GAS-PHASE PHOTOLYSIS
OF
METHYLCYCLOPROPANE AT 1470 AND 1236 d
3219
Gas-Phase Photolysis of Methylcyclopropane at 1470 and 1236 A by Richard D. Doepker Department of Chemistry, University of Miami, Coral Gables, Florida 88124 (Received January 20, 1969)
The photolysis of methyl cyclopropane has been investigated at 1470 and 1236 A in the presence and absence of nitric oxide and oxygen. In addition, a series of experiments was carried out in which HzS was used as a freeradical interceptor, and where Ne was used to increase collisional deactivation. Three primary processes have been postulated, where (C4HB)* represents an internally excited species and [CdHs] a diradical with excess
*
/ \
CHZ-CHZ
-+
[CH&H(CHz)z]
*
(3)
*
internal energy. Although postulation of the [C4Hs] diradical is primarily based on the formation of C2HSand C3H7 radicals, it appears that nearly 90% of the products reported originate through fragmentation of the diradical. Acetylene, allene,and hydrogen decrease with increasing methyl cyclopropane pressure, which is consistent with a secondary decomposition of fragment molecules originating from the diradical.
Introduction Although there have been numerous studies carried out on the vacuum ultraviolet photolysis of hydrocarbons,’>Zthe simple cyclic compounds have received far less attention. Studies in the vacuum ultraviolet region below ionization energy were carried out on cyclopropane,aJ cyclobutane,6 cyclopentane,6and cyclohexane.I-lO With the exception of thermal studies such as Chesick’sll and indirect photochemical studies, of which Cvetanonic and Doyle’s12work is an example, methylcyclopropane has not been investigated. Recently, Scala and Ausloos, studying the photolysis of cy~lopropane,~ postulated the trimethylene diradical as the major source of product formation. Further, Ausloos, Rebbert, and Liaslo reexamined the photolysis of cyclohexane and concluded that C-C cleavage of the.. ring followed by rearrangement of the resulting diradical was the major source of methyl radicals. It is of interest to examine the alkyl-substituted cyclo compounds in an attempt to evaluate the importance of diradical formation, as well as the role of the internally excited fragment molecules on the formation of products. The “simplest” of these systems is methylcyclopropane (MCP).
Experimental Section Irradiation. The light source was an L-shaped, aircooled, electrodeless discharge lamp containing either krypton or xenon a t a pressure of approximately 0.5 Torr. It was operated from a 10-100 W microwave power generator at 2450 MHz (Raytheon Model PGM10x1). A 1-2 mm (thickness) LiF window was attached to the xenon lamp with Torr-Seal (Varian Asso-
ciates). A l-mm CaFz window was used with th,e krypton resonance lamp to eliminate the 1165-A resonance line.13 The cold fingers of the lamps were maintained at - 160” for the xenon and - 195” for the krypton in order to remove water vapor,14 the presence of which would lead to a rather strong emission at wavelengths above 1500 h;. The reaction vessel was either a 1-1. spherical Pyrex bulb or a 155-cc Pyrex cylinder attached directly to the lamp through a standard taper by means of Dow Corning high-vacuum silicone grease. In order to estimate the “relative quantum yields” of ethylene with increasing methylcyclopropane pressure (see Results), irradiations were carried out as reproducibly as possible. The discharge was kept low with constant microwave power while irradiation times were (1) J. R.McNesby and H. Okabe, Advan. Photochem., 3, 157 (1964). (2) J. R. MoNesby, Actions Chim. Biol. Radiations. 9, 36 (1966). (3) C. L. Currie, S.J., H. Okabe, and J. R. McNesby, J. Phys. Chem., 67, 1494 (1963). (4) A,A. Scala and P. Ausloos, J . Chem. Phys., 49,2282 (1968). (6) R.D. Doepker and P. Ausloos, ibid., 43,3814 (1965). (6) R. D. Doepker, S. G. Lias, and P. Ausloos, ibid., 46, 4340 (1967). (7) R.D.Doepker and P. Ausloos, ibid., 42,3746 (1965). ( 8 ) A. Gossauer, 2.Natqforsch., 20a, 594 (1966). (9) R.R.Henta and S. J. Raad, J . Phys. Chem., 71,4096 (1967). (10) P.Ausloos, R.E. Rebbert, and S. G. Lias, ibid., 72,3904 (1968). (11) J. P.Chesick, J . Amer. Chem. SOC.,82,3277 (1960). (12) R. J. Cvetanovic and L. C. Doyle, J . Chem. Phys., 37, 643 (1962). (13) A.H.Laufer, J. A. Pirog, and J. R. McNesby, J . Opt. SOC.Amer., 55,64 (1965). (14) H.Okabe, ibid., 54,478 (1964).
Volume 73, Number 10 October 1969
RICHARD D. DOEPKER
3220 Table I: Effect of NO and Oz on Photolysis of Methylcyclopropane" a t 1470 and 1236 d
_---------
Additive
NO
-~---None
11 46 100 31 109 12 9.0 31 25 a
Oa
(&r"r)
1470
ndd 46 100 0.6 119
A-
(2)
7
4.0 39 100 0.6 98
