ISSN 0018-1439, High Energy Chemistry, 2009, Vol. 43, No. 2, pp. 75–79. © Pleiades Publishing, Ltd., 2009. Original Russian Text © V.K. Mavrodiev, A.S. Vorob’ev, V.M. Yanybin, I.I. Furlei, 2009, published in Khimiya Vysokikh Energii, 2009, Vol. 43, No. 2, pp. 117–121.
GENERAL ASPECTS OF HIGH ENERGY CHEMISTRY
The Mechanism of Formation and Specifics of Fragmentation for Negative Molecular Ions of Allylsilanes V. K. Mavrodieva, A. S. Vorob’evb, V. M. Yanybina, and I. I. Furleia a
Institute of Organic Chemistry, Ufa Scientific Center, Russian Academy of Sciences, pr. Oktyabrya 71, Ufa, 450054 Russia e-mail:
[email protected] b Institute for Physics of Molecules and Crystals, Ufa Scientific Center, Russian Academy of Sciences, pr. Oktyabrya 151, Ufa, 450075 Russia Received April 20, 2008
Abstract—The processes of formation of negative ions by allylsilane molecules were studied by resonanceelectron-capture mass spectrometry, and photoelectron spectra of these compounds were obtained. It was experimentally found that the overwhelming majority of fragment negative ions are produced in the energy range ~6−10 eV. It was shown that the resonance-electron-capture mass spectrum is almost entirely described by one or two series of intershell resonances due to excitation of an electron successively from several higher occupied orbitals to the lower unoccupied π molecular orbital. DOI: 10.1134/S0018143909020015
In [1], we studied some organic compounds of Group IV elements including silanes, such as Me4 − xSiVinx, by means of resonance-electron-capture (REC) mass spectrometry. It was shown that these compounds formed negative ions in several energy ranges and their abundance depended on the efficiency of σ*-π* interaction. Using published data on transmission electron microscopy, UV spectroscopy, and photoelectron spectroscopy (PES), resonances in a lowenergy region (0–4 eV) and a resonance in the energy region of the first π-π* singlet transition (~6.6 eV) were interpreted. However, we failed to establish the nature of resonance states of negative molecular ions with higher energies. In this work, we attempted to reveal the mechanism how vinylsilane and allylsilane molecules 1–14 form negative molecular ions by resonance electron capture in the energy range ~6–10 eV. Earlier, we obtained the photoelectron spectra of these compounds and performed relevant quantum-chemical calculations.
0.4 eV at half maximum, and the electron current was ~1 µA. The electron energy scale was calibrated by the – maximums of curves for the effective yield of S F 6 from –
SF6 (0 eV) and N H 2 from NH3 (5.65 eV). Photoelectron spectra were recorded on an ES-3201 instrument by excitation of molecules with the HeI emission having a quantum energy of 21.21 eV. The experimental procedure is described elsewhere [3]. Quantum-chemical calculations were carried out by the PM3 semiempirical method with the use of the HyperChem7 program. RESULTS AND DISCUSSION The REC spectra of substituted allylsilanes, like those of Me4 – xSiVinx [1], exhibit peaks due to (å-ç)– ions, whose abundance depends on the number of allyl groups in the molecules (see the mass spectra of these compounds in Tables 1 and 2). As the number of allyl – groups increases, the abundance of ë3 H 5 or (Allyl–) and (å-Allyl)– ions increases. For compounds 1–3, which have one allyl group in their structure, (å-ç)– ions have the highest abundance, as in the case of vinylsilanes [1]. The maximal abundance in the spectra of compounds 4–6, which have two or three allyl groups in their molecules, is observed for the Allyl– ion, and the most abundant ions for tetrasubstituted allyl and vinylsilanes are (å-Allyl)– and (å-Vin)–, respectively. In the spectra of allyltrivinylsilane (7), the highest peak likewise belongs to the (å-Vin)– ion; therefore, the behavior of the allyl group in this compound is closer to that of the vinyl, rather than alkyl, group in character. In this
EXPERIMENTAL The following allylsilane derivatives were studied: çMe2SiAllyl (1), Me3SiAllyl (2), Me2PhSiAllyl (3), çMeSiAllyl2 (4), MeVinSi(Allyl)2 (5), çSi(Allyl)3 (6), Vin3SiAllyl (7), Si(Allyl)4 (8), (Cl-CH2)MeSi(Allyl)2 (9), (Cl-CH2)Me2Si(Allyl) (10), ClMe2Si(Allyl) (11), MeSi(Allyl)3 (12), Me3SiCH=CHSiMe3 (13), and Me3SiC≡CSiMe3 (14). Resonance-electron-capture mass spectra were measured with an MI-1201 mass spectrometer modified for operation in the negative ion (NI) detection mode [2]. The electron energy distribution was 0.3– 75
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Table 1. Resonance-electron-capture negative-ion mass spectra of compounds 1–8 Ion (M-H)–
(M-All)–
Compound 1
2
3
4
5
6
7
8
16 (4.1) 100 (7.7)
5.0 (3.6) 100 (8.6)
5.5 (3.8) 100 (8.6)
15 (3.6) 100 (7.7)
16 (3.6) 57 (8.6)
15 (3.3) 58 (8.1)
4.0 (6.1) 3.0 (8.1)
14 (5.7) 8.0 (8.4)
3.0 (2.2) 36 (6.0) 5.0 (8.8) 4.0 (6.4) 27 (8.4)
3.0 (2.4) 27 (5.9) 16 (8.2) 14 (5.9) 58 (8.7)
5.0 (2.2) 52 (5.9) 9.0 (8.2) 6.0 (6.3) 25 (8.6)
5.0 (4.9) 10 (6.3) 11 (8.5) 37 (5.8) 16 (7.3) 3.5 (8.7) 37 (5.3) 100 (6.6) 75 (8.6)
21 (3.3) 6.0 (6.0) 69 (8.1) 4.0 (2.0) 100 (5.9)
6.0 (6.1) 3.0 (8.0)
(M-C2H3)–
6.0 (5.6) 24 (8.6)
–
C2 H 3
10 (6.0) 28 (8.5)
7.0 (3.3) 14 (6.7) 13 (8.7)
–
C6 H 5 (M-CH3)– All–
3.0 (5.7) 5.0 (7.5) 1.0 (6.0) 2.0 (7.9) 1.5 (9.8)
3.5 (6.1) 2.0 (8.0) 0.8 (9.6)
15 (8.9) 7.0 (7.4) 4.0 (6.2) 10 (7.8) 3 (10.4)
10 (6.4) 58 (8.4)
2.4 (6.6) 100 (8.9)
32 (6.1) 100 (8.5)
13 (6.4) 9.0 (8.7)
(M-All-2H)–
context, it should be noted that the role of the 3d orbital of the Si atom in allylsilanes is less significant as compared to the vinyl derivatives. According to the PES data [4], π-σ coupling increases the energy of the higher occupied molecular orbital. Nonetheless, these changes in π-d interaction in vinyl- and allylsilanes barely affect the pattern of the mass spectra of these compounds. It is likely that the effect of coupling is stronger in the lower electronically excited state of allylsilanes than in the ground state [5]. The most abundant peak in the NI mass spectra of chlorinated compounds is that of Cl–. In the case when the chlorine atom is directly attached to the silicon atom, the relevant spectra do not exhibit the peak of Cl– ions at low electron energies, the maximal intensity of the Cl– peak is observed at an electron energy of 7.6 eV. Chlorovinylsilanes are characterized by a similar mass spectrum [6]. Probably, the Si–Cl bond energy in compounds in which the chlorine atom is directly bonded to the silicon, increases to such an extent that the lowelectron-energy dissociation pathway becomes closed because of the coupling of the chlorine p orbital to the unoccupied d orbitals of the silicon atom. This conclusion is consistent with the data obtained by Frost et al. [7] who showed that the chlorine lone electron pair in chlorosilanes is stabilized by dπ-pπ coupling with the
13 (6.2) 68 (8.5) 14 (6.2)
unoccupied d orbitals of the silicon. Strengthening of the Si–Cl σ bond and a weak influence of the halogen atom on the SiH3 σ bonds are observed as a result. In the mass spectra of compounds 9 and 10 in which the chlorine atom is in the β-position to the silicon atom, the maximal intensity of the Cl– peak is detected at an electron energy of 0.9 eV. The abundance of the Cl– ion observed at electron energies of 4.1, 6.0, and ~8.0 eV in the spectra of 9 and 10 is less than 10%; i.e., the mass spectrum of these compounds resembles the REC mass spectrum of halogenated hydrocarbons, the main (Cl–) peak is detected at low electron energies, and the abundance of ions observed at other energies is a few percent or hundredths of percent of the most abundant peak. Analysis of the total ion current curves for vinyland allylsilanes reveals that the REC negative-ion mass spectra show an increase in the ion formation probability at electron energies of 6.3–7 eV, as compared to the energy range 8.5–9 eV, with an increase in the number of vinyl or allyl groups in the molecules. This tendency of vinylsilanes is stronger than that of substituted allylsilanes. The energy of this resonance for vinyl- and allylsilanes is 0.2–0.5 eV below the energy of the corresponding π-π* excited singlet state. Therefore, it may be HIGH ENERGY CHEMISTRY
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Table 2. Resonance-electron-capture negative-ion mass spectra of compounds 9–14 Ion (M-H)–
Compound 9
10
11
12
0.02 (3.1) 0.01 (7.1)
0.04 (3.3) 0.06 (7.1)
1.0 (3.1) 0.7 (8.0)
(M-C2H3)– (M-All)– All–
0.2 (6.0) 0.03 (8.0) 100 (0.9) 1.0 (4.1) 7.0 (6.0) 3.0 (8.2)
Cl–
0.1 (6.1) 0.05 (7.9) 100 (0.9) 6.0 (6.0) 6.0 (7.7)
3.0 (5.8) 3.0 (7.5) 1.0 (5.8) 0.2 (8.4) 28 (6.2) 100 (7.6)
(M-Me)–
4.4 (3.9) 4.0 (5.6) 20 (7.6) 37 (8.8)
1.0 (6.5) 4.0 (8.2) 5.0 (9.6) 8.0 (6.2) 65 (8.2) 100 (9.4)
(M-SiMe3-2H)–
–
14 5.0 (4.9) 69 (8.0) 96 (9.5)
5.0 (6.2) 3.0 (8.3) 2.0 (2.1) 58 (5.9) 11 (6.3) 100 (8.5)
15 (6.3)
(M-SiMe3)–
26 (7.6) 3.2 (9.9) 52 (8.2) 64 (9.3) 100 (10.5)
12 (9.8)
SiM e 3
assumed that this is an electronically excited Feshbach resonance of the intershell type. The parent state of the intershell resonance is the singlet excited state of the molecule, and the extra electron is captured in the fully symmetric Rydberg orbital [9]. To interpret the resonance states of higher energies, let us employ the technique for determination of the spectroscopic states of negative molecular ions [10] based on the simultaneous use of REC negative-ion mass spectrometry and photoelectron spectroscopy. The correlation diagram presents the vertical ionization energies (IEs) and energies of resonance states for some of the vinyl- and allylsilanes examined. Regardless of the number of vinyl or allyl groups in the molecular structure, the upper ionization energy for these compounds will correspond to the removal of an electron from the π-MO. The second ionization energy for trimethylvinylsilane or allyltrimethylsilane corresponds to electron removal from a σ-MO. An increase in the number of vinyl (or allyl) groups in the molecular structure leads to the splitting of the π-MO. The photoelectron spectra and resonance states in the diagram are given on the same energy scale and are stitched for each comHIGH ENERGY CHEMISTRY
20 (3.5) 6.5 (6.3) 68 (8.2)
13
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pound in such a manner that the upper ionization energy coincides with the intershell resonance energy in the range 6.3–7 eV. This arrangement of both scales clearly shows that the energy gap between most resonance states coincides with energy gaps between the ionization energies accurate to within ±0.1–0.2 eV (for convenience of interpretation, the IEs are rounded off to 0.1 eV). For example, the gaps between ionization energies for trimethylvinylsilane of 9.8 π, 10.3 σ, 10.8 σ, and 12.88 σ,π coincide to this accuracy with the gaps between the resonance states observed at energies of 6.6, 7.0, 7.5, and 9.8 eV, respectively. This means that the other, higher energy resonances are intershell resonances as well. They are associated with the excitation of an electron from deeper lying occupied molecular orbitals (OMOs) having the same electron arrangement in the unoccupied molecular orbitals (UMOs) as in the resonance at 6.3–7 eV.1 1 Certain
resonances do not fit to this scheme; presumably, they belong to another series of electronically excited resonance or their corresponding MOs are unresolved by photoelectron spectroscopy.
11.5
7.6
10.85 11.1
HIGH ENERGY CHEMISTRY
12.0
10.85
10.1
9.1 9.25 9.35
9.3
9.1
8.2
7.4
6.8
6.2
13.1
11.3
9.5
9.1
IE
IE
9.0
8.5
8.0
7.4
6.6
13.35
12.1 12.3 12.55
9.75 9.95
9.2
8.7 9.1
8.6
8.0
7.9
7.2
6.1 6.3
RS
HSi(Allil)3
8.9 6.2 9.1
RS
HMeSi(Allil)2
12.9
11.5
10.9
10.1
8.4
8.0
13.0
11.0
10.6
9.9
9.8
IE
10.3
6.3
6.0
RS
IE
9.8
9.0
7.6
6.6 9.8 7.0 10.1 10.4 7.5 10.8
RS
Me3SiVin
8.7 8.9 9.1 9.3 9.4
IE
Si(Allil)4
Correlation diagram of the ionization energies (IE) and resonance states (RS) of allylsilane and vinylsilane molecules.
9.8
7.9 8.0
6.0
9.2
8.8
RS
IE
IE
RS
Me,PhSiAllil
HMe, SiAllil
SiVin4
9.1
8.6 8.8 9.0
7.0
6.6 6.9
6.3
RS
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THE MECHANISM OF FORMATION AND SPECIFICS OF FRAGMENTATION
Thus, comparing the processes of the formation of negative ions by allylsilanes and earlier studied vinylsilanes, we can conclude that they have similar mechanisms of their formation and fragmentation. The overwhelming majority of negative fragment ions of allyland vinylsilanes are produced in the ultraviolet excitation energy range (~6–10eV). The mass spectrum in this energy range is almost entirely described by one or two series of intershell resonances associated with the excitation of an electron from several higher occupied orbitals successively to the lower unoccupied π* molecular orbital. The interaction of the silicon 3d orbital with the π orbitals of the vinyl and allyl groups has a weak effect on the pattern of mass spectra of vinyl- and allylsilanes. Another picture is observed for some chlorinated compounds when the chlorine atom is directly bonded to the silicon atom. In this case, the coupling of the chlorine p orbital to the unoccupied d orbitals of the silicon atom is responsible for the absence of Cl– ions at low electron energies. REFERENCES 1. Furlei, I.I., Mavrodiev, V.K., Salimgareeva, I.M, and Bogatova, N.G., Izv. Akad. Nauk SSSR, Ser. Khim., 1986, no. 3, p. 566.
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2. Khvostenko, V.I., Mazunov, V.A., Fal’ko, V.S., et al., Khim. Fiz., 1982, vol. 1, no. 7, p. 915. 3. Furlei, I.I., Sultanov, A.Sh., Vorob’ev, A.S., et al., Khim. Fiz., 1987, vol. 6, no. 9, p. 1231. 4. Zykov, B.G., Photoelectron Spectroscopy of Organosilicon Compounds, Cand. Sci. Dissertation, Leningrad, 1986. 5. Burshtein, L.Ya. and Shorygin, P.P., Izv. Akad. Nauk SSSR, Ser. Khim., 1988, no. 6, p. 1330. 6. Mass-spektrometriya rezonansnogo zakhvata elektronov: metod i retrospektivnyi obzor (Electron Capture Resonance Mass Spectrometry: Method and Retrospective Review), Ufa, 1987, s. 220. 7. Frost, D.C., Herring, F.G., Katrib, A., McLean, R.N., Drace, J.E., and Weswood, N.R.C., Can. J. Chem., 1971, vol. 49, no. 24, p. 4033. 8. Petukhov, V.A., Zhun’, V.I., Sheludyakov, V.D., and Mironov, V.F., Zh. Obshch. Khim., 1979, vol. 59, p. 1054. 9. Khvostenko, V.I., Vorob’ev, A.S., and Khvostenko, O.G., J. Phys. B: At. Mol. Opt. Phys., 1990, vol. 23, p. 1975. 10. Khvostenko, O.G., Zykov, B.G., Asfandiarov, N.L., Khvostenko, V.I., Denisenko, S.N., Shustov, G.V., and Kostyanovskii, R.G., Khim. Fiz., 1985, vol. 4, no. 10. p. 1366.
