Radiat. Phys. Chem. Vol. 29, No. 5, pp. 393-398, 1987 Int. J. Radiat. Appl. Instrum. Part C Printed in Great Britain. All rights reserved

0146-5724/87 $3.00 +0.00 Copyright © 1987 Pergamon Journals Lid

ESR STUDIES ON THE REACTIONS OF THE CATION RADICALS OF 2'METHYL-2-BUTENE

AND 2,3-DIMETHYL-2-BUTENE IN LOW TEMPERATURE MATRICES

JUN FUJISAWA, SHIN SATO and KAZUO SHIMOKOSHI I

Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan and tDepartment of Chemistry, Tokyo Institute of Technology, Ookayama,

Meguro-ku, Tokyo 152, Japan

(Received 29 September 1986)

Abstract--The reactions of the cation radicals of 2-methyl-2-butene and 2,3-dimethyl-2-butene in low temperature matrices have been investigated by ESR spectroscopy. The cations of both olefins were produced by gamma-irradiation of their dilute solutions in various halocarbons at 77 K. The changes of the spectra were examined by varying the temperature between 77 K and the melting point of each matrix. In the matrices of CCI2FCC12F, CC12FCCIF2, and CCIF2CCIF 2, elevation of temperature to about 100 K gave rise to the conversion of the cations into corresponding allyl radicals. In the case of the CCI2FCCI2F solution of 2,3-dimethyl-2-butene, the charge resonance type dimeric cation was observed as the transient species of the process. On the other hand, the monomer cations remained as they were in the matrices of CCI 4, CCi3F, and CCI3CF 3 upon wanning up to about 145 K. Further elevation of temperature simply resulted in the disappearance of the cations. Possible mechanisms for the formation of the allylic radicals are discussed.

INTRODUCTION

The first extensive ESR study on olefin radical cat- ions has been made by Shida et al. °'2) They employed CCI3F as a rigid matrix medium and found that the temperature effect upon the cation could be studied conveniently without causing the charge neutral- ization reaction. Utilizing this feature, they clarified much concerning the behavior of the isolated olefin cations/') However, little has been learned about the bimolecular reactions of the cations with their neutral parents. Since the cations were stably trapped in a CCI3F matrix, ion-molecule reactions through mo- lecular diffusion did not necessarily take place even in the high temperature range.

In order to overcome this limitation, the concen- trated solutions of some olefins in CCI3F were irra- diated. The formation of the allyl radical was found by Shiotani et al. by this method. °) This result was well interpreted in terms of ion-molecule reaction because the solute dimer or aggregates were likely present in the concentrated solution. However, evi- dence for the presence of the precursor was not obtained, and a possibility of direct radiolysis could not be completely ruled out.

An alternative method which enables the direct detection of ion-molecule reactions is to employ less rigid matrix media which allow the diffusion of the solute molecules to some extent at higher tem- peratures. During the course of our study, ~4) we have found that CCI2FCCI2F and CCIF2CCIF 2 are the suitable matrix media for this purpose as well as

CCI2FCCIF 2 introduced by Iwasaki et aU 5) and suc- cessfully obtained the evidence for the ion-molecule reactions of several olefin cations in these media. ¢6.7) In the present study, these compounds were em- ployed along with CC14, CCI3F, and CCi3CF3 as matrices to explore the fate of the cation radicals of 2-methyl-2-butene and 2,3-dimethyl-2-butene.

EXPERIMENTAL

Commercially available olefins (2-methyl-2-butene and 2,3-dimethyl-2-butene) and halocarbons (CCI4,

CCI3F, CCl3CF3, CCI2FCCI2F, CCI2FCCIF2, and CCIF2CCIF2) from Tokyo Kasei Co. were used with= out further purification. Samples of solid solutions containing small amounts of each olefin (0. I-0.5 tool%) in the various haiocarbons were pre- pared by the standard vacuum technique using a vacuum line with a mercury free pressure gauge (MKS Baratron BHS-AS-100) and then sealed off in a Suprasil quartz tube of 3 mm outer diameter. The samples thus prepared were irradiated at 77 K to a dose of about 0.5 Mrad (1 Mrad --- 104j kg -I) to gen- erate the cation radicals.

The ESR measurement was carried out with a JES FE-2XG spectrometer calibrated with a proton mag- netic resonance probe and a digital frequency counter (Takeda Riken Co. TR-5211C). The sample tem- perature was closely regulated with a temperature controller, and the spectra were recorded at suitable intervals as the temperature was raised progressively from 77 K to the melting point of each matrix.

393

394 JuN FuJls^wA et al.

R E S U L T S

The ESR spectrum of 2-methyl-2-butene cation in a CC12FCCIF 2 matrix has recently been observed by Toriyama e t a l f l ) They reported that the cation gave exceptionally large CH proton coupling (see Table 1) which might not be explained without assuming a strongly twisted structure. We observed similar spec- tra also in the matrices of CCI2FCCI2F and CCIF2CCIF2 at 77 K and assigned them to the cation. The determination of the coupling constants, how- ever, was not straightforward since the cation ex- hibited rather complicated and poorly resolved fea- tures in these matrices.

On the other hand, it was found that the employ- ment of a CCI3CF 3 matrix gave considerably wellr resolved spectrum of the cation in the high tem- perature range (100-143 K). Figure l(a) shows the spectrum observed in this matrix at 113 K. The essential feature of the observed spectrum was fairly well reproduced by the simulation spectrum shown in Fig. 1 (b) except for the small variation of line shapes and intensities. The parameters used for the calcu- lation are a(1H) = 11.0 G, a(3H) = 25.0 G, a(6H) = 2 0 . 6 G , and g - - 2 . 0 0 2 6 (the assignment is shown in Table 1). It is noteworthy that the CH proton coupling constant of the cation in CCI3CF3 is appreciably different from the reported value for the cation in a CCI2FCCIF2 matrix: s) The present value is, however, in good agreement with the CH or CH2 alpha-proton coupling constants of other olefin cat- ions. ~.° This indicates that the observed CH proton coupling constant is rather appropriate for a planar structure of the cation. We tentatively assigned the observed spectrum to the cation radical of 2-methyl-2-butene which undergoes rapid inter- change between the two optimal conformations to give apparent planarity.

With the use of the other matrices, i.e. CC14 and CCI3F, the cation gave somewhat different spectra possibly because of the manifestation of large aniso- tropy for the CH proton. The essential features are,

however, nearly the same as the one observed in a CCI3CF 3 matrix.

The spectra of the cation radical of 2-methyl-2-butene in CCI2FCCi2 F, CCI2FCCIF 2, and CCIF2CC1F 2 changed irreversibly into another com- plicated pattern upon warming to about 100K. Figure 2(a) shows the spectrum observed in CCI2FCCi2F at 123 K. The main feature of the spectrum consists of a nine-line hyperfine multiplet with an average spacing of about 15 G. This suggests the formation of allylic radicals. Shown in Fig. 2(b) is the spectrum calculated by using the h.f.c, con- stants of a (IH)=3.6, 13.3, and 14.0G and a(3H)= 12.2 and 15.4G. These parameters accord well with the h.f.c, constants of the l,l-dimethylallyl radical, t9) The good agreement between Figs 2(a) and (b) suggests that the cation is converted mainly into the 1,l-dimethylallyl radical.

The cation radical of 2,3-dimethyl-2-butene has been studied by several workers ".~°-m and proved to yield ESR spectrum of thirteen equally spaced lines separated by about 17 G. The spectrum in Fig. 3(a) obtained for an irradiated solution of 2,3-dimethyl- 2-butene in CCI3CF 3 at I 13 K is consistent with those previously reported though only nine of thirteen lines can be discerned in the figure. The full set could be obtained with the high gain of instrument and was analyzed by using the parameters of a(12H) = 17.4 G and g = 2.0031, as shown in Fig. 3(b).

Almost identical spectra except for line-width could be observed when other matrices were em- ployed. The magnetic parameters obtained were ranging from 16.9 to 17.4G for the h.f.c, constants and from 2.0030 to 2.0032 for g-values, respectively, as listed in Table 1.

The spectral changes upon warming were also observed for the irradiated solutions of 2,3-dimethyl-2-butene in CC12FCCI2F, CCi2FCCIF 2, and CCIF2CCIF 2. Figure 4(a) shows the spectrum of the species formed in CCI2FCCIF 2 after irreversible change at 108 K. This spectrum was well reproduced by using the h.f.c, constants of a ( IH)= 12.5 and

Table I. Isotropic ESR parameters of the cation radicals of 2-methyl-2-butene and 2,3-dimethyl-2-butene T

h.f.c, constants (G) Temp.

Radical cations Matrix (K) g -fa~:tor a(CH) a(CH3) Ref.

(CH 3)2C---"CH(CH3) + CCI3CF 3 I 13 2.0026 11.0 20.6 25.0

CCI2FCC|2F 77 62.0 16.7 (8) (26.0)" (I 6.8)"

(21.8)"

( C H 3 ) 2 ~ ( C H 3 ) 2 + CCI4 143 2.0031 17.4 CCI3F 77 17.2 (I) CCI~CF 3 123 2.0031 17.4 CCl2FCCI2F 77 2.0030 17.0 CCI2FCCIF 2 77 2.0032 16.9 CCIF2CCIF 2 77 2,0030 17. I In so|ution b 16.6 (10) 3-MW 77 2.0025 16.7 (I 1)

*Alternative coupling constants proposed by Toriyama bln cobaltic afetate-trifluoroacetic acid solution. C3-Methy|pentane.

e t al.

ESR studies on cation radical reactions 395

a

i'

2O G

Fig. 1. (a) ESR spectrum of 2-methyl-2-butene cation in CCI3CF 3 at 103 K. (b) Simulation for (a) using the ESR parameters given in Table 1 and a Gaussian line-width of

AH~ ffi 2.0 G.

b

20 G :

Fig. 2. (a) ESR spectrum of the irradiated CCI,FCCI2F solution of 2-methyl-2-butene at 123 K, showing the feature assigned to l,l,-dimethylallyl radical. (b) Simulation for (a) using the ESR parameters given in Table 2 and a Gaussian

fine-width of AH,~ ffi 1.3 G.

2O G

Fig. 3. (a) ESR spectrum of 2,3-dimethyl-2-butene cation in CCI~CF 3 at II3K. (b) Simulation for (a) using the ESR parameters given in Table 1 and a Gaussian line-width of

AHm, I = 2.5 G.

13.3 G and a(3H) = 3.0, 12.9, and 15.9G (Fig. 4(b)). These values were ascribed to the i,l,2-trimethylallyi radical on the basis of the internal consistency with the coupling constants of other ailylic radicals. (~3)

When CCIF2CCIF 2 was used as the matrix, how- ever, a completely different spectral pattern was observed at 108 K, as shown in Fig. 5. This spectrum consists of at least 17 lines with an average splitting of 7.8 G, which is about one-half of the separation found for the 2,3-dimethyl-2-butene cation. This agrees with the reported spectrum of the charge resonance type dimeric cation observed in the irra- diated 3-methylpentane solution of 2,3-dimethyl- 2-butene at 77K. cm~,~2) Further elevation of tem- perature of the system immediately caused the decay of the dimer cation giving no paramagnetic species.

Of additional interest is the result obtained for the CCI2FCCi2F solution in which both the dimer cation and the allylic radical were observed. An irreversible spectral change of the system upon warming is dem- onstrated in Fig. 6. It can be seen that the dimer cation was initially formed upon the limited warming and then replaced by the l,l,2-trimethylallyl radical with an increase in temperature. This suggests the decomposition of the charge resonance complex into allylic radical and carbonium ion. (14)

DISCUSSION

It is found that the spectra of irradiated solutions of 2-methyl-2-butene and 2,3-dimethyl-2-butene in CCI,FCCI,F, CCi~FCCIF 2, and CCIF, CCIF2 change

396 JUN FUJiSAWA et al.

Table 2. Isotropic ESR parameters of dimethyl and trimethyl-allyl radicals

h.Lc. constants (G) Temp.

Allylic radicals Precursor (K) a(CH) a(CH2) a(CH3) Ref.

(CH3)2CCHCH 2 (CH 3)2C------------------~CH (CH 3) +~ 123 3.6 13.3 12.2 14.0 15.4

(CH 3)2C------CH(CH~) 153 3.56 13.33 12.22 14.06 15.35

(CH3)2CC(CH3)CH 2 (CH 3)2C--"-C(CH3)~ b 133 12.5 3.0 13.3 12.9

15.9

'In a CCI2FCCI2F matrix, bin a CC12FCCIF 2 matrix.

(9)

a

20 6

Fig. 4. (a) ESR spectrum of the irradiated CCI2FCCIF2 solution of 2,3-dimethyl-2-butene at 108 K, showing the feature assigned to l,l,2-trimethylallyl radical. (b) Simu- lation for (a) using the ESR parameters given in Table 2 and

a Gaussian line-width of AHmsl = 1.6 G.

irreversibly at around 100 K from the original features of these cations to the new patterns, whereas the spectra of cations remain as they are in the matrices of CC14, CCI3F, and CCI3CF3 up to about 145 K. In the latter group of matrices, further elevation of temperature resulted in a monotonous decay of the signals due to the cations without giving any new specrum. This uniform decay may be attributed to the neutralization between the cation and halogen anion.

On the other hand, the formation of the new species accompanied by the decay of the cations in

t The stereospccificity of the conversion of olefln cations into allylic radicals was discussed in Ref. (6).

the former group of matrices suggests the in- volvement of other processes. The self-decomposition of the cation is probably not the case because the conversion was observed only in particular matrices and at a temperature characterized not by the solute olefin but by the matrix employed. The ion-molecule reaction between the cation and the parent olefin molecule through diffusion in the softened matrix provides the most plausible explanation for the ob- served results.

For the irradiated solutions of 2-methyl-2-butene in CCI2FCCI2F, CCI2FCCIF 2, and CCIF2CCIF2, the 1,l-dimethylallyl radical was mainly observed upon warming. This radical may be formed by the follow- ing reaction:

(CH3)2C------CH(CH3) + + C5H10

(CH3)2CCHCH 2 + CsH~. (1)

There are additionally two possibilities in forming allylic radicals from the cation; i.e. deprotonation from the cis or trans methyl protons in the C(CH3)2 group may lead to the formation of the cis or t r a n s - l , 2 - d i m e t h y l a l l y l radical.t The spectral pattern expected for these radicals is a septet of quartets, which is different from the observed spectral pattern. Therefore, the present result suggests the de- protonation of a methyl proton from the CH(CH3) group of the cation as the process forming allylic radicals.

2O 6 I

Fig. 5. ESR spectrum of the irradiated CCIF2CCIF2 solution of L3-dimethyl-2-butene at 108 K, showing the feature

assigned to the dimer cation,

ESR studies on cation radical reactions 397

a

C

20 G I "

Fig. 6. ESR spectra of the irradiated CCI2FCCI2F solution of 2,3-dimethyl-2-butene (a) 103 K; (b) at 113 K; (c) at 123 K, showing the irreversible change of the spectra with

temperature

formation of ailylic radicals in solid phase radiolysis of olefins. C~5'~6)

The formation of the charge resonance type dimeric cation observed for the irradiated 2,3-dimethyl-2-butene solution in CC1F,CCIF, and CC12FCCI2F further suggests the involvement of the following reaction:

(CH3)2C-----'C(CH3)~ " + C6H12

[(CH3)2C---C(CH3)2]~. (4)

This reaction is not in conflict with reaction (2) since the successive decomposition of the dimer cation into allylic radical was observed in CCI2FCCI2F at higher temperatures.

[(CH3)2C-C(CH3)z]~"

(CH3):CC(CH3)CH2+C6H~ (5)

Therefore, the dimer cation of 2,3-dimethyl-2-butene may be regarded as an intermediate of the de- protonation reaction (2). This indicates that the deprotonation reaction is well characterized as a bimolecular process. The decomposition reaction (5) has already been discussed by Ichikawa et al. and suggested to involve an intramolecular proton transfer, o2)

Whether the dimer cation of 2,3-dimethyl-2-butene is observed or not depended on the matrix used. The dimer cation was observed in CCI2FCCI2F and CCIF2CCIF2, while in CC1F2CCI2F only allylic radi- cal was observed. Since reaction (4) is considered to be a diffusion controlled process, the temperature at which the reaction occurs depends mainly on the Softening point of each matrix, which is subject to the experimental conditions such as solute concentration and freezing rate. Further investigations are needed for the quantitative estimate for the reactions (4) and (5).

In the case of the irradiated CCI~FCCIF2 solution of 2,3-dimethyl-2-butene, the cation was converted into the 1,1,2-trimethylallyl radical, which is the sole isomer expected to be formed by the deprotonation from the cation and may be considered as due to the reaction shown below:

(CH3)2C---"--C(CH3)~" + C6HI2

--, (CH3)eCC(CH3)CH2 + C6H~. (2)

We have observed similar reactions giving the corresponding allylic radicals also for the cations of propylene and four butene isomers (6~ by using the same method as in the present study. Comparing these results with the present one, the essential pro- cess may consistently be described as the following deprotonation reaction:t

RaRbC-----CRcCH~ --* RoRbCCR~CH2

+H +(Ro~.c=H or CH3). (3)

Such a process has often been assumed to explain the

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