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Low energy electron attachment to brominated ethanes and ethylenes
ELSEVIER
International
Journal
and Ion Processes 149/150 (1995) 123-129
of Mass Spectrometry
andlo”Processes
Low energy electron attachment to brominated ethanes and ethylenes* Takeyoshi
Sunagawa,
Hiroshi
Shimamori*
Fukui Institute of Technology, 3-6-l Gakuen. Fukui 910. Japan Received 3 April 1995; accepted 25 May 1995
Abstract Rate constants have been measured for electron attachment to CHBr2CHBr2, CHBr2CH2Br, CHBr$ZHs, CH2BrCH2Br, CF2BrCH2Br, CF,BrCF2Br, CF3CH2Br, CH,BrCH2F, CHBrCBr*, and CHBrCHBr in Xe buffer gas at room temperature with mean electron energies between 0.04 and 2eV using the pulse-radiolysis microwave-cavity method combined with microwave heating. The rate constant as a function of the mean electron energy is converted to a cross-section as a function of electron energy by an unfolding procedure. In all the compounds, the derived cross-sections show a peak at OeV with no other peaks at higher energies. The effects of molecular structure on the attachment processes are discussed. Keywords:
Brominated
ethanes;
Brominated
ethylenes;
Cross-sections;
1. Introduction
Low energy electron attachment to halogenated compounds, particularly halocarbons containing fluorine atom(s) and/or chlorine atom(s), has been extensively studied for many years [l], since such reactions have practical importance in many fields associated with ionized gases such as gaseous electronics and plasma processing. In contrast, there have been very few investigations of electron attachment to molecules containing bromine or iodine atoms. Measurements of the electron * Dedicated
to Professor
his 60th birthday. * Corresponding author.
David Smith FRS on the occasion
of
Electron
attachment;
Rate constants
energy dependence of the rate constants for brominated aliphatic hydrocarbons [2-41 have shown that the attachment rate constants have maxima at mean electron energies above thermal at room temperature. The rate constants for thermal electrons are very small for all the brominated aliphatic hydrocarbons including CHsBr and C2HSBr, i.e. less than lo-” cm3 molecule-is-i [5-81 at room temperature, though the rate constants increase very rapidly with increasing temperature. Such inefficient electron capture for thermal electrons at room temperature might be part of the reason why so little attention has been paid to bromine-containing molecules. However, a recent study has shown that dibromoethanes [9] and te, rabromoethane [lo] give
0168-l 176/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDZ 0168-l 176(95)04247-4
124
T. Sunagawa. H. Schimamori/International Journal of Mass Spectrometry and Ion Processes 149/150 (1995) 123-129
relatively high rate constants, of the order of lo-* cm3 molecule-’ s-t , for thermal electrons at room temperature. Furthermore, much interest has been attracted by the attachment studies of some bromomethanes and bromoethanes because their transient negative ions produced by thermal electron attachment decompose unimolecularly to form not only Br- but also Brz [9-121, though the details have not been clarified yet. It is obvious that a more systematic study is necessary in order to elucidate the nature of the electron attachment reactions in bromine-containing compounds. The present work is concerned with electron attachment to several bromoethanes and the corresponding fluorine-containing compounds along with some bromoethylenes. The rate constants as a function of the mean electron energy have been measured using the pulse-radiolysis microwave-cavity method combined with the microwave heating technique [13-161. The rate constant data have been converted to cross-sections as a function of the electron energy. The effects of the molecular structure and the nature of the substituted atoms on the attachment cross-sections are discussed.
2. Experimental Details of the experimental apparatus and the measurement procedure have been described previously [13]. The gas sample in a cylindrical quartz cell placed in a two waymode microwave resonant cavity was irradiated by a 3 ns X-ray pulse from a Febetron 706. An X-band microwave circuit with a frequency discriminator unit was used for the determination of the electron density in the cavity. The outputs are amplified and fed to a storage oscilloscope. The heating microwave power was fed through the cavity to increase the electron temperature. The conversion from the measured heating power to the mean
electron energy was achieved by a procedure developed in previous work [ 131. Xe(> 99.999%, Teisan Co.) was used as received. The compounds investigated and their purities are as follows: CHBr2CHBr2 (> 98%), CHBr2CH2Br (> 99%), CHBr2CH3 (> 98%), CH2BrCH2Br (> 99%), CF2BrCF2Br (> 99”/0), CHBrCBr2 (99%), and CHBrCHBr (mixture of cis and trans, > 98%) supplied by Tokyo Kasei Kogyo; CF2BrCH2Br (99%) CF3CH2Br (99%), and CH2BrCH2F (99.9%) from Aldrich. They were used after degassing by freeze-pump-thaw cycles at 77 K. All the measurements were carried out at room temperature (298 & 3 K). In all the measurements a trace amount of sample compound (lower than 10e4 Torr) was added to 70Torr of Xe buffer gas, and at a given microwave heating power the lifetimes were measured for the first-order decay of electrons due to the attachment reaction, e- + RBr -+ negative ions, where RBr denotes a brominated compound. The two-body attachment rate constants (k) were obtained from the lifetimes of the electrons. The negative ion products are primarily assumed to be Br-, but in some cases the formation of Bry has been suggested. This will be discussed in a later section. The rate constants have been converted to cross-sections as a function of the electron energy by an unfolding procedure developed by Christophorou et al. [17]. Since the experimental values of the rate constants are somewhat scattered (see Figs. l-3 below), the values corresponding to a smooth curve through the experimental rate constants are used for the calculation. The calculation procedure was carried out by a microcomputer system. The details of this procedure have been described in a previous paper [15].
3. Results and discussion The electron attachment
rate constants as a
T. Sunagawa. H. Schimamori/International
Journal of Mass Spectrometry
and Ion Processes
149/150 (1995) 123-129
function of the mean electron energy for brominated ethanes, the corresponding fluorinecontaining compounds, and brominated ethylenes are shown in Figs. 1, 2, and 3, respectively. Also shown for comparison are the values at thermal energy for CHBr&HBr* obtained from the threshold photoionization (TPI) method [lo] and those near thermal energy for CH2BrCH2Br and CF2BrCF2Br obtained by the flowing-afterglow Langmuirprobe (FALP) technique [9]. The values for thermal electrons are summarized in Table 1. A steep increase in the rate constant with mean
125
CFzBrCFzBr
I
CHBr2CH3
CFzBrCHzB
CHBrp.ZH$r
0.01
0.1 Mean Electron Energy
.
I
1
j)LS’
’
’
(eV)
CHBr2CHBr2
1
Fig. 2. Rate constants, k, for electron attachment to CF2BrCF,Br, CHrBrCHrF, CF3CH2Br, and CF2BrCH2Br as a function of the mean electron energy: 0, present results; +, Ref. [9] (FALP technique). The full line represents the curve used to derive cross-sections by an unfolding treatment (see text).
CHzBrCHzBr
?
electron energy is evident in data obtained by the FALP method [9]. Since these data have been taken by varying the temperature of the entire gas system, the increase in the rate constants with gas temperature can be ascribed to the increase in the internal (vibrational) energy of attaching molecules. Indeed the activation energies at 70meV and less than 44meV have been derived for CH2 BrCH2Br and CF2 BrCF2Br, respectively [9]. The present thermal values for CF2BrCF2Br and CHzBrCHzBr are in good agreement with those obtained by the FALP technique [9]. On the other hand, the
1O-6
i t 0.01
1
’ j I,‘,’
0.1 Mean Electron Energy
’
I ’
1 (ev)
Fig. 1. Rate constants, k, for electron attachment to CHBrrCHs, CHBr2CHrBr, CHBr+ZHBr,, and CHrBrCHrBr as a function of the mean electron energy: 0, present results; ?? , Ref. [lo] (TPI intercomparison); +, Ref. [9] (FALP technique). The full line represents the curve used to derive cross-sections by an unfolding treatment (see text).
T. Sunagawa, H. Schimamorillnternational Journal of Mass Spectrometry and Ion Processes 149/150 (1995) 123-129
126
1o-7
I
CHBrCHBr h
I
LL
I
1
CHBrCBr2
1o-7
IO-8 /
/
0.01
0.1
Mean Electron Energy
1
(eV)
Fig. 3. Rate constants, k, for electron attachment to CHBrCHBr (mixture of cis and tram) and CHBrCBr, as a function of the mean electron energy: 0, present results. The full line represents the curve used to derive cross-sections by an unfolding treatment (see text).
present value for CHBr2CHBr2 is about twice that obtained from the TPI method [IO] using an intercomparison technique based on the attachment cross-section for CF3Br. Since some of these results using the intercomparison technique are in good agreement with results reported previously by the present laboratory [13,14], the reason for the Table 1 Values of thermal electron attachment rate constants at room temperature and the calculated cross-sections at the electron energy of 0.001 eV Compound
CF2BrCH2Br CHBr2CH3 CHBrzCH,Br CHBr#HBr2 CH2BrCH2F CFsCH2Br CF2BrCH2Br CF2BrCF2Br CHBrCHBr CHBrCBr2 a Ref. [9]. b Ref. [lo].
Rate constant (cm3 molecule-’ 2.5 f 0.2 4.1 f0.1 9.2 f 0.8 1.2:;:; x 1.3 f 0.1 I;5 f 0.2 1.7 f 0.1 1.3 fO.1 1.7&0.2x 1.2 f 0.1
x 10-s x 10-s x lo-’ 10-7 x 10-s x 10-s x 10-7 X 10-7 IO-* x 10-7
Cross-section s-‘) 1.5 x lo-sa
6.9 x lo-* b
1.6 x 10-7a
(cm2) 1.2 2.2 5.4 6.4 6.0 8.7 9.8 6.2 8.7 7.8
x x X x x x x x x x
10-14 10-14 IO_‘4 lo-l4 lo-l6 lo-l5 lo-l4 lo-l4 lo-” lo-l4
discrepancy found here is not clear. Nevertheless, the general features of the electron energy dependence of the cross-sections indicated by the TPI method are in good agreement with the present cross-section data, as discussed later. Most of the compounds show a peak in the rate constant at thermal energy, and the value decreases with the increase in the mean electron energy except that CFzBrCFzBr, CH2Br CH2Br, and CH2BrCH2F seem to exhibit a peak at energies above thermal. For CHBr CHBr the rate constant remains almost constant over the mean electron-energy range. Since the CHBrCHBr supplied was a mixture of cis and trans isomers in an unknown ratio, the measured rate constants must be the sum of the contributions from the respective rate constants for the two isomers. The influence of molecular structure and composition on the magnitude of the rate constants is as follows. For the series of brominated ethanes shown in Fig. 1, the rate constants increase with the number of substituted Br atoms; dibromoethane < tribromoethane < tetrabromoethane. A similar trend can be seen for partially fluorinated bromoethanes (Fig. 2) and for bromoethylenes (Fig. 3). Among the partially fluorinated compounds (Fig. 2), a marked dependence on the number of F atoms is evident for monobromoethanes but there is no significant difference between two dibromoethanes with a different number of F atoms. The rate constants for CHBr2CH3 are larger than those for CH2BrCH2Br, though the difference becomes small at higher energies. It should be noted that similar influences of molecular structure on the magnitude of the rate constants have been shown for various chlorine-containing compounds which have been studied extensively by Christophorou and co-workers [2,18] using the electron swarm method. However, there is a noticeable difference between the Br and Cl systems in that the former show peaks in the rate
T. Sunagawa, H. Schimamori/International
Journal of Mass Spectrometry
constants at near-thermal energy while the latter show maxima at mean electron energies between 0.5 and 1 eV. In addition, we find that the rate constants for the bromine compounds are more than an order of magnitude higher than those for the corresponding chlorine compounds. Shown in Fig. 4 are the cross-sections derived from the measured rate constants. For all the compounds studied, the crosssections show a peak at OeV with no other peaks at higher energies up to about 2eV. The absolute magnitudes of the cross-sections at the peak values are listed in Table 1. Since the values for all the compounds correspond to that at 0.001 eV, the cross-sections at much lower energies may be higher than those cited. There have been no reported cross-
IO-13
section data with which to compare our data, except for CHBr&JHBr, for which the relative values of the cross-sections for Br- formation have been measured by the TPI method [lo]. In Fig. 5, the present cross-sections for CHBr2 CHBr2 are compared with those obtained by the TPI technique. The TPI data shown in the figure have been reproduced by using parameters presented by Alajajian et al. [lo] for the expression of the cross-section function and by normalizing the absolute value to the present one at thermal energy. Although the suggested parameters have a discontinuity at an electron energy of 60meV, the electron 1o-13
1O-14
,o_‘S
3 0.001
0.01
0.01
0.1
1
Electron Energy (eV) Fig. 4. Cross-sections for electron attachment to brominated ethanes and ethylenes as a function of the electron energy derived by unfolding the rate constant data shown in Figs. 1-3.
1
0.1
Electron Energy
IO"' 0.001
127
and Ion Processes 149/150 (1995) 123-129
(eV)
Fig. 5. Cross-sections for electron attachment to CHBrrCHBq as a function of the electron energy derived by unfolding the rate constant data shown in Fig. 1: -, present results; ., Ref. [IO] (TPI). The TPI data is expressed by C(E) = A+-‘/* exp (-&*/A*) + exp(-47)1, where a = 5.57 x IO-* eV”*, X = 4.06 x 10m3eV, y = 8.32 x lo-*eV for 0 5 E 5 60meV and 0.166eV for 60 5 E 5 160meV, and N = 1.649 x IO-i4cm2. The value of N is determined by normalizing the rat constant calculated from the TPI cross-sections to that obtained in the present study at thermal energy. Note that different values for X at E = 60 meV suggested by the original authors cause a discontinuity in the TPI cross-sections.
128
T. Sunagawa, H. Schimamori/International
Journal of Mass Spectrometry
energy dependences of the cross-sections are generally in good agreement with each other. It is necessary to identify the product negative ions. Mass spectrometric measurements for CH2BrCH2Br, CF2BrCF2Br, and CF2BrCH2Br have shown that no parent negative ions are formed. Although the main fragment negative ion is Br-, some (3-20%) of Brz has been detected simultaneously [9,12]. The Brz/Br- product ratio sometimes changes with temperature [9,12]; thus different vibrational energy states of the negative ions may be involved in the formation of Bry and Br-. Nevertheless, since the Brz formation occurs immediately after the electron attachment as does the Br- formation [12], the reaction should be exothermic. Zook et al. [12] have shown from thermochemical considerations that Br, production is indeed exothermic for CF2BrCF2Br and CH2BrCH2Br. Then one can guess that Bry formation is energetically possible for all the compounds studied here. We should note, however, that only Br- formation has been observed for CHBr2CHBr2 [lo]. So, not all the bromoethanes produce Brz ions. If we use a typical value of 2.9eV for the R-Br bond [19-201, we can show that Br- formation is exothermic for all the bromoethanes studied. On the other hand, the production of Br- in the reaction of both cis and trans isomers of the dibromoethylenes may be endothermic, since the heats of reaction can be estimated to be less than 0.28eV and less than 0.25 eV for cis and trans isomers, respectively, assuming their bond dissociation energies to be less than 3.64eV and less than 3.61 eV [20]. However, in both CHBrCHBr and CHBrCBr2, the cross-sections show a peak at OeV (see Fig. 4). If the peak is considered to correspond to Br- production, the production of Br- must be exothermic. It has been suggested by Oster et al. [21] that in electron attachment to fluoroethylenes the additional electron can be captured into one of the unoccupied X* molecular orbitals
and Ion Processes 149/150 (1995) 123-129
(MO) or the high lying c* MO, permitting the temporary anion to decompose by the cleavage of a 0 bond, e.g. CHFCHF(*II) -+ F-(‘S) + CHFCH(*C). If such a consideration is applied to both CHBrCHBr and CHBrCBr*, their cross-sections are predicted to have a peak at a high electron energy. The present results show that the cross-sections have a peak at 0 eV. For such low energy electrons it is therefore reasonable to assume that the additional electron is directly captured into an unoccupied D* MO which may have the character of a local C-Br bond. Wiley et al. [22] have recently suggested that in low energy electron attachment to tetrachloroethylene an electron is captured into one of the two bound excited states of the anion, the lower of which is a g* orbital. The present case may be similar. It is very interesting that the electron energy dependence and the absolute magnitude of the cross-sections for brominated ethylenes are not much different from those for the analogous brominated ethanes (see Fig. 4). It seems that the presence of an unoccupied 7r orbital does not much affect the efficiency of low energy electron attachment.
Acknowledgment
This work has been supported partly by a Grant-in-Aid for Scientific Research on Science of Free Radicals in Priority Areas from the Ministry of Education, Science and Culture, Japan.
References 111L.G. Christophorou,
PI
D.L. McCorkle and A.A. Christodoulides, in L.G. Christophorou (Ed.), Electron-Molecule Interactions and Their Applications, Vol. 1, Academic Press, New York, 1984, Chapter 6. A.A. Christodoulides and L.G. Christophorou, J. Chem. Phys., 54 (1971) 4691.
T. Sunagawa, H. Schimamori/International [3] R.E. Goans and L.C. Christophorou,
[4] [S] [6] [7] [8] [9] [lo] [l l] [12]
Journal of Mass Spectrometry and Ion Processes 1491150 (1995) 123-129
J. Chem. Phys., 63 (1975) 2821. P.G. Datskos, L.G. Christophorou and J.G. Carter, J. Chem. Phys., 97 (1992) 9031. W.E. Wentworth, R. George and H. Keith, J. Chem. Phys., 51 (1969) 1791. K.G. Mothes, E. Schultes and R.N. Schindler, J. Phys. Chem., 76 (1972) 3758. K.M. Bansal and R.W. Fessenden, Chem. Phys. Lett., 15 (1972) 21. E. Alge, N.G. Adams and D. Smith, J. Phys. B, 17(1984) 3827. D. Smith, CR. Herd, N.G. Adams and J.F. Paulson, Int. J. Mass Spectrom. Ion Processes, 96 (1990) 341. S.H. Alajajian, M.T. Bemius and A. Chutjian, J. Phys. B, 21 (1988) 4021. J.J. DeCorpo and J.L. Franklin, J. Chem. Phys., 54 (1971) 1885. D.R. Zook, W.B. Knighton and E.P. Grimsrud, Int. J. Mass Spectrom. Ion Processes, 104 (1991) 63.
129
[13] H. Shimamori, Y. Tatsumi, Y. Ogawa and T. Sunagawa, J. Chem. Phys., 97 (1992) 6335. [14] H. Shimamori, Y. Tatsumi, Y. Ogawa and T. Sunagawa, Chem. Phys. Lett., 194 (1992) 223. [15] H. Shimamori, T. Sunagawa, Y. Ogawa and Y. Tatsumi, Chem. Phys. Lett., 227 (1994) 609. [16] H. Shimamori, T. Sunagawa, Y. Ogawa and Y. Tatsumi, Chem. Phys. Lett., 232 (1995) 115. [17] L.C. Christophorou, D.L. McCorkle and V.E. Anderson, J. Phys. B, 4 (1971) 1163. [18] D.L. McCorkle, I. Szamrej and L.C. Christophorou, J. Chem. Phys., 77 (1982) 5542. [19] W. Tsang, J. Phys. Chem., 88 (1984) 2812. [20] E.C.M. Chen, K. Albyn, L. Dussack and W.E. Wentworth, J. Phys. Chem., 93 (1989) 6827. [21] T. Oster, A. Kuhn and E. Illenberger, Int. J. Mass Spectrom. Ion Processes, 89 (1989) 1. [22] J.R. Wiley, E.C.M. Chen and W.E. Wentworth, J. Phys. Chem., 97 (1993) 1256.
International
Journal
and Ion Processes 149/150 (1995) 123-129
of Mass Spectrometry
andlo”Processes
Low energy electron attachment to brominated ethanes and ethylenes* Takeyoshi
Sunagawa,
Hiroshi
Shimamori*
Fukui Institute of Technology, 3-6-l Gakuen. Fukui 910. Japan Received 3 April 1995; accepted 25 May 1995
Abstract Rate constants have been measured for electron attachment to CHBr2CHBr2, CHBr2CH2Br, CHBr$ZHs, CH2BrCH2Br, CF2BrCH2Br, CF,BrCF2Br, CF3CH2Br, CH,BrCH2F, CHBrCBr*, and CHBrCHBr in Xe buffer gas at room temperature with mean electron energies between 0.04 and 2eV using the pulse-radiolysis microwave-cavity method combined with microwave heating. The rate constant as a function of the mean electron energy is converted to a cross-section as a function of electron energy by an unfolding procedure. In all the compounds, the derived cross-sections show a peak at OeV with no other peaks at higher energies. The effects of molecular structure on the attachment processes are discussed. Keywords:
Brominated
ethanes;
Brominated
ethylenes;
Cross-sections;
1. Introduction
Low energy electron attachment to halogenated compounds, particularly halocarbons containing fluorine atom(s) and/or chlorine atom(s), has been extensively studied for many years [l], since such reactions have practical importance in many fields associated with ionized gases such as gaseous electronics and plasma processing. In contrast, there have been very few investigations of electron attachment to molecules containing bromine or iodine atoms. Measurements of the electron * Dedicated
to Professor
his 60th birthday. * Corresponding author.
David Smith FRS on the occasion
of
Electron
attachment;
Rate constants
energy dependence of the rate constants for brominated aliphatic hydrocarbons [2-41 have shown that the attachment rate constants have maxima at mean electron energies above thermal at room temperature. The rate constants for thermal electrons are very small for all the brominated aliphatic hydrocarbons including CHsBr and C2HSBr, i.e. less than lo-” cm3 molecule-is-i [5-81 at room temperature, though the rate constants increase very rapidly with increasing temperature. Such inefficient electron capture for thermal electrons at room temperature might be part of the reason why so little attention has been paid to bromine-containing molecules. However, a recent study has shown that dibromoethanes [9] and te, rabromoethane [lo] give
0168-l 176/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDZ 0168-l 176(95)04247-4
124
T. Sunagawa. H. Schimamori/International Journal of Mass Spectrometry and Ion Processes 149/150 (1995) 123-129
relatively high rate constants, of the order of lo-* cm3 molecule-’ s-t , for thermal electrons at room temperature. Furthermore, much interest has been attracted by the attachment studies of some bromomethanes and bromoethanes because their transient negative ions produced by thermal electron attachment decompose unimolecularly to form not only Br- but also Brz [9-121, though the details have not been clarified yet. It is obvious that a more systematic study is necessary in order to elucidate the nature of the electron attachment reactions in bromine-containing compounds. The present work is concerned with electron attachment to several bromoethanes and the corresponding fluorine-containing compounds along with some bromoethylenes. The rate constants as a function of the mean electron energy have been measured using the pulse-radiolysis microwave-cavity method combined with the microwave heating technique [13-161. The rate constant data have been converted to cross-sections as a function of the electron energy. The effects of the molecular structure and the nature of the substituted atoms on the attachment cross-sections are discussed.
2. Experimental Details of the experimental apparatus and the measurement procedure have been described previously [13]. The gas sample in a cylindrical quartz cell placed in a two waymode microwave resonant cavity was irradiated by a 3 ns X-ray pulse from a Febetron 706. An X-band microwave circuit with a frequency discriminator unit was used for the determination of the electron density in the cavity. The outputs are amplified and fed to a storage oscilloscope. The heating microwave power was fed through the cavity to increase the electron temperature. The conversion from the measured heating power to the mean
electron energy was achieved by a procedure developed in previous work [ 131. Xe(> 99.999%, Teisan Co.) was used as received. The compounds investigated and their purities are as follows: CHBr2CHBr2 (> 98%), CHBr2CH2Br (> 99%), CHBr2CH3 (> 98%), CH2BrCH2Br (> 99%), CF2BrCF2Br (> 99”/0), CHBrCBr2 (99%), and CHBrCHBr (mixture of cis and trans, > 98%) supplied by Tokyo Kasei Kogyo; CF2BrCH2Br (99%) CF3CH2Br (99%), and CH2BrCH2F (99.9%) from Aldrich. They were used after degassing by freeze-pump-thaw cycles at 77 K. All the measurements were carried out at room temperature (298 & 3 K). In all the measurements a trace amount of sample compound (lower than 10e4 Torr) was added to 70Torr of Xe buffer gas, and at a given microwave heating power the lifetimes were measured for the first-order decay of electrons due to the attachment reaction, e- + RBr -+ negative ions, where RBr denotes a brominated compound. The two-body attachment rate constants (k) were obtained from the lifetimes of the electrons. The negative ion products are primarily assumed to be Br-, but in some cases the formation of Bry has been suggested. This will be discussed in a later section. The rate constants have been converted to cross-sections as a function of the electron energy by an unfolding procedure developed by Christophorou et al. [17]. Since the experimental values of the rate constants are somewhat scattered (see Figs. l-3 below), the values corresponding to a smooth curve through the experimental rate constants are used for the calculation. The calculation procedure was carried out by a microcomputer system. The details of this procedure have been described in a previous paper [15].
3. Results and discussion The electron attachment
rate constants as a
T. Sunagawa. H. Schimamori/International
Journal of Mass Spectrometry
and Ion Processes
149/150 (1995) 123-129
function of the mean electron energy for brominated ethanes, the corresponding fluorinecontaining compounds, and brominated ethylenes are shown in Figs. 1, 2, and 3, respectively. Also shown for comparison are the values at thermal energy for CHBr&HBr* obtained from the threshold photoionization (TPI) method [lo] and those near thermal energy for CH2BrCH2Br and CF2BrCF2Br obtained by the flowing-afterglow Langmuirprobe (FALP) technique [9]. The values for thermal electrons are summarized in Table 1. A steep increase in the rate constant with mean
125
CFzBrCFzBr
I
CHBr2CH3
CFzBrCHzB
CHBrp.ZH$r
0.01
0.1 Mean Electron Energy
.
I
1
j)LS’
’
’
(eV)
CHBr2CHBr2
1
Fig. 2. Rate constants, k, for electron attachment to CF2BrCF,Br, CHrBrCHrF, CF3CH2Br, and CF2BrCH2Br as a function of the mean electron energy: 0, present results; +, Ref. [9] (FALP technique). The full line represents the curve used to derive cross-sections by an unfolding treatment (see text).
CHzBrCHzBr
?
electron energy is evident in data obtained by the FALP method [9]. Since these data have been taken by varying the temperature of the entire gas system, the increase in the rate constants with gas temperature can be ascribed to the increase in the internal (vibrational) energy of attaching molecules. Indeed the activation energies at 70meV and less than 44meV have been derived for CH2 BrCH2Br and CF2 BrCF2Br, respectively [9]. The present thermal values for CF2BrCF2Br and CHzBrCHzBr are in good agreement with those obtained by the FALP technique [9]. On the other hand, the
1O-6
i t 0.01
1
’ j I,‘,’
0.1 Mean Electron Energy
’
I ’
1 (ev)
Fig. 1. Rate constants, k, for electron attachment to CHBrrCHs, CHBr2CHrBr, CHBr+ZHBr,, and CHrBrCHrBr as a function of the mean electron energy: 0, present results; ?? , Ref. [lo] (TPI intercomparison); +, Ref. [9] (FALP technique). The full line represents the curve used to derive cross-sections by an unfolding treatment (see text).
T. Sunagawa, H. Schimamorillnternational Journal of Mass Spectrometry and Ion Processes 149/150 (1995) 123-129
126
1o-7
I
CHBrCHBr h
I
LL
I
1
CHBrCBr2
1o-7
IO-8 /
/
0.01
0.1
Mean Electron Energy
1
(eV)
Fig. 3. Rate constants, k, for electron attachment to CHBrCHBr (mixture of cis and tram) and CHBrCBr, as a function of the mean electron energy: 0, present results. The full line represents the curve used to derive cross-sections by an unfolding treatment (see text).
present value for CHBr2CHBr2 is about twice that obtained from the TPI method [IO] using an intercomparison technique based on the attachment cross-section for CF3Br. Since some of these results using the intercomparison technique are in good agreement with results reported previously by the present laboratory [13,14], the reason for the Table 1 Values of thermal electron attachment rate constants at room temperature and the calculated cross-sections at the electron energy of 0.001 eV Compound
CF2BrCH2Br CHBr2CH3 CHBrzCH,Br CHBr#HBr2 CH2BrCH2F CFsCH2Br CF2BrCH2Br CF2BrCF2Br CHBrCHBr CHBrCBr2 a Ref. [9]. b Ref. [lo].
Rate constant (cm3 molecule-’ 2.5 f 0.2 4.1 f0.1 9.2 f 0.8 1.2:;:; x 1.3 f 0.1 I;5 f 0.2 1.7 f 0.1 1.3 fO.1 1.7&0.2x 1.2 f 0.1
x 10-s x 10-s x lo-’ 10-7 x 10-s x 10-s x 10-7 X 10-7 IO-* x 10-7
Cross-section s-‘) 1.5 x lo-sa
6.9 x lo-* b
1.6 x 10-7a
(cm2) 1.2 2.2 5.4 6.4 6.0 8.7 9.8 6.2 8.7 7.8
x x X x x x x x x x
10-14 10-14 IO_‘4 lo-l4 lo-l6 lo-l5 lo-l4 lo-l4 lo-” lo-l4
discrepancy found here is not clear. Nevertheless, the general features of the electron energy dependence of the cross-sections indicated by the TPI method are in good agreement with the present cross-section data, as discussed later. Most of the compounds show a peak in the rate constant at thermal energy, and the value decreases with the increase in the mean electron energy except that CFzBrCFzBr, CH2Br CH2Br, and CH2BrCH2F seem to exhibit a peak at energies above thermal. For CHBr CHBr the rate constant remains almost constant over the mean electron-energy range. Since the CHBrCHBr supplied was a mixture of cis and trans isomers in an unknown ratio, the measured rate constants must be the sum of the contributions from the respective rate constants for the two isomers. The influence of molecular structure and composition on the magnitude of the rate constants is as follows. For the series of brominated ethanes shown in Fig. 1, the rate constants increase with the number of substituted Br atoms; dibromoethane < tribromoethane < tetrabromoethane. A similar trend can be seen for partially fluorinated bromoethanes (Fig. 2) and for bromoethylenes (Fig. 3). Among the partially fluorinated compounds (Fig. 2), a marked dependence on the number of F atoms is evident for monobromoethanes but there is no significant difference between two dibromoethanes with a different number of F atoms. The rate constants for CHBr2CH3 are larger than those for CH2BrCH2Br, though the difference becomes small at higher energies. It should be noted that similar influences of molecular structure on the magnitude of the rate constants have been shown for various chlorine-containing compounds which have been studied extensively by Christophorou and co-workers [2,18] using the electron swarm method. However, there is a noticeable difference between the Br and Cl systems in that the former show peaks in the rate
T. Sunagawa, H. Schimamori/International
Journal of Mass Spectrometry
constants at near-thermal energy while the latter show maxima at mean electron energies between 0.5 and 1 eV. In addition, we find that the rate constants for the bromine compounds are more than an order of magnitude higher than those for the corresponding chlorine compounds. Shown in Fig. 4 are the cross-sections derived from the measured rate constants. For all the compounds studied, the crosssections show a peak at OeV with no other peaks at higher energies up to about 2eV. The absolute magnitudes of the cross-sections at the peak values are listed in Table 1. Since the values for all the compounds correspond to that at 0.001 eV, the cross-sections at much lower energies may be higher than those cited. There have been no reported cross-
IO-13
section data with which to compare our data, except for CHBr&JHBr, for which the relative values of the cross-sections for Br- formation have been measured by the TPI method [lo]. In Fig. 5, the present cross-sections for CHBr2 CHBr2 are compared with those obtained by the TPI technique. The TPI data shown in the figure have been reproduced by using parameters presented by Alajajian et al. [lo] for the expression of the cross-section function and by normalizing the absolute value to the present one at thermal energy. Although the suggested parameters have a discontinuity at an electron energy of 60meV, the electron 1o-13
1O-14
,o_‘S
3 0.001
0.01
0.01
0.1
1
Electron Energy (eV) Fig. 4. Cross-sections for electron attachment to brominated ethanes and ethylenes as a function of the electron energy derived by unfolding the rate constant data shown in Figs. 1-3.
1
0.1
Electron Energy
IO"' 0.001
127
and Ion Processes 149/150 (1995) 123-129
(eV)
Fig. 5. Cross-sections for electron attachment to CHBrrCHBq as a function of the electron energy derived by unfolding the rate constant data shown in Fig. 1: -, present results; ., Ref. [IO] (TPI). The TPI data is expressed by C(E) = A+-‘/* exp (-&*/A*) + exp(-47)1, where a = 5.57 x IO-* eV”*, X = 4.06 x 10m3eV, y = 8.32 x lo-*eV for 0 5 E 5 60meV and 0.166eV for 60 5 E 5 160meV, and N = 1.649 x IO-i4cm2. The value of N is determined by normalizing the rat constant calculated from the TPI cross-sections to that obtained in the present study at thermal energy. Note that different values for X at E = 60 meV suggested by the original authors cause a discontinuity in the TPI cross-sections.
128
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Journal of Mass Spectrometry
energy dependences of the cross-sections are generally in good agreement with each other. It is necessary to identify the product negative ions. Mass spectrometric measurements for CH2BrCH2Br, CF2BrCF2Br, and CF2BrCH2Br have shown that no parent negative ions are formed. Although the main fragment negative ion is Br-, some (3-20%) of Brz has been detected simultaneously [9,12]. The Brz/Br- product ratio sometimes changes with temperature [9,12]; thus different vibrational energy states of the negative ions may be involved in the formation of Bry and Br-. Nevertheless, since the Brz formation occurs immediately after the electron attachment as does the Br- formation [12], the reaction should be exothermic. Zook et al. [12] have shown from thermochemical considerations that Br, production is indeed exothermic for CF2BrCF2Br and CH2BrCH2Br. Then one can guess that Bry formation is energetically possible for all the compounds studied here. We should note, however, that only Br- formation has been observed for CHBr2CHBr2 [lo]. So, not all the bromoethanes produce Brz ions. If we use a typical value of 2.9eV for the R-Br bond [19-201, we can show that Br- formation is exothermic for all the bromoethanes studied. On the other hand, the production of Br- in the reaction of both cis and trans isomers of the dibromoethylenes may be endothermic, since the heats of reaction can be estimated to be less than 0.28eV and less than 0.25 eV for cis and trans isomers, respectively, assuming their bond dissociation energies to be less than 3.64eV and less than 3.61 eV [20]. However, in both CHBrCHBr and CHBrCBr2, the cross-sections show a peak at OeV (see Fig. 4). If the peak is considered to correspond to Br- production, the production of Br- must be exothermic. It has been suggested by Oster et al. [21] that in electron attachment to fluoroethylenes the additional electron can be captured into one of the unoccupied X* molecular orbitals
and Ion Processes 149/150 (1995) 123-129
(MO) or the high lying c* MO, permitting the temporary anion to decompose by the cleavage of a 0 bond, e.g. CHFCHF(*II) -+ F-(‘S) + CHFCH(*C). If such a consideration is applied to both CHBrCHBr and CHBrCBr*, their cross-sections are predicted to have a peak at a high electron energy. The present results show that the cross-sections have a peak at 0 eV. For such low energy electrons it is therefore reasonable to assume that the additional electron is directly captured into an unoccupied D* MO which may have the character of a local C-Br bond. Wiley et al. [22] have recently suggested that in low energy electron attachment to tetrachloroethylene an electron is captured into one of the two bound excited states of the anion, the lower of which is a g* orbital. The present case may be similar. It is very interesting that the electron energy dependence and the absolute magnitude of the cross-sections for brominated ethylenes are not much different from those for the analogous brominated ethanes (see Fig. 4). It seems that the presence of an unoccupied 7r orbital does not much affect the efficiency of low energy electron attachment.
Acknowledgment
This work has been supported partly by a Grant-in-Aid for Scientific Research on Science of Free Radicals in Priority Areas from the Ministry of Education, Science and Culture, Japan.
References 111L.G. Christophorou,
PI
D.L. McCorkle and A.A. Christodoulides, in L.G. Christophorou (Ed.), Electron-Molecule Interactions and Their Applications, Vol. 1, Academic Press, New York, 1984, Chapter 6. A.A. Christodoulides and L.G. Christophorou, J. Chem. Phys., 54 (1971) 4691.
T. Sunagawa, H. Schimamori/International [3] R.E. Goans and L.C. Christophorou,
[4] [S] [6] [7] [8] [9] [lo] [l l] [12]
Journal of Mass Spectrometry and Ion Processes 1491150 (1995) 123-129
J. Chem. Phys., 63 (1975) 2821. P.G. Datskos, L.G. Christophorou and J.G. Carter, J. Chem. Phys., 97 (1992) 9031. W.E. Wentworth, R. George and H. Keith, J. Chem. Phys., 51 (1969) 1791. K.G. Mothes, E. Schultes and R.N. Schindler, J. Phys. Chem., 76 (1972) 3758. K.M. Bansal and R.W. Fessenden, Chem. Phys. Lett., 15 (1972) 21. E. Alge, N.G. Adams and D. Smith, J. Phys. B, 17(1984) 3827. D. Smith, CR. Herd, N.G. Adams and J.F. Paulson, Int. J. Mass Spectrom. Ion Processes, 96 (1990) 341. S.H. Alajajian, M.T. Bemius and A. Chutjian, J. Phys. B, 21 (1988) 4021. J.J. DeCorpo and J.L. Franklin, J. Chem. Phys., 54 (1971) 1885. D.R. Zook, W.B. Knighton and E.P. Grimsrud, Int. J. Mass Spectrom. Ion Processes, 104 (1991) 63.
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[13] H. Shimamori, Y. Tatsumi, Y. Ogawa and T. Sunagawa, J. Chem. Phys., 97 (1992) 6335. [14] H. Shimamori, Y. Tatsumi, Y. Ogawa and T. Sunagawa, Chem. Phys. Lett., 194 (1992) 223. [15] H. Shimamori, T. Sunagawa, Y. Ogawa and Y. Tatsumi, Chem. Phys. Lett., 227 (1994) 609. [16] H. Shimamori, T. Sunagawa, Y. Ogawa and Y. Tatsumi, Chem. Phys. Lett., 232 (1995) 115. [17] L.C. Christophorou, D.L. McCorkle and V.E. Anderson, J. Phys. B, 4 (1971) 1163. [18] D.L. McCorkle, I. Szamrej and L.C. Christophorou, J. Chem. Phys., 77 (1982) 5542. [19] W. Tsang, J. Phys. Chem., 88 (1984) 2812. [20] E.C.M. Chen, K. Albyn, L. Dussack and W.E. Wentworth, J. Phys. Chem., 93 (1989) 6827. [21] T. Oster, A. Kuhn and E. Illenberger, Int. J. Mass Spectrom. Ion Processes, 89 (1989) 1. [22] J.R. Wiley, E.C.M. Chen and W.E. Wentworth, J. Phys. Chem., 97 (1993) 1256.