- Email: [email protected]
Formation of CF4− from CF4 clusters
Volume 157, number 3
CHEMICAL PHYSICS LETTERS
5 May 1989
FORMATION OF CF; FROM CF, CLUSTERS Johannes LOTTER, Alexander KUHN and Eugen ILLENBERGER Institutftir Phyyikaiische und Thenwtische Chemie der Freien LrniversiiiitBerlin, Takustrasse 3, D-1000 Berlin 33, Germany
Received 25 January 1989; in final form 21 February 1989
Electron attachment to perfluoromethane clusters generates CF, This compound has not previously been observed in the gas phase. The ion yield curve suggests that the initial electron capture proceeds via the first virtual MO in CF,. CFr is then stabilized by intermolecular interaction in a negatively charged complex
1. Introduction In this Letter we report the mass spectrometrical observation of CF; following resonant electron capture by tetrafluoromethane clusters. In previous investigations the formation of negative ions from isolated fluorochloromethanes (including CF,) has been studied by essentially two different mass spectrometric techniques: (a) ion formation by attachment of free electrons [ 1,2] and (b ) negative ion formation by electron transfer in collisions with potassium atoms [3]. It has been shown that free electron attachment to this group of molecules is always associated with the decomposition of the transient negative parent ion (M-) by the well known dissociative attachment reaction e-+M+M-+(M-X)+X-,
(1)
with X being the halogen atom. On the other hand, the charge transfer reaction K-tM+K++M-
In the case of tetrafluoromethane, however, only fragment ions could be detected. We are currently studying negative ion formation following electron attachment to molecular aggregates. We found that all the abovementioned parent anions generated in charge transfer (reaction (2) ) are also observed in the cluster experiment. In addition, we observed CF, and higher aggregates in electron impact of van der Waals complexes of tetrafluoromethane. This is a rather surprising result since previous studies on the isolated compound [4,5 ] have shown that CF, , which is formed by electron capture within a broad resonance around 7 eV, immediately decomposes into F- +CF3 and F+ CF, with remarkable excess translational energy of the products. This was interpreted as being due to the strongly repulsive potential energy surface of CF,- .
The present result, however, indicates that CFq can exist in a configuration which is stable with respect to autodetachment and dissociation.
(2)
may generate the radical parent anion in a stable configuration (with respect to dissociation and autodetachment) provided that such a configuration exists at all. Dispert and Lacmann [ 3 ] studied reaction (2) for M=CC14, CFCl,, CFzClz and CF+ Apart from various negatively charged fragments also known from dissociative attachment, they observed the parent negative ion in the systems CC&, CFC13and CF#&
2. Experimental The new apparatus used in this study will be described in detail elsewhere [ 61. Briefly, a supersonic beam generated by adiabatic expansion is crossed at right angles with an electron beam which is aligned by a small homogeneous magnetic field (B z 100 G) (fig. I). Negative ions arising from the interaction of the molecular beam (containing a distribution of
0 009-2614/89/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )
171
Volume 157, number
3
CHEMICAL
Molecular Beam
I
Filament
Fig. 1. Schematic diagram of the experimental
arrangement.
clusters including the monomer) with the electron beam are extracted from the interaction volume and detected by a commercial quadrupole mass filter. In the final configuration an electron monochromator will be used as electron source. For the present experiment we simply used a heated tungsten filament, the electron beam thus having a broad energy distribution (fwhm 20.5 eV). The CF, clusters were generated by expansion of CF, seeded in Ar ( 1: 10) with a stagnation pressure of 3.7 bar through a nozzle 80 pm in diameter.
3. Results and discussion
Fig. 2 shows negative ion mass spectra taken at an electron energy of approximately 7 eV. Apart from
M = CF, L-5
F-
c
CF,-
2 al E .-
(M-F) -
I$+) (M *CF,)-
B
F2
M-
i
\ WZ)-
0
I
I 100
I
(M2;Fx)-
(M3’F)m
i’ CM,)- :’ I 200
I
I
I
300
Mass (amu) Fig. 2. Negative ion mass spectra obtained pact to aggregates of pertluoromcthane.
172
by 7 eV electron im-
5 May 1989
PHYSICS LETTERS
the fragments known from electron capture by isolated CF4 (F- and CF, ), we additionally observe F, , the parent anion CF, (M- ) and larger complexes. In our previous high-resolution work [ 5 ] it has been shown that isolated CF, captures electrons within two broad and strongly overlapping resonances extending from 4.5 to 10 eV. The lower energy resonance (maximum at 6.8 eV) was interpreted as the electronic ground state of CF,- [ 7 ] formed by the accommodation of the additional electron into the first virtual MO with (C-F) antibonding character. This state immediately decomposes according to CF, +F-
,
+CF,
+F+CF,
,
(3a) (3b)
with high excess kinetic energy in both channels. The resonance of higher energy (maximum at 7.7 eV) was shown to be exclusively coupled with F- formation and low excess translational energy. This electronic state was interpreted as core-excited resonance correlated with F- and the electronically excited CF3 radical. Fig. 3 shows some selected ion yield curves obtained from CF, clusters with the presently used lowresolution configuration. The horizontal scale simply indicates the potential energy difference between the filament midpoint and the reaction volume. The F- ion yield curve then roughly corresponds to that obtained from the isolated compound in the highresolution experiment. Fig. 3 shows that M- is formed within the energy range known from electron attachment to isolated CF4 with the maximum, however, shifted to lower energies. This suggests that electron capture initially generates CF, in a strongly repulsive state within the complex. The ionized aggregate then decomposes by shaking off some CF, (and probably smaller fragment radicals) leaving CF, in its stable configuration. The observation of CF7 does not necessarily imply a positive adiabatic electron affinity for CF, and we rather suggest a situation as depicted schematically in fig. 4. Although the potential energy surface of CF,- is strongly repulsive in the Franck-Condon region it possesses a shallow minimum at considerably larger
5 May 1989
CHEMICAL PHYSICS LETTERS
Volume 157, number 3
E (e\‘1
a 6 F tCF3 F +CF3 F-+
Rc
CF,
c
R (C-F) ,’
Fig. 4. Schematic potential energy curves for isolated CF, and CF; in their electronic ground states.
‘-,, ,.
.‘.
.
CF,
.I
:’
‘.‘,.
Xx,’
, ,_,,. ,. :.
‘,.’ ,,.. i..,
I 1
‘,..
?” ,_(
,-
I 2
;‘.; 7.”
I 3
I 4
I 5
I 6
I 7
I
I
I
8
9
10
Electron Energy [eV] (uncorrected)
->
Fig. 3. Selected ion yield curves obtained from perfluoromethane clusters. For the electron energy scale, see text.
internuclear distances. Electron attachment to isolated CF, then generates fragments with considerable translational energy [ 5,7 1. Electron attachment to CF, clusters, on the other hand, can leave CF7 in the potential well due to effective intermolecular interaction in the ionized complex. Thermodynamically, the ion represents an unstable species since its energy is still above that of the neutral. It is, however, in a configuration where autodetachment cannot occur (within the Franck-Condon principle and the approximation of localized potential energy surfaces this is the case for R-cR, with R, being the crossing point of the potential energy surfaces).
From the present results it is not clear whether the ion represents a tetrahedral CF; with the carbonfluorine bonds weakened significantly from those in the neutral or an ion-molecule complex (CF,.F) -. We note that in a recent study the CF,+ ion could be observed following electron ionization of CF, clusters [ 8 1. Similarly to the negatively charged system studied here, electron or photoionization of CF, does not generate a stable parent cation. In conclusion, it can be seen that electron attachment to clusters of perfluoromethane produces CF; by intermolecular stabilization in the ionized complex. This stabilization is enhanced at lower attachment energies as evident from the CF, ion yield curve in fig. 3.
Acknowledgement We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft (Sfb 337) and the Fonds der Chemischen Industrie.
References [ 1] E. Illenberger, H.-U. Scheunemann and H. Baumglrtel, Chem. Phys. 37 (1979) [2
21.
] H.-U. Schcunemann, E. Illenberger and H. Baumglrtel, Ber. Bunsenges. Physik. Chem. 86 ( 1982) 321. 173
Volume 157, number 3
CHEMICAL PHYSICS LETTERS
[3] I-I. Dlspert and K. Lacmann, Intern. J. Mass Spectrom. Ion Processes 28 ( 1978) 49. [4] P.W. Harland and J.L. Franklin, J. Chem. Phys. 61 (1974) 1621. [5] E. Illenberger, Chem. Phys. Letters 80 (1981) 153.
174
5 May 1989
[ 61 A. Kiihn, J. Lotter and E. Illenberger, to be published. [7] T. Oster, A. Kiihn and E. Illenberger, intern, J. Mass Spectrom. Ion Processes, in press. [8] W. Dcnzcr, G. Hagenow, B. Brutschy and H. BaumgIrtel, J. Phys. Chem. 92 (1988) 6487.
CHEMICAL PHYSICS LETTERS
5 May 1989
FORMATION OF CF; FROM CF, CLUSTERS Johannes LOTTER, Alexander KUHN and Eugen ILLENBERGER Institutftir Phyyikaiische und Thenwtische Chemie der Freien LrniversiiiitBerlin, Takustrasse 3, D-1000 Berlin 33, Germany
Received 25 January 1989; in final form 21 February 1989
Electron attachment to perfluoromethane clusters generates CF, This compound has not previously been observed in the gas phase. The ion yield curve suggests that the initial electron capture proceeds via the first virtual MO in CF,. CFr is then stabilized by intermolecular interaction in a negatively charged complex
1. Introduction In this Letter we report the mass spectrometrical observation of CF; following resonant electron capture by tetrafluoromethane clusters. In previous investigations the formation of negative ions from isolated fluorochloromethanes (including CF,) has been studied by essentially two different mass spectrometric techniques: (a) ion formation by attachment of free electrons [ 1,2] and (b ) negative ion formation by electron transfer in collisions with potassium atoms [3]. It has been shown that free electron attachment to this group of molecules is always associated with the decomposition of the transient negative parent ion (M-) by the well known dissociative attachment reaction e-+M+M-+(M-X)+X-,
(1)
with X being the halogen atom. On the other hand, the charge transfer reaction K-tM+K++M-
In the case of tetrafluoromethane, however, only fragment ions could be detected. We are currently studying negative ion formation following electron attachment to molecular aggregates. We found that all the abovementioned parent anions generated in charge transfer (reaction (2) ) are also observed in the cluster experiment. In addition, we observed CF, and higher aggregates in electron impact of van der Waals complexes of tetrafluoromethane. This is a rather surprising result since previous studies on the isolated compound [4,5 ] have shown that CF, , which is formed by electron capture within a broad resonance around 7 eV, immediately decomposes into F- +CF3 and F+ CF, with remarkable excess translational energy of the products. This was interpreted as being due to the strongly repulsive potential energy surface of CF,- .
The present result, however, indicates that CFq can exist in a configuration which is stable with respect to autodetachment and dissociation.
(2)
may generate the radical parent anion in a stable configuration (with respect to dissociation and autodetachment) provided that such a configuration exists at all. Dispert and Lacmann [ 3 ] studied reaction (2) for M=CC14, CFCl,, CFzClz and CF+ Apart from various negatively charged fragments also known from dissociative attachment, they observed the parent negative ion in the systems CC&, CFC13and CF#&
2. Experimental The new apparatus used in this study will be described in detail elsewhere [ 61. Briefly, a supersonic beam generated by adiabatic expansion is crossed at right angles with an electron beam which is aligned by a small homogeneous magnetic field (B z 100 G) (fig. I). Negative ions arising from the interaction of the molecular beam (containing a distribution of
0 009-2614/89/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )
171
Volume 157, number
3
CHEMICAL
Molecular Beam
I
Filament
Fig. 1. Schematic diagram of the experimental
arrangement.
clusters including the monomer) with the electron beam are extracted from the interaction volume and detected by a commercial quadrupole mass filter. In the final configuration an electron monochromator will be used as electron source. For the present experiment we simply used a heated tungsten filament, the electron beam thus having a broad energy distribution (fwhm 20.5 eV). The CF, clusters were generated by expansion of CF, seeded in Ar ( 1: 10) with a stagnation pressure of 3.7 bar through a nozzle 80 pm in diameter.
3. Results and discussion
Fig. 2 shows negative ion mass spectra taken at an electron energy of approximately 7 eV. Apart from
M = CF, L-5
F-
c
CF,-
2 al E .-
(M-F) -
I$+) (M *CF,)-
B
F2
M-
i
\ WZ)-
0
I
I 100
I
(M2;Fx)-
(M3’F)m
i’ CM,)- :’ I 200
I
I
I
300
Mass (amu) Fig. 2. Negative ion mass spectra obtained pact to aggregates of pertluoromcthane.
172
by 7 eV electron im-
5 May 1989
PHYSICS LETTERS
the fragments known from electron capture by isolated CF4 (F- and CF, ), we additionally observe F, , the parent anion CF, (M- ) and larger complexes. In our previous high-resolution work [ 5 ] it has been shown that isolated CF, captures electrons within two broad and strongly overlapping resonances extending from 4.5 to 10 eV. The lower energy resonance (maximum at 6.8 eV) was interpreted as the electronic ground state of CF,- [ 7 ] formed by the accommodation of the additional electron into the first virtual MO with (C-F) antibonding character. This state immediately decomposes according to CF, +F-
,
+CF,
+F+CF,
,
(3a) (3b)
with high excess kinetic energy in both channels. The resonance of higher energy (maximum at 7.7 eV) was shown to be exclusively coupled with F- formation and low excess translational energy. This electronic state was interpreted as core-excited resonance correlated with F- and the electronically excited CF3 radical. Fig. 3 shows some selected ion yield curves obtained from CF, clusters with the presently used lowresolution configuration. The horizontal scale simply indicates the potential energy difference between the filament midpoint and the reaction volume. The F- ion yield curve then roughly corresponds to that obtained from the isolated compound in the highresolution experiment. Fig. 3 shows that M- is formed within the energy range known from electron attachment to isolated CF4 with the maximum, however, shifted to lower energies. This suggests that electron capture initially generates CF, in a strongly repulsive state within the complex. The ionized aggregate then decomposes by shaking off some CF, (and probably smaller fragment radicals) leaving CF, in its stable configuration. The observation of CF7 does not necessarily imply a positive adiabatic electron affinity for CF, and we rather suggest a situation as depicted schematically in fig. 4. Although the potential energy surface of CF,- is strongly repulsive in the Franck-Condon region it possesses a shallow minimum at considerably larger
5 May 1989
CHEMICAL PHYSICS LETTERS
Volume 157, number 3
E (e\‘1
a 6 F tCF3 F +CF3 F-+
Rc
CF,
c
R (C-F) ,’
Fig. 4. Schematic potential energy curves for isolated CF, and CF; in their electronic ground states.
‘-,, ,.
.‘.
.
CF,
.I
:’
‘.‘,.
Xx,’
, ,_,,. ,. :.
‘,.’ ,,.. i..,
I 1
‘,..
?” ,_(
,-
I 2
;‘.; 7.”
I 3
I 4
I 5
I 6
I 7
I
I
I
8
9
10
Electron Energy [eV] (uncorrected)
->
Fig. 3. Selected ion yield curves obtained from perfluoromethane clusters. For the electron energy scale, see text.
internuclear distances. Electron attachment to isolated CF, then generates fragments with considerable translational energy [ 5,7 1. Electron attachment to CF, clusters, on the other hand, can leave CF7 in the potential well due to effective intermolecular interaction in the ionized complex. Thermodynamically, the ion represents an unstable species since its energy is still above that of the neutral. It is, however, in a configuration where autodetachment cannot occur (within the Franck-Condon principle and the approximation of localized potential energy surfaces this is the case for R-cR, with R, being the crossing point of the potential energy surfaces).
From the present results it is not clear whether the ion represents a tetrahedral CF; with the carbonfluorine bonds weakened significantly from those in the neutral or an ion-molecule complex (CF,.F) -. We note that in a recent study the CF,+ ion could be observed following electron ionization of CF, clusters [ 8 1. Similarly to the negatively charged system studied here, electron or photoionization of CF, does not generate a stable parent cation. In conclusion, it can be seen that electron attachment to clusters of perfluoromethane produces CF; by intermolecular stabilization in the ionized complex. This stabilization is enhanced at lower attachment energies as evident from the CF, ion yield curve in fig. 3.
Acknowledgement We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft (Sfb 337) and the Fonds der Chemischen Industrie.
References [ 1] E. Illenberger, H.-U. Scheunemann and H. Baumglrtel, Chem. Phys. 37 (1979) [2
21.
] H.-U. Schcunemann, E. Illenberger and H. Baumglrtel, Ber. Bunsenges. Physik. Chem. 86 ( 1982) 321. 173
Volume 157, number 3
CHEMICAL PHYSICS LETTERS
[3] I-I. Dlspert and K. Lacmann, Intern. J. Mass Spectrom. Ion Processes 28 ( 1978) 49. [4] P.W. Harland and J.L. Franklin, J. Chem. Phys. 61 (1974) 1621. [5] E. Illenberger, Chem. Phys. Letters 80 (1981) 153.
174
5 May 1989
[ 61 A. Kiihn, J. Lotter and E. Illenberger, to be published. [7] T. Oster, A. Kiihn and E. Illenberger, intern, J. Mass Spectrom. Ion Processes, in press. [8] W. Dcnzcr, G. Hagenow, B. Brutschy and H. BaumgIrtel, J. Phys. Chem. 92 (1988) 6487.