11292
J. Phys. Chem. 1995, 99, 11292- 11296
Heavy Ion Radiolysis of Cyclopentane Laszlo Wojnarovitsi and Jay A. Laverne* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 Received: March 29, 1995; In Final Form: May 2, 1995@
The radiolysis of neat and iodine solutions of cyclopentane has been performed with 1- 15 MeV protons, 5-20 MeV helium ions, 12-31 MeV carbon ions, and 22-32 MeV oxygen ions. The yields of the major products cyclopentene, bicyclopentyl, 1-pentene, and cyclopentyl iodide as well as the yields of a few select minor products have been determined. All products show a decrease in yield with increasing linear energy transfer (LET) of the irradiating particle. Some of these decreases are very large; for instance, the observed drop in the yield of bicyclopentyl from y-rays (LET = 0.14 eV/nm) to oxygen ions (LET = 900 eV/nm) amounts to a net loss of about 4 cyclopentyl radicals/100 eV absorbed energy. This value is 80% of the total cyclopentyl radicals produced in y-radiolysis. There are some indications that the yields of radicals due to decomposition of excited cyclopentane molecules are dependent on the energy and type of irradiating particle.
Introduction
Dame Nuclear Structure Laboratory. The details of the accelerator facility, window assembly, and radiolysis procedure Several groups have investigated the y-ray or fast electron have been described elsewhere.l 6 . l 7 Particle energy incident to radiolysis of cyclopentane, but there has been only one the accelerator exit window was determined by magnetic published reportlo on the heavy ion (3 MeV r'oPo a-ray) analysis to within 0.1%. All irradiations were performed with radiolysis of this compound. Irradiation of cyclopentane with completely stripped ions, and energy loss of the particle in heavy particles of high linear energy transfer (LET = -a/&, passing through the various windows was determined from stopping power) is expected to have a significant effect on standard stopping power tables.I* The doses were calculated product yields because of the increased importance of intratrack from the particle energy incident to the sample, and the charge radical reactions as compared to radical diffusion and reaction was collected from the sample cell and the exit window. The in the bulk medium. The yields of the main products, total energy deposited into the sample was typically about 5 x cyclopentene and bicyclopentyl, are found to be much smaller loi9eV in 20 mL of sample (total dose of -50 krad). with a-particle radiolysis than with y-radiolysis, and at a given Cyclopentane was purified by distillation as described concentration the radical scavenger iodine is found to have much el~ewhere.~ Product analysis of the irradiated samples was less effect on product yields in a-radiolysis. These results are performed within a few days of the radiolysis using a gas similar to that observed in the radiolysis of cyclohexane' [ - I 3 chromatograph-mass spectrometer operated in a selected ion and cycl~octane.'~ However, there are at least two important monitoring mode. Details of the analysis technique were differences between the radiolysis of cyclopentane and that of described in former publications.13.14 the other two cycloalkanes. The viscosity of cyclopentane (0.423 mPa s, 1 mPa s = 1 cP) is much smaller than that of Results and Discussion cyclohexane (0.905 mPa s) and cyclooctane (2.263 mPa s) and therefore diffusion is much faster in cyclopentane than in the Processes Related to C-H Bond Decomposition. The main other compound^.'^ Diffusion affects both the relaxation of the liquid products produced in the radiolysis of neat cyclopentane initial spatial distributions of reactive species and their reaction are cyclopentene and bicyclopentyl. Their yields are shown in rates. Cyclopentane also has a much higher tendency for ring Figures 1 and 2, respectively, as a function of the energy, E,, decomposition than cyclohexane or c y c l ~ o c t a n e . ~The - ~ main of the various heavy ions incident to the sample. Also shown product of this ring decomposition is 1-pentene, but other C5 are the results with 3 MeV 210Poa-particles.I0 The solid lines ring opening products and lower molecular weight scission in these two Figures were obtained by a least-squares fit to the products are also formed. data for each ion. Since the particles used in these experiments The y-radiolysis of cyclopentane at very low doses (-25 are completely stopped in the sample cell, it is the track average krad) was presented in a previous work.9 In the present study yield,I9 Go,which is measured. With all of the heavy ions used the radiolysis of cyclopentane was investigated with 1- 15 MeV here, the yields of both cyclopentene and bicyclopentyl are less protons, 5-20 MeV helium ions, 12-31 MeV carbon ions, and than that found in y-radiolysis, and for a particular ion they 22-32 MeV oxygen ions and compared with the results from appear to increase linearly with increasing particle energy. At y-radiolysis. Experiments with varying concentration of iodine a given energy the yields of both products are found to decrease were used to elucidate the contribution of the cyclopentyl and with increasing particle nuclear charge. other radicals to the overall radiation chemistry of cyclopentane. To a first approximation, the overall effects of particle track structure on the radiation chemistry of hydrocarbons can be Experimental Section assumed to be dependent only on the LET of the irradiating The 'H, "He, "C, and I6O ion irradiations were performed of products LET particle. Of course, in a detailed using the MV vande Graaff Of the is not the sole factor which determines yields. Two particles at the same LET can have completely different radial energy Permanent address: Institute of Isotopes of the Hungarian Academy distributions depending on their charge and energy.1° The of Sciences. P.O. Box 77, Budapest. H-1525, Hungary. resulting differences in the kinetics of the reactive species can Abstract published in Advance ACS Abstracts. June 15, 1995. @
0022-365419512099-11292$09.00/0
0 1995 American Chemical Society
Heavy Ion Radiolysis of Cyclopentane
J. Phys. Chem., Vol. 99, No. 28, 1995 11293
I-._..________ GOCO-y
0'
4 r
12-
6O
I
10
0
20
30
Particle Energy, E, (MeV)
1 00
Figure 1. Track average radiation chemical yields (Go, molecules/ 100 eV) of cyclopentene from neat cyclopentane as a function of initial particle energy (Eo),for protons (m), helium ions (0).carbon ions (A), and oxygen ions (*I, this work; and ?'OPo-a particles (O), ref 10. The dashed line shows the y-radiolysis limit, ref 9. 1.5
6OCo-Y
Track Average LET (eV/nm) Figure 3. Track average radiation chemical yields for the production of cyclopentene and bicyclopentyl in neat cyclopentane as a function of track average LET,, of the various ions. The value of ACsHs represents the net change in cyclopentyl radicals and is equal to 2.0 ( 1 -t /&) times the difference between the yield of bicyclopentyl with y-rays and with a heavy ion. The dashed lines show the y-radiolysis limit, ref 9.
c)
ion
energy (MeV)
LET0 (eV/nm)
cyclopentene
bicyclopentyl
I-pentene
"20-y
1.25
'H
I
0.14 41.6 29.2 17.2 11.0 8.42 131.0 94.9 76.3 64.7 643.8 606.9 573.5 543.0 514.2 888.5 830.5
3.30 2.61 2.5 1 2.52 2.78 2.72 2.25 2.36 2.29 2.36 1.48 1.55 1.60 1.70 1.79 1.43 1.58
1.25 0.82 0.83 0.84 0.95 0.94 0.58 0.68 0.69 0.75 0.32 0.34 0.34 0.36 0.38 0.28 0.35
0.90 0.61 0.63 0.64 0.67 0.65 0.49 0.55 0.52 0.55 0.32 0.29 0.33 0.32 0.31 0.30 0.36
C
a
Q 0
0
.2 s 0.5 6 ,
0
10
'He
30
20
Particle Energy, E, (MeV)
Figure 2. Track average radiation chemical yields (Go, molecules/ 100 ev) of bicvcloDentv1from neat cvclouentane as a function of initid particle'energy (Eo).The symbols are for the track average yields with the same representation as in Figure 1. The dashed line shows the y-radiolysis limit, ref 9.
, . ,
