Nanosecond Pulse Radiolysis of Carbon Tetrachloride

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N a n o s e c o n d Pulse Radiolysis o f

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 1, 2018 | https://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0082.ch023

Carbon Tetrachloride 1

R. COOPER and J. K. THOMAS Chemistry Division, Argonne National Laboratory, Argonne,Ill.60439 A positive ion ofCCl4is observed in the nanosecond pulse radiolysis of pure CCl4. The ion which has an absorption maximum at 4750 A. where εis 2.34X10 , shows an initial rapidfirstorder decay,t = 15 ± 2 nsec. followed by a much slower decay over severalµsec.The slow decay is 4% of the fast decay. Many solutes remove the positive ion forming solute positive ions with spectra in the visible which also exhibit a rapid and slow decay. At the same time, long -lived species are observed with absorptions towards the ultraviolet and these may be caused byClatom/solute com plexes or free radicals. 4

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Tn the y radiolysis of glasses of carbon tetrachloride containing various solutes, the positive ions of these solutes are formed by electron trans fer to the positive ion of the host, (CC1 ) (10). Recent experiments in the γ irradiation at 20 °K. of 3-methylpentane glasses with 1 mole % of CCI4 show that the species Cl " is formed (5). No evidence of the posi tive ion of carbon tetrachloride with an absorption maximum at 4800 A. was obtained at 20 °K., although this species is observed at higher temperatures (5,12). The pulse radiolysis of carbon tetrachloride and solutions of carbon tetrachloride shows that transient species are produced with absorption spectra from 3000 to 7000 A. (3). These species have been assigned to chlorine atom-solute charge transfer complexes (4). No assignment was made to positive ions as in the glassy state. It is possible that the lifetimes of the ions are too short to be ob served by conventional /xsec. pulse radiolysis (14), but may be observed by nanosecond pulse radiolysis techniques. 4

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Present address: Chemistry Department, University of Melbourne, Australia. 351 Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

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RADIATION CHEMISTRY

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Experimental The nanosecond pulse radiolysis technique has been described (8, 14). Carbon tetrachloride was purified as follows: Matheson Research grade C C 1 was dried over anhydrous potassium carbonate for several days, and subsequently distilled, discarding initial and final fractions. However, untreated research grade CCI4 gave identical results to that treated as above. Zone refined naphthalene, anthracene, biphenyl, and NjN^'jN'-tetramethyl-paraphenylenediamine ( T M P D ) were used; pyrene, 1:2 benzanthracene were recrystallized from absolute alcohol, and aniline was purified as described in an earlier paper (6). Normal hexane, cyclohexane, 3-methylpentane, benzene, and toluene were Matheson re search grade; methanol and ethyl alcohol were analytical grade.


4

0.30

0.20 h

Ο

σ u α. Ο

O.IOh

350

400

450

500

550

600

650

\Π\μ— Figure 1.

Spectrum in pure CCl^

Results Pure C C l . Figure 1 shows the spectrum of the species observed directly after a 12 nsec. pulse in degassed C C 1 ; at 4750 A . Ge is measured as 1.88 X 10 . The decay of the species is shown i n Figure 2 and may be divided into two time periods: (a) the decay immediately following 4

4

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Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

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COOPER A N D THOMAS

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Carbon Tetrachloride

the pulse which is first order over three half-lives with t = 15 db 2 nsec. and ( b ) a much slower decay over /xsecs. The long-lived absorption has an identical spectrum to that observed directly after the pulse, show ing that only one species is observed, and that it decays with two different time dependent decay modes; the amount decaying over a short period being twenty-five times greater than that decaying over the longer period. Oxygen or nitrous oxide have no effect on this species. Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 1, 2018 | https://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0082.ch023

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ll 1 11"l 1 1 11 1 1 1111 1

1 1 11 1 1 1ι ι1 ι ι t i l l f 1 1 l!i l t 1 1 1 1

20

Figure 2.

J t

1 1 11 1 1 1 1 11 1 1

1 0 % α bs'n

nanoseconds

Decay of species at 4750 A. in pure CCl

k

Solutions of Aniline or T M P D . Adding T M P D removes the species with a spectrum shown in Figure 1, while the spectrum characteristic of ( T M P D ) is produced. Similarly, aniline also removes the spectrum i n Figure 1 while the (aniline) ion is produced. This spectrum has a peak at 4100 A . and agrees well with that found for (aniline)* in water ( 9 ) , CCU glass ( I I ) , and pure aniline (6). Figure 3 shows that the decrease in the species in Figure 1 is matched by the appearance of the positive ion of aniline. This indicates that the species in Figure 1 is a positive ion: the limiting plateau yield of aniline positive ion is 0.8 at and above 10" M (calculated from c = 12,050 cm. /mole), and if this is equivalent to the yield of the C C 1 positive ion then the extinction coefficient of this ion at 475 τη is 2.34 Χ 10 . The yield of 0.8 for C C 1 is lower than that found in solid C C 1 (11). +

+

2

2

4

4

μ

4

+

4

Solutes that Produce no New Intermediates. Methanol, ethyl alcohol, cyclohexene, cyclohexane, n-hexane, 3-methylpentane, and biacetyl also remove the absorption spectrum attributed to ( C C V ) , but no additional new spectra are observed from 3500 to 6000 A . Millimolar concentrations of these solutes remove the long-lived portion of ( C C 1 ) while 0.1M of all solutes apart from cyclohexane completely removes the short and longlived ( C C 1 ) ; cyclohexane 0.1M increases the decay rate of the positive ion. Adding 10 m M methanol and 20 m M n-hexane decreases the t 4

4

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+

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RADIATION CHEMISTRY

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of the decay of ( C C 1 ) from 15 nsec. to 12 nsec. and 11 nsec, respectively, the kinetics remaining first order. This shows that these solutes are reacting with the positive ion with rate constants of about 1 0 M sec." — i.e., at every collision. 4

+


1 0

.J

I0"

5

I I0"

4

I I0"

I I0"

3

2

[ANILINE]

Figure 3.

- 1

1

L 10'

—

Yield of CCI,/ and (anilinef

Solutes that Produce New Intermediates. B I P H E N Y L . Biphenyl removes the spectrum of the positive ion of C C 1 and produces new transitory spectra, some of which have long lifetimes (/xsec.) while others are short-lived—i.e., 10's of nanoseconds. Figure 4 shows the spectra of the transients produced immediately after the pulse, at 40 nsec. after the pulse, and at 500 nsec after the pulse. The spectra at the end of the pulse and at 40 nsec. after the end of the pulse are calculated as the difference between the total absorption observed and the long-lived absorption (500 n s e c ) . A typical oscilloscope trace illustrates the decay of the spectrum at 5000 A . 4

T O L U E N E A N D B E N Z E N E . Figure 5 shows the spectra produced in 0.1M toluene in C C 1 , the positive ion being removed by the toluene. The spectra are calculated as in Figure 4 and are shown at the end of the pulse, 40 nsec. after the end of the pulse, and at 500 nsec. after the pulse. 4


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COOPER AND THOMAS


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Similar data is shown i n Figure 6 for 0.1M benzene i n C C U , the spectra are shown at the end of the pulse, and at 500 nsec. after the pulse.


ANTHRACENE AND PYRENE.

Both anthracene and pyrene remove the

( C C V ) ion and give new transitory species shown in Figures 7 and 8. In Figure 7 the spectra are shown at the end of the pulse and at 500 nsec. after the pulse, while in Figure 8 the spectra are shown at the end of the pulse minus the long-lived absorption measured at 500 nsec, and as the long-lived absorption.

Figure 4.

Spectra produced in 0.J M biphenyl, and decay of spectrum at 5000 A.

N A P H T H A L E N E A N D 1:2 B E N Z A N T H R A C E N E .

Solutions of naphthalene

and 1:2 benzanthracene, both of which removed the C C l T ion, showed only a long-lived transient with an absorption maximum below 4000 A .


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τ

Figure 5.

II

Γ

Spectra produced in 0.1 M toluene

Discussion The rapid and long-lived decay of the species with spectrum i n Figure 1 is similar to that observed for the decay of negative and positive ions i n water, alcohols, aniline, and cyclohexane (6,13,14). The removal of this species by aniline and T M P D to produce the positive ions of these solutes, confirms that this species is a positive ion. The position of the absorption maximum is similar to that observed i n glasses containing C C 1 (7, 12). The removal of this positive ion by all solutes of lower ionization potential than C C 1 indicates that the positive ions of these solutes may be formed by electron transfer. It is commonly assumed that the negative ion, namely, the electron, is rapidly captured by CCI4 giving CCI4", which may decay rapidly giving CI" and C C 1 . The attractive force between the positive ion ( C C 1 ) and the negative ion (CI" or C C L f ) causes a rapid neutralization of the ions. The time of recombination depends on the separation of the ions, which i n turn depends on the energy of the ejected electron and the rate of thermalization of the elec tron before capture by the C C 1 . A whole span of separations w i l l be pro duced leading to a whole span of recombination times from nanoseconds to /seconds, as is observed experimentally. The resulting kinetics are 4

4

3

4

+

4


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heterogeneous and it is to be expected that solute positive ions might also show a rapid decay followed by a much slower decay, as exhibited by the parent ( C C V ) ion. Other factors as yet unknown also influence the recombination time, for example, the recombination of the positive ions of aniline and T M P D with negative ions i n C H i is much slower than the corresponding reactions of hydrocarbon positive ions such as (biphenyl) , etc. (6). 6

2


+

0.7 0.6 t

(A

Έ 0.5 3

>»

o £ 0.4 η

ιΟ

c

·" 0.3 ο b 0.2 0.1 350

400 450

Figure 6.

500 550

600 650

Spectra produced in 0.1M benzene

For biphenyl, benzene, toluene, anthracene, and pyrene, the above heterogeneous kinetic behavior was observed at 5500 Α., approximately 5500 Α., approximately 5500 Α., 5750 Α., and 6000 Α., respectively. In the case of benzene this agrees quite well with the absorption maximum of 5600 A . quoted i n the literature for the benzene cation ( 2 ) . Anthra cene and pyrene spectra attributed to positive ions have been observed in concentrated sulfuric at about 450 m/x for pyrene, and i n the 300 m/u. and 700 m/x regions for anthracene ( I ) . In the irradiation of CCI* glasses, spectra attributed to positive ions are observed in the 500-600 m/A region for benzene and toluene and i n the 600-700 τημ region for anthracene (10). The agreement between the sulfuric acid and CCI*


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glass data, and the present data are satisfactory. However, i n the case of biphenyl, an absorption attributed to 0 is observed at 550 m/x in the pres ent work, while the C C 1 glass data show peaks at 350 m/x and 700 m/x (10). 2

+

4


ι

Figure 7.

1

1

1

1

1

1

1

Spectra produced in lOmM anthracene

In all cases, a larger and much longer-lived asborption is also ob served in the ultraviolet. Oxygen has no effect on these absorptions and these may be correlated i n the benzene and toluene cases with the Cl-atom-solute complexes proposed by Buhler. However, with the other solutes the maxima are removed much further into the ultra violet. For anthracene the long-lived absorption corresponds to the position of the triplet excited state, but saturating the solution with oxygen only produces a slight enhancement of the natural decay to a fi/o of 1.0 /xsec. A t this concentration of oxygen, approximately 1 0 " M , the quenching of anthracene triplet by the oxygen would have a t\ of about 7 nsec. Hence, the longer t that is observed indicates that the species is not a triplet state of anthracene. A t lower anthracene concen trations this transient actually shows a growth after the pulse which is 2

/2

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dependent on the anthracene concentration. This species may be a radi cal of anthracene produced by the addition of a CI atom.

350 400 450 500 550 600 650 λ (m/x) — Figure 8. Spectra produced in CCl saturated with pyrene h

If the longer-lived species are indeed CI atom-solute complexes, they may be formed at diffusion controlled rates with k — 10 M sec." . In the presence of 0.1M solute these species are observed directly after the pulse, so the CI atoms must be formed in less than 1 nsec. The decay of the positive ion is much longer than this, and it seems that the CI atoms are not formed by the ion-neutralization reaction. 10

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Literature Cited

(1) Aalbersberg, W. Ij., Hoijtink, G. J., Mackor, E. L., Weijland, W. P., J. Chem. Soc. 3049, 3055 (1959). (2) Barachevskii, V. Α., Terenin, A. N., Opt. Spectr. (USSR) (English Transl. 17, 161 (1964). (3) Bühler, R. E., Gäumann, T., Ebert, M., "Pulse Radiolysis," p. 279, Ebert, M., et al. eds., Academic Press, 1966.



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RADIATION CHEMISTRY II

(4) Bühler, R. E., Ebert, M., Nature 214, 1220 (1967). (5) Claridge, R. F.C.,Iyer, R. M., Willard, J. E., J. Phys. Chem. 71, 3527 (1967). (6) Cooper, R., Thomas, J. K., in press. (7) Guarino, J. P., Hamill, W. H.,J.Am. Chem. Soc. 86, 777 (1964). (8) Hunt, J. W., Thomas, J. K., Radiation Res. 32, 149 (1967). (9) Land, E. J., Porter, G., Trans. Faraday Soc. 59, 2027 (1963). (10) Shida, T., Hamill, W. H., J. Chem. Phys. 44, 2375 (1966). (11) Shida, T., Hamill, W. H.,J.Chem. Phys. 44, 2369 (1966). (12) Skelly, D. W., Hamill, W. H.,J.Phys. Chem. 70, 1630 (1966). (13) Thomas, J. K., Bensasson, R. V.,J.Chem. Phys. 46, 4147 (1967). (14) Thomas, J. K., Johnson, K., Klippert, T., Lowers, R., J. Chem. Phys., in press. RECEIVED January 22, 1968. This work was performed under the auspices of the U. S. Atomic Energy Commission.


Nanosecond Pulse Radiolysis of Carbon Tetrachloride

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