The characterization of neutral species formed by electron impact on ethene, propene, isobutene, cis-2-butene and trans-2-butene

International Journal of Mass Spectrometry and Ion Processes, 59 (1984) 295-302 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

295

THE CHARACTERIZATION OF NEUTRAL SPECIES FORMED ELECTRON IMPACT ON ETHENE, PROPENE, ISOBUTENE, CIS-ZBUTENE AND T&AN&ZBUTENE *

BY

GERALD

D. FLESCH

and HARRY

J. SVEC

Ames Laboratory and Department of Chemistry, Iowa State University, Ames, IA 500/l (U.S.A.) (Received

7 February

1984)

ABSTRACT

The principal neutral fragments observed during the bombardment of ethene, propene, isobutene, cis-2-butene and trans-2-butene with 20 eV electrons are (M-H) ‘, (M-H,), (M-CH,)‘, increases energy

(M-CH,), CH;, and (C,H,)‘. The abundance of neutral fragments with molecular weight. The neutral fragments are isomers of lowest

and most are formed in vibrationally

the various fragments

neutral

fragments

are formed

observed

by dissociative

excited

states. The average

is 9.2 + 1.4 eV which

excitation

rather

appearance

is evidence

than dissociative

generated ionization energy of

that the observed

ionization.

INTRODUCTION

Recently we reported the results of characterizing the neutral species produced by electron interactions with some simple alkanes [l]. This report concerns a similar study for some simple alkenes. Other studies of electron/ alkene interactions include a search for metastable electronic states [2], studies of optical emission from fragments [3], a study of the chemical effects in the low energy electron bombardment of propene [4,5], and the observation of electron energy loss spectra [6-81. In this paper we report observations of the neutral species produced, their ionization energies (IEs), and the appearance energies (AEs) of the reactions producing the neutral species when electrons interact with the title compounds.

* Operated

for the U.S. Department

W-7405-Eng-82. Energy Science. 0168-1176/84,‘$03.00

This

research

of Energy

was supported

0 1984

Elsevier

by Iowa State

University

by the Director

Science Publishers

under contract

for Energy,

B.V.

Office

No.

of Basic

296 EXPERIMENTAL

The instrumentation and procedures for this study are the same as those used in the previous work [l]. Briefly, neutral species produced in a pulsed electron beam in the primary ionization chamber of the dual-chambered neutral-fragment mass spectrometer are analyzed in a continuous electron beam of the secondary ionization chamber. The a.c. component of ion current produced in the secondary chamber is due to the neutral species produced in the primary chamber which diffuse into the secondary chamber. A d.c. component is also present due to ionization of sample molecules which scatter into the secondary chamber. When the electron energies used in the two chambers are fixed at optimum values (20 eV primary, 13 eV secondary for these experiments), a neutral fragment spectrum is obtained from the secondary ionization chamber. When the electron energy in the primary chamber is fixed (usually 20 eV) and the electron energy in the secondary chamber is varied, ionization efficiency data for the neutral products are obtained. When the electron energy in the secondary chamber is fixed (usually 13 eV) and the electron energy in the primary chamber is varied, neutral product efficiency (NPE) data are obtained for the reaction producing the neutral product. Threshold energies were evaluated from ionization efficiency data by an extrapolated voltage difference method [9]. The values were determined for individual measurements to the nearest 0.01 eV, but the averages cited were rounded to the nearest 0.1 eV. The electron energy scale for the secondary chamber electron beam was calibrated through evaluation of the ionization efficiency data for the molecular ions of the sample gases which leak into the secondary chamber. The less accurate linear extrapolation method was used to determine thresholds from NPE data because of signal/noise problems near threshold. The values were determined for individual measurements to the nearest 0.25 eV and the averages rounded to the nearest 0.5 eV. The electron energy scale for the primary electron beam was calibrated using the AE of Rydberg atoms of argon [lo]. Research-grade butenes and C.P.-grade ethene and propene were used without further purification. Under typical operating conditions the pressure was estimated to be 10-2-10-3 Torr within the sample gas beam being intercepted by the primary chamber electron beam. Pressure in the secondary chamber due to scattered sample gas was found to be about one sixth that in the primary chamber. Neutral fragment spectra were obtained using 15 I_LAof 20 eV electrons in the primary chamber and 10 ~_LAof 13 eV electrons (20 eV electrons for H,) in the secondary chamber. The choice of 20 eV electrons in the primary chamber was made to avoid complications arising from the generation of

297

supra-excited states of the sample gas molecules. The choice of 13 eV between higher electrons .in the secondary chamber was a compromise energies ordinarily required to increase the a-c. level and lower energies to reduce the d.c. level of the combined a.c.-d-c. ion signals. The lower energies also reduced the interpretive complications arising from dissociative ionization in the secondary chamber. RESULTS

Neutral-fragment spectra were obtained for all the named gases on the same day under identical operating conditions. The spectra obtained were corrected according to I

NFS

=

r OBS

x;

where loBs is an observed ion current, S is the ion gage sensitivity factor for the sample gas, G is’ the observed ion gage pressure of the sample gas, and I NFs is the corrected value for the observed peak in the neutral-fragment spectrum_ These values were then normalized such that Z( I,,,) trans= 100. The normalized spectra of neutral fragments .obtained are listed in Table 1. TABLE

1

Neutral fragment spectra observed during the electron bombardment 20 eV electrons a

m/z 2 15 26 27 28 29 39 40 41 42 53 54 5.5 Total

of simple alkenes with

Compound Ethene

Propene

Isobutene

cis-2Butene

truns-aButene

1.9 0.8 0.6 0.4

3.4 11 0.6 1.3 0.6 0.8 12 16 20

1.8

62

2.1 35 0.1 0.4 0.6 1.7 12 20 0.4 2.5 0.8 2.5 12 88

2.3 26 0.2 1.1 2.3 1.5 14 5.2 6.5 2.5 1.3 26 13 100

4.0 26 0.2 1.3 1.7 1.9 13 5.4 5.0 2.1 1.3 29 13 100

a The energy of the secondary chamber electrons was 13 eV, except for m/z energy was 20 eV. The abundance of rn/.z = 2 is not included in the total.

= 2, where the

298

The values listed for H, were obtained using 20 eV electrons and are not included in the total listed for each gas. The neutral fragment signal at m/z = 41 for the various butenes was found to have two components: C,Hl from C,H;, and C,Hc from C,H,. The data in the table have been corrected for the presence of the fragment ions on the basis of ionization efficiency data. No attempt was made to correct the observed values for individual ionization sensitivities because the observation electron energy (13 eV) was in the region of rapidly changing sensitivities for the ions. The pressure dependence of all the neutral products was linear except for H, from the butenes, where a quadratic pressure dependence was observed. Presumably, for the butenes, H, was formed by recombination of atomic hydrogen. A similar pressure dependence for H, from CH, was observed in the previous study [l]. The IEs determined (primary chamber electron energy fixed, secondary chamber electron energy varied) for the various neutral products are listed in Table 2. The uncertainties listed represent the reproducibilities obtained for three or more determinations. Second upward breaks due to dissociative

TABLE

2

Ionization energies (eV) of the neutral fragments observed during the electron bombardment of simple alkenes with 20 eV electrons m/z

Ethene 2 15 26 28 39 40 41 42 54 55 56

IE/Species

Compounds

14.7kO.3 9.8 + 0.1 11.5*0.2 10.0+0.1

Propene 14.6*0.3 9.6 +O.l 10.9kO.l 10.4kO.l 7.4 * 0.4 10.1 + 0.3 7.7 f 0.1

Isobutene

c&2Butene

tram-2-

15.1 & 0.2 9.4 + 0.2 10.9 rfi0.2 11.0+0.2 9.7 f0.3 10.1 f 0.4

14.8 +0.2 9.4t-0.1 lLl+o.l 10.5 + 0.2 8.7 f 0.3 10.1+ 0.2 8.OkO.2 10.6 f0.1 9.4+0.2 8.4 f 0.6 8.0 + 0.4 8.5 f 0.1

14.9 f 0.3 9.3 f 0.2 ll.0f0.2 10.6f0.3 8.9f0.4 10.0* 0.3 7.7 10.3 10.8 + 0.2 9.6 f 0.2 9.2f0.2 7.8 & 0.2 8.4 + 0.2

d

10.9 + 0.2 d

8.0 f 0.4 8.9 + 0.3 8.6 f 0.1

’

Butene 15.4/H, a 9.8/CH, a 11.4&H, = 10.5/C, H, ’ 8.7/CH,CCH b lO.O/CH ,CCH Z a 8.1/CH,CHCH, a 11.4 e 9.7/C, H, a 9.1/CH,CHCHCH, 7.9/CH,C(CH,)CH, 9.2 ’

a b

“Ref. 11. b Ref. 12. ’ IE = ionization energy. d Not evaluated. e The appearance energy for the C, Hz fragment from C, H, is 11.4 eV (iso) ’ or 11.3 eV (cis)a. f The ionization energy for C,H, is 9.2 eV (iso) a or 9.1 eV (cis or trans) a.

299

ionization were observed in the ionization efficiency data for m/z = 41 from the butenes. For isobutene, the quality of data for m/z = 42 and the initial upward break for m/z = 41 were unsatisfactory for evaluation. The AEs determined (secondary chamber electron energy fixed, primary chamber electron energy varied) for the various neutral fragments produced are listed in Table 3. The large uncertainties reflect the difficulties of obtaining reproducible data when the a.c. component of the ion signal (neutral product) becomes a smaller and smaller fraction of the total a.c.+ d.c. signal emerging from the secondary chamber. TABLE

3

Appearance energies (eVj of neutral fragments observed during the electron bombardment simple alkenes a

m/z

Compound Propene

15

26 28 39 40 41 54 55

of

8.5 rfr1 9 +1 9.5 + 1

Isobutene

c&ZButene

tram-2-

9 fl 11 fl 11.5fl 8 +1 9.5 + 1 11.5fl 9.0 + 1 11.5*1

8 9 12.5 8 9.5 10 8 7.5

7.5% 1 9 +1 10 fl 8.5 + 1 8.5+1 10.5fl 7.5 f 1 7.0fl

1f1 +1 + 1 +l + 1 fl fl f 1

Butene

a NPE data could not be evaluated for the neutral fragments from ethylene or for H, for the other alkenes because the signal/noise ratio was so poor near threshold.

DISCUSSION

The neutral-fragment spectra listed in Table 1 are distinctly different from the positive-ion spectra of the title gases at 13 or 20 eV. Thus, artifacts due to dissociative ionization of the title gases in the secondary chamber, or due to passage of ions from the primary chamber past the potential barriers in the dual ion source are considered to be negligible. As expected, the abundances of CH; and (M-CH,)’ and the total abundance of neutral fragments increase as methyl groups are added to the ethene structure. The presence of significant amounts of C,H; in the spectra for propene and the butenes indicates the stability of this radical and the complexities of the fragmentation reactions occurring. There is also a significant abundance of C,H,, but the detection sensitivity for C,H, (IE = 11.4 eV) is low due to the 13 eV electrons used to measure the spectra.

300

The majority of the neutral species ( > 90% of those observed) consisted of the fragments (M-H)‘, (M-H,), (M-CH,)‘, {M-CH,), CH;, and C,H;. No signal for M was observable at 13 eV because the d.c. signal for M+ was so large [IE(C,H,) = 10.5 eV, IE(C,H,) = 9.7 eV, IE(i-C,H,) = 9.2 eV, IE(cis-C,H,) = 9.1 eV, and IE (truns-C,H,) = 9.1 eV] that the a.c. and d.c. components were not separable by the narrow band amplifier used for the measurements. It is cIear that the neutral-fragment spectra of the cis- and [runs-butenes are indistinguishable. This is consistent with the evidence in ionic fragment spectra that C,Hg’ isomers equilibrate to a mixture of interconverting structures before fragmentation [13-151. However, it is equally clear that the neutral fragment spectrum for isobutene is different from the other butenes at m/z = 15, 40, 41, and 54. The isobutene structure yields significantly different proportions of CH; and (M-CH,)’ than do the cis- and trans-butenes, and the loss of CH, is preferred over the loss of H, in isobutene while the opposite is true for the other butenes. These differences in fragmentation indicate that isobutene does not isomerise to the same extent in the neutral fragmentation processes observable in these experiments as it does in ionic fragmentation processes. The values for IE listed in Table 2 are in good agreement with the reference values associated with the radicals and molecules identified in the last column of the table, except that, in general, the individually observed experimental values are less than those of the reference values. This indicates that the neutral species are formed in excited states. For example, the values for H,, CH;, and C,H, are less than their reference values by 0.6 + 0.2,. 0.3 + 0.2, and 0.3 of:0.2 eV, respectively. These differences compare favorably with the energy values for the first vibrational stretch for H, (0.54 eV) [16], the first symmetrical stretch for CH; (0.34 eV) [17], and the first carbon-carbon stretch in &HZ (0.24 eV) [18]. Such excitation energies in the neutral species have been observed previously [1,19,20]. The sample gas molecules are detectable as neutral species in the secondary chamber on!& if they are excited as a result of interactions with the pulsed primary chamber electron beam. Neutral species were observed at m/z = 28 for ethene, m/z = 42 for propene, and KU/Z= 56 for the butenes at electron energies befow their ground-state IEs. The IEs determined were less than their ground-state IEs by 0.5 + 0.2 eV. Such IE values are reasonable for one or two quanta of internal energy in the form of vibrational energy. The higher energy threshold values for the relatively abundant species observed for the butenes at m/z = 41 are the evidence that these species are fragment ions from excited C,H, rather than parent ions from C,H;. These values are 0.6 + 0.1 eV less than the reference values for the AEs of the fragment ions from C,H,. This difference is in excellent agreement with the

301

values for the excitation energies of C,H, cited above. The AEs listed in Table 3 are less than the values expected if the source of the neutral fragments were due to dissociative ionization, but are greater than expected for simple dissociation. Most of the values fall in the range of energies for which threshold energy loss spectra [6] have been observed for these gases. Presumably the electronic states to which the molecules are efficiently excited in the primary chamber are more energetic than is necessary to dissociate the molecule and the excess is carried off as kinetic and/or vibrational energy. The IEs discussed above support this view. It is expected that the values for the AEs for comparable fragments from the three butenes should be equal since equivalent bonds are broken in the various fragmentation processes. The values listed in Table 3, except for m/z = 55, are indeed reproducible within about one electron volt. The wide range of values observed for m/z = 55 is not explainable at this time. The average value for the AEs for (M-H)‘, (M-H,), (M-CH,)‘, (M-CH,), CH;, and C,H, for this study is 9.0 f 1.3 eV, while the similar value for the alkanes [l] is 13.3 + 1.0. Since the energies of the bonds broken in the alkenes and alkanes are not that dissimilar, the lower value for the alkenes is supporting evidence that the fragments reported here are formed in dissociative excitation reactions. CONCLUSION

The neutral fragments produced by the electron bombardment of the simple alkenes are predominantly (M-H)‘, (M-H,), (M-CH,)‘, (M-CH,), CH; and C,H;. The fragments are the isomers of lowest IE and may have internal excitation energies of up to 0.6 *eV. The abundance of the neutral fragments increases as the molecular weight increases. The observation of more heavy fragments than light fragments in this study is similar to that made in our earlier study [l], but contrary to observations made by others [21--231. The low AEs for the fragmentation reactions and the non-isomerisation of isobutene indicate that ionic dissociation is not a significant source of the neutral fragments observed in these experiments. An ion source of new design is under construction and should be useful in resolving the apparent discrepancies concerning the source of the neutral fragments observed. ACKNOWLEDGEMENT

We thank Cynthia Olson for collecting summer student trainee in 1983,

some of the data while she was a

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