Structure of Radicals Produced by y-Radiolysis. Part 2. Radicals Derived from 5-Membered Alicyclic Molecules

RON E. LINDER AND A. CAMPBELL LING' Depcrrrr?ietir o J C l ~ e ~ ~ i i s t r y , We.\( L'irgit~ia Ut~ioersit?.. Morgcttiro~~~t~. W . V . 36506

Received December 29, 1971'

Data are presented for the results of ;.-radiolysis on cyclopentane, methyl cyclopentane, cyclopentene, and cyclopentadiene, held as solutes in adamantane matrices. Data from this particular matrix isolation technique are in accord with liquid phase studies for cyclopentane and cyclopentene, and indicate that tertiary hydrogen loss occurs in methyl cyclopentane, and that cyclopentadiene yields at least three radicals. Two of the radicals derived from cyclopentadiene have been identified as the C,H,. entity with five equivalent protons, and the

. - allylic-type radical formed by hydrogen atom addition to molecular C,H,. 0 Les rcsultats de la radiolyse ;I du cyclopentane, mcthycyclopentane, cyclopentene, et cyclopentadiene,

retenus comme solutes dans des matrices adamantane, sont prksentes. Cette technique particuliere d'isolement par matrice donne des resultats en accord avec ceux de la phase liquide pour le cyclopentane et cyclopentene. et montre que la perte de I'hydrogene tertiaire se produit dans le methylcyclopentane tandis que le cyclo- pentadiene conduit au moins a trois radicaux. Parmi ceux ci deux ont tte identifies: c'est I'entitC C,H,. avec

n cinq protons ~quivalents et le radical de type allylique forme par addition d'hydrogene atomique sur C,H, moleculaire. [Traduit par le journal]

Canadian Journal of Chemistry. 50. 3982 (1972)

Introduction As part of an investigation involving radio-

lysis of five-membered heterocyclic molecules it was felt that data from equivalent alicyclic compounds would prove useful. Cyclopentane has been studied previously by Fessenden and Schuler (1) in the liquid phase using continuous flow radiolytic production techniques below room temperature and by Fischer (2) using chemical generation in the liquid phase. The major radical was shown to be that resulting from C-H bond cleavage with a structure that is either planar or rapidly inverting in a "chair- boat" vibration analogous to that undergone by cyclohexane. In this context, i t is interesting to note that the cyclohexyl radical resulting from radiolysis of cyclohexane does not exhibit four equivalent protons at the b-position relative to the unpaired electron and cannot be planar ( 1 ) . Ohnishi and Nitta (3) have shown that radiolysis of cyclopentadiene as a crystalline solid at 77 OK leads to the vroduction of two different radicals, one of wiich was identified as the C,H,- radical with five equivalent -

'To whom correspondence should be addressed. 'Revision received July 18, 1972.

protons, but the second radical could not be identified from the observed e.s.r. spectrum. Similarly, Kuri and co-workers (4) obtained a broad, and poorly resolved, e.s.r. spectra following radiolysis of cyclopentadiene at 77 O K

and assigned i t to two different radical species, one of which was probably the C,H,. radical. Pyrolysis experiments on ferrocene were shown to lead to the production of C,HS. radicals ( 5 ) , although radiolysis and photolysis of metallo- cene solutes in glassy 2-methylpentane matrices at 77 "K gave no evidence for C,H,- radical production (6). However, photolysis of cyclo- pentadiene itself as a crystalline solid at 77 "K does lead to C,H,. generation, but no other radical (7). Analogously, photolysis of a dilute solution of hexachloro-cyclopentadiene in car- bon tetrachloride in a flow systeill leads to the production of C,CI,. radicals and no others (8). In contrast to these predominantly solid phase studies, Krusic and Kochi (9) studied the chemical generation of C,H,. from cyclo- pentadiene in the liquid state at low temperature. E.s.r. data for the ;,-radiolysis of cyclopentene in adamantane has now been reported by Wan and co-worker ( 1 0).

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L l N D E R A N D L I N G : STRUCTURE OF RADICALS. P A R T 2 3983

Experimental Cyclopentane (CP), cyclopentene (CPE), methyl cyclo-

pentane (MCP), cyclopentadiene dimer, and adamantane, were obtained from commercial sources in the purest grade available and used as received. Cyclopentadiene (CPDE) was prepared from its dimer by standard distillation pro- cedures, collected a t - 78 "C, degassed, and stored at -78 "C on a vacuum line. Incorporation of CP, CPE, and MCP, into the adamantane matrix was done by multiple recrystalli- zation of adamantane crystals from small aliquots (cu. 25 ml) of solvent (CP, CPE, and MCP. respectively), using fresh portions of the solvent each time (3-4 repetitions). Incorporation of CPDE into adamantane required a slightly different technique due to the risk of dimerization a t room temperatures and above. Adamantane was recry- stallized from methyl cyclohexane3 to remove solvent impurities contained in the cavity on receipt from the vendor. It was transferred to a coniainer on the vacuum line and degassed, using an ice-bath to restrict sublimation. Freshly prepared CPDE from its storage reservoir (at 195 OK) on the vacuum line was transferred to the ada- mantane vessel using liquid N, to condense the vapor and allowing the reservoir to warm to 273 OK. The container with adamantane was allowed to warm from 77 to 273 OK, thus permitting the adamantane to dissolve in the CPDE liquid. After 60 min the CPDE was transferred out of the adamantane container to a new vessel, and fresh CPDE transferred into the adamantane from the reservoir. Using standard vacuum line techniques and controlling the temperature with ice, Dry-Ice, and liquid N,, the adaman- tane was thoroughly washed four to five times with CPDE monomer, where at no time was the temperature allowed to exceed 0 "C for liquid CPDE monomer. so curtailing dimer formation. All adamantane samples containing the requisite solute were dried in an oven at ca. 35-40 "C for 15 min, and then irradiated at room temperature in a Pyrex tube utilizing a "Co source (dose rate 3.5 x 10" eV g-' min-' via Fricke solution dosimetry). Irradiated adamantane samples containing solute were transferred to quartz e.s.r. tubes and monitored at room temperature using a Varian E-3 e.s.r. spectrometer (typically; 0.5-50 mW power, 0.5-2.0 G modulation amplitude, 16 min scans at 3390 100 G). Saturation effects were noted for radicals derived from CPDE as low as 10 mW, typical :)-doses were cu. eV g- ' , radical lifetimes varied

3Recrystallization of adamantane from methyl cycIo- hexane followed by p-radiolysis in the usual manner does not lead to any recognizable e.s.r. spectrum attributable to methyl cyclohexane itself. The only noted feature is a broad singlet, typical of polycrystalline e.s.r. studies, and characteristic of I-adamantyl radicals, rather than of 2-adamantyl radicals which lead to distinctly different signals (11). It is interesting to note that sublimation techniques do not necessarily lead to adamantane samples free from impurities. A very characteristic e.s.r. spectrum is obtained by radiolysis of adamantane directly from samples supplied by the vendor. This characteristic spectrum persists even after sublimation, and is attributed to trapped solvent molecules used in the synthesis of adamantane originally.

widely but were all in excess of 2 h. Particularly for radicals derived from CPDE, serious degeneration of the low power spectrum was noted after only 90-120 min. Re-irradiation of the same adamantane sample invariably led to shorter lifetimes, more rapid degeneration, and poorer resolution, of the observed e.s.r. spectrum. Computer simulations of e.s.r. spectra were carried out by a modified program supplied by Quantum Chemistry Program Exchange4 util- izing an IBM-360175 computer system and Cal-Comp Plotter accessory.

Results and Discussion Experimental spectrum, stick spectrum, and

computer simulated spectrum for ?-irradiated MCP are shown in Fig. 1. Low power and high power e.s.r. spectra of ;*-irradiated cyclopentad- iene together with the spectrum obtained from 11-radiolysis of cyclopentene are shown in Fig. 2. Hyperfine coupling constants for the radicals studied are shown in Table 1. Resolution of spectra using adamantane as the trapping matrix is adequate for unambiguous assign- ments (line widths in the range 0.8-3.0 G) , but line widths in general are inferior to those ob- tained in the liquid state (e.g., 0.08 G (9), 0.5 G and less (1, 12).5

Cyclaj~c~ntnne and Cj~clopentene These data from the solid state at room

temperature agree well with that previously recorded for low temperature liquid phase studies (see Table 1 ). In the case of CP, tempera- ture dependent studies in adamantane matrices between 77 and 373 "K proved ambiguous, and did not resolve the question of whether the radical was planar o r undergoing vibratory inversion through the ring center.

Metlijvl Cycloj~entane Data obtained are consistent with tertiary

hydrogen loss as previously postulated for alkane molecules (13). The e.s.r. spectrum being assigned to a radical

dH,-cH,-c-cH,--&H2, I

CH3

coupling constants are given in Table 1 . Again,

4Quantum Chemistry Program Exchange, Indiana Uni- versity, Bloomington, Indiana. Program originally written by C. S. Johnson and M. K. Ahn of Yale University, QCPE Catalog Number 83.

'R. W. Fessenden and R. H. Schuler (14) report isotropic spectra in SF, matrices with I9F line widths cu. 2G.

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CANADIAN JOURNAL O F CHEMISTRY. VOL. 50, 1972

FIG. I. Stick spectrum: (a), experimentally observed spectrum; (b), computer simulated spectrum; (c), for y-irradiated methyl cyclopentane in an adamantane matrix a t room temperature.

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LINDER AND LING: STRUCTURE OF RADICALS. PART 2 3985

temperature dependent studies were unable to answer the question as to whether the radical is planar, or rapidly inverting a t room temperature.

C.vclopc~r7tcidiene The experimental data are given in Table 1

and in Fig. 2. Power saturation experiments demonstrated that above approximately 20 mW, the e.s.r. spectrum of one radical could be enhanced significantly a t the expense of others. The spectrum obtained a t 32 mW is shown in Fig. 20, and exhibits a well defined sextet with intensities close to the theoretical 1 : 5 : 10: 10: 5: 1 required for five equivalent protons, and a coupling constant of 6.1 -t 0.1 G. The signifi- cant features of the underlying spectrum, both at high power and low power, are matching sets of doublets to the outside of the sextet. This is characteristic of the spectrum obtained from the radiolysis of cyclopentene, as demonstrated by the published spectrum of Wan and co-worker (10) and the experimentally obtained spectra in this work (displayed in Fig. 2c). Comparison on a peak-by-peak basis, via direct overlay of one spectrum on top of the other, shows complete agreement between the cyclopentenyl spectrum and the peaks in the low-power observed spectrum from cyclopentadiene. How- ever, this low-power spectrum displays more peaks than can be accounted for by the sum of the cyclopentadienyl spectrum and the cyclo- pentenyl spectrum. There are, therefore, at least three radicals produced by 7-radiolysis of cyclo- pentadiene in adamantane matrices. Both of these observed and assigned radicals can be accounted for simply; the cyclopentadienyl radical C,H,. by loss of a hydrogen atom from the -CH,- group in the five-membered ring, and the cyclopentenyl allylic structure

'.-.' 0 FIG. 2. Experimental e.s.r. spectra for y-irradiated cyclopentadiene and cyclopentene in adamantane matrices at room temperature: (a) high power spectrum of cyclo- by addition of a hydrogen a pentadiene (32 mW); (b) low power spectrum of cyclo-

of c~clopentadiene a t one of the double bonds pentadiene (2 mW); (c) spectrum of the allylic-type radical (as shown in the scheme below). derived from cyclopentene in adamantane (2 mW).

1 I

HID( POWER SCAN

/- ( I

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1 ' , I

L O W POWER SCAN

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3986 CANADIAN JOURNAL OF CHEMISTRY. VOL. 50. 1972

TABLE 1. E.s.r. coupling constants for radicals derived from 5-membered alicyclic molecules

Couplingconstantsobserved't Coupling constants reported Radical (G) ( G ) Reference

0 a,= 22.5 a , = 34.0 u, = 21.48 a,, = 35.16 1 21.05 34.86 2

a,,, = 23.0 a,, = 33.0

'9 GHz instrument tk0.2 G. tk0 . l G.

It is tempting to assign the third radical to the

structure &H,-CH,-CH-CH,-CH, (i.e.. the cyclopentyl radical) corresponding to hy- drogen atom addition at the other end of the double bond, but there are insufficient data from the e.s.r. spectrum to support this hy- pothesis. Hydrogen atom addition to alkenes in conventional matrices is well documented, and Wan6 reports that y-radiolysis of cyclooctate- traene in adamantane leads to two radicals, one of which can be explained by hydrogen atom addition to one of the double bonds in this molecule. Finally, in an attempt to elucidate further the mode of decay of adamantane isolated species, the rate of decay of the sextet attributed to the C,H,- radical was determined over a period of 110 h at room temperature. Radical concentrations, as measured by the height of the high-field central line of the sextet, were determined for fourteen different times between 0 and 110 h. Plots of concentration

6Private communication from J.K.S. Wan of Queen's University, Kingston, Ontario.

against time (correlation coefficient 0.979,), log (concentration) against time (correlation co- efficient 0.980,), and concentration-' against time (correlation coefficient 0.979,), were all reasonably linear, with a half-life for the radical species of ca. 142 h at room temperatures. It is not possible, therefore, to draw any meaningful conclusions concerning the order of the kinetic rate, since data spanned less than one half-life for the process.

One of us (R.E.L.) thanks the National Aeronautics and Space Administration for a pre-doctoral fellowship; we thank the Research Corporation of New York for partial support of this work, the Chemical Engineering Department of this University for use of their facilities, and Professor J. K. S. Wan of Queen's University, Kingston, Ontario, for a chance to read a preprint of his concerning the radiolysis of cyclooctatetraene in adamantane matrices.

1. R. W. FESSENDEN and R. H. SCHULER. J. Chem. Phys. 39, 2147 (1963).

2. H. FISCHER. J. Phys. Chem. 73,3834 (1969). 3. S. - I . OHNISHI and I . NITTA. J . Chem. Phys. 39, 2848

(1963). 4. S. ARAI, S. SHIDA, Y. YAMAGUCHI, and Z. KURI. J.

Chem. Phys. 37, 1885 (1962).

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LINDER A N D L I N G : S T R U C T U R E OF RADICALS. PART 2 3987

5. P. J. ZANDSTRA. J . Chem. Phys. 40, 612 (1964). 10. D. R. GEE and J. K. S. WAN. Can. J. Chem. 49, 20 6. A. CAMPBELL LING. J . Inorg. Nucl. Chem. 34, 2978 (1971).

( 1 972). 11. G . J . HYFANTIS and A. CAMPBELL LING. TO be 7. G . R. LIEBLING and H. M. MCCONNELL. J. Chem. published.

Phys. 42, 3931 (1965). 12. R. W. FESSENDEN and S. OGAWA. J. Am. Chem. Soc. 8. F. GRAF and Hs. H. GUNTHARD. Chem. Phys. Lett. 86, 3591 (1964).

7, 25 (1970). 13. S. W. KANICK, R. E. LINDER, and A. CAMPBELL LING. 9. P. J . KRUSIC and J. K. KOCHI. J. Am. Chem. Soc. 90, J . Chem. Soc. (A), 2971 (1971 ).

7155 (1968). 14. R. W. FESSENDEN and R. H. SCHULER. J. Chem. Phys. 45, 1845 (1966).

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