to the Interpretation of Mass Spectrum of Hexaethoxydisiloxane Linked scans and M+1, M+2 isotopic effects

The aim of this article is the study of the fragmentation reactions of hexaethoxydisiloxane initiated by electronic impact in the ionization chamber of a double focusing mass spectrometer. The initiation center of fragmentation reactions is established by quantum calculations. The daughter ions of hexaethoxydisiloxane molecular ion are obtained by linked scan B/E. The primary fragmentation ions with the masses 341, 297, 296 and 269 were obtained experimentally by B/E linked scan by radical induced fragmentation reactions. The eliminations of neutral fragments such as hydrogen, acetaldehyde, ethylene and water from the primary ions and the obtained ions by consecutive elimination reactions were emphasized experimentally by the B/E(1-E) 1/2 linked scan by charge induced reactions.


Hydrolysis -Condensation Reactions
The chromatogam of monomers, dimers, trimers and tetramers of TEOS is presented in figure 1 Scan numbers, structural formulae, structural codes and molecular weights of identified TEOS dimers are presented in table 1. The author developed a procedure [1,14] for the mass spectra interpretation of some silicon alkoxides and their transesters and oligomers obtained by the sol-gel process. This procedure was applied in previous works [14][15] and in the present work for the interpretation of mass spectrum of hexaethoxydisiloxane.
Mass spectrum of an organic substance, as well as a TEOS dimer such as hexaethoxydisiloxane is the result of a series of unimoleculare consecutive and competitive chemical reactions, which constitutes a pattern of fragmentation [10][11][12][13].

Experimental part
The development of GC-MS method for TEOS dimers determination in sol-gel processes consisted in establishing of the GC separation conditions of the analyzed components and the establishing of the optimal MS parameters ( Table 2).
For the interpretation of mass spectrum TEOS dimer, the linked scans were used to study the metastable ions obtained in the first region without field (FFR1): 1) The B/E linked scan: This method of scanning allows obtaining daughter ions m 2 + from a preset precursor ion m 1 + . The condition for obtaining the daughter ion is B/E = constant, where B is the magnetic field, and E is the electric field applied in the magnetic and electric sectors of the mass spectrometer. 2) The (B/E)(1-E) 1/2 linked scan: It is used to obtain the ions which lose small molecules with a preset mass (e.g. water, ethanol, ethylene, acetaldehyde, etc.).
The M+1, M+2 isotopic effects for the obtained ions were calculated by MS Interpreter software from Perkin Elmer.

Results and discussions
The ionization process The mass spectrum of hexaethoxydisiloxane to the resolution R = 1000 is presented in Table 3. The molecular ion m/e 342 obtained in the electron impact ionization 70 eV has an intensity of 3.9%.
Quantum calculations by MOPAC 7 for neutral and ionized TEOS dimer molecules ( fig. 3) show that the  Legend: molecular ion is encoded M +. ; the primary fragmented ions are thicken in the gray background cells; the ion intensity is expressed as % of basic ion normalized to I = 100. initiation center of fragmentation reactions is located on the oxygen atom of an ethoxy group; net negative charge on this atom decreases from -0.459 to -0.321. The siloxane bond is strong in the ionized dimer molecule by charge delocalisation due to +I inductive effect of the silicon atom and -I inductive effect of the oxygen atom. Another argument concerning the existence of the molecular ion and some fragmentation ions are the M+1 and M+2 isotopic effects measured experimentally compared to the theoretical ones.
According to the isotopic distributions of the atoms that compose an alcoxide as TEOS and its oligomers as hexaethoxydisiloxane, the strongest M+1 and M+2 isotopic effects are due to silicon atom 29 Si and 30 Si with natural concentration 4.67 % and 3.10 %, respectivelly, followed by the M+1 isotopic effect of carbon atom 13 C (1.1 % ) [1]. The M+2 isotopic effect of the silicon atom is selectivelly in the case of silicon alkoxides and their oligomers without halogen atoms.
There is a good agreement between experimental M+1, M+2 isotopic effects for some obtained ions and those calculated theoretically (Table 4); for example is a good agreement between experimental values (1 % and 0.4 %) in Table 4 and those calculated theoretically by MS Interpreter software (Table 5)     The ion m/e 296 obtained by reaction (4) stabilized by inductive effect is intense although it is a radical cation. Part of ions m/e 296 with a sufficient internal energy decompose further. A very probable reaction is induced by radical, with acetaldehyde elimination, according to equation (5). (5) The cation radical with m/e 252 has the intensity I = 5.1 % and even mass as transposition parent ion with m/e 296.
One reaction pathway to obtain the daughter ion m/e 269 of molecular ion m/e 342 is the elimination of a fragment with mass 73 uam; that mean elimination of an ethyl radical and a molecule of acetaldehyde.

Eliminations of neutral molecules from primary ions
The elimination of a molecule of ethanol can be also evidenced experimentally by linked scans (B/E)(1-E) 1/2 . In figure 5.a-e are shown molecular ions and fragmentation ions of hexaethoxy-disiloxane alongside its methoxy transesters TR1D to TR4D that eliminate ethanol. It finds that the elimination of ethanol decrease with increasing transesterification. The elimination of ethanol is an intense reaction for molecular ions of species TEOS dimer and methoxypentaethoxydisiloxane (TR1D) that have six and five ethoxy groups.
The fragmentation ions of hexaethoxydisiloxane that eliminate acetaldehyde and ethylene by liked scans B/E(1-E) 1/2 are shown in figure 6 and 7 respectivelly. Thus there can be written, according to the experimental data obtained by linked scan (B/E)(1-E) 1/2 , the reaction pathways to obtaining the ion with mass m/e 139 from the ion with m/e 297 (daughter ion of molecular ion m/e 342) by succesive eliminations of ethylene and water according to the reactions (6).
The fragmentation ion with m/e 123 is obtained on the same reaction pathway until they obtained the ion with m/ e 185 by elimination of acetaldehyde and water according to the reactions (7).
The ion H 3 O 5 Si 2 with m/e 139 is confirmed by the M+1 isotope effect; experimental value of this effect is 1.8 % (Table 2), and the theoretical calculated value is 1.7 %. The experimental M+1 isotopic effect of ion H 3 O 4 Si 2 with m/e 123 has the value I=1.5 % as against of theoretical value I=1.3 %.
An argument for partial double ionization of molecular ion, at two ethoxy groups from the two silicon atoms, is the presence in the TEOS dimer spectrum of high intensity (7) ion C 2 H 5 + at m/e 29 (I = 44.2%). One reaction pathway to obtain the ion with m/e 142 (I=1.7 %) is elimination of ion C 2 H 5 + from double charged ion 171 ++. (342/2) with intensity I=3.7 % (Table 3).
Reaction pathways for obtaining mass spectrum of hexaethoxydisiloxane are summarized in Table 6.

Conclusions
The aim of this article was the study of the fragmentation reactions of hexaethoxydisiloxane initiated by electronic impact in the ionization chamber of a double focusing mass spectrometer. Hexaethoxydisiloxane as TEOS dimer with the structural formula (C 2 H 5 O) 6 Si 2 O and molecular weight M=342, was obtained in sol-gel process by hydrolysis-condensation reactions.
The author developed in a previous work [14] a procedure for the mass spectra interpretation of some silicon alkoxides and their transesters and oligomers obtained by the sol-gel process. This procedure was applied in present work for the interpretation of mass spectrum of hexaethoxydisiloxane.
Quantum calculations for neutral and ionized TEOS dimer molecules established that the initiation center of fragmentation reactions is located on the oxygen atom of an ethoxy group.
The primary fragmentation ions at m/e 341, 297, 296 and 269 were obtained experimentally by B/E linked scan, by radical induced fragmentation reactions, as the daughter ions of hexaethoxydisiloxane molecular ion.
The eliminations of neutral fragments such as hydrogen, acetaldehyde, ethylene and water from the primary ions and the obtained ions through consecutive elimination reactions were emphasized experimentally by the B/E(1-E) 1/ 2 linked scan by charge induces reactions.
Thus there can be written 35 fragmentation pathways for the primary events and the eliminations of neutral According to the isotopic distributions of the atoms that compose an alcoxide as TEOS and its oligomers as hexaethoxydisiloxane the strongest M+1 and M+2 isotopic effects are due to silicon atoms 29 Si and 30 Si, followed by the M+1 isotopic effect of carbon atom 13 C.
There is a good agreement between experimental M+1, M+2 isotopic effects for some obtained ions and those calculated theoretically; for example is a good agreement between experimental values (1 and 0.4 %) and those calculated theoretically (0.9 % and 0.4 %) for the molecular ion of hexaethoxydisiloxane with m/e 342 and intensity 3.9%.
The interpretation of a mass spectrum is important for the assigning of the structure of a molecular species with unidentified or wrong registered mass spectrum.