Radiation Chemistry-II

33 The Reaction of Nitrous Oxide with Excited Molecules in the Radiolysis and Downloaded via UNIV OF TEXAS AT EL PASO on October 27, 2018 at 07:30:13 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Photolysis of Liquid Alkanes R. A . H O L R O Y D Atomics International Division of North American Rockwell Corp., Canoga Park, Calif. 91304

In the 1470-A. photolysis of cyclohexane-nitrous oxide solu tions, nitrous oxide reacts with excited cyclohexane mole cules to form nitrogen and oxygen atoms. The reaction of N O with photoexcited 2,2,4-trimethylpentane molecules is much less efficient than with cyclohexane. In the radiolysis of these solutions, G(N ) is the same for different alkanes at low (=< 5 mM) N O concentrations. At higher concentra tions, G(N ) from the radiolysis of cyclohexane is greater than G(N ) from the radiolysis of 2,2,4-trimethylpentane solutions. The N yields from 2,2,4-trimethylpentane are in excellent agreement with the theoretical yields of electrons expected to be scavenged by N O. The yield of N in the radiolysis of cyclohexane which is in excess of that formed from electrons is attributed to energy transfer from excited cyclohexane molecules to nitrous oxide. 2

2

2

2

2

2

2

2

N

itrous oxide has been used extensively in radiation chemical studies (1, 16, 17, 18, 19, 20, 21, 22, 23) to assess the importance of ionic processes. The reaction of nitrous oxide with the electron results i n the formation of nitrogen, Reaction 1.

β- + Ν 0 - > Ν + 0 ' 2

2

(1)

Kinetic studies of the competitive reactions of other electron scavengers support this hypothesis (18, 20). In the radiolysis of solutions of nitrous oxide in alkanes, reactions with other intermediates must be considered. Radicals, hydrogen atoms, and positive ions can be eliminated (5, 20), but a reaction with excited molecules is possible. It has been reported 488

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

33.

HOLROYD

Liquid Alkanes

489

recently that photoexcited cyclohexane molecules transfer energy to ben zene and cyclohexane (9, 25). Therefore, it is reasonable to expect that dissociative energy transfer to nitrous oxide can also occur (8). To assess the importance of such energy transfer in the radiolysis, solutions of nitrous oxide in several alkanes were photolyzed at 1470 Α., and the results were compared with the radiolysis of these solutions. Experimental The hydrocarbons (Phillips research grade) were passed through a dried silica gel column before use. Nitrous oxide (Matheson Co.) was used as received. Solutions were prepared by adding a known amount of nitrous oxide to a degassed sample of alkane. For photolysis at 1470 A . a combination X e arc and reaction cell (15) was used. The light flux through the sapphire exit window was about 8 Χ 10" Einsteins/sec. The actinometer used was the photolysis of cyclohexane for which φ ( Η ) has been shown to be 1.0 (25). In the cell the solutions were in contact with the sapphire window. Solutions were photolyzed for — 20 min., and the conversion was approximately 0.1%. The solutions were stirred during photolysis and cooled to 13 ± 2°C. to prevent evaporation. The concentration of nitrous oxide i n the liquid phase was calculated from the Bunsen coefficients of 3.3 for cyclohexane and n-hexane and 3.8 for 2,2,4-trimethylpentane (26). The radiation cells were made from 9-mm. o.d. borosilicate glass tubing. The concentration of nitrous oxide was calculated by assuming that the amount in the gas phase was negligible since the solutions filled approximately 90% of the cells. The samples were thermostatted at 35°C. and irradiated for one hour in a C o y-ray facility at a dose rate of approximately 1.3 X 10 e.v. liter" sec." . Ferrous sulfate dosimetry was used. Since we wished to compare different alkanes, samples were prepared in pairs; one sample of the pair always contained cyclohexane and the other some other alkane. Each sample of each pair contained the same amount of nitrous oxide. After photolysis or radiolysis, products volatile at —196°C. were first transferred to a calibrated volume, and the pressure was recorded. This gaseous fraction was then analyzed by gas chromatography on a Linde 5A molecular sieve column. The liquid fraction was analyzed on a 2-meter Poropak Q column for cyclohexanol, on a 30% β,/Γ-oxydipropionitrile + 10% A g N 0 column for cyclohexene, and on a D o w 11 silicone column for dicyclohexyl. 9

2

G 0

19

1

1

3

Results and Discussion Photolysis. The principal products of the photolysis of pure liquid cyclohexane are hydrogen and cyclohexene; dicyclohexyl is a minor prod uct (9, 25). If nitrous oxide is present, the yield of hydrogen is reduced (Table I), and nitrogen is formed (Figure 1). These effects are compli mentary since ψ (Ho) + Φ ( Ν ) = 1. The nitrous oxide does not affect 2


490

RADIATION CHEMISTRY

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φ ( Ο ι Η ) but does reduce φ ( Ο Η ι ) although not by as much as φ ( Η ) . The major oxygenated product formed is cyclohexanol and φ(Ο ΗιιΟΗ) — φ(Ν )/2. 2

22

β

0

2

β

2

Table I.

Photolysis of Cyclohexane—Nitrous Oxide Solutions Quantum Yields

[N O] mM g

0 16 28 50 48 (+ 5 % benzene) 71 99 144 β

H

N

1.0 .84 .81 .78

0 .18 .22 .28

.86 .80 .80 .73

.32 .65 .67 .57

.04 .40 .42 .50

.37

.013

.68

.055

2

C Hio

2

6

e

—

—

C

12^22

.06 .06 .06 .07

C H OH 6

n

0

—

.13

—

—

—

.18

—

.26

—

Cyclohexene yield decreases somewhat with conversion.

[N 0]mM 2

Figure 1. Quantum yield of nitrogen in the 1470-A. photolysis of alkane-^nitrous oxide solutions at 13°C. Yields are revive to φ(Η ) = 1 for liquid cyclo hexane photolysis at 1470 A. Ordinate is concentra tion of N 0 in moles/liter 2

2

Ο Cyclohexane Q n-Hexane • 2,2,4-Trimethylpentane


33.

Liquid Alkanes

HOLROYD

Table II.

491

Photolysis of Alkane—Nitrous Oxide Solutions Quantum Yields Alkane

[ N 0 ] mM

N

H

CH+

2,2,4-Trimethylpentane

0 17 30 63 95

0 .015 .02 .05 .08

.11 .07 .08 .07 .05

.21 .21 .19 .19 .20

2,2-Dimethylbutane

0 68

0 .04

.07 .08

.30 .28

n-Hexane

29 49 68

.09 .14 .16

.93 .87 .81

2

g

2

Four other alkanes were photolyzed in the presence of nitrous oxide principally to determine the yield of N . In all four cases φ ( Ν ) was less than for cyclohexane solutions (Table II and Figure 1). For cyclopentane and 2,2-dimethylbutane φ ( Ν ) at 68 m M N 0 is only 0.04, which is 10% of the yield of N for cyclohexane. For 2,2,4-trimethylpentane solutions φ ( Ν ) is also low, and φ ( Ο Η ) , a major product, is un affected by N 0 . n-Hexane seems to be an intermediate case with respect to No formation; like cyclohexane φ ( Ν ) + φ ( Η ) = 1 here too. 2

2

2

2

2

2

4

2

2

2

There are several possible explanations which need to be considered for the formation of N in the photolysis of cyclohexane-nitrous oxide solutions. These include direct absorption of vacuum ultraviolet light by nitrous oxide, photoionization of the solvent followed by electron attach ment by nitrous oxide, and reaction of nitrous oxide with either excited cyclohexene or excited cyclohexane molecules. Of these possibilities only the last explanation—reaction of excited cyclohexane molecules with nitrous oxide—is important. 2

Photolysis of nitrous oxide at 1470 A . in these solutions can be dis counted since the molar absorption coefficient for cyclohexane is 10,000 M' c m . (10) and that for nitrous oxide about 2 5 0 0 M c m . (13). Preferential absorption by nitrous oxide would occur if nitrous oxide were adsorbed on the sapphire window, but this is discounted since the nitrogen yield is a function of the alkane present. 1

-1

1

-1

A t longer wavelengths, near the absorption edge of cyclohexane (>1750 Α . ) , the molar absorption coefficient of nitrous oxide is greater than that of cyclohexane. Nitrogen might be formed from the direct photolysis of nitrous oxide at these wavelengths, but the wavelength would have to be one at which cyclohexane also absorbs. This long wave length photolysis cannot be an important source of N for the following 2


492

RADIATION CHEMISTRY II

reasons. (1) If photolysis at a longer wavelength occurred, then all the photoproducts of cyclohexane would be reduced together by nitrous oxide. This is not the case, for while the yield of dicyclohexyl is unaffected by nitrous oxide, a 50% reduction in φ ( Η ) is observed. (2) If long wavelength photolysis occurred, then N formation should occur to a similar extent in all alkanes since the alkanes studied have grossly similar absorption spectra (12), yet the nitrogen yields vary from alkane to alkane. (3) The arc emits principally 1470 A . light. 2

2

Photoionization of the hydrocarbon followed by dissociative electron attachment (Reaction 1) should be considered since the ionization poten tial of a molecule is less in the liquid phase than it is in the gas phase. F o r hydrocarbons the ionization potential is 1 to 1.5 e.v. less in the liquid phase (24). The photon energy at 1470 A . is about 1.4 e.v. below the gas-phase ionization potentials of cyclohexane and 2,2,4-trimethylpentane (14). Some ionization may therefore occur, but the efficiency of this process is expected to be low. Photoionization is eliminated as a source of N for the following reasons. (1) If photoionization occurred and the electron reacted with nitrous oxide, then O " would be formed. It has been shown in the radiolysis of cyclohexane-nitrous oxide solutions that subsequent reactions of O " result in the formation of cyclohexene and dicyclohexyl (J, 16, 17) and very little cyclohexanol (16, Table III). In the photolysis nitrous oxide reduces the yield of cyclohexene and does not affect the yield of dicyclohexyl. This indicates that O " is not formed in the photolysis, and consequently N does not result from electron capture. (2) A further argument against photoionization is that cyclo hexane and 2,2,4-trimethylpentane have comparable gas-phase ionization potentials but exhibit quite different behavior with respect to No formation. 2

2

The intermediates in the photolysis of cyclohexane which might react with nitrous oxide to form N are hydrogen atoms, excited C H i , and excited C H i . Hydrogen atoms can be eliminated on the basis that φ ( Η ) is only 0.14 (25) and that the rate of reaction of Η atoms with nitrous oxide is too slow (5). The cyclohexene which is formed in the photolysis, Reaction 3, could initially have as much as 7 e.v. excess energy and could conceivably sensitize the decomposition of nitrous oxide. Such a reaction would produce N but would not affect the yield of hydrogen. Since φ ( Η ) is reduced by nitrous oxide, excited cyclohexene cannot be the main source of N . 2

6

6

0

2

2

2

2

Excited cyclohexane molecules are produced in the primary process, Reaction 2 (9, 25). If N were formed by energy transfer from C H i * to 2

e

2

(2) (3)


33.

HOLROYD

Liquid Alkanes

493

C H * + N 0 -» N + Ο + C H e

1 2

2

2

e

(4)

1 2

nitrous oxide, Reaction 4, this would cause a reduction in φ ( Η ) and φ(Ο Ηιο) since Reaction 4 competes with Reaction 3. It is concluded that energy transfer to nitrous oxide is the principal source of N . If the oxygen atom generated by Reaction 4 is in an excited singlet state, it would be expected to react readily with cyclohexane by insertion into a carbon-hydrogen bond (6), Reaction 5. This is the case since cyclohexanol is formed but its yield is only 50% of ( N ) . Apparently other 2

β

2

2

Ο + C H e

1 2

—» ( C H — Ο — H ) * e

n

50% -> C H O H 6

(5)

n

50% —» other products products are also formed in this reaction. N o other products which might result from Reaction 5 were found in this study. However, nitrous oxide does not reduce φ ( Ο Η ι ) quite as much as it does φ ( Η ) (see Table I ) . Thus, a product of Reaction 5 may be cyclohexene, which is formed by elimination of H 0 from the intermediate. β

0

2

2

Table III.

Radiolysis of Alkane-Nitrous Oxide Solutions

0

2,2,4-Trimethylpentane

Cyclohexane

mM

G(N )

GiC H OH)

1.3 6 11 21 44 73 138 162 310

0.65 1.0 1.2 1.45 2.01 2.44

—

—

2.0 1.8 2.2 2.0 2.6

0.95 0.90 0.86 0.89 0.75

0.64 1.13 1.36 1.95 2.56 3.10

4.4 3.8 3.5 3.5 3.7

3.07 3.71

1.9 0.9

0.75 0.68

3.82 4.90

2.5 2.1

2

e

n

G(excess N) t

—

0.2 0.2 0.2 0.4 0.4 0.6 0.8 0.9 1.9

0.21

—

0.36 0.39

—

0.52

"Yield in molecules/100 e.v.

The mechanism consisting of Reactions 2, 3, and 4 requires that [φ(Ν )] depend linearly on [ N 0 ] ~ \ Equation I, where φ is the quantum yield of excited molecules formed in Reaction 2. 2

_ 1

2

2

1/φ(Ν ) = 1/φ + * / * 4 f c [ N 0 ] 2

2

s

2

(I)

The data are plotted in Figure 2. The ratio k /k was evaluated from this plot to be 28 ± 4 M " at 10 °C. Comparison of this value with the corresponding ratio for benzene (25) indicates nitrous oxide is a more effective quencher than benzene. 4

s

1


494

RADIATION CHEMISTRY

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In other alkanes the reaction of excited molecules with nitrous oxide is less important, and i n some cases it may not occur at all. In n-hexane the N yields are about one-half what they are i n cyclohexane. In other alkanes such as 2,2,4-trimethylpentane the yields of N were quite low. A small yield could be attributed to one of the other effects discussed, but if it is attributed to excited alkane molecules, then energy transfer to nitrous oxide is much less important i n 2,2,4-trimethylpentane than i n cyclohexane. 2

2

50

_L

100 M

150

-i

200

Figure 2. Kinetic plot of Equation I for the photolysis of cyclohexane-nitrous oxide solutions. Abscissa is l/tfNi); Ordinate is l/[N O] in M z

1

Radiolysis. The photochemical experiments suggest that i n the radiolysis a reaction of nitrous oxide with excited molecules would be expected i n cyclohexane but should be less important i n 2,2,4-trimethylpentane. The radiolysis results (Figure 3 and Table III) show that at nitrous concentrations less than 10 m M , where reactions of excited molecules are unimportant, G ( N ) is the same for cyclohexane and 2,2,4trimethylpentane solutions. A t concentrations of nitrous oxide from 20 to 160 m M , G ( N ) from cyclohexane solutions is greater than G (No) from 2,2,4-trimethylpentane solutions, and the excess yield increases with the concentration of nitrous oxide. [The nitrogen yields reported here for the concentration range 5-200 m M are i n good agreement with those reported by Sherman (20)] Nitrous oxide reduces G ( H ) from cyclohexane (16, 17, 18, 20, and Table I I I ) , but it has little effect on G ( H ) and G ( C H ) from 2,2,4-trimethylpentane. 2

2

2

2

4


33.

HOLROYD

Liquid Alkanes

495

To account for the N yields, it is assumed that nitrous oxide scavenges electrons i n both 2,2,4-trimethylpentane and cyclohexane but also scavenges excited molecules i n cyclohexane. The electron is a precursor of much of the H formed i n the radiolysis of cyclohexane, but i n 2,2,4trimethylpentane most of the hydrogen is formed i n processes which do not involve reactions of scavengeable electrons. Freeman (7) has calculated from theoretical considerations the yields of electrons expected to be scavenged by nitrous oxide as a function of concentration. The total yield of electrons is assumed to be 3 electrons/100 e.v. The number of electrons which react with nitrous oxide at each concentration is then calculated from the initial spatial separation of the charges and the probability of reaction of the electron with nitrous oxide for each separation. The calculated values for cyclohexane are shown as the solid line i n Figure 3. Since the dielectric constants are similar, the theory would predict the same values for 2,2,4-trimethylpentane if the spatial distribution of charges were similar and the mobility and diffusion coefficient of the electron were the same i n the two liquids. The yields of nitrogen for 2,2,4-trimethylpentane are i n excellent agreement with the theoretical values, suggesting that the principal reaction of nitrous oxide in this case is electron scavenging. The divergence of G ( N ) above 200 m M is caused partly by energy absorption i n the nitrous oxide directly. 2

2

2

Figure 3. Yields of nitrogen in molecules/100 e.v. as a function of nitrous oxide concentration in cyclohexane, O, and in 2,2,4-trimethylpentane ·. Solid line represents theoretical yields of electrons scavenged by nitrous oxide (7) For cyclohexane solutions the yield of nitrogen exceeds that caused by electron scavenging (as given by theory or by the 2,2,4-trimethylpentane results). This excess yield can be reasonably accounted for if nitrous oxide reacts with some cyclohexane derived intermediate which


496

RADIATION CHEMISTRY

II

is concluded to be excited molecules (Reaction 4). The reasons for this conclusion are given below. (1) The excess N is formed in the radiolysis in the concentration region where reactions of excited molecules occur. The excess nitrogen yields were treated like the photolysis results in order to calculate fc /fc . A plot of 1/G (excess N ) vs. [ N 0 ] was found to be linear, and from the values of slope and intercept k /k was found to be 1 4 M at 35°C. This value is in reasonable agreement with the photolysis results, con sidering that the rate of energy transfer may vary with temperature. Apparently the excited molecules formed in the radiolysis have properties similar to photoexcited molecules. The yield of these excited molecules, calculated from this plot is 1.2 =b 0.2 molecules/100 e.v. [This is a mini mum value for the total number of excitations produced by the radiation. Additional excited molecules are expected from neutralization reactions, and it is not known how efficiently higher excited states are converted to this excited state.] 2

4

2

3

_ 1

2

4

1

3

(2) The reaction of excited molecules with nitrous oxide provides an explanation for the formation of cyclohexanol in the radiolysis. The yields of cyclohexanol measured in this study are given in Table III. Somewhat lower yields were reported by others: Blackburn and Charlesby ( I ) report G ( C H n O H ) + G ( C H O ) < ^ 0.1 at nitrous oxide con centrations between 15 and 130 m M , and Sagert and Blair (16) found G ( C H n O H ) = 0.25 at nitrous oxide concentrations between 150 and 250 m M . As is shown in Table III, G ( C H n O H ) is approximately onehalf the yield of excess nitrogen formed. In the photolysis the quantum yield of cyclohexanol is one-half φ ( Ν ) . Thus, it is reasonable that i n the radiolysis the cyclohexanol is formed by attack of singlet oxygen atoms on cyclohexane, Reaction 5. e

6

1 0

e

e

2

(3) Benzene, a known quencher of excited molecules, reduces G ( N ) in cyclohexane solutions but not significantly in 2,2,4-trimethylpentane solutions. In the photolysis of cyclohexane, benzene reduces the extent of decomposition as a result of energy transfer, Reaction 6 (9, 25). Further, i n the photolysis of cyclohexane-nitrous oxide solutions, 2

C e H * + C H —> C H 12

e

e

e

1 2

+ C H e

e

(6)

benzene reduces the yield of N formed (Table I) because benzene com petes with nitrous oxide for the excited cyclohexane molecules. In the radiolysis of 88 m M nitrous oxide in cyclohexane, adding 5 % benzene reduces G ( N ) by 0.5 molecules/100 e.v. or 70% of the excess yield at this concentration of nitrous oxide. In the radiolysis of 88 m M nitrous oxide in 2,2,4-trimethylpentane, addition of 5 % benzene reduces G ( N ) only 5 % . The reduction of G ( N ) in cyclohexane by benezene can be 2

2

2

2


33.

HOLROYD

Liquid Alkanes

497

explained by energy transfer from excited cyclohexane molecules to benzene which competes with transfer to nitrous oxide. Conclusions Evidence has been presented for the formation of excited cyclo hexane molecules which survive long enough to transfer energy to nitrous oxide. The observed lifetime is 14/fc sec, which corresponds to — 10" sec. if Reaction 4 is diffusion controlled. However, energy transfer may occur faster than the diffusion rate since i n this case a solvent molecule is excited. F o r example, photochemical studies have shown that for quench ing of an excited benzene molecule in liquid benzene the quenching rate constant can be as large as 1 0 M sec." (11). li k were of this magni tude, then the lifetime of C H i * would need to be only -— 10" sec. to account for the results. On the other hand, it has been argued by Burton et al. (2, 3, 4) that the lifetime of singlet excited cyclohexane molecules is too short ( ^ 1 0 ~ sec.) to be observed. This conclusion is based largely on the behavior of scintillators containing fluorescent solutes dissolved in cyclohexane. It is important at this point to emphasize that at solute concentrations close to millimolar (which are usually employed i n scintillator studies) excited cyclohexane molecules cannot be detected by nitrous oxide. Higher con centrations of both nitrous oxide and benzene (-25) are required to ob serve energy transfer. This study confirms that in the radiolysis of cyclo hexane, solutes at millimolar concentrations interact mainly with the electron and not with excited molecules. 9

4

U

- 1

1

4

10

e

2

13

Acknowledgments I am indebted to W . M . Bowe, Jr. for his experimental assistance and to J. M . Warman for several stimulating discussions.

Literature Cited (1) Blackburn, R., Charlesby, Α., Nature 210, 1036 (1966). (2) Burton, M., Ghosh, Α., Yguerabide, J., Rad. Res. Suppl. 2, 462 (1960). (3) Burton, M., Z. Electrochem. 64, 975 (1960). (4) Burton, M., Mol. Cryst., in press. (5) Dainton, F. S., Sills, S. Α.,Proc.Chem. Soc. 1962, 223. (6) DeMore, W., Raper, O. F.,J.Chem. Phys. 46, 2500 (1967). (7) Freeman, G. R.,J.Chem. Phys. 46, 2822 (1967). (8) Holroyd, R. Α.,J.Phys. Chem. 72, 759 (1968). (9) Holroyd, R. Α., Yang, J. Y., Servedio, F. M., J. Chem. Phys. 46, 4540 (1967). (10) Lipsky, S., private communication. (11) Lipsky, S., "Physical Processes in Radiation Biology," L. Augenstein, ed., p. 215, Academic Press, New York, 1964.


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RADIATION CHEMISTRY

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(12) Lombos, Β. Α., Sanvageau, P., Sandorfy,C.,Chem. Phys. Letters 1, 42 (1967). (13) McNesby, J. R., Okabe, H., Advan. Photochem. 1, 184 (1964). (14) Price, W. C., Bralsford, R., Harris, P. V., Ridley, R. G., Spectrochem. Acta 14, 45 (1959). (15) Radnoti, D., Eisel, E., Yang, J. Y., Rev. Sci. Instr. 37, 970 (1966). (16) Sagert, Ν. H., Blair, A. S., Can. J. Chem. 45, 1351 (1967). (17) Sato, S., Yugeta, R., Shinsaka, K., Tereo, T., Bull. Chem. Soc. Japan 39, 156 (1966). (18) Scholes, G., Simic, M., Nature 202, 895 (1964). (19) Seki, H., Imamura, M., Bull. Chem. Soc. Japan 38, 1229 (1965). (20) Sherman, W. V.,J.Chem. Soc. 1966 A, 599. (21) Sherman, W. V.,J.Am. Chem. Soc. 88, 1567 (1966). (22) Sherman, W. V.,J.Phys. Chem. 70, 2872 (1966). (23) Ibid., p.667. (24) Vermeil, C., Matheson, M., Leach, S., Muller, F., J. Chim. Phys. 61, 598 (1964). (25) Yang, J. Y., Servedio, F. M., Holroyd, R. Α., J. Chem. Phys. 48, 1331 (1968). (26) Yen, L. C., McKetta, Jr., J. J.,J.Chem. Eng. Data 7, 288 (1962). RECEIVED December 29, 1967. Work supported by Research Division of U. S. Atomic Energy Commission.


Radiation Chemistry-II

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