J . Phys. Chem. 1989, 93, 2123-2129
2123
Matrix Infrared Study of Low-Energy Electron Impact on Hydrocarbons Sefik Suzert and Lester Andrews* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 (Received: April 20, 1988)
Bombardment of argon/hydrocarbon samples with low-energy electrons (50-150 eV) during condensation onto a 12 K CsI window produced neutral fragments and rearrangement products. Electron impact on C2H6created C2H4, C2H2,C2Hs, and C2H3radicals and CH4. Ethylene and C2D4produced mainly C2H2and C2D2,whereas CH2CD2gave C2H2,C2HD, and C2D2,as well as cis- and frans-HDC=CDH, due to rearrangements. Electron impact on vinyl chloride (and bromide) produced hydrogen-bonded acetylene-HCI (and HBr) complexes and chloroacetylenes (and bromoacetylenes), whereas acrylonitrile produced the hydrogen-bonded C2H2-HCN complex and cyanoacetylene, as well as rearrangement to H2C= C=C=N-H based on strong N-H stretching and antisymmetric C=C=C stretching bands at 3449 and 1789 cm-I. Electron impact on 1,l-dichloro- and cis-l,2-dichloroethyleneproduced acetylene, hydrogen-bonded HCI-chloroacetylene complex, and dichloroacetylene, as a result of CI2, HCI, and H2 elimination, respectively. The most important conclusion from this matrix isolation electron-impact study is that neutral molecules and fragments were the most abundant products, and no evidence for charged fragments could be observed for these hydrocarbons.
Introduction The use of electron impact for creating excited molecules, fragments, and ions is very common in mass spectrometry and related fields. Although these techniques are extremely sensitive, they only detect ions, and neutral molecular products and fragments are not observed. Optical techniques can detect charged and uncharged species, but their sensitivity is very low when compared to ion detection techniques. The matrix isolation technique offers a solution to the inferior sensitivity of optical methods since the fragments can be collected for very long periods (4-10 h). In a recent study,' infrared spectra were observed for OH radicals, and OH- and Ar,H+ ions produced by electron impact of dilute A r / H 2 0 gaseous mixtures during condensation on a 12 K CsI window. This demonstrated the feasibility of creating and observing ionic (both positive and negative) as well as neutral fragments all in one matrix sample. Use of low-energy electrons (50-200 eV) was essential to provide sufficient electron density (50-200 pA/cm2) without heating the sample. The present work describes observation of various neutral fragments using electron impact of hydrocarbons and halogenated hydrocarbons. Proton radiolysis of halocarbons has been carried out in this laboratory;2 these experiments produced positive and negative ions as well as neutral fragments. Both positive and negative molecular ions have also been studied by using reactive atoms, vacuum-UV photolysis, and dissociative electron attachment on various
TABLE I: Major Product Bands (cm-') Observed in Solid Argon upon Electron Impact of Hydrocarbons C,Hn C,Dn C,Ha CH,CD, C,Dd identification"
3341 3302 3298 3288 3284
3298 3284 3130
2587 244 1 2438
+ On
leave from Middle East Technical University, Ankara, Turkey.
0022-3654/89/2093-2123$01.50/0
2442 2438
1439 1340 1335 1331 1304
1331
1298 1042 993
1042 99 1
948 91 1 906 900
91 1 906 900
736
Experimental Section A thermionic electron source built from stainless steel and ceramic insulating parts with a thin (4-7 mil) tungsten wire filament was used; the details have been given earlier.' Electrons emitted from the filament were directed toward a stainless steel ring on the CsI window. Energy and current density of electrons could be independently varied. Typically 50-eV electron energy and 50-pA current measured on the ring were used. The geometrical arrangement was fixed such that light emitted by the filament did not fall onto the CsI window; electrons, however, could be bent onto the window by the applied positive voltage on the ring. The argon matrix samples containing Ar/reagent = 400-200/1 were continuously bombarded by electrons during condensation at 12 K. Gaseous hydrocarbons were obtained in lecture bottles from Matheson, their deuteriated counterparts from MSD Isotopes, and liquid samples from Aldrich. Samples were prepared by using standard manometric techniques. The samples were deposited at a rate of approximately 2 mmol/h for 2 h with the electron source off and additional 2-4 h with the electron source on. Spectra were recorded on a Nicolet 5 DXB FTIR spec-
3298 3288
736
887 846 797 736
721 704 54 1
682 54 1
54 1
533
519 395 'A, E, and M refer to acetylene, ethylene, and methane, respectively. MA refers to mixed acetylene C2HD, CE and TE to cis- and trans-dideuterioethylene,and ER to ethyl and V to vinyl radicals.
trometer using 2-cm-I resolution; however, the band positions are accurate to within 1 cm-I. Spectra were background corrected. (1) Suzer, S.; Andrews, L. J . Chem. Phys. 1988, 88, 916.
(2) Andrews, L.; Grzybowski, J. M.; Allen, R. 0. J . Phys. Chem. 1974, 79, 904. (3) Andrews, L. Spectroscopy of Molecular Ions in Noble Gas Matrices. In Molecular Ions; Miller, T. A,, Bondybey, V. E., eds.; North-Holland: Amsterdam, 1983. (4) McDonald, S. A,; Andrews, L. J . Chem. Phys. 1979, 70, 3134. (5) Prochaska, F. T.; Andrews, L. J . Am. Chem. SOC.1977, ZOO, 2107. (6) Andrews, L.; Johnson, G . L.; Kelsall, B. J. J . Phys. Chem. 1982,86,
3374.
(7) McDonald, S . A,; Johnson, G. L.; Keelan, B. W.; Andrews, L. J . Am. Chem. SOC.1980, 102, 2897.
0 1989 American Chemical Society
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Suzer and Andrews
The Journal of Physical Chemistry, Vol. 93, No. 5. 1989
0 3400
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3000
2800
WAVENUMBERS
1600
1400
1200
1000
800
WAVENUMBERS
600
400
CCM-l>
Figure 1. Infrared spectrum of an Ar/C2H6= 200/1 mixture (a) without electron impact, (b) with deposition and electron impact (50 PA) for 3 more h, and (c) with deposition and electron impact (100 FA) for 3 more h. A refers to acetylene (C2Hz),E to ethylene (CzH4), M to methane (CH,), ER to ethyl radical (CzH,), and V to vinyl radical (C2H3).
Results Matrix electron-impact experiments will be described for several hydrocarbon systems. C2H6and C2D6.After argonlethane samples were deposited with low-energy electrons, new product bands were prominent in all regions of the spectra. Absorptions in the C-H stretching regions around 3300 and 3200 cm-’ and others at 1439, 1331, 1305,948,736, and 533 cm-’ were major product bands. Increase of the current density increased the intensity of these bands as well as introducing other weaker features as shown in Figure 1 and listed in Table I. The spectrum of C2D6upon electron impact also revealed bands in the C-D stretching region and at 991, 72 1, 542, and 396 cm-I. Annealing the matrix slightly decreased the intensity of the products as well as precursor bands. C2H4,C2Hf12,and Cf14 Electron impact on ethylene produced a group of bands in the C-H stretching region around 3300 cm-I, similar to the ethane products, a doublet at 1335 and 1331 cm-’ and another band at 736 cm-’ as well as several weaker bands at 1380, 1356, and around 910 and 700 cm-I. Corresponding product bands in l,1-C2H2D2and CzD4were observed. Figure 2 shows the spectrum of 1,l-CH2CD2before and after the electron impact; t h e major product bands are collected in Table I together with the ethane products. C,H$ ( X = Cl, Br, and CZV). Spectra of monosubstituted ethylenes under electron impact are richer when compared to ethylenes. C2H3C1produced several bands in the C-H stretching region: a strong band at 2764 cm-I and others at 2109, 1343,834, 815, 751, 737, 630, and 606 cm-l as well as weaker features through6ut the spectrum. Figure 3 shows the spectrum of C2H3C1 before and after electron impact. Spectra of C2H3Brand C2H3Cl are very similar; the spectrum of C2H3Bris not reproduced here. The spectrum of acrylonitrile, C2H3CN,is somewhat different (8) Illenberger, E.; Baumgartel, H.; Suzer, S . J . Electron Spectrosc. 1984, 33, 123.
TABLE 11: Relative Intensities of the Ion Abundances in the 70-eV Electron Impact of Ethane13 ion re1 int ion re1 int C2H6+ 26 C*H4+ 100 C2H5+ 22 CzHz+ 23 TABLE I11 Major Product Bands (em-’) Observed upon Electron Impact of CIH3X (X = CI, Br, or CN) during Condensation with Excess Argon at 12 K C2H3CI CzH3Br C2HlCN identification’ 3563 * 3449 * (N-H stretch) 3326 3322 3315 C2HX (A and Y) 3303 3303 3303 CZHZ 3289 3289 3289 C2H2 3282 3282 3282 C2H2-HX (A’) C2H2-HCN (HCN’) 3234 2764 CZHZ-HCI (HCI’) C2H2-HBr (HBr’) 2467 2269 Y (C=N) * 2206 * 2173 * 2125 C2HX (C=C stretch) 2109 2087 2071 * 1789 1344 1343 C2H2-HX 1318 Y 1199, 815 HCCl (CC) * 1045 * 889, 802 ? 834 794 C2H2-HX (A’) 752 750 753 CzH2-HX (A’) 737 737 737 * 585 605 605 503 CzHX (A and Y) “New bands (asterisks) are most possibly due to H2C=C=C= Y is H-C=C-C=N, and ? denotes unidentified fragments.
H-H,
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 2125
Low-Energy Electron Impact on Hydrocarbons
4A
Y
d l O b
TE
I
II
I
d t
DA
I
\V/
Y
J 3400
3100
2600
2500
2200
lS00
WAVENUMBERS
1600
1300
1000
400
700
CCM- 1 >
Figure 2. Infrared spectrum of an Ar/CH2CD2= 200/1 mixture (a) without electron impact and (b) with electron impact during deposition. A refers to acetylene (C2H2),MA refers to mixed acetylenes (C2HD), and DA refers to deuterioacetylene (C2D2);TE refers to trans- and CE to cis-di-
deuterioethylenes.
/
HC 1
I CA
I cc
I
3400
3200
3000
zeoo
WAVENUMBERS
2600
1100
900
700
500
WAVENUMBERS
CCM- 1 >
Figure 3. Infrared spectrum of an Ar/C2H3Cl = 200/1 mixture (a) without electron impact and (b) with electron impact during deposition. A' refers to the perturbed acetylene and HCI' to hydrogen chloride in the of acetylene-hydrogen chloride complex. CA denotes chloroacetylene, and CC refers to chlorocarbene HCCI. The arrow in b points the position of uncomplexed HCI.
Suzer and Andrews
2126 The Journal of Physical Chemistry, Vol. 93, No. 5, 1989
Y
/
HCN
A'
I
Y
co I
* *
I
(b>
I
l*l I
* t
Y
*
*
w
r c Y
Y
I1
I
I
d t
a
3500
3300
3100
2300
2100
1900
1700
1500
1300
WAVENUMBERS
SO0
1100
700
500
CCM-1)
Figure 4. Infrared spectrum of an Ar/C2H,CN = 200/1 mixture (a) without electron impact and (b) with electron impact during deposition. A' and HCN' refer to the acetylene-HCN complex. Y refers to cyanoacetylene and CO and W identify CO and water impurities. Bands denoted by asterisks are believed to be due to the rearrangement product CH2=C=C=N-H as discussed in the text. from those of vinyl chloride and bromide and is shown in Figure 4. Strong product bands at 3563,3449, and 1789 cm-' were the major differences, with other bands corresponding to those observed in C2H3C1and C2H3Br. All the major product bands for these three monosubstituted ethylenes are collected in Table 111. Electron impact on C6H5C1 was carried out for comparison purposes, and only weak features could be observed even at high electron current densities. C2H2C12. Electron impact on 1,l and cis- 1,2 isomers of dichloroethylene produced several product bands, which were similar in both isomers and are given in Table IV. Similarity of the spectra for two isomers negated carrying out a similar study on the trans-1,2 isomer or higher substituted ethylenes. Figure 5 shows the spectrum of 1,1-C2H2C12before and after electron impact, which is representative of the dichloroethylene studies.
Discussion Product absorptions will be identified and electron-impact dissociations and rearrangements will be considered. c2H6 and c2D6. Assignment of the major product bands was straightforward with the aid of already published data.612 Accordingly, electron impact on ethane produced mainly the neutral species C2H4, C2H2, C2HS radical, and CH4 with no evidence for charged species. Even the Ar,H+ band at 903 cm-', which is one of the strongest features in the electron-impact IR spectrum of water,' could not be detected with any appreciable intensity. Increase of the current to 200 pA almost quantitatively converted all the C2H6 precursor to products. Similarly deuteriated ethane produced C2D4,C2D2,C2DSradical, and CD4. Although
TABLE IV: Major Product Bands (cm-I) Observed upon Electron Impact of C2H2CI2during Condensation in an Argon Matrix 1,1-C2H2C12 cis-C2H2C12 identification 3326 3314 3303 3289 3283 3106 3086 2782 1301 1201 995, 992, 989 910 85 1 827
3326 3314 3303 3289 3283 3106 3086 2782 1201 995, 992, 989 910 827 787
759 736 697 617 605
760 136
?
t-CzH2CI2 (TE) c - C ~ H ~ C(CE) I~ C2HCI-HC1 (CA') C2HC1-HC1 (CA')
617 605
it is difficult to make any meaningful comparison between the intensity of observed IR bands and their concentrations, we conclude that the production of C2H, (C2D4)and C2H2(C2D2) was more efficient than that of either the ethyl radical or methane. This leads us to propose that neutral diatomic H2elimination via either one step (process 1) or two steps (process 2) is more fae-
(9) Manceron, L.; Andrews, L. J . Phys. Chem. 1985, 89, 4094. (10) Jacox, M. E. J . Phys. Chem. Rej: Data 1984,13,945, and references therein. (1 1) Golike, R.; Mills, I.; Crawford, B. L. J . Chem. Phys. 1956, 25, 1267. (12) Crawford, B. L., Jr.; Lancaster, J. E.; Inskeep, R. G. J . Am. Chem. SOC.1953, 21, 618.
H-CZC-CI (CA) C2HCI-HCI (CA') C2H2 (A) C2H2 (A) C2H2 (A) t-C,H,CI, (TE) c-CzH2C12 (CE) C2HCI-HC1 (HCI') c - C ~ H ~ C(CE) I~ t-C,H,CI, (TE) C2C12 (DC) Z-C2H2CI2 (TE) c-C~H~C (CE) I~ t-CZH2C12 (TE) 1 ,l-C2H2CI,
-
+ C2Hs
-H2
C2H4
(1)
-2H2
e-
+ C,H6
C2H2
(2)
e-
+ C2H6 2C2H5
(3)
Low-Energy Electron Impact on Hydrocarbons
i3
HC 1
'T
1
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 2127 /
C A'
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4
/
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o
d
Cb)
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t
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.-
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WAVENUMBERS
2600
1450
1250
850
1050
650
WAVENUMBERS
450
250
CCM- 1>
Figure 5. Infrared spectrum of an CH2CC12= 200/1 sample (a) without electron impact and (b) with electron impact during deposition. A refers to acetylene; CA' and H-CI' to chloroacetyleneand hydrogen chloride complexed by hydrogen bonding. CE and TE refer to cis- and trans-1,2-dichloroethylene, CHCI=CHCI, respectively. The arrow denotes the position of uncomplexed HCI.
vorable than is the bond rupture (process 3). The production mechanism of CH4, evidenced by the weak band at 1304 cm-', is more complex and may be mainly due to secondary processes. Carbon-carbon bond rupture would produce two C H 3 radicals, which upon diffusion and reaction with C2H6 precursor can produce C2H5radical and methane. Failure to detect any ionic species suggests that molecular elimination or simple bond rupture are the favored channels upon electron impact and that ionization channels 2re less favorable for these systems. It must be noted that charged species are more difficult to trap in solid argon; however, the fact that no infrared evidence was found for charged species, which are in fact expected to be very strong absorbers, clearly demonstrates that neutral products dominate. A crude comparison with the mass spectrum of ethane is noteworthy at this point. Table I1 gives the ion intensities observed under 70-eV electron impact of ethane.I3 Ethylene cation, C2H4+, is the most abundant ion, followed by the parent cation, C&+. Acetylene cation, C2H2+,is present to only 1/5 of the ethylene ion. Although a meaningful comparison with our spectrum requires a knowledge of absolute IR intensities both in the C-H stretching and bending regions, acetylene, C2H2,is by far the most abundant species, which simply reflects that the neutral distribution under electron impact is not at all correlated with the ion distribution. Hence, extracting information about neutrals by using mass spectrometric results must be done with extra caution. In this regard matrix IR data can be helpful. C2H,, CH2CD2,and C2D4. Assignment of the major product bands was again straightforward with reference to published Accordingly, electron impact on ethylene and deuterioethylene produced mainly C2H2 and C2D2. 1,l-Dideuterioethylene produced again C2HZ,C2Dband C2HDwith comparable intensities; however, cis- and trans- 1,2-dideuterioethylenewere also observed in substantial and comparable yields after electron (13) Cornu, A.; Massot, R. Compilation of Mass Spectral Data: Heyden and Sons: London, 1966.
impact. Production of acetylene and isotopic acetylenes is the result of H,, HD, or D2 elimination:
e-
e-
+ C2H4
e-
+ C2D4
-HZ
C2H2
(4)
C2D2
(5)
- +
+ C2H2D2
-D2
-HD,Hz,Dz
C2H2 CzHD + C2D2
(6)
Production of cis- and trans-dideuterioethylenewas surprising since we started with 1,l-dideuterioethylene only. Observation of these mixed isotopic ethylenes indicates that electron impact on the hydrocarbons produces a manifold of excited states, which subsequently lead to (i) fragmentation as well as (ii) return to the ground state following rearrangement e-
+ AB
- -+ [AB*]
A
AB
B
(fragmentation) (isomer rearrangement)
Molecules must be sufficiently excited to overcome the energy barrier for isomerization; hence when they return to the ground state all the previous knowledge is lost, and the production of fragmentation as well as nonfragmentation channels will be governed by thermodynamic factors. The nonfragmentation channels, of course, lead to no new products in the spectra of C2Ha, C$6, C2H4, and C2D4 since there are no differences in the precursors and the products. It makes quite a difference in dideuterioethylene since the nonfragmenting products HDC=CHD (cis and trans) are different from their precursor, CH2=CD2. No C2HD3or C2H3Dproducts were found, which means that secondary exchange processes ( H C2H2D2 C2H3D D) are unimportant, and the electron-impact products in these experiments arise from a single precursor molecule. Finally, a sharp multiplet at 900, 906, and 91 1 cm-' can be identified as arising from the vinyl free radical as can isotopic counterparts at 887, 797, and 704 cm-*.I4 The observation of weak vinyl radical bands
+
-
+
2128
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989
in the present experiments with both ethane and ethylene shows that H-atom elimination processes contribute to the overall spectrum, but these processes play a minor role compared to molecular H2 elimination. C2H3Cland C2H3Br. Assignment of the product bands was straightforward using the results of matrix vacuum-UV photolysis on these system^.^,^ The major products again are C2H2-HX and C2HX.15 However, the acetylene and the hydrogen halide products are not isolated molecules in the matrix but are complexed together through hydrogen bonding. The structure of the complex was deduced to be T-shaped in the original infrared studies7 and by later microwave work:I6 X
e-
+ H~C=CHX
e-
+ H2C=CHX
-
I
Y H-&C-H
-H2
H-C=C-X
(7)
(8)
No free acetylene or free H X was observed, as is indicated in Figure 3. Apparently electron impact gives sufficient energy to break the bonds, but the products are entrapped in the quickly forming matrix cage. Observation of chloro- and bromoacetylene must proceed via a similar H2 elimination process, and now the question arises whether or not the eliminated H2 is also complexed with the remaining fragment, chloroacetylene (or bromoacetylene). Since Hz cannot be observed directly in IR spectra and due to the extremely weak interaction between H2 and chloroacetylene, its presence cannot be inferred from the product spectrum, and the question cannot be answered unambiguously. Note that a similar question can be raised for the ethanes and ethylenes. Again no unambiguous answer can be given, but one might argue that H 2 would not be captured as efficiently due to its very low molecular weight. Two of the other bands at 815 and 1198 cm-' are very close to bands assigned to chlorocarbene; the intensity distribution (8 15 cm-I stronger and 1199 cm-' weaker) is also in agreement with the reported spectrum.I0 Hence, these bands are probably due to chlorocarbene. The production of HCCl must be via a double-bond rupture process somewhat different from others: e-
-
+ C2H3C1
HCCl
+ CH2
e-
+
HzC=CH-CeN
+ H2C=CH-C=N
-
-H2
H-C=C-H
H-C=C-C=N
CH3CN in this 1aborat0ry.l~Jacox has assigned18bands at 2038, 999, and 690 cm-I to keteneimine produced by excited argon-atom reaction with CH$N and N H reaction with C2HZ,which are also common to the electron-impact experiments with CH3CN. Observation of the 3449-cm-' N-H stretching fundamental suggests that electron impact induced a 1,3-sigmatropic rearrangements of the type e-
+ H2C=CH--CrN
(10)
(1 1)
In this respect, the -CN group behaves like a halogen. However, additional product bands were observed at 3563,3449,2206,2173, 2125, 1789, 1045, 889, 802, and 585 cm-'. The strong band at 3449 cm-I is in the.N-H stretching region where a strong band was observed at 3461 cm-I in similar electron-impact studies with
e-
+ H2C=CC12
H2C=C=C=N-H
(1 2)
-H2
CI-C=C-Cl
(13) CI
I
e-
+
cis-HCIC=CHCI
-
-
H
I
H-CeC-CI
(14)
-a*
H-C=C-H (1 5) Again whether or not the eliminated H 2 in (1 3) or C12 in (1 5) is complexed with the remaining fragment cannot be answered, but H-C1 is definitely hydrogen bonded to monochloroacetylene, as evidenced by the perturbed H-CI and chloroacetylene bands. In addition to elimination, rearrangement reactions also contribute:
-
e-
+ HZC=CCl2
e-
+ cis-HClC=CHCl-
cis-HClC=CHCl
+ trans-HCIC=CHCl (16)
trans-HClC=CHCl
+ H2C=CCI2
(17) Since 1,2- and 1,l -elimination are observed on electron impact, we may ask which process is favored: H
\
/
CI
/
-
H-CeC-CI
(18)
-
H---CEC-CI
( 1 9)
-HCI
H /c=c\c, H\ ,C=C,,c,
(14) Jacox, M. E.; Olson, W. B. J. Chem. Phys. 1987,86,3134. Shepherd, R. A.; Doyle, T. J.; Graham, W. R. M. J . Chem. Phys. 1988, 89, 2738. (15) Hunt, G. R.; Wilson, M. L. J . Chem. Phys. 1967, 34, 1301. Turrell, G. C.; Jones, W. D.; Maki, A. J. Chem. Phys. 1957, 26, 1544. (16) Legon, A. C.; Aldrich, P. D.; Flygare, W. H. J . Chem. Phys. 1981, 75, 625. (17) Bohn, R.; Andrews, L., unpublished results.
-
A similar process has been characterized in an electron-impact study of CH3CN.I7 On the basis of the spectra of keteneimine's and allene,I9 strong antisymmetric C=C=N and C=C=C stretches are expected near 2038 and 1955 cm-', respectively. However, interaction will force these apart, and the new bands at 2206 and 1789 cm-' are assigned to these motions for HzC3NH. (It is possible for the 2125-cm-I band rather than the 2206-cm-' band to be contributed by H2C3NH.) The 3563-cm-I band is assigned as the overtone of the 1789-cm-I band (2 X 1789 = 3578 cm-I). The 1045-cm-' band is appropriate for a C=N-H bending mode and one of the 889- or 802-cm-I bands is probably due to the CHI wagging mode (838 cm-' for allene).I9 The remaining bands, particularly in the 2000-2300-~m-~region, could be contributed by radical species formed by H-atom detachment from the precursor. C6H5Cl. Failure to observe any strong absorptions for fragmentation products similar to the ones observed in ethane and ethylene is consistent since the diatomic elimination would lead to destruction of the aromatic character of the benzene ring and hence is not favored. This further supports the hypothesis that diatomic elimination is the most favored channel for the various electron-impact fragmentation processes. 1,1-C2H2Cl2and cis-1,2-C2H2C12. Assignment of the major product bands is facilitated by the previous vacuum-UV photol y ~ i sand ~ , ~other matrix isolation work on dichloroacetylene.20 The major products now are H-C=C-H, H-CrC-CI, C1CEC-CI, and HCI, and again H-C=C-Cl and HC1 are complexed together in the matrix cage.
(9)
It is interesting that the corresponding process is not observed for either 1,l- or 1,2-dichloroethylene, as will be discussed shortly. C2H3CN. As for vinyl chloride and vinyl bromide, electron impact and acrylonitrile produced C2H2 complexed with HC=N, as evidenced by the perturbed acetylene and H-C=N bands,I7 and cyanoacetylene15 was also formed:
e-
Suzer and Andrews
H
-HCI
CI
(18) Jacox, M . E. Chem. Phys. 1979.43, 157.
(19) Patten, K. 0.. Jr.; Andrews, L. J . Am. Chem. SOC.1985, 107, 5594. (20) Klaboe, P.; Klaboe-Jensen, E.; Christensen, D. H.; Johnsen, I. Specrrochim. Acta 1970, 26A, 1567.
J. Phys. Chem. 1989, 93, 2129-2133
Our spectral observations suggest that each is equally likely. In fact, observation of H-CEC-H from I , 1-dichloroethylene is a good illustration of the principle that all possible products are produced, as found for the deuteriated ethylenes. Electron-Impact Process. The main body of experimental evidence provided in this work indicates that electron bombardment causes primarily fragmentation and rearrangement reactions. These reactions must take place on the matrix surface during the condensation process where the gas density is high, since irradiating the sample by electrons after the matrix was formed gave no appreciable change in the spectra observed. Furthermore, when electrons were directed across the gas stream away from the matrix, no products were observed. In the production of fragments and rearrangement products, the matrix must play an important role either (i) by providing a higher molecular density or (ii) by providing the third body to dissipate the extra energy such that rearrangement reactions can occur. Certainly, the matrix is essential for trapping the fragments together in the same cage after the initial rupture of the bonds. All of this evidence suggests that electron impact is most effective just before the fragments are frozen into the matrix and not in the gas phase nor after the matrix is formed. In this respect it is appropriate to compare the matrix electron-impact results with high-pressure mass spectrometry results and to state that the majority of products formed by electron impact in a high-density gas are the neutral species and that ions are produced in much smaller concentrations. Conclusions In this work, we have described the use of I R absorption spectroscopy to study the well-known electron-impact process on dilute argon/hydrocarbon gaseous mixtures during their con-
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densation onto a CsI window at 12 K. The electron-impact process takes place on the sample surface where the condensing gas density is high. Surprisingly the major products were neutral species; although some ion formation cannot be ruled out, no significant spectroscopic evidence was found for trapped ions.21 Diatomic molecular elimination seems to be the most favorable fragmentation channel. Both 1,l- and 1,2-elimination were observed from isotopic and substituted ethylenes. Fragments trapped together in the matrix cage formed complexes as evidenced by perturbed IR absorptions; in this respect electron impact is very similar to vacuum-UV photolysis. In addition, rearrangements leading to various isomers were observed by electron impact including 1,lC2HZD2to cis- and trans-CHDCHC and a 1,3-sigmatropic hydride shift in acrylonitrile to give the H2C==C=C=N-H species. The most important conclusion from this matrix isolation study of electron impact on hydrocarbons is that netural molecules and fragments were the major products, and charged fragments were not observed here.
Acknowledgment. We gratefully acknowledge financial support from the Thomas F. and Kate Miller Jeffress Memorial Trust and the communication of results on vinyl radical by W. R. M. Graham before publication. Registry NO. C&74-84-0; C2D6, 1632-99-1;CzH4, 74-85-1; CH2CD2, 6755-54-0; CzD4,683-73-8; C2H3C1,75-01-4; C2H3Br,593-60-2; C2H3CN,107-13-1; 1,1-C2H2Cl2,7 5 - 3 4 ; cis-C2H2CI2,156-59-2; c6HSCI, 108-90-7. (21) A weak product bandt0 at 697 cm-I in vinyl chloride experiments could be due to HCI,, but its position on the shoulder of a strong vinyl chloride band at 712 cm-' requires caution for this identification.
Rigidochromism as a Probe of Gelation and Densification of Silicon and Mixed Aluminum-Silicon Aikoxides John McKiernan,t Jean-Claude Pouxvie1,t Bruce Dunn,tq* and Jeffrey I. Zinkt** Department of Chemistry and Biochemistry and the Department of Materials Science and Engineering, University of California, Los Angeles. Los Angeles. California 90024 (Received: June 7, 1988)
The emission maximum of ReC1(CO)3bpy blue shifts as a function of increasing rigidity of the surrounding matrix. This rigidochromic effect provides a sensitive means of probing the structural changes that occur in the sol-gel process. Two sol-gel systems, tetraethoxysilane (TEOS) and a mixed aluminosilicate system (ASE), are studied. The magnitudes of the shifts of the luminescence maximum of the rigidochromic probe molecule were established in relevant test systems by using fluid and frozen solid ethanol and TEOS as test matrices. Shifts of about 2500 cm-' were found. The changes in the luminescence were then followed during the sol-gel-solid transformation of the aluminosilicate and TEOS systems. In the former, the emission maximum monitors the initial partial rigidification during aging and further follows the subsequent rigidification during drying. In the TEOS system, the emission of the probe does not shift during aging but exhibits a large change during drying. These contrasting results show that on the molecular level the two systems have quite different structural properties. The aluminosilicate gel contains small pores that trap the dye molecules and large pores that enable the solvent to diffuse. The small pores partially contract during aging without macroscopic changes of the gel. Further shrinkage occurs during drying. In contrast, in the TEOS system the ReC1(C0)3bpy molecules are not encapsulated into the gel and instead remain in the interstitial liquid phase. The molecules adsorb on the silica surface of the pore walls at the last stage of the drying.
The sol-gel process is a synthesis technique recently developed to prepare gels, glasses, and ceramic powders.' Metal alkoxides (formula M(OR),, where M is Al, Si, Ti, . . ., and R is an organic group) undergo hydrolysis (eq 1) followed by polycondensation reactions in solution at room temperature (eq 2). When the rates M(0R) H20 M(0H) ROH (1) M(0H) + M(0H) M-0-M + H20 (2)
+
-+
+
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Department of Chemistry. *Department of Materials Science.
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of the reactions are well controlled, the solution becomes increasingly more viscous as an inorganic polymer M-0-M grows. Eventually the solution turns into a transparent amorphous solid. At this stage, the material is a rigid gel that still contains water and organic solvents. Subsequent drying at room temperature or 70 'C removes most of the organic molecules. The resulting (1) For general references on the sol-gel process, see Glass and Glass Ceramics From Gels. J . Non-Cryst. Solids 1986, 82, and Better Ceramics Through Chemistry II; Brinker, C . J., Clark, D. E., Ulrich, D. R., Eds.; MRS Symposium, Vol. 73.
0 1989 American Chemical Society
