EDWARD K. C.LEE
2804
Benzene Photosensitization of Cyclopentanone and Cyclopentanone-2-t
by Edward K. C. Lee Department of Chemistry, University of California, Irvim, California (Received January 16, 1967)
Two primary energy-transfer processes have been found in the benzene photosensitization of cyclopentanone at 2537 A. Singlet energy transfer produces an excited cyclopentanone which decomposes to give CO and two CzH4 or CO and c-C4Hswith a quantum efficiency of 0.45 f 0.09 and has an effective quenching cross section of 1.6 A2. On the average, the excited singlet benzenes transfer about 15 kcal/mole of energy into vibrational excitation modes of the cyclopentanones in addition to about 84 kcal/mole of electronic energy required for the process. Triplet energy transfer produces an excited cyclopentanone (triplet) which rearranges to give 4-pentenal with a quantum efficiency of 0.40 f 0.08, and it is speculated that triplet benzene probably has a lifetime longer than 0.3 psec. I n the benzene-cyclopentanone system, small amounts of methylcyclopropane, l-butene, propylene, and ethylene are produced by the benzene photosensitization of the 4-pentenal which is the product of the primary photosensitization. Distribution of tritium radioactivity among various photochemical products has been measured to clarify the mechanism of decomposition.
Introduction Photochemically excited cyclopentanone decomposes in the gas phase to yield primarily carbon monoxide, ethylene, and cyc1obutane.l A mechanism involving a diradical intermediate has been suggested to account for these decomposition products.’J Under certain experimental conditions, however, 4-pentenal was found to be an important rearrangement product13 and the yield of 4-pentenal increased with the increasing pressure of inert gases used. This result and the lack of sensitivity to the presence of 0 2 have been interpreted to mean that the photochemical intermediate is a vibrationally excited sinqlet c y c l ~ p e n t a n o n e . ~In~ ~the liquid-phase photolysis, 4-pentenal is the predominant product, and it has been shown to arise primarily through a triplet mechanism.6 Recently, gas-phase photochemical decomposition studies with trans-2,3dimethylcyclopentanone6 and cis- and trans-2,6-dimethylcyclohexanone7 have shown nonstereospecific formation of cis- and trans-1,2-dimethylcyclobutane and cis- and trans-1,2-dimethylcyclopentane,respectively, involving diradical intermediates. Fluorescence and phosphorescence emissions from cyclopentanone have been observed in the condensed phase,s8 but fluorescence emission alone was observed The Journal, of Physical Chemistry
in the gas phase.8b Therefore, decomposition mechanism involving a triplet intermediate in the liquidphase photolysis5 is not inconsistent with the above spectroscopic observation, although that in the gasphase photolysis is q u e s t i ~ n a b l e . ~I,n~ the work reported here, an attempt is made to show evidence for the role of a triplet cyclopentanone intermediate. The well-studied benzene photosensitization t e c h n i q ~ e ~ ~ ’ ~ has been used in promoting cyclopentanone molecules (1) S. W. Benson and G. B. Kistiakowsky, J . Am. Chem. SOC.,64, 80 (1942). (2) F. E.Blacet and A. Miller, ibid., 79, 4327 (1957). (3) (a) R. Srinivasan,ibid., 81,1546(1959); (b) ibid., 83,4344(1961); (c) ibid., 83, 4348 (1961). (4) For review see R. Srinivasan, Advan. Photochem., 1, 83 (1963); J. G. Calvert and J. N. Pitts, Jr., “Photochemistry,” John Wiley 1966,Chapter 5. and Sons, Inc., New York, N. Y., (5) P. Dunion and C. N. Trumbore, J. Am. C h m . SOC.,87, 4211 (1965). (6) H.M. Frey, Chem. Ind. (London), 947 (1966). (7) R. L. Alumbaugh, G. 0. Pritchard, and B. Rickborn, J. Phya. Chem., 69, 3225 (1965). (8) (a) S. R. La Paglia and B. C. Roquitte, ibid., 66,1739 (1962): (b) Can. J . Chem., 41, 287 (1963). (9) H.Ishikawa and W. A. Noyes, Jr., J . Am. Chem. SOC.,84, 1502 (1962); J . Chem. Phys., 37, 583 (1962). (10) W. A. Noyes, Jr., and I. Unger, Advan. Photochem., 4, 49 (1966).
BENZENE PHOTOSENSITIZATION
OF
CYCLOPENTANONE AND CYCLOPENTANONE-2-l
to an excited singlet or triplet state in the gas phase, since both singlet-singlet and triplet-triplet energytransfer processes are exoergic. The over-all kinetic behavior of these two distinct cyclopentanone intermediates has been compared with respect to pressure dependence of yield, quantum efficiency, and deactivation process. Some aspects of intersystem crossover of the excited singlet benzene is discussed below. The use of cyclopentanone-2-t as a tracer molecule helped to identify the important secondary photolysis products in the system. Experimental Section A spinning band column was used to distill cyclopentanone (CP) obtained from Chemical Procurement Laboratories, Inc., and the middle third fraction, bp 129", was collected. The purity was checked by gas chromatographic analysis using a thermal conductivity detector and no impurity was detected; the purity was greater than 99.8% at the limit of detection (-0.2%) of impurity with the instrument used. 4-Pentenal (PA) was prepared h y the photolysis of cyclopentanone in the liquid phase3a and it was purified by preparative gas chromatography. The purity as checked by gas chromatography was -99%. Tracer quantity of cyclopentanone-2-t (CPT) was prepared by an isotopic-exchange technique similar to that used for the preparation of cyclopentanone2,2,5,5-d4." A mixture of 1.56 g of the purified cyclopentanone, 0.100 mg of tritiated water (10 mcurie of HTO obtained from New England Nuclear Corp.), and 0.100 ml of the aqueous solution 0.1 M in NaC2H302and 10% in NaCl was degassed and sealed in a glass tube in vacuo. Following an equilibration for 2 days at 100' , the contents were combined with 6 ml of dry diethjl ether. The exchange labeled cyclopentanone fraction was collected a t 128-129" and was repeatedly dried over Drierite (CaS04) in ~(zcuo. A residual amount of diethyl ether in the sample was pumped off on a vacuum line a t -55". The chemical purity check by gas chromatography showed 1 .O% of ether as thi: oiily impurity in the exchange labeled cyclopentanone. The radiochemical purity check was made by photolyzing the cyclopentanone-2-t in the gas phase and analyzing the products by radio gas chromatography.12 Cpon photolyses at 3130 and 2537 A the impure preparation gave trace amounts of tritium-labeled propane, n-butane, and acetylene in addition to ethylene and cyclobutane. The first three products listed, not to be found in the photolysis of pure cyclopentanone, gradually diminished in photochemical yields as successive stages of purification proceeded. The finally purified cyclopentanone-2-t had
2805
a specific activity of 0.15 mcurie/mmole. In a few runs cyclopentanone-2-t, with a specific activity of 0.03 mcurie/mmole, was used. Zone refined grade benzene, "99.999Y0 pure," obtained from Litton Chemicals and Baker Analyzed reagent grade benzene were tried after degassing. They showed no detectable difference in photosensitization and fluorescence. In all the runs reported here, only zone refined grade benzene was used. Ethylene, propane, cis-2-butene, and trans-2-butene were of Phillips Research grade, and they were used after several freeze-pump-thaw cycles had been employed for their purification. Samples were handled on a mercury-free and greasefree glass vacuum line equipped with Viton-A diaphragm valves. Two photolysis cells were used: an 89.7ml cylindrical quartz cell (GQ) with two flat end windows 50 mm in 0.d. and 50 mm in length with a small freezing tip a t the bottom and a 528-m1 cylindrical Vycor cell (FV) with a window blown flat at one end 50 mm in 0.d. and 300 mm in length. A T-shaped quartz fluorescence cell with a 25-mm long side arm was 65 mm long and equipped with three Suprasil flat end windows 25 mm in diameter. All the photolysis runs were made at room temperature (23-25") with two photochemical lamps. The most frequently used lamp was a helical quartz mercury resonance lamp (Hanovia SC-2537) operated with a current of 27 ma for 2537-A photolysis. A Corning CS-7-54 filter cut out radiation at wavelengths below 2300 A. A Bausch and Lomb high-intensity grating monochromator (Model No. 5 with a 200-w mercury arc lamp, SP-200) was used for 3150-A photolysis. For the fluorescence measurement, an unfiltered and collimated beam 13 mm in diameter from a small mercury resonance lamp (Pen-Ray Model 11, SC-le from Ultra-Violet Products, Inc.) was used as an excitation source. The fluorescence emission from benzene was scanned manually with a Bausch and Lomb monochromator and its intensity was monitored with an RCA 1P28 photomultiplier and a recording electrometer.13 The dark current was lower than 4 X 10-9 amp under typical operating conditions. Pressure was measured to an accuracy of k 0 . 3 mm with a Wallace-Tiernan gauge on the vacuum line. Usually a metered amount of gas in a calibrated volume (11.7 ml) was frozen into the sample cell and the (11) A. Streitwieser, Jr., R. H. Jagow, R. C. Fahey, and 5. Suzuki.
J. Am. Chem. SOC.,80, 2326 (1958). (12) J. K.Lee, E. K. C. Lee, B. Musgrave, Y-N. Tang, J. W. Root, and F. S. Rowlsnd, Anal. Chem., 34, 741 (1962). (13) H.V. Malmstadt, R. hl. Barnes, and P. A. Rodriguez, J. Chem. Educ., 41, 263 (1964).
Volume 7 1 , Xumber 9 August 1967
EDWARD K. C. LEE
2806
~
Table I : Hydrocarbon Product Distribution vs. Cyclopentanone-24 Pressurea
Run no.6
A-440 A-437 A-434 A-435 A-438 B-329 A-439 A-442 B-324 B-331d C-321 C-3 17 C-319 C-318
Pressure of CPT, mm
CaHaT, x 10'
0.3 0.4 0.5 0.7 1.0 1.3 1.5 2.0 2.0 3.0 3.0 4.0 4.4 5.0
11.1 13.9 13.7 16.1 28.0 27.4 36.6 37.9 36.6 45.1 54.8 62.2 67.4 70.4
Observed radioactivity, countc-C~HIT, Methylcyclo-
x
10'
6.9 8.6 8.7 10.0 17.8 17.4 23.2 23.6 23.2 28.7 35.5 40.5 43.3 45.5
propanst
576 f 40 679 f 40 501 f 40 452 f 40 376 f 40 203 f 30 298 f 30 133 f 30 94 =t30 28 f 30 79 f 30 47 f 30 21 f 30 0 f 30
DirectC yield, x 10'
0.4 0.6 0.7 1.0 1.5 1.6 2.2 3.0 2.5 3.8 5.0 6.7 7.4 8.4
+
(ClHaT c-CIHIT Sensitised yield, x 10'
17.6 21.9 21.7 25.1 44.3 43.2 57.6 58.5 57.3 70.0 85.3 96.0 103.3 107.5
QY
0.053 0.067 0.066 0.076 0.13 0.16 0.18 0.18 0.21 0.25 0.23 0.26 0.28 0.29
a The photolysis was carried out at benzene pressure of 2.5 mm in the quartz cell (GQ) for 3.00 min, using a 2537-A Hg resonance line. As monitored by the quantum counting method, 3.29 x 106, 2.77 x 106, and 3.70 X 106 radioactivity counts correspond to unit quantum yield for series A, B, and C, respectively. It was computed from the result of run no. B-334 (CPT = 5.0 mm, t = 3.0 min, CzHaT = 4145, and c-C4H7T = 2573), assuming that 7% of the incident radiation was absorbed by 2.5 mm of benzene in the sensitized runs. ciS-CdH8 (0.5 mm) was also present.
'
pressure of the gas in the cell was calculated from the known volume expansion factor. The content in the photolysis cell was frozen into a 10.7-ml sample loop a t liquid nitrogen temperature for subsequent hydrocarbon analysis. All hydrocarbon products were transferred into the loop with better than 99% recovery efficiency, since nonradioactive C3Hs (10 mm in 10.7 ml volume) was commonly added as a carrier gas for the trace amount of photolysis products. The gas sample loop was connected to the gas chromatographic carrier stream and then the sample was injected. For 4-pentenal and cyclopentanone analysis, the content ip the photolysis cell was frozen into a small capillary tubing at liquid nitrogen temperature and about 10-pl of liquid 2-methylpentane was added to the content as a carrier. The sample liquid was then withdrawn with a 10-pl syringe for an injection into a gas chromatographic column. The tritium-labeled hydrocarbon products were analyzed by conventional radio gas chromatography12 and the macroscopic product yield was determined with a thermistor detector by thermal conductivity measurement. The separation of hydrocarbon products was made at 23" on a 60-ft long or a 22-ft long and 0.25-in. 0.d. column packed with 30-50 mesh Chromosorb P (HNDS) coated with 35% by weight of dimethylsulfolane. The separation and analysis of 4-pentenal and cyclopentanone were made on a gas chromatograph with a hot wire thermal conductivity detector using a The Journal of Physical Chemistry
15-ft long and 0.25-in. 0.d. column packed with 30-60 mesh Chromosorb G (DMCS AW) coated with 5.0% by weight of Carbowax 20M a t BO". The operational intensity of the mercury resonance lamp was measured by potassium ferrioxalate actinometry.l4 It was approximately 4 X 10l6 photons/ sec over an area approximately 12 cm2. The actual quantum counting of the excited benzene was monitored through the use of an actinometry technique developed by Cundall, et uZ.;lS the triplet benzene quantum yield is known to be 0.63 in the benzene-photosensitized isomerization of cis-2-C4H8to trans-2-C4H8. I n every series of runs, one or two quantum counting monitor runs were made with benzene pressure identical with that in the series of interest. The yield of truns-2C4H8was determined by gas chromatographic analysis using a 60-ft dimethylsulfolane column.
Results The hydrocarbon product distribution as a function of cyclopentanone-2-t pressure when the benzene pressure is at 2.5 mm is shown in Table I. The photochemical conversion was less than 3% in all of the runs. The yields of C2H,T and c-C4H,T monotonically increase with increasing CPT pressure, while the yield of (14) C. G. Hatchard and C. A. Parker, Proc. Roy. SOC.(London) A235, 518 (1956); A220, 104 (1953). (15) R.B. Cundall, F. J. Fletcher, and D. G. Milne, Trans. Faraday SOC.,60, 1146 (1964).
BENZENE PHOTOSENSITIZATION OF CYCLOPENTANONE AND CYCLOPENTANONE-2-1
2807
Table 11: Variation of 4Pentenal (PA) Yields with Cyclopentanone (CP) Pressure
Run no.”
Pressure of CP, mm
A-5 A-6 A-8 A-416 A-419 A-421 A-423 A- 1 A-4 B-9 B-10 B-12 B-15 B-17 B-18 B-19 B-21 C-38 c-39 C-41
0.05 0.07 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.5 0.7 0.9 1.2 I .5 1.7 2.0 2.5 3.0 4.0 5.0
Yield -Relative PA
0.20 0.22 0.27 0.23 0.26 0.33 0.36 0.40 0.24 0.20 0.29 0.25 0.20 0.30 0.31 0.31 0.13 2.08 1.70 1.64
yielda-
100 PA/
CP
3.18 4.10 5.70 6.92 7.78 12.1 15.4 19.5 10.7 3.62 6.59 7.34 6.74 12.3 15.8 18.9 10.6 43.3 43.1 65.3
(PA
+ CP)
5.9 5.1 4.5 3.2 3.2 2.6 2.3 2.0 2.2 5.2 4.2 3.3 2.9 2.4 1.9 1.6 1.2 4.6 3.8 2.4
of PA, mm
N C
of PA
0.0030 0.0036 0.0045 0.0048 0.0064 0.0065 0.0069 0.0070 0,0088 0.026 0.029 0.030 0.035 0.036 0.032 0.032 0.030 0.138 0.152 0.120
0.063 0.063 0.063 0.059 0.061 0.061 0.061 0.060 0.060 0.022 0.022 0.022 0.022 0.024 0.024 0.024 0.024 0.016 0.016 0.016
0.047 0.057 0.071 0.081 0.10 0.11 0.11 0.12 0.15 0.12 0.13 0.14 0.16 0.15 0.13 0.13 0.12 0.086 0.095 0.075
QY
Photolysis runs were 30 min long and with a 2537-A Hg resonance line. series A: P c ~ H =~ 0.50 mm and Vycor cell (FV); series Pc~B =~ 0.50 mm and quartz cell (GQ); series C: P c ~ = H 2.5 ~ mm and quartz cell (GQ). * Ratio of molar thermal conductivity responses of PA to C P is 1.03 f 0.03 and relative yields are calculated from the measured peak area and the thermal response factor. Product yield expressed in millimeters a t an assumed unit quantum efficiency as monitored by the quantum counting method.
B
=
methylcyclopropane-t decreases with increasing CPT pressure. It will be shown later that methycyclopropane-t is not a primary product but a secondary product. The observed yields of C2H3T and c-CdHVT have two origins: direct photolysis of CPT at 2537 A1-3J6 and benzene photosensitization of CPT at 2537 A. The direct photolysis yield can be computed from the experimental result of CPT photolysis in the absence of benzene and this calculated yield can be subtracted from the observed over-all yield in order to evaluate the sensitization yield. The yield of 4-pentenal (PA) as a function of C P pressure is shown in Table 11. The integrated area under the gas chromatographic peak has been used to evaluate the percentage of PA as 100 (PA)/(PA CP). The yield of PA was calculated in units of pressure by multiplying the percentage of PA with the initial pressure of CP. “N” represents the quantum counting of the singlet benzene produced in the system and it was also expressed in units of pressure for convenience. The quantum yield of PA was then simply calculated as the yield of PA divided by “N.” The direct photolysis yield of PA was so small in the pressure range studied3 that almost all of the observed yield is attributable to the sensitization yield. The over-all
+
accuracy of the quantum yield evaluation should be about 10-15%. The quantum yield of PA monotonically increases with increasing pressure of CP up t o about 1 mm and then the trend reverses. It is significant to note that the pressure dependence of the quantum yield of 4-pentenal is clearly different from that of ethylene and cyclobutane. The observed fluorescence emission of benzene in the gas phase a t 2.5 mm pressure showed the kind of highpressure intensity distribution that is reported in the literature.l’J8 Only relative emission intensities for the high-pressure spectrum were measured to illustrate the pressure-dependent fluorescence quenching by cyclopentanone. The results are shown in Table 111. I n series A, the benzene emission intensities which give rise to the photocurrent were recorded before and after the addition of a given pressure of cyclopentanone. I n series B, the benzene emission intensity was measured once in the beginning of the series and after each addition of cyclopentanone leading t o a higher pressure (16) R.F. Klemm, D. N. Morrison, P. Gilderson, and A. T. Blades, Can. J . Chem., 43, 1934 (1965).
(17) C.K. Ingold and C. L. Wilson, J . Chem. Soc., 941 (1936). (18) G. B. Kistiakowsky and C. S. Parmenter, J . Chem. Phys., 42, 2942 (1965).
Volume 71, Number 9 August 1967
2808
EDWARD K. C. LEE
Table I11 : Benzene Fluorescence Quenching by Cyclopentanone" Pressure of CP, mm
Table IV : Effect of CPentenal on the Benzene Photosensitization of Cyclopentanone-2-to Run no.
io,
10-0 amp
0 1.0 2.0 3.0 4.1 6.1 8.6
7.8 7.8 7.8 7.5 7.2 7.3 7.3
0 0.8 1.8 2.4 3.0 4.0 6.2 7.5
2.8
...
...
... ... ...
... ...
i, 10-0 amp
262
261
iO/i
Pressure, mm
Series A 7.8 5.8 4.7 3.8 3.2 2.2 1.8
(1.00) 1.34 f 0.06 1.66 f 0.08 1.96 i 0.09 2.25 f 0.11 3.32 i 0.16 4.06 i 0.19
Series B 2.8 2.3 1.7 1.7 1.4 1.2 1.0 0.8
(1.00) 1.2 1.6 1.6 2.0 2.3 2.9 3.5
All runs were made a t CeHe pressure of 2.5 mm using 2537-A excitation. The emission intensity was measured with a 13-mp band pass centered a t 280 mp, where peak emission occurs. Dark current has been subtracted: series A, dark current = 0.6 X 10-9 amp a t 640 v; series B, dark current = 3.5 X 10-9 amp a t 900 v.
CsHe CPT PA
0.50 0.10 ... 0.10 c-C4H7T,counts 6473 f 90 6080 f 90 c-CpHs, area counts 1110 f 100 1019 f: 100 Yields relative to cyclobutane = 1.00 1.8 1.8 CrHsT (2.9) (3.2) (C*Hdb Methylcyclopropane-t 0.23
