J . Phys. Chem. 1984, 88, 605-612
605
Infrared Multiple-Photon Decomposition of Cyclopentene James 0. Shoemaker and Robert W. Carr, Jr.* Department of Chemical Engineering and Materials Science, University of Minnesota. Minneapolis, Minnesota 55455 (Received: May 25, 1983)
The infrared multiple-photon decomposition of cyclopenteneyields cyclopentadieneas the major hydrocarbon product (>95%). The other products are 1,4-pentadieneand cis- and trans-l,3-pentadiene, along with trace amounts of fragmentation products. The 1,3-pentadienes have not been previously reported in cyclopentene pryrolysis. Typical conversions obtained by focussing the laser beam into the sample ranged from less than 1% to as much as 20% per lo3 pulses with estimated beam waist fluences of approximately 150 J/cm2, indicating that cyclopentene excitation is strongly bottlenecked. No products were detected with collimated beams of 1 J/cm-2 and after 1.7 X lo4 pulses. Reaction product yields increase with increasing wavelength in the fundamental absorption band centered at 1048 cm-'. Excitation originating in the Q branch of this transition results in very small yields. Total C5 yields increase rapidly with increasing fluence at 1029 and 1033 cm-'. A simple model of dissociation in which the focused beam geometry is a circular hyperboloid of revolution predicts fluence-yield behavior that is consistent with experimental observations. Conversion of cyclopentene decreases with increasing cyclopentene pressure between 0.01 and about 0.5 torr, and thereafter increases rapidly with increasing pressure. On the other hand, addition of N2,up to about 200 torr, causes total C5yields to decrease monotonically over the entire pressure range. A limited number of experiments were done with 1-methylcyclopentene. At the same wavelength (1019 cm-I), pressure, fluence, and weak field absorbance, dehydrogenation of 1-methylcyclopentene gives yields that are as much as 20 times greater than yields from cyclopentene.
Introduction High-power infrared lasers can be used to induce unimolecular reactions in polyatomic molecules by multiple-photon excitation to energies greater than the reaction threshold. Unimolecular processes that have been observed include bond dissociations, molecular eliminations, and isomerizations. In this paper we report a study of the infrared multiple-photon decomposition (IRMPD) of cyclopentene, which has not previously been reported. Cyclopentene is an excellent candidate for IRMPD studies. The thermal decomposition has been extensively studied, and the Arrhenius activation energy and preexponential factor are reliably known.' The major thermal reaction channel is unimolecular H2 elimination to yield cyclopentadiene.
Although cyclopentene is a relatively large organic molecule which might be expected to have a reasonably high reaction probability per pulse, it is also a relatively stiff molecule. The results suggest that, in spite of its size, cyclopentene requires large laser energies to obtain reasonable yields per pulse, and that pumping the vibrations corresponding to the reaction cooridinate provides no advantage. That vibrational state density is a strong influence on yield is corroborated by experiments with 1-methylcyclopentene. It was also of interest to see if higher energy reaction channels that are not competitive with H2 elimination by thermal excitation are accessible by infrared laser excitation. Multiple-channel reactions are well-known in IRMPD. The cis- and trans-1,3pentadienes found here have not been reported in any of the thermal work to date.
Experimental Section Cyclopentene has an infrared fundamental absorption band at 1048 cm-' and a weaker band at about 975 cm-', both of which can be excited by C 0 2laser lines. Furthermore, the 1048-cm-' band falls on a window in the cyclopentadiene infrared absorption spectrum. Detailed analyses of the cyclopentene vibrational spectrum are a ~ a i l a b l e ,showing ~,~ that the 1048-cm-' band corresponds to CH, motions that may lead directly to the transition state for 3,5 elimination of H,, the dominant thermal reaction path.' Although the purpose of the initial study was to characterize the reaction products and the effects of experimental parameters on the product yield, cyclopentene provides an opportunity to study some interesting features of IRMPD. The dominant products of infrared laser-induced reactions usually come from the lowest energy reaction channel of the ground electronic state. Thus cyclopentadiene and hydrogen are the expected products from cyclopentene. It was of interest to see whether pumping the CH2 motion of 1048 cm-' would facilitate hydrogen elimination. (1) (a) D. W. Vanas and W. D . Walters, J . Am. Chem. SOC.,70, 4035 (1948); (b) G. I. Mackay and R. E. March, Can. J . Chem., 48, 913 (1970); (c) D. A. Knecht, J . Am. Chem. Soc., 95, 7933 (1973); (d) K. 0. King, In?. J . Chem. Kine?., 10, 117 (1978). (2) L. M. Sverdlov and E. N. Krainov, Opt. Spectrosc. (USSR),6 , 214 . . (1959). (3) J. R. Villarreal, J. Laane, S. F. Bush, and W. C. Harris, Spectrochim. Acta, Part A , 35, 311 (1979).
The radiation from a Lumonics 101-2 grating tuned TEA pulsed CO, laser, operated multimode, was passed through a circular aperture of 1.99-cm2 area. The laser energy was calibrated with a Scientech 36-001 disk calorimeter and Model 362 power/energy meter. During experiments pulses were counted and integrated by electronics of our own design which are discussed el~ewhere.~ The fluence per pulse which passed through the aperture ranged from about 0.4 to about 1.0 J/cm2 for the P3*line of the 9-pm band at 1029 cm-', which was the primary line used to irradiate cyclopentene. Laser pulse energies in this paper are reported as fluences (J/cm2) before focusing. Fluences at the focus were estimated to be at least 100 J/cm2. The laser radiation was focused by a 40-cm focal length ZnSe lens through the center of a 2.5 cm X 30 cm Pyrex reaction cell with NaCl or KCl windows, and greaseless glass-teflon vacuum valves. The cells were placed such that the focal point of the lens was a t midpoint on the centerline of the cell. The beam diameter a t the windows was then about 0.5 cm. N o visible luminescence was observed with a dark-adapted eye either from the laser beam hitting the cell window or the cell volume itself in any experiment. Light absorption measurements were carried out on the evacuated cells and the lens itself to allow back-calculation of fluence entering the cell from measurements made of fluence leaving the cell. A conventional glass vacuum system with a mercury diffusion pump was used for sample preparation. Chemical Samples Co. - -- __. - - __ ___ -- (4) J 0 Shoemaker and R-W Farr,. Jr ,Re" Scr Insfrum, 53, 1086 (1 982)
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0022-3654/84/2088-0605$01.50/00 1984 American Chemical Society
606
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984
Shoemaker and Carr 0.1
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cyclopentene (99.9%) was used without further purification and degassed, as were all liquid samples, by a freeze-pump-thaw technique and then stored at room temperature under its own vapor. Vapor was withdrawn as needed. Chemical Samples Co. dicyclopentadiene (99%) was distilled to cycl~pentadiene.~Fresh distillate was always degassed and used at once. Aldrich Chemical Co. 1-methyl-1-cyclopentene(96%) was prepared, stored, and then used in the same manner as cyclopentene. Pressure in the prepared cell was measured with an MKS Baratron Type 222 0-10-torr pressure transducer with a digital display accurate to 1 mtorr. In experiments including a nitrogen bath gas a measured pressure of the cyclopentene was condensed into a cold finger and isolated. After evacuation the desired pressure of nitrogen was introduced. The system was reisolated and the cyclopentene allowed to interdiffuse with the nitrogen in the cell. After irradiation, typically 1000 laser pulses, the cell was pressurized with ambient air by momentarily opening the valve. The gaseous products were analyzed with a Hewlett-Packard 5730A gas chromatograph using a flame ionization detector and Hewlett-Packard 3390A reporting integrator. Usually three or four analyses were obtained from a single experiment. The column used was a 2-m, I / & 0.d. aluminum tube packed with 20 wt% dimethylsulfolane on 60/80 chromosorb. It was assumed that all the C5 products had the same fid response. The relative response of the fid to cyclopentene and cyclopentadiene was
( 5 ) "Organic Synthesis", Vol. 36,Wiley, New York, 1956, p 33.
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Flgure 4. Dependence of yield per lo3 pulses upon fluence at p = 50 mtorr. Solid line, linear least-squares fit to high fluence data: 0, 1029 cm-', slope of line, 1.7; A, 1033 cm-', slope of line, 2.2.
measured to be 1.00 f 0.005. All products were identified by GC-FT/IR and GC-MS techniques. Cocondensation of product gases from multiple experiments of a high, 5-10 torr, pressure of cyclopentene with degassed acetone provided an injectable liquid sample. A small set of GC-FTIR analyses was performed on a temperature-programmed silica gel column to identify products lighter than C5.
Results The infrared absorption spectrum of cyclopentene between 900 and 1100 cm-I is shown in Figure 1. Cyclopentene was photolyzed at the seven indicated wavelengths distributed over two absorption bands. When the laser beam was not focused, no reaction products could be detected, even with more than 24 000 pulses and 0.62 J/cm2 at 1048 cm-I, and more than 16000 pulses and 1.06 J/cm2 at 1033 cm-'. However, with focused radiation several reaction products were found. They were positively identified by GC/MS and GC-FTIR to be cyclopentadiene, 1,4-pentadiene, cis- and trans- 1,3-pentadiene, 1,3-butadiene, propyne, ethylene, acetylene, and ethane. A trace amount of a substance that was most probably methane based on GC retention time was also seen, but the FTIR spectrum was not sufficiently resolved for positive identification. Cyclopentadiene comprised more than 95% of the reaction products under most reaction conditions except at higher pressures of cyclopentene. An adequate material balance was only obtained when hydrogen was assumed to be formed in amounts equal to
IR Multiphoton Decomposition of Cyclopentene
The Journal of Physical Chemistry, Vol. 88, No. 3, 1984 607
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Figure 5. Dependence of C5 yield per lo3 pulses upon cyclopentene pressure at 1029 cm-’ and 0.74 J/cm2. the cyclopentadiene yield. Since the analytic techniques used were not capable of detecting hydrogen, this conclusion was not confirmed. The products having fewer than five carbons were only formed in trace amounts (50.5%) and were not quantitated. In fact, they were unobservable at the lower pressures and typical conversions of 10% or less. Thus the three pentadienes accounted for virtually all of the minor products observed. A few experiments were done with 1-methyl- 1-cyclopentene at 1029 and 1019 cm-I, on the blue side of an absorption band with I , , , 1000 cm-I. Figure 2 shows the relevant portion of the IR spectrum. Analysis of reaction products by GC-FTIR permitted tentative identification of trace amounts of propyne and 2-methyl-l,3-butadiene, as well as positive identification of a small amount of cyclopentadiene (-3-7%). Two G C peaks suspected of being methylcyclopentadienes were found, but unfortunately positive identification was not possible because published IR spectra for these species could not be found. The spectra did not correspond with published spectra for any of the methylpentadienes. Spectra were qualitatively consistent with the molecular structure of methylcyclopentadiene, which is the major product of the methylcyclopentene thermal decomposition.6 The G C retention time is also consistent with this assignment. We tentatively conclude that the major products from IRMPD of methylcyclopentene are methylcyclopentadiene and, by inference, hydrogen. In cyclopentene photolysis no dependence of the composition of C5 products on fluence or extent of reaction could be established. Experiments carried out to greater than 90% conversion at 1033 cm-I gave pentadiene to cyclopentadiene ratios that were not significantly different from values found at the smallest conversions (
