Electron transfer from 2-methyltetrahydrofuran radical to tetracyanoethylene at low temperatures

Radiat. Phys. Chem. Vol. 15, pp. 377-382 Pergamon Press Ltd., 1980.

Printed in Great Britain

ELECTRON T R A N S F E R FROM 2-METHYLTETRAHYDROF U R A N RADICAL TO T E T R A C Y A N O E T H Y L E N E AT LOW T E M P E R A T U R E S M. HOSHINO, S. ARAI and M. IMAMURA The Institute of Physical and Chemical Research, Wako-shi, Saitama 351, Japan (Received 22 February 1979; in revised form 14 May 1979) Abstract The formation mechanism of tetracyanoethylene(TCNE) radical anions in irradiated 2-methyltetrahydrofuran(2-MTHF) solutions of TCNE has been studied at low temperatures using matrix isolation and pulse radiolysis techniques. TCNE radical anions are produced not only by electron attachment but also by electron transfer from 2-MTHF radicals to TCNE molecules. The latter process (electron transfer) was found to proceed much more slowly than the former process. On the basis of the diffusion controlled reaction theory and free volume theory, the bimolecular rate constant of the electron transfer reaction can be expressed as follows k = 6.7 x 109T1/2exp [-450/(T - 75)]dm3mol-I s-I. Electron transfer from 2-MTHF radicals to pyrene, 1,3-dinitrobenzene, and 1,4-dicyanobenzene has not been observed.

INTRODUCTION 2-MTHF has been frequently used as a solvent in photochemistry and radiation chemistry, because it forms a transparent glassy solid at low temperatures. "-5) For instance, Hamill and his coworkers ~4) have used 2-MTHF solutions at 77 K in order to study a number of radical anions of organic molecules produced by irradiation with ionizing radiations. In radiation chemistry of 2-MTHF itself, a number of interesting papers have been published on the absorption spectrum, physical structure, and chemical reactions of the trapped electron in 2-MTHF at 77 K : 6-~°) Neutral radicals produced from 2-MTHF upon irradiation have also been examined by ESR. "H4) These studies are mainly concerned with the identification and formation mechanism of the radicals in the solid state at 77 K. The identified radicals are A and B; the former is produced by a rapid ion-molecule reaction of a cation radical of 2-MTHF, the latter by intramolecular proton transfer from the same cation radical. "Lt2~

o

o

o

Radical A

Radical B

Radical C

I H

Radical B may change into radical A by intermolecular proton transfer at temperatures higher than 77 K Radical B + 2-MTHF ~ Radical A + 2-MTHFH +, where 2-MTHFH + represents a protonated 2M T H F molecule. Ling and Kevan have suggested the formation of radical C in the y-radiolysis of adamantane-dj6 containing 2-MTHF at 77K. "4~ Recently Murabayashi et al. tm have investigated the radicals produced in the y-radiolysis of fluid 2-MTHF at 195 K using a spin-trapping technique. They concluded that radicals A and C are the principal products in the liquid phase. The chemical behavior of these radicals has not yet been well determined, although they should inevitably play an important role in radiolysis whenever 2M T H F is used as a solvent. In the present study, we have carried out the pulse radiolysis and rigid matrix experiment of 2-MTHF solutions containing TCNE, which has a strong electron affinity of 2.9eV. °6) The results indicate that the 2-MTHF radical transfers an electron to a TCNE molecule at a diffusion controlled reaction rate. This electron transfer is one of the important processes leading to the formation of T C N E radical anions.

377

378

M. HOSHINO et al. A

EXPERIMENTAL 2-MTHF supplied from Wako Pure Chemical Industries was purified by fractional distillation. The distillate was stored on N a - K alloy under vacuum. TCNE sublimed several times was kindly offered from Dr. Ogasawara of Hokkaido University. Pyrene was purified by recrystallizing three times from ethanol solutions. 1,4-Dicyanobenzene and 1,3-dinitrobenzene supplied from Wako Pure Chemical Industries were used without further purification. A Mitsubishi Van de Graaff accelerator was used as a source of pulsed electron beams. The pulse width and energy were 1.0/~s and 2.6MeV, respectively, unless otherwise stated. The apparatus for low temperature pulse radiolysis was the same as described elsewhere. "7) The temperature of the sample was controlled by the passage of cold nitrogen gas. The fluctuation in temperature was less than ---2°C. The irradiation cell for the rigid matrix experiment had an optical path-length of 2mm. The samples were directly immersed in liquid nitrogen in a Dewar vessel with optical windows during y-irradiation and subsequent spectroscopic measurements. The absorption spectra of trapped intermediates were recorded on a Cary 14 R spectrophotometer. The dose rate of -/-rays from 6°Co was 58600 rad/min. In order to examine chemical changes which occur at elevated temperatures, the irradiated samples were warmed by withdrawing for a few minutes from liquid nitrogen and then immersed again in liquid nitrogen for the purpose of optical measurements. The temperature of the sample was controlled by lengthening gradually the duration of the warm-up process and checking the spectral change every time. The temperature was not measured. RESULTS (a) M a t r i x isolation F i g u r e 1 s h o w s t h e optical a b s o r p t i o n s p e c t r a o b t a i n e d b y t h e 3,-radiolysis o f t h e 2 - M T H F solution of 1 × 10 -2 mol d m -3 T C N E at 77 K. T h e dotted line s p e c t r u m w a s o b s e r v e d b e f o r e w a r m i n g t h e i r r a d i a t e d s a m p l e a n d is a s c r i b e d to T C N E

0.8

O D 0.6 0.4

ii",' ' "-" ~ :~ ,:,

0.2

350

400 450 WAVELENGTH ( r i m )

500

FIG. 1. Absorption spectra for the irradiated 2-MTHF solution of 1 . 0 x l 0 - 2 m o l d m -3 TCNE. Dotted line; measured at 77 K, solid line; measured at 7 7 K after warming the sample for two minutes. The solution was irradiated at 77 K with v-rays to a dose of 2.9 x 105 rad.

n~ -

re

0

50

°I

75

u) ,~ 100

£

25

[

5o

O m m

75

I

( 800

i

200us

us

I

100

FIG. 2. Absorption signals as a function of time on pulse irradiation of the 2-MTHF solution of 0.011 moldm -3 TCNE at 113 K. The pulse width was 1.0/xs. The upper formation curve was monitored at 425 nm, the lower decay curve at 700 nm.

radical anions. The absence of absorption due to trapped electrons indicates that electrons are completely captured by TCNE molecules at the concentration used here. The solid line spectrum was observed after warming the sample for 2 min. The absorption ascribed to the TCNE radical anion was increased 1.72 times in intensity without any change in spectral shape. Since trapped electrons do not exist in the sample, the increase of the concentration of the TCNE radical anion implies that the other reducing species produced by y-radiolysis should transfer an electron to a TCNE molecule. The relative yield in radical anions from the reducing species to that from the electrons is estimated to be at least 0.72. The radical anions disappeared completely by warming the sample for 6 min. (b) Pulse radiolysis Figure 2 shows the formation curve and the decay curve monitored at 425 and 700 nm, respectively, in the pulse radiolysis of the 2-MTHF solution of 0.011 mol dm -3 TCNE at l 1 3 K . As shown in the figure, TCNE radical anions are formed in a rapid process within 20 Its after the pulse and in a slow process, over 0.5 ms. On the other hand, the absorption of the solvated electron disappears within 20 its. Therefore, the slow formation of the TCNE radical anion is not due to the reaction of a solvated electron with a TCNE molecule. The slow formation should be caused by the reaction of the other reducing species with a TCNE molecule, being similar to the one observed upon warm-up in the matrix isolation study. The slow formation process fits a first-order kinetic law

Electron transfer from 2-methyltetrahydrofuran radical

379

2.0 Q.-I:/t D}

1 xlo ~ T,n

1.0 ~.

O------

~xlo 3

1.0

2.0 (TCNE] X I 0 2 m o t d m "s

i

1.0

2.0 [TCNE] X 10 2 m o l dn~ 3

Fro. 3. Dependence of K (=k[TCNE]) on TCNE concentration. The temperature of the solutions was kept at ll3K.

FIG. 4. (D®- Dt°)/D° as a function of TCNE concentration (see text). D= is the optical density (at 425 nm) of the solution after the fast and slow formation processes of TCNE- are over. D° is the optical density when only the fast process is over. Therefore, (D®-Dt°)/Dt° corresponds to the ratios of the yield of TCNE- produced by the slow process to that produced by the fast process.

In ( C ~ - C , ) = - K t + constant, where C® and C, represent the concentrations of the T C N E radical anion at an infinite time and at time t, respectively. K is the product of the bimolecular rate constant k and the T C N E concentration. The dependence of K on the T C N E concentration was examined in the range from 2.2 x 10-4 to 1.1 x 10-2 tool dm -3. An excellent linear relation was obtained as shown in Fig. 3. The bimolecular rate constant was evaluated to be 5 x l 0 s dm 3 tool-' s - ' at 113 K. The value of K is markedly dependent on temperature as shown in Table 1. A temperature rise of only 10 K causes K to increase by an order of magnitude. Similar results have been frequently observed in kinetic studies at temperatures near the glass transition point of a solvent. "'''a'~9> The ratios of the yield of the T C N E radical anion produced by the slow process to that produced by the fast process are plotted as a function of the T C N E concentration in Fig. 4. The ratio attains unity as the concentration increases, indicating that the yield in anions from the reducing species is almost equal to that from the electrons. Figure 5 presents the oscilloscope traces of the absorptions at 490 and 1000 nm when the 2-MTHF solution of 1.0 x 10-2 moi dm -3 pyrene was irradi-

ated at 113 K with electron pulses. The absorption at 490 nm is assigned to the pyrene radical anion. The 'absorption due to the soivated electron was monitored at 1000nm in this case, because the pyrene radical anion had a relatively large absorption at 700 nm. The major amount,of the radical anion is formed instantaneously with the pulse, while a small amount is formed concurrently with the decay of the solvated electron. There is no slow formation of the anion radical as observed for TCN E. All pyrene radical anion may be for-

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TABLE 1. TEMPERATURE DEPENDENCE OF K IN THE 2M T H F SOLUTION OF 4.0 X 10 -3 m o l d m -3 T C N E

Temperature (K)

113

123

133

K(s ')

1.9xlos

2.5x104

1.6xl0 s

FIG. 5. Absorption signals as a function of time on pulse irradiation of the 2-MTHF solution of 1.0 x 10-2 mol dm -3 pyrene at 113 K. The pulse width was 1.0/zs. The long tail in the 1000-nm signal is caused by the absorption due to the pyrene radical anion.

380

M. HOSHINO et al.

med by the reactions of mobile and solvated electrons with pyrene molecules. The slow formation of the solute radical anion has not been observed for 1,4-dicyanobenzene and 1,3-dinitrobenzene. DISCUSSION (a) Mechanism of the formation of TCNE radical

anions In the previous papers "5'2°) the radiolytic mechanism of 2-MTHF has been proposed as follows

(1)

2-MTHF,,~2-MTHF .++ e,.,

(2)

e,, -~ e, or es,

(3) 2-MTHF* + 2-MTHF --, 2-MTHF" + 2-MTHFH ÷,

(4)

2-MTHF,~-H + 2-MTHF,

(5)

H + 2-MTHF~ H2 + 2-MTHF',

(6)

2-MTHF" +em ---,2-MTHF,

where era, e, and e, refer to mobile, trapped and solvated electrons, respectively. The fast formation of the TCNE radical anion is obviously due to the reaction of TCNE molecule with electrons (7)

e, ore~ + T C N E ~ T C N E .

On the other hand, the matrix isolation and pulse radiolysis studies have revealed that the reducing species other than electrons produces TCNE radical anions in a subsequent warm-up or slow process. In the mechanism described above, the possible candidates for the reducing species are H, 2-MTHF-, and 2-MTHF'. Atomic hydrogen, however, can not contribute to the slow formation

(8)

cH3.

O

(9)

of the TCNE radical anion, because of its fast reaction with 2-MTHF. In fact, the ESR signal of atomic hydrogen has not been detected in irradiated 2-MTHF at 77 K. (') Hager and Willard (2" suggested that a mobile electron produced 2HTHF- by reaction (6), although the anion was not detected directly. The reaction can explain a marked decrease in the yield of the trapped electron when 2-MTHF was irradiated to an extremely large dose. Since the mobile electron also reacts with TCNE in the present system, the amount of 2-MTHF- produced should decrease with increasing concentration of TCNE. If the slow formation of the TCNE radical anion is caused by MTHF-, the contribution of the process becomes smaller at high concentrations of TCNE. In contrast with the expectation, the relative yield of the TCNE radical anion produced by the slow process approaches unity at higher concentrations of TCNE, as shown in Fig. 4. Therefore, 2 - M T H F is not responsible for the slow formation. The relative yield in anions from the reducing species to that from the electrons was found to be 0.72 in matrix isolation and 1.0 in pulse radiolysis. The former value is less reliable, because some portion of TCNE radical anions might be lost during warming the sample. Judging from the large yield of TCNE- from the reducing species, we conclude that MTHF', one of the main primary products in the radiolysis of MTHF, corresponds to the reducing species. The formation reaction of the TCNE radical anion is the electron transfer between 2-MTHF" and TCNE. Two kinds of 2-MTHF" are produced from radical cations of 2-MTHF as mentioned earlier. Both radicals have an unpaired electron at the carbon atom adjacent to the oxygen atom and their ionization potentials may be close to each other. We consider that both radicals react with TCNE in a similar manner

HH~

• TC E

O H~CH3 , H~CHa O H+~HCH3 ÷O

GH3j

+ TCNE-

2-MTHF

" OH3. H--C H3 . 2-M'"FH" 0

0

Electron transfer from 2-methyltetrahydrofuran radical Reaction (8) may be followed by reaction (9). 2-MTHF" is also produced by reactions (4) and (5). When the contribution of the reactions is taken into account, the relative yield of 2-MTHF" to the electrons amounts to 1.15. "*'2°~ The relative yield of T C N E anions from electrons to that from 2-MTHF" becomes unity at higher concentrations of TCNE, where both electrons and 2-MTHF" may be scavenged almost completely by TCNE. On the basis of the occurrence of reactions (7) an]d (8) the observed yeild indicates that the amount of MTHF" produced is equal to that of electrons. This result is consistent with the mechanism in the radiolysis of 2 - M T H F mentioned earlier, where 2-MTHF" is mainly produced from 2 - M T H F r.. At lower concentrations of T C N E the relative yield is larger than unity, as shown in Fig. 4. This fact means that some portion of the electrons recombine with positive ions without the formation of T C N E - . On the other hand MTHF" may react efficiently with T C N E even at the low concentrations examined. (b) T e m p e r a t u r e d e p e n d e n c e o f b i m o l e c u l a r rate constant

By applying the usual Arrhenius expression to the data of Table 1, the activation energy for the bimolecular reaction of a T C N E molecule with 2-MTHF" is calculated to be 6.4kcal mo1-1. The pre-exponential factor is evaluated to be 1.2 × 10 TM dm 3 moi -I s -I, which seems to be enormously large. In addition, these Arrhenius parameters lead to a rate constant of 2.6x 1013dm3mol - ' s -1 at room temperature. The value apparently exceeds the diffusion controlled rate constant. The alternative expression is based on the diffusion controlled reaction theory ~22~and the free volume theory, t23) The diffusion controlled rate constant is (10)

k = 4¢rcranDN 1000

where crab stands for the distance between molecules A and B in collision. D is the sum of the diffusion coefficients of both molecules and N is the Avogadro number. Cohen and Turnbull a3~ have shown that the diffusion coefficient can be written as

(11) where a

381

specified. To is the temperature at which the free volume disappears. F r o m equations (10) and (11) the following equation can be derived

(12)

k = A T '/2 exp [ B / ( T - To)],

where A =4¢rcranNa/lO00 and B =/3. The temperature dependence of the rate constant obtained in the present study is well interpreted by equation (12), if 7 5 K is taken as To.m~ The actual rate constant is expressed in the following form

(13) k = 6.7 x 109 T m exp [-450/(T - 7 5 ) ] dm 3 moi-' s -I. According to the equation, the rate constant at 300K is estimated to be 1.6x 10~°dm3mol-ls -I, which lies in a reasonable range. The present type of electron transfer, i.e. the electron transfer from a neutral radical to a neutral molecule, has been scarcely known in chemical kinetics. However, it occurs at a diffusion controlled rate when the electron affinity of an acceptor molecule is high enough. A similar formula (equation (14)) has been used for the decay of the solvated electron in alcohols, aqueous solutions, and 2 - M T H F ~u'lsJg) (14)

k = A ' exp [ B ' / ( T - To)].

The temperature dependence of equation (12) is essentially governed by the exponential term, because T I/2 is relatively insensitive to temperature. Therefore, equation (12) is practically the same as equation (14).

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D = a T l/2 exp [/3[(T - To)],

and /3 are constant if a solvent is

79, 1513. 8. G. C. DISMUKES, S. L. HAGER, G. H. MORINEand J. E. WILLARD,J. chem. Phys. 1974, 61,426. 9. T. ICrtIKAWA,H. YOSrnDA and K. HAYASHI, Bull. Chem. Soc. Japan. 1975, 48, 2685.

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