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perdeuterated n-alkane mixtures at 77 and 195 K
Radiat. Phys. Chem. Vol. 17, pp. 241-246, 1981 Printed in Great Britain.
0146-57241811040241-06502.0010 Pergamon Press Ltd.
ON THE PRESENCE OF SCAVENGABLE DEUTERIUM ATOMS IN y-IRRADIATED OLEFIN/PERDEUTERATED n-ALKANE MIXTURES AT 77 AND 195 K OLA CLAESSON and ANDERS LUND The Studsvik Science Research Laboratory, S-611 82 Nyk6ping, Sweden
(Received 3 February 1980) Abstract--In order to examine the presence of primary deuterium atoms in systems of normal hydrocarbons, irradiated mixtures of 1,7-octadiene-hLJn-octane-d,, and l-decene-h~n-decane-d2z have been studied by ESR at 77 and 195 K. The presence of deuterium atom adddition radicals indicates that scavengable deuterium atoms are formed during irradiation at these temperatures. The yield of these primary atoms in the 1-decene-h~oln-decane-d2z system has been estimated with the help of a simple hydrogen abstraction model and the measured yields of the different radicals. G(D) is found to be 1.3-+0.5. Complementary measurements of the hydrogen gas yield have been performed.
INTRODUCTION THE QUESTION of whether hydrogen atoms are present in the radiolysis of solid hydrocarbons at cryogenic temperatures has been under much debate ever since the early work of Timm and Wiilard.t~ In this work, they obtained evidence for trapped hydrogen atoms at 4 K only in the case of irradiated methane. From this and similar observations they were led to the conclusion that the free radicals formed in the radiolysis of solid hydrocarbons were not produced by elimination of hydrogen atoms. Gillbro and Lund, t2~ when studying the ndecane-h22/n-decane-d22 system, found that the ratio of protiated radicals to deuterated radicals exceeded the molar ratio by a large factor. The presence of a hydrogen atom scavenger was found not to affect the isotopic distribution of the decyi radicals formed. As a result of this, and having the results of Timm and Willard~*~in mind, a hydrogen atom abstraction model was considered unlikely. Instead, preference was given to an explanation consisting of some form of excitation energy transfer. At about the same time, Miyazaki and Hirayama t3~ found that photolytically-produced hydrogen atoms selectively abstracted another hydrogen atom from the solute iso-butane in the solvent neopentane at 77 K. Wilkey and Wiilard<4~ found evidence of a low activation energy for the abstraction of hydrogen atoms from C - H bonds by D atoms in 3-methylpentane-dl4 containing 1% C - H bonds as an isotopic impurity.
A knowledge of the isotopic distribution of the hydrogen gas produced in the radiolysis of mixtures of n-decane-h22/n-decane-d22 was thought to make it possible to discern between the excitation transfer and the hydrogen abstraction model in this system. Experiments along these lines were carried out, t5'6~ but the results were inconclusive and could be explained by either of the two models. Recently Iwasaki et al. tv~ demonstrated that hydrogen atoms are able to abstract hydrogen from a C-H bond at cryogenic temperatures between 10 and 30 K in the methane/ethane system. In view of these findings it was considered interesting to establish the presence of D atoms in a normal hydrocarbon system. For this purpose an ESR study of perdeuterated n-hydrocarbon single crystals doped with a hydrogen atom scavenger, an olefin, has been performed. EXPERIMENTAL The deuterated chemicals used, n-decane-d22 and noctane-dis were both purchased from Merck, Sharp and Dohme and stated to contain 99 atom-% D. They were not purified further. The n-decane-hz~used was from two sources. One, from Merck of 99 mole-% purity was passed through columns of aluminium oxide and silica gel, respectively, followed by fractional distillation in an argon atmosphere. The second was obtained by fractional distillation in an argon atmosphere of Fluka's 99 mole-%, olefin free n-decane-h22, l-decene-h2o and 1,7octadiene-h,4 of 98 mole-% purity from Fluka were used as received. Two types of samples were prepared.
241
242
OLA CLAESSON and ANDERS LUND
(i) For the measurement of hydrogen gas: Solutions of 1-decene-h20 in n-decane-h22 were prepared. Samples (1 ml) in Pyrex ampoules were degassed using freezepump-thaw cycles on a vacuum line. The ampoules were then sealed. (ii) For ESR measurements: Solutions of 1-deceneh2o in n-decane-d20 and 1,7-octadiene-h~4 in n-octane-d~8 were prepared. The samples were deaerated and single crystals were grown in 4mm o.d. Suprasil tubes, as already describedJ s) The samples were irradiated at 77 and 195 K in an AEC 220 6°Go y-source. Measurement of the dose rate by Thermo Luminescence Dosimetry gave 0.21 Mrad h t The doses given to the samples were 0.8-1.3 Mrad. The hydrogen gas yields were measured on a Toepler pump in a way which is described in Ref. 9. In order to extract all hydrogen gas, the samples were melted and refrozen to 77 K prior to the measurements. A cold trap kept at 77 K was used to avoid organic substances from entering the measuring part of the system. The ESR measurements were made on a Varian E-9 spectrometer operated at 9.27GHz. A dual cavity was used, so that the sensitivity of the spectrometer could be monitored with a standard sample (Mn 2+ in SrO) inserted into one of the cavities during the experiment. The samples were kept at 77 K, and the microwave power was kept between 0.01 and 0.1 roW. The intensities were determined by integrating the spectra. Absolute concentrations were determined relative to a frozen solution of lmM Cu[EDTA], using the equation (1)
C = C .L.m__~x.lr l~mrl~"
Here C stands for concentration, I for integrated intensity, m for the mass of the sample and I for the length of the frozen sample in its tube. The index x denotes the sample to be measured, and the index r the Cu[EDTA] reference substance. Relative concentrations were obtained in the following way. An ESR line which could be assigned to one radical only, having no contributions from other radicals, was integrated. The result was then multiplied by a factor that takes into account the number of transitions in that radical. This value was then compared with the related value of another radical, which had been derived in the same way. RESULTS
1. Assignment of the radical ESR spectra As we are interested only in the types of the radicals formed, we here refrain ourselves from a detailed analysis. For such an analysis, see for example, Ref. 8. Single crystal spectra of samples of 5% 1.8octadiene-h~4 in n-octane-d~8, irradiated at 77 and 195 K are shown in Fig. I. A stick diagram analysis of the 195 K spectrum (Fig. la) consists of two radicals. One is the CH~D(~H(CHzhCH~---CH2 radical with coupling constants ao = 3.5G, a, = 28G (2H), az = 33G (2H) and a3 = 25G (IH). The couplings a,, a2 and a~ are assigned to the methyl, the methylene and the a - p r o t o n s , respectively The other is the C H 3 C H ( C H 2 ) 4 C H = C H 2 radical
with coupling constants a~ = 27G (3H), a2 = 31G (2H) and a3 = 24G (IH). The 77 K spectrum has the features of the C H 2 D ( ~ H ( C H 2 h C H = C H : radical but the intensity is lower than at 195 K. The central parts of both of these spectra are attributable to the "CaDI7 radical. The single crystal spectrum of a sample of 1% l-decene-h2o in n-decame-d22 irradiated at 195 K is shown in Fig. 2. The small single lines on the wings are caused by the CH3(~H(CH2)6CH~CH2 radical, and the triplet structure is part of the CH2D(~H(CH:)TCH3 spectrum. The central part is that of the "CtoD21 radical. The 77 K spectrum is similar, but again, the signals on the wings has a lower intensity than at 195 K. In both these spectra there is no sign of lines from radicals formed in a sample of pure n-decene-h~o.
2. Radical concentration measurements The total radical concentration, and the concentrations of the CH2(~H(CH2)6CH=CH2, CH2D(~H(CH2CH3 and "C~oD2~ radicals at different compositions and temperatures in the system n-decane-d2Jn-decene-h2o are given in Table I. In Table 2 the concentration of the CH2DCH(CH2)4CH~---CH2 radical relative to the total radical concentration in a 5% mixture of 1,8-octadiene-h~4 in n-octane-d~8 at three different temperatures are shown. The scatter of the data within a series of measurements is 10% although the uncertainty in one m e a s u r e m e n t is about 50% as calculated from equation (1).
3. Hydrogen gas yield Measurements of the yield of hydrogen gas in the system n-decane-h2o/n-decane-h22 are summarized in Table 3. The measurements were carried out at 77 and 195 K and with some different compositions. The scatter of the data is about 10%. DISCUSSION The solutes used in this investigation, 1,7octadiene-h,4 and 1-decene-h2o, are of the type of c o m p o u n d s known to function as scavengers of hydrogen atoms. The two types of radicals found in the 1,7-octadiene-h~4/n-octane-d,8 system, are both formed by an addition reaction, (2) (X) + CH2 = CH(CH2)4CH = CH3 CH2X(~H(CH2)aCH = CH2. Where X denotes a hydrogen or a deuterium atom. The yield of the hydrogen atom addition radical is
On the presence of scavengable deuterium atoms
243
(a)
:
25G
I
I I I
, I
,,l[i
, IJ~l,
I Jl,,
i,
(b) FIG. 1. A single crystal ESR spectrum of 5% 1,-octadiene-ht4 in n-octane-dins. (a) Irradiated and measured at 77 K. to) Irradiated at 195 K and measured at 77 K. The stick plots of the radicals CH21~H(CH2)4CH==CH2 (above) and CH3CH(CH2)4CH~----CH (below) are drawn with an intensity ratio of 10/1.
244
OLA CLAESSONand ANDERSLUND
t
25 G
I
Fl6. 2. Single crystal ESR spectrum of 1% I-decene-h2o in n-decane-dn. Irradiated at 195K and measured at 77K. TABLE I. RADICAL YIELDSIN THE
Sample
n-DECENE-h20]n-DECANE-d22sYSTEM.t
T/~
GR
GD
GH
GRD
CloD22
77
3.7
1% CIoH20 in CIOD22
77
3.2
0.04
0.06
3.1
5% CIoH20 in C10D22
77
3.2
0.13
0.19
2.9
C10D22
195
3.9
1% C10H20 in C10D22
195
3.9
0.64
0.05
3.2
5% C10H20 in C10D22
195
3.5
0.78
0.Ii
2.6
"I'GR is the total yield of all radicals, Go the yield of the CH2D(~H(CH2)TCH3 radical, GH the yield of the CH2 = CH(CI-I2)6CHCH3radical and jGRDis the yield of the . CloD21radical.
TABLE 2. THE INTENSITY OF THE CH2D(~ H(CH2hCH~----'CH2 RADICAL (R I) RELATIVE TO THE TOTAL RADICAL CONCENTRATION IN 5 % OCTADIENE-hIJn-OCTANE-dr8 AS A FUNCTION OF TEMPERATURE
I/K 77 143 195
I (RI)/I (tot) 0.03 0.05 0.12
low compared with that of the deuterium addition radical, viz. about 8%. Deuterium atoms are produced under the action of radiation by the
overall reaction (3)
C8D1~-- • C8D,7 + D.
The hydrogen atoms originate either from isotopic impurities in the solvent, or from decomposition of the solute. The relative yield of the deuterium addition radical in the 1,7-octadiene-h,Jn-octane-d,8 system increases with increasing temperature, as can be seen in Table 2. This would indicate that reaction (2) has an activation energy; unfortunately, this could not be determined as the data in Table 2 do not give a linear Arrhenius plot. The three types of radicals observed in the I-decene-h2o/n-decane-dn system can be ac-
On the presence of scavengable deuterium atoms
the temperature interval 77-195 K. A rough estimation of the yield of these atoms need to be made. One estimate is G ( D ) = 0.8 which is the yield of the deuterium atom addition type of radical, CH2Dt~H(CH2)7CH3, at 195 K (Table 1). This value should be a lower limit, since the scavenging is likely to be incomplete as a result of other competing process such as reaction (6). At 77 K the added decene is inefficient as a deuterium atom scavenger, so that here competing reactions are more significant than at 195 K. Another estimate has to be based on a proposed reaction scheme. A simple scheme for the ndecene-h2o/n-decane-d22 system is as follows. Deuterium atoms are produced by reaction (3) and this is followed by reactions (4)-(6). It should be noted that this reaction scheme predicts a constant radical concentration when the amount of additive is low; Table 1 shows this to be the case within experimental uncertainties. G(D) is equal to half
TABLE 3. YIELDS OF HYDROGENGASIN PURE n-DECANE-d22 AND IN THE n-DECANE-h2o/n-DECANE-h~ SYSTEM, AT 7 7 K
AND 195 K T/K
Sample
G
CIOD22
77
1.8
CIOH22
77
3.5 ± 0.3
It CIOH20 in CIOH22
77
3.0
5% ClOH20 in CIOH22
77
2.5
CIoH20
77
0.8
CIOD22
195
2.7
C10H22
195
4.2
1% CIoH20 in CIOH22
195
4.0
5% CioH20 in CIOH22
195
3.6
CIoH20
195
1.3
counted for by the processes (4)
D + CH2 = CH(CH2)7CH3
(5) (6)
245
~- C H 2 D C H ( C H 2 ) T C H 3
[ ~ CH2 = CH(CH2)6t~HCH3 + HD D + C10D22 '
> C,oD2,
Table 1 shows that reaction (4) is temperature dependent but that reaction (5) is not. Moreover, the radical formed by reaction (5) is found in a yield relative to the total radical yield which is equal to the mole fraction of 1-decene-h2o in the mixtures. This latter result is quite surprising when compared with the n-decane-h2dn-decane-d22 system, where, at a mole fraction of n-decane-h22 of 0.01, the ratio "C,oH2d'C,oD2,+'CIoH2~ is 0.31 at a temperature of 77 K. t2~ According to a recent investigation ~°) selective abstraction in mixtures like n-decane-h2dn-decane-d22 occurs because the reaction (7) D + CH3(CH2)sCH3-'-CH3(CH2)TCHCH3 + HD takes place more readily than reaction (6) at 77 K. Our results show that an olefinic group lessens the selectivity. In fact, reactions (4) and (7) occur with about the same rate constant at 77 K. This explains why the ratio of protiated to deuterated decyl radicals in the n-decane-h2dn-decane-d22system is unaffected by addition of a small amount of 1decene-h2o, a) It seems that primary deuterium atoms are present in these systems, and are able to react in
÷ D2.
the total radical yield and this averages to 1.8-+ 0.2 as estimated from the data of Table 1. This must be an upper limit since we discard contributions to the ESR absorption from other types of radicals than those mentioned in the reaction scheme. It is known, for instance, that radicals formed in pairs contribute to the ESR spectrum of the pure ndecane-d22"Lm We arrive therefore at an estimate of G(D) of somewhere between 0.8 and 1.8, i.e. 1.3 _+ 0.5.
The data of Table 3 show that the yield of hydrogen gas decreases on addition of n-deceneh2o. This is caused by a reaction of type (4), where D is replaced by H. This reduces the amount of free hydrogen atoms, and consequently suppresses abstraction reactions of type (5). If reaction (5) was the only reaction taking place, the yield of hydrogen gas would be half the yield of aikyl radical. The data of Tables 1 and 3 show, however, that the hydrogen gas yield from n-decane-d22 at 195 K exceeds half the yield of alkyl radicals by an amount of 0.8. This yield can be attributed to a unimolecular process. Gfiumann and Reipso "3> found a yield of 0.97 for a corresponding unimolecular reaction in deuterated hexane at 203 K. At 77K, Tables 1 and 3 show that the yield of
246
OLA CLAESSONand ANDERSLUND
hydrogen gas in n-decane-d~2 is half the yield of alkyl radical, indicating that all hydrogen gas is formed by a bimolecular process. It should be remembered that the large experimental uncertainties in the measurement of the absolute yields of the radicals makes a comparison with other absolute yields precarious. A measurable quantity of hydrogen gas is produced from l-decene-h2o, (see Table 3). If hydrogen atoms were formed, they should be captured at the double bond and so produce alkyl radicals. No such radicals were observed. The values G = 0.8 and (3 = 1.3 are thus unimolecular yields. This in accordance v~ith previous findings. "4) We finally comment on the interpretation of the selective formation of protiated radicals in an ndecane-d2~ crystal doped with n-decane-h22. From the data of Gillbro and Lund ~2) we can estimate the G-value of .C~oHz~ radicals to be 1.8 in a sample containing 5% n-decane-h22. Subtracting the yield of radicals formed by direct radiolysis of CtoH22 one obtains a yield G = 1.5 to be caused by secondary effects. This agrees reasonably well with the estimate in the present paper, 0.8-< G < 1.8 for D atoms reacting in secondary reactions and so supports the hypothesis "°~ that the indirect yield can be attributed to a selective reaction
between D atoms and CIoH22 molecules to produce •CIoH2, radicals and HD gas. REFERENCES 1. D. TIMM and J. WILLARD,J. phys. Chem. 1969, 73, 2403. 2. T. GILLBROand A. LUND, Chem. Phys. Lett. 1974, 27, 300. 3. T. MIYAZAK1and T. HIRAYAMA,J. phys. Chem. 1975, 79, 566. 4. D. D. WtLKEY and J. E. WILLARD,J. chem. Phys. 1976, 64, 3976. 5. T. MIYAZAKI,Radiat. Phys. Chem. 1977, 10, 219. 6. O. CLAESSONand A. LUND, Chem. Phys. Lett. 1977, 47, 155. 7. M. lWASAKI,K. TORIYAMA, H. MUTO and K. M. NUNOME, Chem. Phys. Lett. 1978, 56, 494. 8. T. GILLBROand A. LUND, Rad[at. Phys. Chem. 1976, 8, 625. 9. O. CLAESSONand A. LUND, Chem. Phys: 1978, 35, 63. 10. T. M~YAZAKI, J. KASUGA~, M. WADA and K. KINUGAWA,Bull. chem. Soc. Japan 1978, 51, 1676. 11. T. GILLBROand A. LUND, Chem. Phys. Lett. 1975, 34, 375. 12. M. IWASAKLK. TORIYAMA,H. MUTOand K. N~NOME, Chem. Phys. Lett. 1976, 39, 90. 13. T. GAUMANNand B. REIPSO,In Radiation Chemistry-II. Advances in Chemistry Series (Edited by R. F. Gould), Vol. No. 82, p. 447, American Chemical Society Publications, Washington, 1968. 14. S. YA. PSHEZHETSKH, A. G. KOTOV, V. K. MmINCHUK, V. A. ROGINSKII and V. I. TUPIKOV, E P R o/ Free Radicals in Radiation Chemistry, p. 159. Wiley, N e w York, 1974.
0146-57241811040241-06502.0010 Pergamon Press Ltd.
ON THE PRESENCE OF SCAVENGABLE DEUTERIUM ATOMS IN y-IRRADIATED OLEFIN/PERDEUTERATED n-ALKANE MIXTURES AT 77 AND 195 K OLA CLAESSON and ANDERS LUND The Studsvik Science Research Laboratory, S-611 82 Nyk6ping, Sweden
(Received 3 February 1980) Abstract--In order to examine the presence of primary deuterium atoms in systems of normal hydrocarbons, irradiated mixtures of 1,7-octadiene-hLJn-octane-d,, and l-decene-h~n-decane-d2z have been studied by ESR at 77 and 195 K. The presence of deuterium atom adddition radicals indicates that scavengable deuterium atoms are formed during irradiation at these temperatures. The yield of these primary atoms in the 1-decene-h~oln-decane-d2z system has been estimated with the help of a simple hydrogen abstraction model and the measured yields of the different radicals. G(D) is found to be 1.3-+0.5. Complementary measurements of the hydrogen gas yield have been performed.
INTRODUCTION THE QUESTION of whether hydrogen atoms are present in the radiolysis of solid hydrocarbons at cryogenic temperatures has been under much debate ever since the early work of Timm and Wiilard.t~ In this work, they obtained evidence for trapped hydrogen atoms at 4 K only in the case of irradiated methane. From this and similar observations they were led to the conclusion that the free radicals formed in the radiolysis of solid hydrocarbons were not produced by elimination of hydrogen atoms. Gillbro and Lund, t2~ when studying the ndecane-h22/n-decane-d22 system, found that the ratio of protiated radicals to deuterated radicals exceeded the molar ratio by a large factor. The presence of a hydrogen atom scavenger was found not to affect the isotopic distribution of the decyi radicals formed. As a result of this, and having the results of Timm and Willard~*~in mind, a hydrogen atom abstraction model was considered unlikely. Instead, preference was given to an explanation consisting of some form of excitation energy transfer. At about the same time, Miyazaki and Hirayama t3~ found that photolytically-produced hydrogen atoms selectively abstracted another hydrogen atom from the solute iso-butane in the solvent neopentane at 77 K. Wilkey and Wiilard<4~ found evidence of a low activation energy for the abstraction of hydrogen atoms from C - H bonds by D atoms in 3-methylpentane-dl4 containing 1% C - H bonds as an isotopic impurity.
A knowledge of the isotopic distribution of the hydrogen gas produced in the radiolysis of mixtures of n-decane-h22/n-decane-d22 was thought to make it possible to discern between the excitation transfer and the hydrogen abstraction model in this system. Experiments along these lines were carried out, t5'6~ but the results were inconclusive and could be explained by either of the two models. Recently Iwasaki et al. tv~ demonstrated that hydrogen atoms are able to abstract hydrogen from a C-H bond at cryogenic temperatures between 10 and 30 K in the methane/ethane system. In view of these findings it was considered interesting to establish the presence of D atoms in a normal hydrocarbon system. For this purpose an ESR study of perdeuterated n-hydrocarbon single crystals doped with a hydrogen atom scavenger, an olefin, has been performed. EXPERIMENTAL The deuterated chemicals used, n-decane-d22 and noctane-dis were both purchased from Merck, Sharp and Dohme and stated to contain 99 atom-% D. They were not purified further. The n-decane-hz~used was from two sources. One, from Merck of 99 mole-% purity was passed through columns of aluminium oxide and silica gel, respectively, followed by fractional distillation in an argon atmosphere. The second was obtained by fractional distillation in an argon atmosphere of Fluka's 99 mole-%, olefin free n-decane-h22, l-decene-h2o and 1,7octadiene-h,4 of 98 mole-% purity from Fluka were used as received. Two types of samples were prepared.
241
242
OLA CLAESSON and ANDERS LUND
(i) For the measurement of hydrogen gas: Solutions of 1-decene-h20 in n-decane-h22 were prepared. Samples (1 ml) in Pyrex ampoules were degassed using freezepump-thaw cycles on a vacuum line. The ampoules were then sealed. (ii) For ESR measurements: Solutions of 1-deceneh2o in n-decane-d20 and 1,7-octadiene-h~4 in n-octane-d~8 were prepared. The samples were deaerated and single crystals were grown in 4mm o.d. Suprasil tubes, as already describedJ s) The samples were irradiated at 77 and 195 K in an AEC 220 6°Go y-source. Measurement of the dose rate by Thermo Luminescence Dosimetry gave 0.21 Mrad h t The doses given to the samples were 0.8-1.3 Mrad. The hydrogen gas yields were measured on a Toepler pump in a way which is described in Ref. 9. In order to extract all hydrogen gas, the samples were melted and refrozen to 77 K prior to the measurements. A cold trap kept at 77 K was used to avoid organic substances from entering the measuring part of the system. The ESR measurements were made on a Varian E-9 spectrometer operated at 9.27GHz. A dual cavity was used, so that the sensitivity of the spectrometer could be monitored with a standard sample (Mn 2+ in SrO) inserted into one of the cavities during the experiment. The samples were kept at 77 K, and the microwave power was kept between 0.01 and 0.1 roW. The intensities were determined by integrating the spectra. Absolute concentrations were determined relative to a frozen solution of lmM Cu[EDTA], using the equation (1)
C = C .L.m__~x.lr l~mrl~"
Here C stands for concentration, I for integrated intensity, m for the mass of the sample and I for the length of the frozen sample in its tube. The index x denotes the sample to be measured, and the index r the Cu[EDTA] reference substance. Relative concentrations were obtained in the following way. An ESR line which could be assigned to one radical only, having no contributions from other radicals, was integrated. The result was then multiplied by a factor that takes into account the number of transitions in that radical. This value was then compared with the related value of another radical, which had been derived in the same way. RESULTS
1. Assignment of the radical ESR spectra As we are interested only in the types of the radicals formed, we here refrain ourselves from a detailed analysis. For such an analysis, see for example, Ref. 8. Single crystal spectra of samples of 5% 1.8octadiene-h~4 in n-octane-d~8, irradiated at 77 and 195 K are shown in Fig. I. A stick diagram analysis of the 195 K spectrum (Fig. la) consists of two radicals. One is the CH~D(~H(CHzhCH~---CH2 radical with coupling constants ao = 3.5G, a, = 28G (2H), az = 33G (2H) and a3 = 25G (IH). The couplings a,, a2 and a~ are assigned to the methyl, the methylene and the a - p r o t o n s , respectively The other is the C H 3 C H ( C H 2 ) 4 C H = C H 2 radical
with coupling constants a~ = 27G (3H), a2 = 31G (2H) and a3 = 24G (IH). The 77 K spectrum has the features of the C H 2 D ( ~ H ( C H 2 h C H = C H : radical but the intensity is lower than at 195 K. The central parts of both of these spectra are attributable to the "CaDI7 radical. The single crystal spectrum of a sample of 1% l-decene-h2o in n-decame-d22 irradiated at 195 K is shown in Fig. 2. The small single lines on the wings are caused by the CH3(~H(CH2)6CH~CH2 radical, and the triplet structure is part of the CH2D(~H(CH:)TCH3 spectrum. The central part is that of the "CtoD21 radical. The 77 K spectrum is similar, but again, the signals on the wings has a lower intensity than at 195 K. In both these spectra there is no sign of lines from radicals formed in a sample of pure n-decene-h~o.
2. Radical concentration measurements The total radical concentration, and the concentrations of the CH2(~H(CH2)6CH=CH2, CH2D(~H(CH2CH3 and "C~oD2~ radicals at different compositions and temperatures in the system n-decane-d2Jn-decene-h2o are given in Table I. In Table 2 the concentration of the CH2DCH(CH2)4CH~---CH2 radical relative to the total radical concentration in a 5% mixture of 1,8-octadiene-h~4 in n-octane-d~8 at three different temperatures are shown. The scatter of the data within a series of measurements is 10% although the uncertainty in one m e a s u r e m e n t is about 50% as calculated from equation (1).
3. Hydrogen gas yield Measurements of the yield of hydrogen gas in the system n-decane-h2o/n-decane-h22 are summarized in Table 3. The measurements were carried out at 77 and 195 K and with some different compositions. The scatter of the data is about 10%. DISCUSSION The solutes used in this investigation, 1,7octadiene-h,4 and 1-decene-h2o, are of the type of c o m p o u n d s known to function as scavengers of hydrogen atoms. The two types of radicals found in the 1,7-octadiene-h~4/n-octane-d,8 system, are both formed by an addition reaction, (2) (X) + CH2 = CH(CH2)4CH = CH3 CH2X(~H(CH2)aCH = CH2. Where X denotes a hydrogen or a deuterium atom. The yield of the hydrogen atom addition radical is
On the presence of scavengable deuterium atoms
243
(a)
:
25G
I
I I I
, I
,,l[i
, IJ~l,
I Jl,,
i,
(b) FIG. 1. A single crystal ESR spectrum of 5% 1,-octadiene-ht4 in n-octane-dins. (a) Irradiated and measured at 77 K. to) Irradiated at 195 K and measured at 77 K. The stick plots of the radicals CH21~H(CH2)4CH==CH2 (above) and CH3CH(CH2)4CH~----CH (below) are drawn with an intensity ratio of 10/1.
244
OLA CLAESSONand ANDERSLUND
t
25 G
I
Fl6. 2. Single crystal ESR spectrum of 1% I-decene-h2o in n-decane-dn. Irradiated at 195K and measured at 77K. TABLE I. RADICAL YIELDSIN THE
Sample
n-DECENE-h20]n-DECANE-d22sYSTEM.t
T/~
GR
GD
GH
GRD
CloD22
77
3.7
1% CIoH20 in CIOD22
77
3.2
0.04
0.06
3.1
5% CIoH20 in C10D22
77
3.2
0.13
0.19
2.9
C10D22
195
3.9
1% C10H20 in C10D22
195
3.9
0.64
0.05
3.2
5% C10H20 in C10D22
195
3.5
0.78
0.Ii
2.6
"I'GR is the total yield of all radicals, Go the yield of the CH2D(~H(CH2)TCH3 radical, GH the yield of the CH2 = CH(CI-I2)6CHCH3radical and jGRDis the yield of the . CloD21radical.
TABLE 2. THE INTENSITY OF THE CH2D(~ H(CH2hCH~----'CH2 RADICAL (R I) RELATIVE TO THE TOTAL RADICAL CONCENTRATION IN 5 % OCTADIENE-hIJn-OCTANE-dr8 AS A FUNCTION OF TEMPERATURE
I/K 77 143 195
I (RI)/I (tot) 0.03 0.05 0.12
low compared with that of the deuterium addition radical, viz. about 8%. Deuterium atoms are produced under the action of radiation by the
overall reaction (3)
C8D1~-- • C8D,7 + D.
The hydrogen atoms originate either from isotopic impurities in the solvent, or from decomposition of the solute. The relative yield of the deuterium addition radical in the 1,7-octadiene-h,Jn-octane-d,8 system increases with increasing temperature, as can be seen in Table 2. This would indicate that reaction (2) has an activation energy; unfortunately, this could not be determined as the data in Table 2 do not give a linear Arrhenius plot. The three types of radicals observed in the I-decene-h2o/n-decane-dn system can be ac-
On the presence of scavengable deuterium atoms
the temperature interval 77-195 K. A rough estimation of the yield of these atoms need to be made. One estimate is G ( D ) = 0.8 which is the yield of the deuterium atom addition type of radical, CH2Dt~H(CH2)7CH3, at 195 K (Table 1). This value should be a lower limit, since the scavenging is likely to be incomplete as a result of other competing process such as reaction (6). At 77 K the added decene is inefficient as a deuterium atom scavenger, so that here competing reactions are more significant than at 195 K. Another estimate has to be based on a proposed reaction scheme. A simple scheme for the ndecene-h2o/n-decane-d22 system is as follows. Deuterium atoms are produced by reaction (3) and this is followed by reactions (4)-(6). It should be noted that this reaction scheme predicts a constant radical concentration when the amount of additive is low; Table 1 shows this to be the case within experimental uncertainties. G(D) is equal to half
TABLE 3. YIELDS OF HYDROGENGASIN PURE n-DECANE-d22 AND IN THE n-DECANE-h2o/n-DECANE-h~ SYSTEM, AT 7 7 K
AND 195 K T/K
Sample
G
CIOD22
77
1.8
CIOH22
77
3.5 ± 0.3
It CIOH20 in CIOH22
77
3.0
5% ClOH20 in CIOH22
77
2.5
CIoH20
77
0.8
CIOD22
195
2.7
C10H22
195
4.2
1% CIoH20 in CIOH22
195
4.0
5% CioH20 in CIOH22
195
3.6
CIoH20
195
1.3
counted for by the processes (4)
D + CH2 = CH(CH2)7CH3
(5) (6)
245
~- C H 2 D C H ( C H 2 ) T C H 3
[ ~ CH2 = CH(CH2)6t~HCH3 + HD D + C10D22 '
> C,oD2,
Table 1 shows that reaction (4) is temperature dependent but that reaction (5) is not. Moreover, the radical formed by reaction (5) is found in a yield relative to the total radical yield which is equal to the mole fraction of 1-decene-h2o in the mixtures. This latter result is quite surprising when compared with the n-decane-h2dn-decane-d22 system, where, at a mole fraction of n-decane-h22 of 0.01, the ratio "C,oH2d'C,oD2,+'CIoH2~ is 0.31 at a temperature of 77 K. t2~ According to a recent investigation ~°) selective abstraction in mixtures like n-decane-h2dn-decane-d22 occurs because the reaction (7) D + CH3(CH2)sCH3-'-CH3(CH2)TCHCH3 + HD takes place more readily than reaction (6) at 77 K. Our results show that an olefinic group lessens the selectivity. In fact, reactions (4) and (7) occur with about the same rate constant at 77 K. This explains why the ratio of protiated to deuterated decyl radicals in the n-decane-h2dn-decane-d22system is unaffected by addition of a small amount of 1decene-h2o, a) It seems that primary deuterium atoms are present in these systems, and are able to react in
÷ D2.
the total radical yield and this averages to 1.8-+ 0.2 as estimated from the data of Table 1. This must be an upper limit since we discard contributions to the ESR absorption from other types of radicals than those mentioned in the reaction scheme. It is known, for instance, that radicals formed in pairs contribute to the ESR spectrum of the pure ndecane-d22"Lm We arrive therefore at an estimate of G(D) of somewhere between 0.8 and 1.8, i.e. 1.3 _+ 0.5.
The data of Table 3 show that the yield of hydrogen gas decreases on addition of n-deceneh2o. This is caused by a reaction of type (4), where D is replaced by H. This reduces the amount of free hydrogen atoms, and consequently suppresses abstraction reactions of type (5). If reaction (5) was the only reaction taking place, the yield of hydrogen gas would be half the yield of aikyl radical. The data of Tables 1 and 3 show, however, that the hydrogen gas yield from n-decane-d22 at 195 K exceeds half the yield of alkyl radicals by an amount of 0.8. This yield can be attributed to a unimolecular process. Gfiumann and Reipso "3> found a yield of 0.97 for a corresponding unimolecular reaction in deuterated hexane at 203 K. At 77K, Tables 1 and 3 show that the yield of
246
OLA CLAESSONand ANDERSLUND
hydrogen gas in n-decane-d~2 is half the yield of alkyl radical, indicating that all hydrogen gas is formed by a bimolecular process. It should be remembered that the large experimental uncertainties in the measurement of the absolute yields of the radicals makes a comparison with other absolute yields precarious. A measurable quantity of hydrogen gas is produced from l-decene-h2o, (see Table 3). If hydrogen atoms were formed, they should be captured at the double bond and so produce alkyl radicals. No such radicals were observed. The values G = 0.8 and (3 = 1.3 are thus unimolecular yields. This in accordance v~ith previous findings. "4) We finally comment on the interpretation of the selective formation of protiated radicals in an ndecane-d2~ crystal doped with n-decane-h22. From the data of Gillbro and Lund ~2) we can estimate the G-value of .C~oHz~ radicals to be 1.8 in a sample containing 5% n-decane-h22. Subtracting the yield of radicals formed by direct radiolysis of CtoH22 one obtains a yield G = 1.5 to be caused by secondary effects. This agrees reasonably well with the estimate in the present paper, 0.8-< G < 1.8 for D atoms reacting in secondary reactions and so supports the hypothesis "°~ that the indirect yield can be attributed to a selective reaction
between D atoms and CIoH22 molecules to produce •CIoH2, radicals and HD gas. REFERENCES 1. D. TIMM and J. WILLARD,J. phys. Chem. 1969, 73, 2403. 2. T. GILLBROand A. LUND, Chem. Phys. Lett. 1974, 27, 300. 3. T. MIYAZAK1and T. HIRAYAMA,J. phys. Chem. 1975, 79, 566. 4. D. D. WtLKEY and J. E. WILLARD,J. chem. Phys. 1976, 64, 3976. 5. T. MIYAZAKI,Radiat. Phys. Chem. 1977, 10, 219. 6. O. CLAESSONand A. LUND, Chem. Phys. Lett. 1977, 47, 155. 7. M. lWASAKI,K. TORIYAMA, H. MUTO and K. M. NUNOME, Chem. Phys. Lett. 1978, 56, 494. 8. T. GILLBROand A. LUND, Rad[at. Phys. Chem. 1976, 8, 625. 9. O. CLAESSONand A. LUND, Chem. Phys: 1978, 35, 63. 10. T. M~YAZAKI, J. KASUGA~, M. WADA and K. KINUGAWA,Bull. chem. Soc. Japan 1978, 51, 1676. 11. T. GILLBROand A. LUND, Chem. Phys. Lett. 1975, 34, 375. 12. M. IWASAKLK. TORIYAMA,H. MUTOand K. N~NOME, Chem. Phys. Lett. 1976, 39, 90. 13. T. GAUMANNand B. REIPSO,In Radiation Chemistry-II. Advances in Chemistry Series (Edited by R. F. Gould), Vol. No. 82, p. 447, American Chemical Society Publications, Washington, 1968. 14. S. YA. PSHEZHETSKH, A. G. KOTOV, V. K. MmINCHUK, V. A. ROGINSKII and V. I. TUPIKOV, E P R o/ Free Radicals in Radiation Chemistry, p. 159. Wiley, N e w York, 1974.