Robert B. Shortridge,Jr.,
1936
and Edward K. C.
Lee
Photochemistry of Cyclohexanone. I I Second I
Robert 6.Shortridge, Jr.,* and Edward K. C. Lee* Department of Chemisfry, University of California, Ifvine, California 92664
(Received January 15, 1972)
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The results of the vacuum ultraviolet photolyses (174.5 and 193.1 nm) of cyclohexanone involving the *(act* n) and ~ ( U C O * n) transitions are reported. Substantial product yields of C2&, C s H ~ and , c-CsH6 produced a t low pressures are indicative of new photodecomposition channels which become available at high energies, because these C2 and Cs products Rad not been observed previously (or in insignificant amounts if observed) in photolyses at A,, 2253.7 nm where the 3(a* n ) transition is involved. For the mixture of 1.0 Torr of cyclohexanone and 1.8 Torr of 0 2 , the observed C2, Cs, and CS hydrocarbon product ratios are 1.0:0.3:1.0 a t A,, 174.5 nm whereas they are 0.4:O.l:l.O at A,, 193.1 nm. A mechanism involving trimethylene and acyltrimethylene biradicals as well as acylpentamethyiene biradicals (via a-cleavage) is invoked in order to explain the observed product distribution from the short wavelength photodecomposition. Furthermore, in the photolysis of cyclohexanone-2-t an excessive amount of C2H4 is produced indicating possible importance of the P-cleavage process by which the triz C H ~ - C ( = O ) - ~ H Tor CK-CHT-C(=O)-CH2 (presumably “singlet” species) tium-labeled biradical competes for ring closure and pressure stabilization us. its unimolecutar decomposition. The primary dissociating precursors are postulated to be the vibrationally hot SO* and SI* states produced uia rapid internal conversion of the S3 and Sz states. +-
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Introduction The gas-phase photochemistry of cyclohexanone excited in its T* 9- n absorption region has been studied by numerous research groups in the past 40 years,3-8 and it has been reviewed extensively. Two main photoproducts formed in the (na*)state photochemistry of cyclohexanone are (1) decarbonylation l o give I-pentene or cyclopentane plus CQ and (2) photoisomerization to 5-hexenal involving an intramolecular H atom migration
CH(S:) + -+ 5-hexenel (2) For cyclohexanone, three other high-energy absorption n, ace* n, and a* bands corresponding to the gco* T electronic transitions are known.10 A study of photolysis in i t s fourth excited singlet state (a* T ) , involving short vacuum uv excitation A,, 5 147.0 nm, has been reported recently.11 While the above study was in progress, we initiated a study of the photochemical kinetics of cyclohexanone excited to its second and third excited states, GCO* n a t A,, 195 nm and act* n at A,, 175 nm. The present paper deals with this long vacuum uv photolysis of cyclohexanone in the gas phase, with the hope of determining the primary photoproduct distributions and the changes in the primary physical processes effected by the high photoexcitation energy. In analogy with the long vacuum uv phGtslysis studies12J3 of cyclobutanone, we expected to observe higher decomposition/ stabilization (D/S) ratios and to observe new photodecomposition channels.
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Experimental Section Chemicals. Cyclohexanone ( aker and Adamson) was used after drying with Drierite (CaSO4) under vacuum, and gas chromatographic analysis showed no detectable impurity. Approximately 1 ml of this cyclohexanone was The Journal of Physical Chemistry, Vol. 77, No. 78, 5973
tritium labeled at the CY position, as described elsewhere.14 The specific activity of this sample was determined to be -2 Ci/mol. The propylene and one of the samples of propane used in this study were of Phillips Pe troleum’s Research Grade. The second sample of propane puwas of Instrumental Grade from Matheson (99% rity). Dry oxygen of 99.5% purity (USP Grade) from the National Compressed Gas Co. and Air Products was used. Dry,oil-free helium, Ki-Pure nitrogen (99.96% by volume) from Liquid Carbonic, and ultrahigh (99.97%) purity methane (Matheson) were also used in this study. Vacuum and Photochemical Apparatus. Most of the samples were handled on the grease-free and mercury-free combination glass and metal vacuum line which was described previousXy.lb Additionally, a new vacuum line was used to fill samples requiring very low pressures of ketone and radical scavengers. It employed 4-mm in-line glass Teflon valves (Fisher and Porter) in order to minimize the absorption of the ketones. Pressures could be accurately measured using a capacitance manometer (MKS Baratron Type 77 electronic pressure meter with 77H-30 pressure head). A 82.9-m1 cell with two Supracil quartz windows (2 in. a d . ) and a Teflon needle valve (4 mm, Fischer and Porter), and a 479-m1 cell with two Suprasil windows and a Teflon valve were used. The absorption spectra of the l(rrco* *- n) and I ( U C C * n) electronic transitions of cyclohexanone, which were obtained by Udvarhazi and EE-Sayed,lOa indicate that the carbon resonance line (193.1 nm) should efficiently cause excitation to the ~ ( c r ~ : O * n) state, while the nitrogen resonance lines (174.5 and 174.3 nm) should effect excitation to the P(aCC* n) state, Accordingly carbon and nitrogen resonance flow lamps were constructed similar to those described by Davis and Braun.16 The desired radiation was produced by allowing the mixtures of either 1% CH4 or 1% Nz in helium to flow continuously through a lamp that was positioned in the waveguide of a Raytheon microwave power generator (Model PGM-10, 85 W of con-
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1937
Photochemistry of Cyclohexanone
tinuous wave power output at 2450 MHz with a UG 435A/U waveguide flange accessory). A 0.3-m McPherson vacuum ultraviolet monochromator was used to check the spectral purity of the carbon and nitrogen atom resonance lamps at the 200-nm region. The spectral purity of the longer wavelength region was also checked, and revealed some impurity radiation in the 220-300-nm region with both lamps. The intensities of both lamps are rated in the l O I 5 photon/sec region.16 In the operation of the carbon resonance lamp, the photolysis cell was typically placed about 7 cm from the Suprasil window of the lamp with a convex silica lens of 100-mm focal length placed in the light path immediately in front of the cell window. This arrangement effectively filtered out the 165.7-nm resonance line and passed only the 193.1-nm line. However, no such spacing was maintained during the operation of the nitrogen resonance lamp, inasmuch as the 174.5-nm resonance line is the only radiation produced between the Suprasil cutoff (- 165 wm) and the X
