Contributions to the Interpretation of Mass Spectra of the TEOS Methoxy-Transesters. Linked Scans, Accurate Mass Measurements and M+1, M+2 Isotopic Effects

The aim of this article is the study of the fragmentation reactions of the methoxy-transesters of tetraethoxysilane (TEOS), with structural formula (CH3O)x(C2H5O)ySi, initiated by electronic impact in the ionization chamber of a double focusing mass spectrometer. The initiation centers of fragmentation reactions for the neutral and ionized TEOS methoxy-transesters were established by quantum calculations of net atomic charges. The ions of the mass spectra of the TEOS methoxytransesters were obtained experimentally by the B/E and B/E(1-E) linked scans. Thus, there can be written the fragmentation pathways for the primary events by B/E linked scan (cleavage of methoxy and ethoxy groups) and eliminations of neutral molecules (acetaldehyde, ethylene, formaldehyde, molecular hydrogen) by B/E(1-E) linked scan. Accurate mass measurements at high resolution (5000), double bond equivalent (DBE) for the molecular ions and fragmentation ions of the TEOS methoxytransesters and M+1, M+2 isotopic effects were used to confirm their identification. The reaction pathways and accurate mass in mass spectrum of dimethoxydiethoxysilane (TR2M) are detailed.


. GC-MS method and optimization parameters
For the interpretation of the TEOS mass spectrum, the linked scans were used to study the metastable ions obtained in the first region without field (FFR1): The B/E linked scan: This method of scanning allows obtaining daughter ions m2 + from a preset precursor ion m1 + . 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.
The chromatograms for TEOS monomers in reaction mixtures with parental and nonparental solvents, are shown in Figure 1 [1,3].
The mass spectra of the TEOS methoxy-transesters are like to that of TEOS but contain molecular and fragmentation ions with 14 units less which correspond to the difference between the mass of the methyl and ethyl groups (Figure 2a

Contributions to the interpretation of mass spectra of the TEOS methoxy-transesters
In this work, we present a study of the reactivity of these molecular species obtained in the sol-gel process by interpreting their mass spectra.
For the easier viewing of exposure, the mass spectra of the TEOS methoxy-transesters (TR1M-TR4M) are given in Tables 1-4 as table-mass-intensity pairs; the intensity is expressed as a percentage relative to the basic peak normalized to I = 100.

The ionization process and obtaining molecular ions
The ionization process for the TR1M-TR4M molecular species takes place according to equation (5) by eliminating a nonparticipant electron of the oxygen atom of a methoxy or ethoxy group.
An argument in favor of this premise is the net atomic charge calculations performed with the MOPAC 7 program for these species before and after ionization compared to the same process for TEOS as the reference system. Figure 3.a-e. shows net atomic charges for (a) tetraethoxysilane (TEOS), (b) -(e) the TEOS methoxy-transesters and their molecular ions obtained in the process of ionization by electron impact at 70 eV. Charge centers are located on the oxygen atoms in the ethoxy and methoxy groups [1].
Quantum calculations with the MOPAC 7 program also provided other significant data for the methoxy-transesters of TEOS before and after ionization: dipole moments and ionization potentials (   After the electron impact at 70 eV, the reaction center is located on the oxygen atom of a methoxy or ethoxy group and leads to molecular ions with comparable formation heats; for example for the TR1M transester, the difference between the formation heats for the TR1M + ion (1) with the reaction center on the ethoxy group and the TR1M + ion (2) with the center of reaction on the methoxy group is only 0.4 kcal/mol (Figure 3b).
The initiation center for fragmentation reactions is located at TEOS on the ethoxy group, and at tetramethoxysilane (TMOS) on the methoxy group. Tetramethoxysilane can be considered as the second reference system along with TEOS. For the other methoxy-transesters (TR1M-TR3M) the charge location can be produced, as shown in Figure 3b and in Table 6, on both the ethoxy and the methoxy groups.
The preponderance of the two reaction centers in molecular ion obtained by electronic impact is dependent on the relative ratio of the ethoxy groups and the methoxy groups.
In all the molecular ions obtained from TEOS to TMOS (Figure 3a-e), charge delocalizations occur due to the electropositive nature of the silicon atom (in particular) and of the carbon atom relative to the oxygen atom. The highest stabilization by charge delocalization occurs in tetramethoxysilane (Figure 3e) which exhibits a very intense molecular ion of 58.2% of the base peak ( Table 5).
The dipole moments and ionization potentials for the ionic species have higher expected values compared to the corresponding neutral species (Table 6).

Primary events. Cleavages of  bonds
The fragmentation ions obtained directly from the molecular ion by cleavage of the  bonds, can be clearly demonstrated experimentally by the B/E linked scan, which gives the daughter ions for the molecular ions of the TEOS methoxy-transesters.  The same fragmentation reactions occur in the other methoxy-transesters with the specification that the weight of the elimination reaction of the methoxy group is increasing as the degree of transesterification increases, resulting in the obtaining of the basic ion in the mass spectrum of TMOS at m/e 121 (Table 5).
All fragmentation ions in Tables 3-5 obtained in primary processes are thickened in grey cells and the molecular ions of the TEOS methoxy-transesters in yellow cells.
The elimination of a methyl group is an intense reaction for the species containing the ethyl groups because after the cleavage a stabilized oxication is formed by the resonance effect with the formation of a carbocation as in the case of TEOS, according to the reaction (6). Thus, for the TEOS, TR1M and TR2M species, the M-15 is the basic peak in the mass spectrum, and for the TR3M species with one single ethyl group, the M-15 ion is very intense (85.4%) ( methyl groups, the intensity of the M-15 ion is low (7.2%) due to obtaining an oxication that can only be stabilized by hydrogen transposition more difficult to perform ( Table 5).
The elimination of two methoxy groups simultaneously occurs for the TR2M-TR4M species (Table 2-5) and was discussed for TMOS by Wheeler [2].
The ion of m/e 90 has the intensity I=32.3% in the mass spectrum of the tetramethoxysilane (TMOS) ( Table 5; in grey cell) and can be obtained by simultaneous ionization of two oxygen atoms, due to lower shielding of the oxygen atom to the methoxy groups compared to ethoxy groups, at electron impact in ionization chamber of mass spectrometer. For tetramethoxysilane we have the reaction (7): The obtained cation with the m/e 90 is expected to readily eliminate a CH3O • radical to obtain a stable siliconiu ion: The simultaneous elimination of a methoxy group and an ethoxy group through the same mechanism described by equation (7) occurs only in the TR1M-TR3M species with mixed alkoxy groups and the ionic intensity increases from 1% for the TR1M specie to 7.9% for the TR3M specie with the increase in the number of methoxy groups (Table 7; in grey cells).
The elimination of an ethyl group is more intense when the number of ethoxy groups in the TEOS methoxy-transesters is higher and the ionic intensity increases from 3.6% for the TR3M specie to 22.9% for the TEOS specie and does not occur for the TMOS specie containing only methoxy groups (Table 7; in grey cells).
Primary processes with cleavages of  bonds and eliminations of H • , CH3 • , C2H5 • , CH3O • and C2H5O • radicals can be an indication of the weight of the two mentioned reaction centers. For example, the elimination of a methoxy group is proportional to the number of methoxy groups in the discussed species. Thus, the ionic intensity obtained from molecular ion by removing a CH3O • radical increase from 2.2% for TR1M to 4.3% for TR2M and to 7% for TR3M. For the TMOS molecular species containing only methoxy groups, the removal of the CH3O • radical is an intense reaction as expected by leading to the base ion in the mass spectrum with the m/e 121 (Table 7; in grey cells). The comparative intensities of CH3O + and C2H5O + cations in the mass spectra of the TR1M-TR4M species (Tables 3-5) are specified in Table 8 (grey cells).

Eliminations of neutral molecules from primary ions and from those obtained by consecutive elimination reactions
Emphasizing the fragmentation ions eliminating small molecules is accomplished by (B/E)(1-E) 1/2 linked scans. The chromatograms and the mass spectra with fragmentation ions of TR1M-TR4M and TEOS which eliminate acetaldehyde, ethylene and formaldehyde are shown in Figures 5, 6, 7 respectively.
The elimination of acetaldehyde occurs mainly from the primary ions of the TEOS, TR1M-TR3M species and is even more intense as the number of ethoxy groups is higher. In Figure 5a-d, all fragmentation ions of TEOS and TR1M-TR3M that eliminate acetaldehyde are shown.
The fragmentation ions of the TEOS, TR1M and TR2M species that remove ethylene by linked scan (B/E)(1-E) 1/2 are shown in Figure 6.a-c. The ethylene elimination reactions are the more intense as the number of ethoxy groups in the studied species increases.
The formaldehyde removal reactions are dominant in tetramethoxysilane (TMOS) as found in the literature study [1]. These elimination reactions are experimentally confirmed by linked scan (B/E)(1-E) 1/2 and are shown in Figure 7

Separation of fragmentation ions at high-resolution. Accurate mass and isotopic effects for the ions in the mass spectra of the TEOS methoxy-transesters
The accurate mass measurements and M+1, M+2 isotopic effects of the ions in the mass spectra of TEOS, TEOS dimer and TEOS cyclic tetramer were presented in previous works [13][14][15][16][17]; considerations on accurate mass and isotopic effect measurements are the same for the TEOS methoxy-transesters.
A strong argument on the existence of ions in the mass spectra of molecular species TR1M-TR4M is the accurate mass measurements at high resolution R=5000 at a pre-set error of the 10 mili mass unit (mmu). From Tables 9-12 it is found that for virtually all ions in the mass spectra of these molecular species we have differences of less than 10 mmu between the observed and calculated masses.
An argument in favor of the existence of an ion in the mass spectrum is the M+1, M+2 isotopic effects measured experimentally compared to theoretically calculated values. Thus, Tables 9-12 (in grey cells) show M+1, M+2 isotopic effects of molecular ions and the most intensive fragmentation ions in the mass spectra of the TR1M-TR4M (TMOS) species. A good concordance between the experimental values of the M+ 1, M+ 2 isotopic effects and those calculated with the ISO dedicated program [18] is found.
The values of the double bond equivalent (DBE) for the ions of the TEOS methoxy-transesters are also shown in Tables 9-12.    Removal of a CH3O • radical is a characteristic reaction of the TEOS methoxy-transesters. The arguments of the existence of the ion obtained by the reaction (9) are shown in the gray frame and are extracted from Table 10.

Elimination of neutral molecules. Elimination of formaldehyde
The elimination of formaldehyde is typical for methoxy transesters and for alkoxides with methoxy group; for the TR2M molecular species, formaldehyde removal is experimentally demonstrated by the (B/E)(1-E) 1/2 linked scan; among the fragmentation ions removing CH2O is also the ion with m/e 165 (Figure 7.c) which is confirmed by the exact measurement of its mass and the equivalent of double bonds shown in the grey background and extracted from Table 10.
The ions of the mass spectra of the TEOS methoxy-transesters were obtained experimentally by the B/E and B/E(1-E) 1/2 linked scans The argument for the existence of two possible reaction sites for the oxygen atom of the methoxy group or the ethoxy group was obtained by calculating the net atomic charges with the MOPAC program 7.
The primary fragmentation reactions and the neutral molecule eliminations (formaldehyde, acetaldehyde, ethylene, molecular hydrogen) were written based on information obtained experimentally by the B/E and B/E(1-E) 1/2 linked scans.
The elimination of two methoxy groups simultaneously occurs for the TEOS methoxy-transesters with 2-4 methoxy groups. An example is the ion of m/e 90 in the mass spectrum of tetramethoxysilane (TMOS). This ion can be obtained by ionization of two oxygen atoms by the simultaneous elimination of two methoxy groups from the molecular ion.
The strong argument for the existence of TEOS methoxy-transesters ions was based on accurate masses measurements at high resolution (R=5000). It is found that for virtually all ions in the spectra of these molecular species we have differences less than10 mili mass unit (mmu) between the observed and calculated masses.
An argument in favor of the existence of an ion in the mass spectrum is the M+1, M+2 isotopic effects, where the main contribution is of the 29 Si and 30 Si isotopes of silicon. A good concordance between the experimental values of the M+ 1, M+ 2 isotopic effects and those calculated with the ISO dedicated program is found.
The reaction pathways in the mass spectrum of dimethoxydiethoxysilane (TR2M) and accurate mass values are presented in detail.