Gamma radiolysis of C6F6, product formation

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Radiat. Phys. Chem. Vol. 38, No. 4, pp. 399-405, 1991 Int. J. Radiat. AppL lnstrum., Part C Printed in Great Britain

GAMMA RADIOLYSIS OF

C6F6,

PRODUCT FORMATIONS"

N O R M A N H. SAGERT,I~ JACQUES C. LEBLANC, l D O N A L D D. WOOD, ~WALTER KREMERS, 2 JOHN B. W~SrMOaZ3 and WAYNED. BUCHANNON3 i Research Chemistry Branch and 2Radiation Applications Research Branch, AECL Research, Whiteshell Laboratories, Pinawa, Manitoba, Canada ROE IL0 and 3Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2 (Received 14 September 1990; in revised form 18 December 1990) Abstract--The Yradiolysis of perfluorobenzene (PFB) has been studied at a dose rate of about 26 Gy' sand at total doses up to 105Gy. Radiolyses were carried out in fluorine-passivated nickel cells in the absence of air. There were no significant gas yields, but higher molecular weight products were observed and characterized by combined gas chromatography and mass spectrometry (GC/MS). These higher molecular weight products included decafluorobiphenyl(DFBP), but more highly fluorinated dimers were produced with higher yields. Higher oligomers were formed in significant yields, and the trimer was especially prominent. Polymers with molar masses up to and exceeding 1500 (which corresponds to octamers) were observed by GC/MS, although their yields were small. The yield of all polymers totalled 1.7 molecules of PFB consumed for each 100 eV absorbed. This result is comparable to yields measured by earlier workers at much higher doses and dose rates.

INTRODUCTION Perfluorobenzene (PFB) and related fluids have high thermal stability, but earlier studies have shown that their radiation stability is only moderate (Florin et al., 1960; Khramchenkov, 1966; MacKenzie et al., 1965; Bloch and MacKenzie, 1969; Sutcliffe and McAlpine, 1973). They attach electrons in both the gas (Chowdhury et al., 1986; Wentworth et al., 1987) and liquid phases (Sagert et al., 1969; Shoute and Mittal, 1987; Symons et al., 1977) and electron mobility and electron-hole recombination have also been studied in them (Nyikos et al., 1980; Van den Ende et al., 1982). However, no attempt seems to have been made to identify the initial products from the radiolysis of these fluids, and to measure their yields. Earlier studies were done at high doses and dose rates, and the radiation stability of the fluids was inferred from the G value for the disappearance of the PFB itself. This work is an attempt to use modem gas chromatography combined with mass spectrometry (GC/MS) to identify some of the initial products and measure their yields.

purity was better than 99.9% as determined by gas chromatographic analysis. After distillation, the PFB was dried over activated molecular sieve 4A. Decafluorobiphenyl (DFBP) was obtained from Alfa Products, Thiokol/Ventron Division. Irradiations were carried out in nickel cells, which had been passivated by exposure to fluorine gas. The PFB was measured into these cells, and degassed on a vacuum line using the freeze-pump-thaw technique. Samples were irradiated using 6°Co y rays in an AECL Gammacell 220 at the ambient temperature of this source ('-,50°C). The dose rate in PFB was 9.48 x 1019eV.ml-l.h-I (2.608Gy.s - l ) in October 1988 as determined by Fricke dosimetry.

Analytical techniques

For selected samples, gas yields were measured using a gas extraction line. However, most products were not sufficiently volatile to be determined in this way and they were analyzed using one of two G-C/MS systems. Preliminary identifications and most quantitative work were done using a system consisting of a Hewlett-Packard (HP) 5890 gas chromatograph connected to an HP 5970 Mass Selective Detector. A EXPERIMENTAL phenyl methyl silicone-coated capillary column was used in a temperature-programmed mode. The Materials, preparations and irradiations library of mass spectra included in the software of the PFB was obtained from the Aldrich Chemical HP 5970 was of little use in identifying products. Company. It contained a small amount of pentaSome quantitative work was also done on another fiuorobenzene, which was removed by distillation HP 5890 gas chromatograph fitted with an electron using a spinning-band column. The fractiondistilling capture detector. between 80.45 and 80.55°C was collected and its Most product identification work was done using a VG 7070E-HF double-focusing mass spectrometer operated in the electron impact mode (70 eV) at a tlssued as AECL No. 10412. :[:Author to whom correspondence should be addressed. resolution of 1500. The mass spectrometer was 399

NORMANH. SAOERTet al.

400

coupled to an HP 5890 gas chromatograph, and the column and temperature programming was similar to that used with the HP system. RESULTS AND DISCUSSION

Product identification A typical gas chromatogram of the radiolysis products, generated by plotting the total ion current as a function of time, is shown as Fig. 1. This chromatogram is for an absorbed dose of about 120 kGy. Two products, labelled B and C in Fig. 1, are the most abundant single products, but another half a dozen products are formed in relatively large yield. For this system, few authentic standards were available, and the most useful one, DFBP, had a retention time equal to that of the peak labelled A. Mass spectra of some selected products, taken on the VG GC/MS, are shown in Figs 2-7. Figure 2 shows a mass spectrum of the product giving rise to peak A in Fig. 1. It is very similar to published mass spectra of DFBP, both as published in the literature (Majer, 1961) and as found in the library of the mass spectrometer software. Since, in addition, the gas chromatographic retention time for this peak is identical to that of authentic DFBP, it is reasonably safe to identify this product as DFBP. Figures 3 and 4 show mass spectra of the products giving rise to two GC peaks preceding that of DFBP. The mass spectrum of the first product (peak D in Fig. 1), shown as Fig. 3, gives ions up to m/z 410, which is surprisingly high for a compound with such a short retention time. An ion of m/z 410 corresponds I

I

to CI2F~, that is, the molecular ion of DFBP which has incorporated two extra fluorine molecules. A very abundant ion m/z 205 is assigned as C6F;-. The mass spectrum of the second product (peak E in Fig. 1), shown as Fig. 4, gives an abundant molecular ion, m/z 372, corresponding to Cm2F~, arising from DFBP, with one added fluorine molecule. Compared to Fig. 3, this spectrum, which resembles those of products with similar retention times, also shows strong peaks for losses of small fragments from the molecular ion and a much weaker C6 F+ ion peak. For example, the base peak is 303, which corresponds to C . F ~ . Apparently, on this column the more highly saturated molecules are eluted earlier than more aromatic molecules with the same number of carbon atoms, even though the aromatic products are considerably lighter. The products formed with the largest yields correspond to those labelled B, C and F in Fig. 1. A mass spectrum of the product labelled C is shown as Fig. 5. The molecular ion appears to be 558, corresponding to C~s F~s. This, and similar products, show a very strong base peak at 353, corresponding to CI2F~. Thus these products are likely trimer species partially saturated with two fluorine molecules. The dimer fragment ion seems especially stable. The intermediate-molecular-weight products, labelled G through I in Fig. 1, give rather similar mass spectra, with variations in the abundance of the molecular ion. They can be represented by the mass spectrum of product I, shown as Fig. 6. This product has a molecular ion at 706, corresponding to C24 Ffi, and thus represents a tetramer having two fluorine

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m/z Fig. 2. Mass spectrum of gas chromatographic peak from PFB radiolysis identified as DFBF (peak A in Fig. l). molecules above that required for aromaticity. The initial part of the mass spectrum resembles that of products B and C, suggesting that they are structurally similar. The GC peaks preceding product I have molecular ions of 744, which is most likely C24F~4. With these products, peaks at 539 (CIsF~7) and 501 (C~sF~5) are prominent. For the tetramers, as for the dimers and trimers, the more highly 205

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fluorinated products are eluted from the GC column first. The products shown as J and K in Fig. 1 are a different type of product again, and the mass spectrum for K is shown as Fig. 7. The heaviest observable ion is 725, which must be still a fragment ion, probably C24F~3. The base peak is 668, corresponding to C24F~0. Thus this product may be a

Spectrum of Peok "D"

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NORMANH. SAGERTet al.

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tetramer as well, but with a different structure. Not shown are mass spectra obtained from trace products with elution times greater than that of product K. Some of these products had molar masses greater than 1500, and probably correspond to octamers. Their mass spectra all have 169 (C3F +) as the base peak.

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Determination o f yields Figure 8 shows the formation of products A, B and C as a function of absorbed dose up to a dose of about 25 kGy. The formation of product A (DFBP) is linear with dose and, from calibration with external standards, gives a yield of 0.0465 molecules/100 eV.

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7-Radiolysis of C6F6, product formation -'-T

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m/z Fig. 6. Mass spectram of gas chromatographic peak from PFB radiolysis identified as fluorinated tetramer (peak I in Fig. 1). monomer unit incorporated into a product molecule, an estimate of the total product yield can be made. This is shown in the curve labelled Z of Fig. 8. The "total polymer" production is fairly linear with dose over this dose range. The curve labelled ? includes all the products detected and some of the later peaks in

The plots for the formation of products B and C are less linear. Since these products are not conclusively identified, and thus cannot be compared with known standards, it is not possible to make accurate measurements of the yields. However, if the detector sensitivity can be taken as constant for each

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the gas chromatogram do become relatively more important at later times even though their areas as shown in Fig. 1 are relatively small. Thus the nonlinearity of peaks B and C is not strongly reflected in curve I . Figure 9 shows the formation of "total polymer" as a function of dose, measured using the total ion current method on the HP GC/MS. The initial yields are about 1.7molecules/100eV and correspond to molecules of PFB consumed, not to actual molecules of polymer produced. When the electron capture detector was used, the "total polymer" yield was similar. These yields are minimum polymer yields, and are very close to those measured earlier (MacKenzie et al., 1965) at much higher doses and dose rates. From the shape of the curves for the formation of products B and C, which have been shown to be 2.0

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fluorinated trimers, it would seem that these products are initial, not secondary, products in the sense that they are not formed by the radiolysis of products. We cannot determine whether they are initial products in the sense that they are formed directly in the initial radiation events. They do appear to be the dominant products at the smallest doses used. By analogy with benzene radiolysis, the expected major prodcuts might have been DFBP and, perhaps, perfluorocyclohexadiene or some other simple fluorinated product. However the fluorinated trimers appear to form instead. It is difficult to imagine single chemical events or a series of straightforward reaction steps leading to these trimers. However, bearing in mind the collective nature of electron transport in this system (Nyikos et al., 1980; Van den Ende et al., 1982), concerted events leading to these products may occur. The formation of all the products up to K seems to be initial in the sense used here, but the larger products observed could be secondary products in the sense that they are formed by radiolysis of products. To test whether the trimers and tetramers really were initial rather than secondary products, in the sense noted above, samples of PFB containing 1.01 × l 0 - 4 moles of DFBP per liter were irradiated. The yields of trimers and tetramers were virtually identical to those from samples without the added DFBP. This is additional evidence that these products are not secondary products since, if DFBP were a precursor, its addition should have increased their yields. The photochemistry of PFB has been studied in some detail (Hailer, 1967; Suijker et al., 1986). There is a good body of knowledge about the various singlet and triplet excited states. It is also well established that the dewar benzene analogue of PFB can be a major product. However, from our radiolysis studies there is no way of knowing whether the products arise during ion recombination, or from excited states formed directly, nor from which excited state (or exciton) each product arises.

I

Acknowledgements--We wish to thank the staff of the

Radiolysis

Reactor Laboratory at the Whiteshell Nuclear Research Establishment for their assistance in determining the gas yields and G. G. Haacke for assistance with the dosimetry.

P01ymer" Y i e l d s

REFERENCES

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Bloch F. W. and MacKenzie D. R. (1969) Radiolysis of cyclic fluorocarbons. II. Perfluoroaromatics at elevated temperatures. J. Phys. Chem. 73, 552. Chowdhury S., Grimsrud E. P., Heinis T. and Kebarle P. (1986) Electron affinities of perfluorobenzene and perfluorophenyl compounds. J. Am. Chem. Soc. 108, 3630. Florin R. E., Wall L. A. and Brown D. W. (1960) Gamma irradiation of hexafluorobenzene.J. Res. Nat. Bur. Stand. 64A, 269. Hailer I. (1967) Kinetics and mechanism of the photochemical valence tautomerization of hexafluorobenzene. J. Chem. Phys. 47, I 117.

~-Radiolysis of C6F~, product formation Khramchenkov V. A. (1966) Formation of high boiling products of radiolysis by irradiation of mixtures of hexafluorobenzene with perfluorocyclohexane and perfluorononane. Atomnaya Energiya 21, 375. MacKenzie D. R., Bloch F. W. and WiswaU R. H. Jr (1965) Radiation chemistry of some cyclic fluorocarbons. J. Phys. Chem. 69, 2526. Majer J. R. (1961) Mass spectrometry of fluorine compounds. Adv. Fluorine Chem. 2, 55. Nyikos L., Van den Ende C. A. M., Warman J. M. and Hummel A. (1980) High mobility excess electrons in the electron-attaching liquid hexafluorobenzene. Y. Phys. Chem. 84, 1154. Sagert N. H., Reid J. A. and Robinson R. W. (1969) Radiolysis of cyclohexane with electron scavengers. VII. Perfluorocyclohexane and perfluorobenzene as electron scavengers. Can. J. Chem. 47, 2655. Shoute L. C. T. and Mittal J. P. (1987) Absorption and conductivity studies on the transients generated in the

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gamma-radiolysis of perfluorobenzene in rigid matrices. Radiat. Phys. Chem. 30, 105. Suijker J. L., Huizer A. H. and Varma C. A. G. O. (1986) Picosecond spectroscopy in study of the photoinduced isomerization of hexafluorobenzene. Laser Chem. 6, 333. Sutcliffe H. and McAlpine I. (1973) The radiation chemistry of polyflunrinated organic compounds. Fluorine Chem. P.ev. 6, 1. Symons M. C. R., Selby R. C., Smith I. G. and Bratt S. W. (1977) ESR studies on the structure of C6F~ anions. Chem. Phys. Lett. 48, 100. Van den Ende C. A. M., Nyikos L., Warman J. M. and Hummel A. (1982) Mobifity, reaction kinetics and optical absorption spectrum of the excess electron in pure C6F6 and admixtures with non polar liquids. Radiat. Phys. Chem. 19, 297. Wentworth W. E., Limero T. and Chen E. C. M. (1987) Electron affinities of hexafluorobenzene and pentafluorobenzene. J. Phys. Chem. 91, 241.