Paramagnetic Species Produced by γ Irradiation of Organic CompoundsB. Smaller and M. S. Matheson Citation: J. Chem. Phys. 28, 1169 (1958); doi: 10.1063/1.1744362 View online: http://dx.doi.org/10.1063/1.1744362 View Table of Contents: http://jcp.aip.org/resource/1/JCPSA6/v28/i6 Published by the American Institute of Physics. Additional information on J. Chem. Phys.Journal Homepage: http://jcp.aip.org/ Journal Information: http://jcp.aip.org/about/about_the_journal Top downloads: http://jcp.aip.org/features/most_downloaded Information for Authors: http://jcp.aip.org/authors

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THE JOURNAL OF CHEMICAL PHYSICS VOLUME 28. NUMBER 6 JUNE. 1958

Paramagnetic Species Produced by 'Y Irradiation of Organic Compounds*

B. SMALLER AND M. S. MATHESON

Argonne National Laboratory, Lemont, Illinois (Received December 19, 1957)

Some alkanes, olefins, alcohols, ethers, and a few other organic compounds have been irradiated with C0 60 'Y rays at low temperatures. The frozen irradiated samples have been examined for para­magnetic resonance, and the observed spectra in several cases have been identified with specific radicals. The methyl, ethyl, and allyl radicals are among those identified. The results in each case have been considered in relation to the present knowledge of the radiation chemistry of the com­pound. The mode of radical formation in many cases appears to be quite specific. Thus, in normal alkanes the major process for radical formation is by loss of a specific H atom. Hyperconjugation is used to interpret the resonance spectra.

INTRODUCTION

FREE radicals or other odd electron fragments have often been postulated as intermediates in the de­

composition of organic compounds by ionizing radiation. Although recent researches1 •2 indicate that much of the decomposition in aliphatic hydrocarbons, for example, occurs by nonradical processes, it still is important for formulating mechanisms in radiation chemistry to identify the radicals that are formed. If several organic compounds are irradiated and resulting radicals are identified, one may hope generalizations can be drawn concerning modes of bond rupture.

For the identification of radicals the method of para­magnetic resonance is very powerful, and has been ap­plied extensively by Gordys-6 and others in the identifi­cation of paramagnetic species in such x-irradiated or­ganic compounds as carboxylic acids, alcohols, amines, amides, and mercaptans. At present it is necessary to stabilize very reactive free radicals by forming them in rigid media in order to achieve concentrations detectable by their paramagnetic resonance. In the present work this required irradiation and measurement at 77°K. However, the results should still have some significance for condensed phase irradiations at higher temperatures.

The identification of a given resonance spectrum with a definite radical also has importance apart from the specific radiation study for two reasons. First, the known spectrum can be used to identify the radical in more complex situations, and secondly, the resonance spec-

* Based on work performed under the auspices of the U. S. Atomic Energy Commission.

I Weber, Forsyth, and Schuler, Radiation Research 3, 68 (1955).

2 L. Dorfman, (Symposium on Radiation Chemistry of Hydrocarbons, 132nd Meeting of American Chemical Society, New York, September, 1957), finds part of H2 is eliminated from CH. or C2H. by nonradical processes. (A complete list of pertinent papers would be quite extensive.)

3 Gordy, Ard, and Shields, Proc. Nat!. Acad. Sci. U. S. 41, 983 (1955).

• Gordy, Ard, and Shields, Proc. Nat!. Acad. Sci. U. S. 41, 996 (1955). .

6 C. F. Luck and W. Gordy, ]. Am. Chern. Soc. 78, 3240 (1956).

6 W. Gordy and C. G. McCormick, ]. Am. Chern. Soc. 78, 3243 (1956).

trum furnishes significant information about the orbital of the odd electron.

For reasons such as those discussed above, we have irradiated some hydrocarbons and certain other organic compounds with C0 60 'Y rays, and examined the result­ing paramagnetic resonances at 9350 Mc/sec. The present paper presents the results of this work.

EXPERIMENTAL

Materials

The compounds studied were obtained from commer­cial sources and were used in general without further purification. Materials used were as follows: Phillips Pe­troleum Company "AA" research-grade methane, eth­ane, n-hexane, and cyclopentane of 99.60, 99.99, 99.81, and 99.98 M% minimum purity, respectively; Phillips pure-grade, n-nonane, neopentane 99 M% minimum purity; Matheson Company instrument-grade propane and isobutane; Matheson CP butane, ethylene, and propylene; Matheson cyclopropane, cyclohexane, allyl alcohol, dimethyl ether, methyl chloride, ethyl chloride, cis-1 ,2-dichloroethylene, (pract) a-trioxymethylene; Mallinckrodt absolute methanol; U.S. Industrial Chem­icals Company absolute ethanol; Merck isopropyl alcohol and diethyl ether; Eastman n-propanol, n-oc­tadecane (pract), and n-octacosane; Fisher Scientific Company purified ethylene chloride; Ethyl Corporation tetramethyllead (ca 99% pure), tetraethyl lead (com­mercially pure), and n-tetrapropyl lead (technical); lead (commercially pure) and n-tetrapropyllead (tech­nical) ; well-purified dimethyl mercury given by Richard Bernstein (University of Michigan); and Marlex-50 (Phillips) given by A. A. Miller of General Electric Company.

Preparation of Samples All samples were thoroughly degassed on a vacuum

line by pumping on samples frozen with liquid nitrogen, and then thawing with no pumping. After four or five cycles of freezing, pumping, and thawing the samples were sealed off while frozen with liquid nitrogen. The sample tube used for methane is shown in Fig. 1. The Wood's metal and constriction B were present only in

1169

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1170 B. SMALLER AND M. S. MATHESON

l To vacuum lino

A

B

Woods metal

Graded seal

Ordinary fused silica 5mm 0.0.

High purity corning fused silica 4 or 5 mm O. O. thin walled

FIG. 1. Sample tube for irradiations.

the methane sample tubes, and were used to avoid de­~omposition of the methane during sealing off. In seal­mg off methane (vp, 10 mm at nOK) the tube of Fig. 1 was shut off from the vacuum line and heated lightly near the Wood's metal. The Wood's metal melted and sealed constriction B. After the metal cooled the tube was opened again to the pumps and sealed off at A. Samples of the compounds which are gaseous at room te~perature were collected by connecting the gas cyhnder to the vacuum line and then condensing the gas in the sample tube after the tube and the vacuum system we~e thoroughly ev~cuated and flushed with gas. C.ondensatlOn was accomphshed with a Dewar of liquid mtrogen, the liquid nitrogen level being somewhat be­low the bottom of the tube. Liquids were pipetted into the sample tubes which were then attached to the va­cuum line. Care was taken to avoid condensation of moisture on or in the tubes. During pumping the rest ~f t~e v~cuum line was separated from the pumps by a hqUld mtrogen cooled trap and a tube containing gold foil to trap out oil and mercury. Samples (except for t~e gases) were sealed off at about 5X 10-5 mm pressure WIth the sample at liquid nitrogen temperature.

The metal alkyl compounds were stored in bulbs at - 80°C on a vacuum line in a hood and distilled into irradiation tubes as required.

Irradiations

For 'Y irradiations in liquid nitrogen, up to 6 samples w,ere ~et vertically in a cylindrical arrangement of 6-cm dIam m a Dewar (9-cm Ld.). Centered in the circle of samples was another Dewar of 4-cm o.d. To start an

irradiation the C0 60 source (2700 curies) was lowered into the central Dewar so that the samples were 3 cm from the center of the source (approx. 1.6X1017 ev/g min). Without the center Dewar the source (and sup­porting rod) was chilled by the liquid nitrogen in the outer Dewar and iced up so that it could not be raised. Th~ irradiation chamber used has been previously de­scnbed. 7 All samples except methane were irradiated and measured at nOK.

Because of the heat losses, this arrangement could not be .used with. liquid helium. For irradiation in liquid hehum a specIal glass Dewar with a long neck was fitted into another glass Dewar. The outer Dewar was filled with liquid nitrogen and the inner one with helium. The sample tubes were placed in the inner Dewar at the side closest the source, which was located just outside the outer Dewar. The samples were somewhat further from the source in this setup. For measurement of resonance spectra, a sample was quickly transferred from the radiation Dewar to the Dewar associated with microwave equipment. When a sample was transferred from liquid helium to liquid hydrogen, solid hydrogen froze about the sample, and this was taken as evidence that samples did not warm appreciably during transfer.

Irradiation with Light

For sample irradiation by light an all-fused silica Dewar was constructed. The lower end of the inner tube was essentially square (1 X 1 cm) in cross section with two windows opposite each other of plane polished fused silica sealed on. The plane polished fused silica windows on the outer tube of the Dewar were cemented on, and the outer section of the Dewar was connected to the inner section of the Dewar by a ground joint, so that the Dewar could be disassembled for cleaning. An H-4 mercury arc was used with a quartz lens to focus light on the usual sample tubes which were simply im­mersed in the liquid nitrogen in the Dewar. The sample tubes contained quickly frozen (in liquid nitrogen) 2% by volume degassed solutions of Pb(CHa)4 or Pb(C2H s)4 in EPA or 3-methyl pentane. Filters selected light of 2800 to 3400 A for Pb(CHa)4 irradiation and 3000 to 3400 A for Pb (C2Hs) 4. It was found necessary to pump on the liquid nitrogen (resulting temp. "'63°K) ~o produce paramagnetic species. Possibly, the light mtroduced too much heat into the sample for the trap­ping of radicals at n°K.

Resonance Detection Equipment

The paramagnetic resonance detection system was a conventional microwave system (9350 Mc/sec) utiliz­ing a reflection cavity in a static magnetic field. The cavity was constructed as part of a low-temperature Dewar system. This equipment has been previously de-

7 Blomgren, Hart, and Markheim, Rev. Sci. Instr. 24, 298 (1953). Chamber of Fig. 3 combined with turret containing 2700 curie source.

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PAR A MAG NET I C RES 0 NAN C E IN j - I R R A D I ATE D COM PO U N D S 1171

scribed8 as has also the dual modulation system9 used. About 2X 1012 spins of 1, 1-diphenyl-2-picrylhydrazyl are detectable at 77°K.

RESULTS AND DISCUSSION

I. Hydrocarbons

Because of the complexity of spectra obtained with higher hydrocarbons, it was considered quite important to establish the spectra characteristic of the simpler radicals such as methyl. Gordy and McCormick6 ob­tained a quartet from zinc dimethyl x-irradiated at 77°K and assign a total spread of 70-80 gauss to the methyl resonance. In our experiments irradiation of methane at liquid helium temperature, was followed by measurement of the resonance spectrum at liquid hy­drogen temperature with the results shown in Fig. 2. This spectrum was not detected at 4°K apparently be­cause of saturation effects. Evidence for a paramagnetic species was found on the oscilloscope when the sample irradiated at 4°K was placed in the spectrometer for examination at 77°K. However, such a signal decayed completely in a few minutes. This rapid disappearance of radicals in methane at 77°K is to be expected since WaughlO finds self-diffusion in solid methane is rapid at about 60oK. It seems rather certain that the quartet of lines found in irradiated methane is due to methyl radical for the following reasons: First, since the hy­drogen nucleus has a spin of t, a methyl radical can have a total nuclear spin of I z= t and should yield a quartet with relative line intensities of 1: 3: 3: 1, similar to the ratios in Fig. 2. Second, methyl radical can be formed simply from methane by breaking a single C-H bond, while it is difficult to postulate other radicals easily formed from methane which would have three equiva­lent hydrogen atoms. Further, although ion-molecule reactions appear to be important in the gas-phase radiol­ysis of methanell producing major amounts of hydrogen in a molecular process, a considerable fraction of the reaction produces hydrogen atoms and methyl radi­cals.2 •12 Third, hydrogen atoms (the normal H doublet of 506 gauss is observed) are found in an amount com­parable to that of the methyl radical. Other entities such as CH3+ would be expected to be nonparamagnetic and not observable. In methane and all other com­pounds irradiated (except methyl chloride) the reso­nance pattern centered near g = 2.0 (Ho in figures) .

In order to confirm the above identification of methyl radical, dimethyl mercury was irradiated and measured at 77°K. The spectrum in the lower part of Fig. 2 was

8 Delbecq, Smaller, and Yuster, Phys. Rev. 104, 599 (1956). 9 B. Smaller and E. Yasaitis, Rev. Sci. Instr. 24, 991 (1953). 10 J. S. Waugh, J. Chern. Phys. 26, 966 (1957). 11 (a) D. P. Stevenson and D. O. Schissler, J. Chern. Phys.

23, 1353 (1955). (b) D. O. Schissler and D. P. Stevenson, ibid. 24, 926 (1956). (c) F. W. Lampe, J. Am. Chern. Soc. 79, 1055 (1957).

12 Meisels, Hamill, and Williams, J. Chern. Phys. 25, 790 (1956).

..J -0: Z

'" iii

..J -0: Z

'" '"

1-+--\---\---'.----\1 M etho ne

-TIME

H

Dimethyl mercury

FIG. 2. Top: Paramagnetic resonance measured at 200K in methane irradiated 3 hr at 4°K. Total spread, 80 gauss. Line width, 3.5 gauss. Peak heights, obs., 1.01 :2.84 :3.00: 1.16; theor. for methyl, 1: 3: 3: 1.

Bottom: Paramagnetic resonance in dimethyl mercury ir­radiated (40 min) and measured at 77°K. Total spread, 76 gauss. Line width, 10.8 gauss. Peak heights, obs. 1.30: 3.00: 2.74:0.73. Solid line, second derivative of absorption. Dashed line, magnetic field. Both vs time.

measured within a few hours after irradiation and is similar to that obtained with methane.13 The broadening of the lines may be attributed to the difference in en­vironment. (After standing 45 days at 77°K the irradi­ated sample showed a triplet on top of a broad pattern.) Again, we conclude the radical giving the quartet is almost certainly methyl. Our results taken with those of Gordy establish the resonance spectrum of methyl radical.

The next alkane in the normal paraffin series was ir­radiated and its spectrum measured at 77°K. The result­ing complex spectrum (Fig. 3) appears to be a quartet of triplets. Such a result can arise if the odd electron interacts equally with three hydrogens and to a some­what lesser extent interacts with two other equivalent hydrogens. The radical most likely to be formed in irradiated ethane which fits these requirements is the ethyl radical. The greater density of the electron at the nuclei of the three hydrogens farthest from the site of the free valence can be accounted for in terms of hyper­conjugation. The expected electron distribution in the ethyl radical includes the main structure (a) as well as

13 In reference 6 Gordy and McCormick found a 5-line pat­tern in Hg(CH 3)2 irradiated at 77° K with 40 kv x-rays.

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1172 B. SMALLER AND M. S. MATHESON

by hyperconjugation the structures (b) and to a lesser extent structures (c).

HH

(a) H:C:C·

HH

H H (b) H·C::C (c)

H H

HH H:C:C:

HH

Gordy and McCormick6 examined x-irradiated Hg(C2H 5) 2 at nOK and found a symmetrical sextet with a total spread of 130 gauss, which they attributed to the ethyl radical. It is possible that they did not re­solve the individual components because of line broad­ening in Hg(C2H5h For example, in Fig. 2, it is seen that the lines of the methyl radical spectrum are very much broader in the Hg(CHa)2 environment than in the methane. A similar environmental effect in going from ethane to Hg (C2H 5) 2 could broaden the pattern of Fig. 3 into a sextet. Also, note that the total spread in Table I is 115-120 gauss.

Additional support for the interpretation that the pattern obtained in ,),-irradiated ethane is that due to ethyl radical is obtained by irradiating ethylene or ethyl chloride. The spectra obtained with these two compounds are identical with that shown for ethane in Fig. 3 (see Table I). The most likely radical which would be a common intermediate for all three com­pounds is the ethyl radical.

The relative intensities in Fig. 3 are not in agreement with the theoretical ratios 1:2:3:1:6:3:3:6:1:3:2:1, expected for ethyl radical if all line widths are the same. The outermost lines especially, appear weak. Because of the inertia of the recording system it fails to respond correctly to low-amplitude signals.

For ethane and all higher saturated hydrocarbons the hydrogen doublet was relatively weak with respect to the alkyl radical portion of the spectrum. As all ir­radiations except that for methane were carried out at nOK, it is possible that at this temperature the small H

..J <I Z C>

<fl H

-TIME

FIG. 3. Paramagnetic resonance in ethane irradiated 20 min and measured at 77°K. Spread from peak 2 to peak 11-79 gauss, or av separation peaks in quartet 2,5,8,11-26 gauss. Line width, 3.7 gauss. Solid line, second derivative of absorption. Dashed line, magnetic field. Both vs time.

TABLE 1. Lines observed in ethane, ethyl chloride, and ethylene.

Line (Fig. 3) Ethane Ethyl Chloride Ethylene

1 3396.8" 3399.4" 2 3378.5 3378.7 3378.8" 3 3372.2 3373.8 4 3358.1 3359.6 5 3352.6 3352.8 3352.6 6 3346.7 3347.2 7 3331. 2 3332.7 8 3326.2 3326.3 3325.0 9 3320.1 3320.1

10 3306.3 3306.6 11 3299.3 3299.8 3298.2 12 3281. 0 3280.3

• Position of lines in gauss. The relative fields are more accurate than the absolute values. All 12 corresponding lines were observed in ethylene but 8 were not measured with the proton resonance equipment, because of the low intensity of the spectrum. Suggested triplets are: 1. 2, and 4; 3,5, and 7; 6. 8, and 10; 9, 11, and 12.

atoms are not trapped by solid hydrocarbons, but mostly recombine or otherwise react except for a frac­tion reaching the fused silica wall to be trapped there.14

To test this hypothesis, cyclohexane and n-nonane cyl­inders were irradiated in glass tubes and then transferred under liquid nitrogen to unirradiated fused silica tubes for measurement. No atomic hydrogen was found in the samples irradiated bare, although when cyclohexane or n-nonane was irradiated in the usual fused silica tube, an easily measurable hydrogen doublet was obtained.

Irradiated propane gives the spectrum shown in Fig. 4. Actually with longer irradiations two additional weak peaks can be observed on either side of the pat­tern shown, so that we believe this to be an 8-line spec­trum (175-gauss spread), which then must be due to 7 approximately equivalent H atoms in the radical. (The theoretical intensities are 1: 'l': 21: 35: 35: 21: 7: 1 so the end lines should be relatively weak.) The isopropyl

radical CH3CHCH3 explains the pattern obtained if the odd electron molecular orbital includes approxi­mately equal amounts of s atomic orbital contribution from each H atom. Note that the H atoms on the car­bons adjacent to the central carbon are contributing approximately equally with the H on the central car­bon. Scission of the propane to give ethyl and methyl radicals might occur to a small extent, with their pat­terns hidden by the main pattern found in propane. Gordy15 suggests that seven closely spaced doublets of 160 gauss spread observed in irradiated DL-valine may be due to the isopropyl radical. However, our 8-line spectrum, if due to a single species, requires 7 nearly equivalent hydrogens, and the isopropyl radical seems

14 Livingston, Zeldes, and Taylor, Discussions Faraday Soc. 19, 166 (1955).

15 W. Gordy, "Electron Spin Resonance in the Study of Radiation Damage," Gatlinburg Symposium on Information Theory in Biology, October 29-31, 1956, p. 21 (to be pub­lished by Pergamon Press).

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PAR A MAG NET I C R E ~S 0 NAN C E I N r - I R R A D I A TE D COM P 0 U N D S 1173

the only radical to fit the requirements which would originate in a simple process.16

n-Butane, on irradiation, gives an odd line pattern, and this seems to rule out any appreciable carbon­carbon bond rupture to form methyl or ethyl radicals which give even line spectra. It may be noted that the central pair of peaks for methyl and ethyl radicals have separations of 27 gauss, while the butane line separa­tions are 29 gauss, so that the methyl and ethyl peaks should fall between the lines found for butane. In Fig. 4 (bottom), 'Y-irradiated butane shows 7 lines, the end lines being barely detectable. If this spectrum is due to one radical, the most likely species is that obtained by removing an H atom from either carbon 2 or carbon 3, which are equivalent. The resulting radical CHa-CH2-

CH-CHa would have 6 hydrogens which would inter­act strongly, and perhaps approximately equally with the odd electron as required. Here the spectrum shows that through hyperconjugation, the hydrogens on the carbons adjacent to the carbon of the free valence are involved in the hyperfine splitting.

.J .. z '" (J)

\ \ \ \ \ \ \ \ \ \ \ \ \ \ , \ \ ...

.....

I 1, 11\ IVrl I I) 11"1,

/ ' I 1/ / "-

H / ~ / / HO/I I I / -TIME

Propane

H

Butane

FIG. 4. Top: Propane irradiated 20 min. Spread for Speaks, 175 gauss. Average peak separation, 25 gauss. Line width, 11.S gauss. Peak heights, obs., ?:4.S:19.0:35.0:34.3:21.6: 5.S:?, theor. for 1=7/2.1:7:21:35:35:21:7:1.

Bottom: Butane irradiated 20 min. Spread for 7 peaks, 176 gauss. Average peak separation, 29 gauss. Line width, 12.S gauss. Peak heights, obs., ?:9.5:16.9:20.0:17.6:10.2:?; theor. for 1=6/2,1 :6:15:20:15:6:1. Solid line, second derivative of absorption. Dashed line, magnetic field. Both vs time.

16 L. Gevantman and R. R. Williams, Jr. [J. Phys. Chern. 56, 569 (1952) ] find more isopropyl radical than n-propyl but more methyl than either. However, our lower temperature and condensed phase could tend to simplify processes.

.J .. z '" (J)

'\ '\ '\ '\ '\ -\ '\ '\ '\

1\ '\ '\ I , /\

t'-. III , ",71 'II " V -

II '-1 v / 'f / 7 7 "J / 1/ 1\ I

-TIME

Cyclohexane

H

Cyclapentone

FIG. 5. Top: Cyclohexane irradiated 20 min. Total spread for 6 peaks-105 gauss. Average peak separation-21 guass. Line width outer peaks, 13.S gauss. Peak heights, theor. for cyclo­hexyl, 1:5: 10: 10:5: 1.

Bottom: Cyclopentane irradiated 17 hr. Total spread for Speaks, 164 gauss. Average peak separation, 24 gauss. Line width, 10.6 gauss. Peak heights, obs., 3.76:14.S:23.4:35.0: 35.0:25.9:15.2:3.21; theor. for Slines, 1:7:21:35:35:21:7:1. Solid line, second derivative of absorption. Dashed line, magnetic field. Both vs time.

McCauley and Schulerl7 have irradiated liquid butane at 25° with 12 scavenger finding twice as much sec-butyl as n-butyl radical. Methyl and ethyl radicals are also found in somewhat lesser amounts. At liquid nitrogen temperatures one supposes that the sec-butyl radical is favored even more strongly over other possibilities.ls

In an attempt to obtain the spectrum of a -CH2-

CH-CH2-type radical, cyclohexane and cyclopentane were irradiated with results as shown in Fig. 5.19 The six lines of the cyclohexane spectrum do not have the relative intensities expected for the above radical, and the peak separation is less than for compounds below

believed to yield -CH2-CH-CH2-. Possibly this

17 (a) C. E. McCauley and R. H. Schuler, J. Am. Chern. Soc. 79, 400S (1957). (b) The results of Keenan, Lincoln, Rogers, and Burwasser [J. Am. Chern. Soc. 79, 5125 (1957)] in liquid butane (-30 to _500 C) also indicate more sec­butyl than n-butyl radicals.

18 For example in liquid n-hexane H. A. Dewhurst (private communication) finds dimer and products indicating frag­mentation. However, irradiation at liquid nitrogen tempera­ture produced essentially only dimer.

19 H. A. Dewhurst [J. Chern. Phys. 24, 1254 (1956)] inter­prets results as showing preferential rupture of C-H bonds with maintenance of ring system when liquid cyclohexane is irradiated.

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1174 B. SMALLER AND M. S. MATHESON

..J <0: Z

'" '"

1\ \ \ \ \ \ \ \ \ \ \ \ \ \ \n-Hexane\

"\---\ \ \' \ \ \ \

_11

J/ ~/I

I VI II Ii 'II I 11 I I I L' I V; v I -'1 I

II I I I I I J I /

\\\"-\\ \ \ \

\ .i ~ \ \ \ t-. \ \ \ -~- \ \' \ \\ \ 1 n-Nonane \

:'--l

t- I I III' "

/"'\ / / /1 N 'r-/1 I/, I II 1/ I I/) J I

I 'I IU I 'I I /1 1 I I III I ,ijHo I l{ I I /

- TIME

FIG. 6. Top: n-hexane. Total spread, 171 gauss.

H

Bottom: n-nonane. Total spread, 166 gauss. Both irradiated 20 min. Solid line, second derivative of absorption. Dashed line, magnetic field. Both vs time.

is a 5-line pattern doubled with the weak outer peaks hidden by a broad underlying resonance.

With cyclopentane 8 lines, not 6 or 10, were obtained whether the irradiation was 20 min or 17 hrs, suggesting 7 H atoms are possibly involved. This result is not un­derstood at present. The spectra obtained with cyclic compounds, including the complex cyclopropane spec­trum, all showed a long relaxation time.

For examples of longer members of the n-alkane series, n-hexane and n-nonane were irradiated and the resulting resonances observed. With moderate resolu­tion the n-hexane spectrum was essentially identical with that shown for butane in Fig. 4, and the n-nonane showed only a very minor difference in appearance. However, with better resolution, n-hexane and n-nonane give the spectra in Fig. 6, whereas the n-butane pattern is unchanged from the simple 7-line structure of Fig. 4. There seems to be little evidence in Fig. 6 for an appre­ciable amount of an even line structure. This is surpris­ing, since one might expect some removal of hydrogen from the middle of the chain, especially in n-nonane. Possibly a considerable fraction of the radicals present

is of the type obtained in n-butane (-CH2-CH-CIIs) with the complexity caused by the nonequivalence of hydrogens or to the presence of other radicals such as . CH2-CH2- but not . CHa or . CH2CHa.20 Since n-

20 M. Magat and R. Viallard, [J. chim. phys. 48, 385 (1951)] find that in the mass spectrometer for normal paraffins C-C rupture is less likely at the ends and that a maximum occurs for C 3-C. bond rupture.

nonane seemed not to be long enough to give a spectrum characteristic of a polymethylene chain, the paraffins n-octadecane, n-octacosane and Marlex-50 (linear polyethylene) were irradiated. The polyethylene gave the simplest result (Fig. 7), giving a six-line spectrum after 20 min irradiation and a similar increasingly stronger pattern of absorption up to 17 hr exposure. The Marlex-50 is reported to have 1.5 double bonds per 1000 carbon atoms and less than 1.5 methyl groups per 1000 carbons, so it seems very probable that the results correspond to those to be obtained for a polymethylene chain.

The octadecane and octacosane were passed through silica gel at 65°C to remove any unsaturates that might be present. The n-octadecane gave a 6-line spectrum for 20 min or 20 hrs exposure (Fig. 7), but for intermediate irradiations the central part of the spectrum became rather complex. It is presumed that the complexities may arise from impurities, while Fig. 7 (bottom) shows the result obtained when a long normal paraffin loses an H atom from the middle of the chain. After 20 min irradiation n-octacosane gave 8 lines (top, Fig. 8-with greater amplification the outer lines can be made quite strong) rather similar to the pattern in cyclopentane. With longer irradiations the spectrum became more complex, finally simplifying to six lines with some struc­ture (bottom, Fig. 8). The octacosane may possibly

..J <0: Z

'" '"

\ \ .. \ \ \ \ \\ ~\ \ \ \ \\ 1\\

r r - +;--.../ I I '/ I. /

I 1/ V 'J II ! '1 I

I I I I -TIME

Marlex- 50

H

\ \

n- Octadecane

_\

~L-

/ I

/

FIG. 7. Top: Marlex-50 irradiated 17 hrs. Spread for 6 peaks, 154 gauss. Average line width, 16 gauss. Average peak separa­tion, 31 gauss. Peak heights, obs_ 2.31 :4.88:9.56: 10.0:4.84: 2.36.

Bottom: n-octadecane irradiated 20 hrs. Spread for 6 peaks, 165 gauss. Average line separation, 33 gauss. Peak heights, obs., 3.24: 6.36: 10.0: 9.50: 6.08 :3.30. Solid line, second deriva­tive of absorption. Dashed line, magnetic field. Both vs time.

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PARAMAGNETIC RESONANCE IN ,-IRRADIATED COMPOUNDS 1175

contain some molecules with a methyl side chain,21 in which case if the radiation effect were highly specific the original 8 lines could be accounted for by -CH2

C (CHa) -CH2-. Although the results with cyclopen-

tane have not been accounted for, it seems probable that the radical -CH2-CH-CH2- gives a six-line

paramagnetic resonance spectrum of about 160-gauss spread.

Two examples of branched, saturated hydrocarbons have been irradiated and examined; they are isobutane and neopentane. Results are shown in Fig. 9 and are quite complex. The neopentane pattern consists of an odd line spectrum and an even line spectrum in which the lines are narrower. Simple removal of a hydrogen to

give a (CHah CCH2 radical would most likely give a simple triplet and may account for part of the three central peaks of the odd line pattern. Although the normal paraffins give little evidence of C-C bond rup­ture, branched hydrocarbons are more susceptible to fragmentationl7b ,22 at least in the mass spectrometer.20

If a . CHa splits off, the resulting tertiary butyl radical

-' <t Z C>

en \ \ \ \ \

-\ \ '\ " \ \ \ \' \ \ \ \1 \ \

n \J " JV

II, '1-,

/ / '/ I / ,j V / / / / /',

/ / / / / -TIME

n-Octacosane 20 min. Irrad.

H

17 hrs. Irrad.

FIG. 8. Top: n-octacosane irradiated 20 min. Spread for 8 peaks, 182 gauss. Average peak separation, 26 gauss. Peak heights, obs., ?:9.3:22.5:33.4:35.0:22.4:10.1:?

Bottom: n-octacosane irradiated 17 hr. Spread for 6 lines, 157.3 gauss. Average peak separation 31 gauss. Line width, 14 gauss. Peak heights, obs., 3.87: 7.38: 10.0:9.55 :6.82 :3.50. Solid line, second derivative of absorption. Dashed line, mag­netic field. Both vs time.

21 M. R. Pulver (Eastman Organic Chemicals Department), private communication.

22 Methyl group branches increase the CH4 yield in hydro­carbons. E. Collinson and A. J. Swallow, Chern. Revs. 56,471 (1956).

-' <t Z C>

en

-TIME

H

FIG. 9. Top: Neopentane irradiated 20 min. Even narrow line spectrum: Average line separation, 23 gauss. Peak heights, obs., ?:? :34 :95: 109: 126 :90:34:?:?; theor. for 1=9/2, 1 :9: 36:84:126:126:84:36:9:1. Line width-3.7 gauss. Odd line spectrum: Average line separation, 24 gauss. Peak heights, obs., 0.26:0.92 :2.00:0.77 :0.27, theor. for (CHa)a-C-CH2,

0: 1:2: 1 :0. Line width, 4.3 gauss. Bottom: Isobutane irradiated 20 min. Total spread between

peaks indicated by vertical lines, 162 gauss. Solid line, second derivative of absorption. Dashed line, magnetic field. Both vs time.

should yield a lO-line spectrum. Only 8 lines are ob­served in the even line structure, but since the theoret­ical intensities for (CHahC, are 1:9:36:84:126:126: 84:36:9: 1 the outermost lines would be very difficult to observe. The narrowness of the lines would indicate exactly equivalent H atoms as in t-butyl radical. The 4 methyl lines might possibly be hidden under the t­butyl pattern (23-gauss separation of lines). Complex fragmentation yielding isopropyl radical would give 8 lines, but the isopropyl spread for 8 lines is 175 as com­pared to 160 for the 8 even lines in neopentane. Without substantial rearrangement of the molecule, an odd line spectrum of more than 3 lines is difficult to account for.

The isobutane spectrum, except for the central peak, may be an even line pattern. Perhaps, t-butyl radical is present as the tertiary hydrogen is known to be particu­larly easily removable.2a The appearance of the isobu­tane spectrum changes as the dose is increased.

Two unsaturated hydrocarbons have been irradiated, ethylene and propylene. Ethylene, as previously noted, gives a spectrum identical with that obtained from

23 In reference 17(b) isobutane irrad. at -30 to -50 0 C gives more fragmentation than the n-butane and more (CHa)z­CHCHz than (CHa)aC"

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1176 B. SMALLER AND M. S. MATHESON

...J ~ ~---r---+.~H+~~----H z ~ ~--1---~~~r+~~--H

H

-TIME

FIG. 10. Propylene irradiated 20 min. Total spread, 62 gauss. Average line separation, 15.5 gauss. Line width, 8.2 gauss. Solid line, second derivative of absorption. Dashed line, mag­netic field. Both vs time.

ethane (Fig. 3). If this pattern is due to the ethyl radical, hydrogen atom addition to ethylene must occur. No hydrogen atom doublet was observed in ethylene or propylene, indicating that these scavenged H atoms which might otherwise be trapped on the silica wall.

The other fragment, perhaps, H 2C=CH, either disap­pears (possibly by combination with itself or H atoms or by polymerization) or is not observable.

Propylene gives a 5-line pattern but the line separa­tion is 15 gauss (Fig. 10) as compared with 25 gauss ob­served in propane or 29 gauss in n-butane, so that the separation is definitely less than obtained with saturated hydrocarbons. The 5 lines are beautiful confirmation of the usual picture of the allyl radical as a superposition of the structures (a) and (b). 24

(a) CH2=CH-CH2 (b) CH2-CH=CH2•

In this radical the middle hydrogen causes no resolvable splitting of the lines.

The results with the normal paraffin hydrocarbons irradiated and measured at low temperatures suggest some generalizations. The radicals observed are formed by loss of a single hydrogen atom. (Processes leading to direct formation of molecular products do not concern us in this study.) Further, in spite of the high energies available from the secondary electrons during irradia­tion, specific hydrogen atoms are preferably lost from the molecule. Relatively, little C-C bond rupture oc­curs, although the C-C bond is weaker than the C-H bond. The preferential loss of H atoms may be due to the "cage effect" in that H atoms can escape, whereas larger fragments cannot move through the rigid ma­terial. Also the higher frequency of vibration for C-H

2' W. E. Moffitt and C. A. Coulson, [Trans. Faraday Soc. 44, 81 (1948)] have calculated that for the radicals of : type CH2(CHhn-1CH2, the free valence is fairly evenly distributed on odd indexed carbons with none on even carbons. ' ,

Lefkovits, Fain, and Matsen [J. Chern. Phys. 23, 1690 (1955)] calculate nearly the same energy 'with structures (a) and (b) as with a more complete treatment.

would favor C-H rupture, when energy sufficient to rupture either C-C or C-H is available. For highly branched hydrocarbons some C-C bond breaking ap­parently does occur.

It is of course possible that many of the radicals ob­served are not formed by rupture of an initially excited or ionized molecule. An H atom must generally be formed with considerable kinetic energy and could ab­stract an H atom (the reaction must be selective) from a nearby molecule.26 Such a process would also account, at least in part, for the absence of H atoms in the organic solid.

II. Oxygenated Compounds

A number of oxygenated compounds have also been irradiated and examined. In a sense, the simplest of these is 1,3, 5-trioxane for which a doublet after irradi­ation was expected. The species responsible for this

doublet would be the radical CHOCH20CH20 formed L-_____ I

by loss of an H atom. However, a triplet superimposed on another resonance seems to be present. Possibly this . . is the radical formed by ring rupture, CH20CH20CH20 yielding a triplet and a singlet. Since C12 and 0 16 do not have nuclear magnetic moments, contributions from the atomic orbitals of these atoms to the molecular orbital of the odd electron do not affect the hyperfine splitting.

For methyl alcohol and ethyl alcohol, resonances are obtained similar to those obtained by Luck and Gordy.5

-TIME

Methanol

H

Dimethyl ether

FIG. 11. Top: Methanol irradiated 90 min. Total spread, 36 gauss. Average line separation, 18 gauss.

Bottom: Dimethyl ether irradiated 21 min. Total spread 35 gauss. Solid line, signal. Dashed line, magnetic field. Both vs time.

26 Gibson, Ingram, Symons, and Townsend [Trans. Faraday Soc. 53, 914 (1957)] find that OH radicals abstract H atoms selectively from isopropanol.

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PARAMAGNETIC RESONANCE IN ,{-IRRADIATED COMPOUNDS 1177

We find the total spread for the methanol triplet 36 gauss and for the ethanol quintet 89, while Luck and Gordy obtain 30 and 93 gauss, respectively (see Figs. 11 and 12). However, we prefer to assign these reso-. . nances to CH20H and CHaCHOH rather than to (CHz)+ and (C2I4)+. Partially, our preference is based on the results of radiation chemistry in the liquid state. McDonell and Gordon26 and Skraba, Burr, and Hess27

find ethylene glycol and hydrogen the major products in methanol with 'Y rays and C14 betas respectively, in agreement with the interpretation that . CH20H and . H are formed. In the case of ethyl alcohol, McDonell and Newton28 find Hz and![CH(CHa)OH]z as::chief products and Burr29 has shown that the hydrogen atom removed by irradiation comes from the carbinol carbon.

Dimethyl and diethyl ethers give spectra on irradia­tion similar to methanol and ethanol respectively. This indicates the hydroxyl hydrogen is not involved appre­ciably in the spectra obtained with the two alcohols. The probable radicals are . CH20CHa and CHa-

CHOCH2CHa.

..J <l Z ~

tii \ \ \ \

\ \\\ \ \ \ \ \ \ \

-- 1\

I f-./ .~ r-r:::-:

V /, 1/ / / # / /

/ '/ / / / / lr / /

-TIME

Ethanol

H

Oiethyl ether

FIG. 12. Top: Ethanol irradiated 21 min. Spread for 5 peaks, 89.3 gauss. Average line separation, 22 gauss. Solid line, second derivative of absorption.

Bottom: Diethyl ether irradiated 20 min. Spread 5 peaks, 86 gauss. Average line separation, 22 gauss. Solid line, signal. Dashed line, magnetic field. Both vs time.

26 W. R. McDonell and S. Gordon, J. Chern. Phys. 23, 208 (1955).

27 Skraba, Burr, and Hess, J. Chern. Phys. 21, 1296 (1953). 28W. R. McDonell and A. S. Newton, J. Am. Chern. Soc.

76, 4651, (1954). 29 J. G. Burr, Jr., J. Am. Chern. Soc. 79, 751 (1957).

...J <l Z ~

Ul

.i _\ -'" \ \ \ J,. ~ \ \ \ l

n-Propanol

\ \ ~ \ \ \ - P 1\

f11- I \V' ~

J 1""'- -~ 1/

1 1 'I / / 1 1 1 / / 1

! / / / .L .L

H

r--T-"---''\-----\--+--\---\-l Isoproponol

,-TIME

FIG 13. Top: n·Propanol irradiated 20 min. Spread 5 peaks, 80 gauss. Average line separation, 20 gauss.

Bottom: Isopropanol irradiated 20 min. Spread 7 peaks, 115 gauss. Average line separation, 19.5 gauss. Solid line, second derivative of absorption. Dashed line, magnetic field. Both vs time.

The patterns obtained with n-propyl alcohol and iso­propyl alcohol are shown in Fig. 13. The ll-propyl com­pound gives 5 (perhaps . CHzCH2CH20H) or possibly 7 peaks (CHsCHCHzOH). The latter may fit the results . of reference 28 on this alcohol. The expected 4-line spec-trum (CHaCH2CHOH) was obtained photochemically . in reference 25. Isopropyl alcohol gave the expected 7-line spectrum. The heights are incorrect for CHa-

C (OH) CHa, but annealing intensifies the 3 and 5 lines so that this radical is believed present.

Allyl alcohol after short irradiations gave a 5-line spectrum (total spread 66 gauss) quite similar to that for propylene (62 gauss, Fig. 10). Thus it would appear that the OH in this one case is removed either initially or subsequently by abstraction by an atom or radical. In ref. 25 allyl alcohol gave a quartet, using H 20 2 and uv. However, an OH could well be expected to abstract

an a-hydrogen to give H 2C= CH-CHOH, rather than abstracting an OH. With better resolution the five lines showed structure possibly caused by other species.

III. Chlorinated Compounds

The ethyl chloride spectrum as discussed previously was identical with that obtained from ethane or ethylene. Unfortunately, irradiation of cis-l ,2-dichloro­ethylene and ethylene dichloride did not prove informa­tive, since only the broad peaks of Fig. 14 were obtained.

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1178 B. SMALLER AND M. S. MATHESON

.J .. z C>

VI

\ \ \

" "'-

/ /

/

\ \ \ \ \ \ \ ~ \

i\..

" -i'-J " "

" " / / / / / /

/ / /

Ethylene dichloride

H

1-\---\--\----'< ____ --\--+---1 c i $- 0 i ch I oret hy Ie ne

-TIME

FIG. 14. Top: ethylene dichloride irradiated 21 min. Line width, 75 gauss.

Bottom: cis-dichloroethylene irradiated 80 min. Line width, 80 gauss. Solid line, signal. Dashed line, magnetic field. Both vs time.

The results obtained with methyl chloride (Fig. is) are quite complex, involving an asymmetric pattern. Whereas ethyl chloride appears to lose a chlorine atom by ejection or abstraction, the behavior of methyl chloride does not seem to be similar.

IV. Tetraalkyl Lead Compounds

Lead tetramethyl and lead tetraethyl were irradiated pure with "I rays and in a 2% solution in a frozen glass with uv light. Each compound gave the same resonance spectrum with uv irradiation as with "I irradiation, ex-

.J .. z C> U)

-TIME

H

FIG. 15. Methyl chloride irradiated 40 min. Solid line, second derivative of absorption. Dashed line, magnetic field. Both vs time.

cept that only in the "I-ray case was the normal hydrogen doublet observed. The dominant spectra obtained with the two compounds did not correspond to methyl or ethyl radicals. The isotopes in lead are mainly of zero nuclear spin, so that in most fragments the lead should not complicate the spectrum, but a less intense and more complex pattern could arise from fragments with lead of nonzero nuclear spin.

ACKNOWLEDGMENTS

We wish to express our gratitude for generous gifts of the tetraalkyl lead compounds from Mr. Richard Scales and Dr. Roy Sugimoto of Ethyl Corporation; of dimethyl mercury from Dr. Richard Bernstein of the University of Michigan; and of Marlex-SO and n­octacosane from Dr. A. A. Miller of General Electric Company. We wish to thank Dr. William Hayes who carried out some resonance measurements, J\fr. E. L. Yasaitis and Mr. E. C. Avery for resonance equipment maintenance and aid, and Mr. William Mulac for carry­ing out many irradiations. Also we wish to express our gratitude to Dr. O. C. Simpson for very helpful advice and encouragement.

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