A. K. DHINGRA AND R. D. KOOB
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at present be deferred, it is possible to speculate as to the identity of the second temperature-dependent deactivation process. Since the quenching substituents of indole for the series of derivatives considered here correspond to parent compounds which are effective as electron scavengers, and since electron scavengers as a class are effective quenchers of the fluorescence of indole der i v a t i v e ~ ,one ~ possible mechanism for intramolecular quenching is electron capture by the quenching group, perhaps involving direct contact of the side chain with the indole ring. The substitution of DIO for HzO suppresses the primary quenching process to a significant extent. This is responsible for the large isotope effect observed for indole and probably for those indole derivatives in which extensive intramolecular quenching does not occur. In addition, the presence of a charged a-amino group in immediate proximity to the indole ring appears to enhance the isotope effect. This is not the case if the amino group is separated from the indole ring by one or more residues, as in gly-trp. One possible explanation is that the charged a-amino group serves as a proton donor and that proton quenching of the excited indole is
an important factor in such cases. Apart from this, the intramolecular quenching processes for this series of derivatives do not appear to show much isotopic dependence. Consequently, the magnitude of the isotope effect generally decreases with decreasing quantum yield. Finally, the persistence of intramolecular quenching, although to a diminished extent, in nonpolar media deserves comment. If model 2 is correct, it would be expected that the primary quenching process would be largely suppressed under these conditions, while the strictly internal quenching may persist, although modified by the different medium.21
Acknowledgment. The authors recognize the able technical assistance of Mr. Theodore Lutins, Mr. Richard Kolinski, and Mr. Ross Bolger. We also thank Dr. Rufus Lumry and Dr. Gary Pool for some very helpful discussions. This work was partially supported by ONR Grant KO. NR108-815. (21) NOTEADDEDIN PROOF. The values cited in Tables I, 11, and 111 are baaed upon an assumed value of 0.14 for Q26 for tryptophan in H z O . ' ~ - ' ~ Should this value be revised, the values of Q,(YO, and a1 would be altered, but not El.
Methylene Produced by Vacuum-Ultraviolet Photolysis. 11. Propane and Cyclopropane by A. K. Dhingra and R. D. Koob Department of Chemietry, North Dakota State University, Fargo, North Dakota
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(Received June 89, 1970)
Propane, argon, and oxygen are used as additives to investigate the reactions of methylene produced by the vacuum-ultraviolet photolysis of propane (123.6 nm and 147.0 nm), cyclopropane, and cis- and truns-1,Z dimethylcyclopropane (-165 nm). With the exception of propane at 147.0 nm, insertion of CHz(lAJ into propane to yield butanes accounts for at least 60% of the total methylene yield. Similarly, with the above exception, no CH@,-) appears to arise from the primary photodecomposition of the source molecule. The relative yield of insertion product obtained in the photolysis of propane shows a definite wavelength dependence. However, the relative rate of reaction of methylene with argon in competition with propane is wavelength independent. The rate of reaction of methylene with argon relative to propane found for our systems is similar to that found in other steady-state systems, but is different by an order of magnitude from recent flash photolysis results. Contrary t o a suggestion of other workers, it does not appear necessary to postulate a trimethylene diradical in the primary process in the photolysis of cyclopropane.
Introduction Recently, we have reported studies of methylene produced by the vacuum-ultraviolet photolysis of propane a t 123.6 nm.' We noted at that time the similarity between the reactions of methylene produced from this source and methylene produced from the more conThe Journal of Phgsical Chemistry, Vol. 74, No. 96,I970
ventional sources, ketene and diazomethane. These studies have since been extended to Propane at 147 nm and cyclopropane at approximately 165 nm. The results of these studies are presented here. (1)
R. D. Koob, J . ~ h y 8 Chem., . 73, 3168 (1969).
METHYLENE PRODUCED BY VACUUM-ULTRAVIOLET PHOTOLYSIS Experimental Section Materials. Propane and cyclopropane were obtained from Air Products and Chemicals, Inc. Propane was research grade. cis- and trans-1,2-dimethylcyclopropane were obtained from Chemical Samples Co. and were used without further purification. Glc examination of these substituted cyclopropanes showed no butane or butene impurities. Both propane and cyclopropane were purified by gas chromatography until impurity levels were below 10 ppm. For cyclopropane this required at least two successive purification cycles. The hydrocarbons were then dried over Drierite and vacuum distilled to storage bulbs. Argon used was Air Products Ultra High Purity grade. Oxygen was Linde CP. Both were used without further purification. Lumps and Cells. Rare gas resonance lamps, similar to those described by Ausloos and Lias,2 were used for the photolysis. All lamps were filled on a mercury-free vacuum line capable of achieving pressures less than 1 X Torr (Veeco discharge gauge). For the propane photolysis, lamps were gettered with titanium gettering assemblies and were greater than 98% chromatically pure in the region between 105 and 200 nm (AlcPherson 0.3-m vacuum monochromator). LiF windows were used for both krypton and xenon lamps. For the cyclopropane work, a water impurity was intentionally left in a krypton filled lamp, This lamp gave an intense water emission spectrum, Figure 1. Two lamp-cell configurations were used to study propane photolysis. The first was a “T” shaped lamp with windows at each end of the crossbar. The win-
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Wavelellgth A
Figure 1. The many-line spectrum is the water emission spectrum in this region. The absorption spectrum of propane begins around 162 nm. The absorption spectrum of cyclopropane begins near 171 nm. The overlay shows the importance of the water 165-nm line when cyclopropane is photolyzed through a propane filter.
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dows looked into individual sample cells. After a short break-in period, the ratio of light intensity entering each of the two cells was constant. Thus, one cell with constant sample conditions was used as an external standard to which runs ma.de in the other cell could be compared. The second lamp-cell configuration consisted of an LiF window clamped between two O-ring joints and sealed vacuum tight. One such joint formed the discharge area of the lamp and the second was attached to a stopcock and served as a sample cell. This configuration was used for sample pressures greater than 1 atm. Propane-cyclopropane mixtures were irradiated in a two-compartment cell. The first compartment had a path length of approximately 1.5 cm and was filled with 200 Torr of propane. The second compartment was filled with a propane-cyclopropane mixture. Such an arrangement assured that only the cyclopropane component of the mixture was actually undergoing photolysis. A “water” lamp was used in these experiments. The nature of the light absorbed by the cyclopropane can be deduced from Figure 1. Here the absorption spectra of propane and cyclopropane overlay the emission spectrum of the “water” lamp. Only those wavelengths which lie between the onset of the cyclopropane absorption and the onset of the propane absorption contribute to the photolysis. Oxygen was added to all reaction mixtures in amounts equal to 10% of the total hydrocarbon pressure. The oxygen is intended to serve as a free radical scavenger. Absence of products in the five and six carbon range indicate that it is performing this function. In all experiments, photolysis was carried to less than 0.1% conversion of parent to product. All analyses were done by gas chromatography (FID) on a 20-ft, 20% (w/w) squalane column maintained at room temperature. Results Table I lists the observed isobutane t o normal butane ratio for all systems examined. The values of this ratio obtained by Halberstadt and hlcNesby3 in a propane-ketene-oxygen system and by Johnson, Hase, and Simons4 in a propane-diazomethane-oxygen system are also included. Within experimental error these values are equal. Correcting for the number of hydrogens of each type in propane we obtain 3k2/kl = 1.2. (Reactions 1 and 2 are found in the Discussion section below). Figure 2 is a plot of the product ratio [CzHs]/[C4HIO] vs. the reactant mixture ratio (Ar)/(CIH8). Experimentally, these numbers were obtained in two ways: (2) P. Ausloos and 9 . G. Lias, Radiat. Res. Rev., 1, 75 (1968). (3) M. L. Halberstadt and J. R. McNesby, J . A m e r . Chem. Soc., 89, 3417 (1967). (4) R. L. Johnson, W. L. Hase, and J. W. Simons, J . Chem. Phys., 52, 3911 (1970).
The Journal of Physical Chemistry, Vol. 74, N o . 26, 1970
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A. K. DHINGRA AND R. D. KOOB
Table I : Relative Rates of Insertion of Methylene into Primary and Secondary Bonds of Propane as a Function of the Source of the Methylene
