 4-10" I< range by mounting the esr sample tube in a doublewalled quartz tube, 11 mm o.d., 5 mm i d . , and 55 cm long, inserted through the sample opening of the Varian 4531 multipurpose esr cavity. The annulus of the tube was evacuated. The portion of the tube below the cavity passed through a tight-fitting 10-cm thick polyurethane stopper into a 2-1. dewar of liquid helium mounted in a Styrofoam box filled with liquid nitrogen. Controlled application of gaseous helium pressure to the surface of the liquid helium, by means of a second tube through the stopper, caused liquid helium to rise in the dewar tube sufficiently to maintain the sample a t 4" during photolysis. With less pressure, a flow of evaporating helium such as to provide sample tempem tures in the 4-10°K range was maintained. Measurements of rapid growth and decay of H atoms were made by setting the magnetic field of the esr spectrometer at the value of one of the lines of the hydrogen doublet and allowing the chart recorder to plot the change in concentration as a function of time continuously. When tubes of liquid 3MP are immersed in liquid nitrogen at 77"K, the 3RIIP-hl4 or 3X1P-d~changes to a glass with a viscosity6 of about 1012.5 P. Usually this glass is uncracked but cracks when the temperature is subsequently lowered. Such cracking is often accompanied by fracturing of the quartz sample tube. The fracturing can usually be avoided if cracking of the matrix glass is induced at 77°K by repeated partial melting and refreezing, before lowering the temperature further. Tritiated 3MP [3&!tP-d14 (3H) or 3pVIP-I214 ( 3H)] was prepared6 with specific activities of about 14 Ci g-' by gently agitating 1-ml samples of degassed 3MP with 10 Ci of Tzgas in the presence of 0.5 g of pre-degassed Raney nickel at 60" for 48 hr. Essentially all of the tritium entered organic combination. Esr sample The Journal of Physical Chemistry
A. LONGAND JOHN E. WILLARD
tubes containing the high specific activity 3R9P were safely handled outside the tritium hood during esr examination by encasing them in 4 mm o.d., 3 mm i.d. Kel-F tubing turned from 0.25-in. 0.d. tubing. The Kel-F was sealed at each end by forcing it into the closed end of a glass test tube, while the latter was gently heated in a gas flame. The Kel-F is sufficiently transparent to radiation from the mercury lamp at wavelengths absorbed by the H I so that the rate of growth of the trapped H atom signal from the photolysis of HI-3;\IIP-d14samples in such tubes is 30% or more of the rate without the Kel-E'. The 3MP-dl4, obtained from Merck Sharp and Dohme, showed an optical density at 200 nm of ca. 0.2 for a 5-mm thickness of a degassed sample. After passage through a silica gel column, its optical density was less than 0.05. Such treatment did not change the properties with respect to trapping H atoms.
Results Yields of Trapped H atoms, D Atoms, and Radicals. The quantum yields of trapped H atoms and of 3MPd13 radicals produced during the first few seconds of photolysis of 1 X lop3 mole fraction (mf) of H I in 3hIP-d14at 20,30, and4O"K are unaffected by temperature (within the experimental error of &lo%). This is shown by measurement of esr signal heights, corrected for the inverse proportionality of the esr sensitivity to temperature. This implies that the ratio of prompt geminate recombination of H and I, to abstraction of D by hot H atoms, to thermalization H followed by trapping, is independent of temperature over this range. The areas under the integrated first derivative signals for H and 3MP-d13radicals after a few seconds irradiation indicated a quantum yield for trapped H production of about 0.03, assuming that the quantum yield of the 3NIP-d13radicals is O.2.'l8 If the quantum yield of radical production is 0.2, the rate of light absorption in our experiments without filters was about 10'4 photons sec-l in the volume of the sample which was in the sensitive portion of the esr tube (Le., ca. 0.1 ml). The rate of consumption of HI was about 0.3%/min of illumination. The initial rates of trapped H atom production are proportional to the incident light intensity (as varied by varying the position of the lamp and as determined from the rate of radical production), as expected. For illuminations of longer than a few seconds at 20-40"1
