Raman spectroscopy and DFT calculations of intermediates in the hydrolysis of methylmethoxysilanes

Journal of Molecular Structure 1023 (2012) 204–211

Contents lists available at SciVerse ScienceDirect

Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc

Raman spectroscopy and DFT calculations of intermediates in the hydrolysis of methylmethoxysilanes Martin D. Bennett a, Christopher J. Wolters a, Kurt F. Brandstadt b, Mary M.J. Tecklenburg a,⇑ a b

Department of Chemistry, Central Michigan University, Mt. Pleasant, MI 48859, USA Dow Corning Corporation, Midland, MI 48686, USA

h i g h l i g h t s " Methoxysilanes and methylmethoxysilanes were hydrolyzed at neutral pH. " All intermediates in the sequential hydrolysis steps were detected by Raman. " DFT structures and frequencies are a good model for the experimental vibrations.

a r t i c l e

i n f o

Article history: Available online 12 June 2012 Keywords: Methylmethoxysilane Silanol Hydrolysis Intermediates Raman spectroscopy DFT calculations

a b s t r a c t A series of methylmethoxysilanes (tetramethoxysilane, methyltrimethoxysilane, dimethyldimethoxysilane, and trimethylmethoxysilane) underwent hydrolysis and condensation in aqueous solutions at circum-neutral pH while the reaction was monitored by Raman spectroscopy. The SiO and SiC stretches of the silanes were vibrationally coupled so that a single intense peak was observed in that region of the Raman spectra. The SiO/SiC stretch was assigned for all of the solvated reactants and silanol intermediates and products of each hydroysis step in solutions of low ionic strength and neutral pH. Assignments compared favorably with vibrational frequencies calculated by a DFT method. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Organosilicones are used in a plethora of products and processes, giving desired material properties for coatings, adhesives, and lubricants. In addition, living organisms produce vast quantities of silica under conditions that are mild in comparison to the high-energy methods required for industrial processing of silicon dioxide into the great variety of silicone based materials. Bioprocessing of silicon-functional materials has the potential to reduce the environmental impact of industrial silicone production. Feasibility of enzyme catalysis of organosilicon reactions is under investigation, specifically hydrolysis and condensation to produce siloxanes [1–4]. Classical methods for detection of silica in aqueous solutions are non-specific (colorimetric molybdic acid method [5]) or time consuming (gas chromatography [3,6] and 29Si-NMR spectroscopy [1]). Vibrational spectroscopy (Raman and FTIR) is ideal for monitoring organosilanes and silicones as it is rapid and can be done in situ. ⇑ Corresponding author. Tel.: +1 989 774 3078; fax: +1 989 774 3883. E-mail address: [email protected] (M.M.J. Tecklenburg). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2012.06.008

Recent studies by FTIR include characterization of the surface of silica aerogels [7] and monitoring organosilicate inner layer dielectric films in microprocessors [8]. In a review of the Raman spectra of a series of methylmethoxysilanes [9] the SiO and SiC peaks were mutually assigned to a very strong polarized peak at 590–630 cm 1. This was supported by the calculated potential energy distributions [10–12]. The symmetric SiO stretch in tetramethoxysilane [13] is in the same spectral region (640 cm 1). Several analytical methods, including Raman spectroscopy [14–17], have been used to characterize the products of hydrolysis and condensation of alkoxysilanes in aqueous and alcoholic solutions. Ultimately, the siloxane vibrations grow in intensity (SiOSi sym. stretch, primarily below 600 cm 1) replacing the SiO/SiC stretch. The most thorough studies have been carried out on tetramethoxysilane. During hydrolysis, the individual intermediates were identified by comparatively analyzing the 29Si-NMR and Raman spectra over the same time intervals [18,19]. The SiO symmetric stretches of the products during the hydrolysis of tetramethoxysilane shifted to higher wavenumbers as the lower mass hydroxide groups replaced the methoxy groups. In the hydrolysis of tetramethoxysilane the fully hydrolyzed silanol (silicic acid,

205

M.D. Bennett et al. / Journal of Molecular Structure 1023 (2012) 204–211

Si(OH)4) was not observed, presumably, because the silanol intermediates underwent condensation to siloxanes before the concentration of Si(OH)4 became detectable. In a comprehensive study, the vibrational frequencies of the methylmethoxysilanes were calculated using several basis sets [20,21]. Excellent agreement between experimental and calculated frequencies was obtained from the DFT/B3LYP method with the aug-cc-pVDZ basis functions, except for the SiO/SiC symmetric stretch of the silicon tetrahedron in which the calculated frequencies were 20–30 cm 1 less than found experimentally. They also found good agreement with experimental frequencies using the DFT/B3LYP method with a 6-31G⁄ basis set after scaling the force field according to the SQMFF theory [22,23] before calculating the frequencies. Calculations were also carried out on the fully hydrolyzed products of methylmethoxysilanes but not the intermediate, partially hydrolyzed silanes. Experimental frequencies were not available for the intermediates except for the hydrolysis of tetramethoxysilane. The aim of this work is to identify the sequentially hydrolyzed intermediates and products in the hydrolysis of methylmethoxysilanes in conditions which stabilize hydrolysis intermediates against condensation long enough for detection by Raman spectroscopy. DFT calculations of the structure and vibrational frequencies of the symmetric SiO/SiC stretch of the intermediates will support the assignments.

16000

2. Experimental 2.1. Materials Trimethylmethoxysilane (#SIT8566.0), dimethyldimethoxysilane (#SID4123.0), and methyltrimethoxysilane (#SIM6560.0) were purchased from Gelest Inc. (Tullytown, PA). Tetramethoxysilane (#34,143-6) was purchased from Sigma–Aldrich (St. Louis, MO). All were used as received without further purification.

2.2. Instrumentation Raman spectra were collected with a near IR dispersive Raman spectrometer (Kaiser Optical System RXN1: single monochromator with holographic transmissive grating, Invictus 785 nm diode laser and Andor back illuminated CCD detector). The laser was fiber optically coupled to a sample probe with a 10 objective focused into a cylindrical vial. The spectra were acquired through the side of glass reaction vials, focusing on the aqueous layer. In the 400– 800 cm 1 region, we found less background scattering from ordinary glass than from quartz. The laser power at the sample was 250 mW and the acquisition time varied from 5 to 25 s for the hydrolysis reactions. An initial background spectrum, before silane addition, was subtracted from all subsequent Raman spectra.

A

Time (min) 383 343 223 163 116 42 29 23 11 8 6 4 0

14000

Counts

12000 10000 8000 6000

6.0

4000 400

600

800

1000

Raman Shift

B

4000

1400

1200

1000

852 806 741 695 671 652

1410

1162 1085 1065

1461

1198

Counts

1017

800

652

671

695

633 633

12000

8000

0

1400

Counts

16000

1200

(cm-1)

Raman Shift (cm-1) 432

329

563

600

400

Raman Shift (cm-1) Fig. 1. Raman spectra of methyltrimethoxysilane (MTMOS) as it hydrolyzes in water. (A) A 3-D view and (B) an overlayed view. Note the different x-axis direction in the 3-D figure in order to reveal the small peaks growing on the high wavenumber side of the 633 cm 1 peak. Peaks at 1017 and 1461 cm 1 are assigned to the methanol byproduct.

206

M.D. Bennett et al. / Journal of Molecular Structure 1023 (2012) 204–211

A

6000

Time (min) 60 52 47 43 34 29 22 16 13 5 4 3 0

Counts

5000

4000

3000

60 2000

400

600

800

1000

Raman Shift

1200

1400

B

696 673

731

1017

786

647

Counts

6000

647

4000

Raman Shift (cm-1)

1400

1200

1096

2000

840 817 786 731 696 673

1451

1116

1461

1199

Counts

0

(cm-1)

1000

800

590

600

400

Raman Shift (cm-1) Fig. 2. Raman spectra of tetramethoxysilane (TMOS) as it hydrolyzes in water. (A) A 3-D view and (B) an overlayed view. Note the different x-axis direction in the 3-D figure in order to reveal the small peaks growing on the high wavenumber side of the 647 cm 1 peak. Peaks at 1017 and 1461 cm 1 are assigned to the methanol byproduct.

2.3. Hydrolysis of methylmethoxysilanes The hydrolysis reactions of the methylmethoxysilanes were formulated by preparing 1.0 M solutions in water. The water was purified with a reverse osmosis system and deionized with a Barnstead D0803 high capacity column. The pH of the purified, nonbuffered water was seven and the reactions were carried out at room temperature (ca. 22 °C). Following initial vigorous shaking, the two phases quickly separated and the first spectrum of the aqueous layer was acquired within 1 min and additional spectra were obtained every minute. Over time the methoxysilane phase partitioned into the water as the silane in the aqueous phase was hydrolyzed until only the aqueous phase was left. As the rate of hydrolysis decreased, the acquisition interval was gradually increased to an hour or more until the SiO/SiC peaks disappeared and the solutions began to gel. Since the solubility of the methoxysilanes was relatively low in water, the solvation and rate of hydrolysis of the methoxysilanes in the aqueous solutions were observed to be dependent on the mixing procedure. Ideally, a small gauge (>25) needle attached to a syringe with a capacity for the entire aliquot was used to introduce the methoxysilanes into the aqueous solutions. The injection was made as quickly as possible and the needle was rapidly removed from the solution in order to minimize the inadvertent loss of

sample. The two-phase aqueous/methoxysilane solutions were capped immediately and vigorously shaken in order to fully disperse and promote solvation of the unhydrolyzed silane. If the solution was not mixed thoroughly the concentrated silane droplets were observed to gel. 2.4. Solubility and frequency calculations Supplemental information on the samples was obtained by calculating water solubility and vibrational frequencies. The estimates of the water solubility values were calculated with the EPI Suite shareware (version 3.11 [24]) available from the Environmental Protection Agency (EPA). The WSKOWWIN model estimates the log (Kow), octanol–water partition coefficient, by the fractionation method. An empirical relationship relates log (Kow), molecular weight, and/or the melting point of the molecule to the solubility of the molecule in water. The estimated solubilities were above 3000 ppm for all methylmethoxysilanes, easily detectable by Raman spectroscopy, except for trimethylmethoxysilane (ca. 300 ppm). The solubilities were even higher for the intermediate hydroxysilanes so they all were expected to be detectable by Raman spectroscopy. The estimated vibrational frequencies were obtained by performing electronic structure calculations using density functional

207

M.D. Bennett et al. / Journal of Molecular Structure 1023 (2012) 204–211

30000

A

Time (min) 384 320 278 172 134 116 82 63 43 28 17 11 9 0

Counts

25000 20000 15000 10000

6.5

5000 1400

1200

1000

800

600

0

400

Raman Shift (cm-1)

635 622

1400

1200

1000

793 755 696

878

1065

1265

5000

Raman Shift (cm-1)

487

1017 1161 1115

1415 1405

15000

1469 1453

Counts

25000

Counts

649

635 622

649

B

800

385

356

546

600

400

Raman Shift (cm-1) Fig. 3. Raman spectra of dimethyldimethoxysilane (DMDMOS) as it hydrolyzes in water. (A) A 3-D view and (B) an overlayed view. Peaks at 1017 and 1461 cm to the methanol byproduct.

theory (DFT). The B3LYP functional was used with a standard Gaussian basis set, 6-31G, implemented within the Spartan ‘04 software (Wavefunction, Irvine, CA) to optimize the structures and compute theoretical vibrational frequencies. Final structures were at equilibrium as they had no imaginary frequencies. The symmetric SiO/SiC stretch was identified by observing animations of the normal modes. An empirical scaling factor was derived that minimized the differences between the calculated and measured SiO/SiC symmetric stretch frequencies of all 14 molecules in the study.

3. Results and discussion The stability of the intermediates in the aqueous hydrolysis reaction of methoxysilanes is dependent upon experimental variables such as pH, concentration, ionic strength and temperature [25]. In this study, each methoxysilane was hydrolyzed in water (deionized, not buffered, pH 7). Preliminary experiments in phosphate buffered aqueous solutions of differing ionic strengths showed that the hydrolysis rate increased such that few of the intermediates were observable in the 1 min time between spectral acquisitions. In fact, at pH 7 the methoxysilanes should be stable against hydrolysis since acid or base catalysis is required. While the highly purified water we used was pH 7, the neat methylmethoxysilanes, which were used as received, probably contained trace

1

are assigned

amounts of hydrochloric acid from their synthesis, starting from chlorosilanes. The residual acid is the likely catalyst source but low enough concentration that the reaction progressed over hours. Since the rate of the reaction was slowest when the silanes were prepared in deionized water at neutral pH, all of the individual hydrolysis intermediates could be identified. Analysis of the complete set of vibrations was not possible because the short-lived intermediates were detected in a complex solution consisting of the original silane, each of the partially and fully hydrolyzed silanes (silanols), condensation products (dimers, trimers, etc.), as well as a growing concentration of the by-product, methanol. However, the intense, isolated Raman peak for the symmetric stretch of the silicon tetrahedron, the combined SiO/SiC stretch could be identified for each molecule in the whole series of hydrolysis intermediates and products.

3.1. Hydrolysis of methylmethoxysilanes A representative set of Raman spectra acquired during the hydrolysis of methyltrimethoxysilane (MTMOS) in water, are shown in Fig. 1A in a 3-D format while Fig. 1B shows an overlay of the same spectra with peaks identified. A large peak was observed at 633 cm 1 that increased and, subsequently, decreased with time. Several peaks rise and fall on the higher energy side

208

M.D. Bennett et al. / Journal of Molecular Structure 1023 (2012) 204–211

20000

A Time (min) 10080. 990 750 510 270 140 30 5 0

Counts

15000

10000

5000

400

600

800

1000

Raman Shift

1200

168

0

1400

(cm-1)

B 20000

Counts

619 619

15000

1400

1200

1000

777 758 695

1017

890 851

5000

1116

126 8

10000

1465 1451 1412

Counts

Raman Shift (cm-1)

800

600

400

Raman Shift (cm-1) Fig. 4. Raman spectra of trimethylmethoxysilane (TMMOS) as it hydrolyzes in water. (A) A 3-D view and (B) an overlayed view. Note the different x-axis direction in the 3-D figure in order to reveal the small peaks growing on the high wavenumber side of the 619 cm 1 peak. Peaks at 1017 and 1461 cm 1 are assigned to the methanol byproduct.

of the peak (640–750 cm 1). The peaks at 1017 and 1461 cm 1 are due to methanol, a byproduct of hydrolysis and condensation. The hydrolysis reaction is slow enough that all of the intermediates can be identified as they appear in time. Sequentially, peaks appear at 633, 652, 671, and 695 cm 1 (Fig. 1B, inset). The 633 cm 1 peak was assigned to the SiO/SiC symmetric stretch of the aqueous solvated methyltrimethoxysilane based on its assignment to 627 cm 1 in the pure liquid (unsolvated) [9,26]. The SiO/SiC symmetric stretches of the sequential intermediates, mono(652 cm 1), di-(671 cm 1) and tri-hydroxylmethylsilanes 1 (695 cm ), were observed during the hydrolysis reaction. Since the other peaks in the region (741, 806, and 852 cm 1) follow the same temporal pattern as the 633 cm 1 peak, they are assigned to the SiO/SiC antisymmetric stretch (806 cm 1) and the methyl rocking vibrations (741 and 852 cm 1) of solvated methyltrimethoxysilane (as compared to the spectrum of neat methyltrimethoxysilane) [9]. The 563 cm 1 peak that appeared late during the hydrolysis reaction can be assigned to the SiOSi stretch of a siloxane condensation product. It compares favorably with the strong peak observed at 558 cm 1 in the spectrum of hydroxy-endblocked polydimethylsiloxane [27]. Solutions of tetramethoxysilane (TMOS) were hydrolyzed under the same conditions as methyltrimethoxysilane. In water (Fig. 2), all of the intermediates during the hydrolysis reaction were detected sequentially in time. The SiO symmetric stretch vibration of aqueous tetramethoxysilane was assigned to 647 cm 1. In addition, the individual hydrolyzed products were assigned: mono(673 cm 1), di-(696 cm 1), tri-(731 cm 1), and tetrahydroxysubstituted silane (786 cm 1, silicic acid). In review, these

assignments matched published Raman spectral assignments of the intermediates during the hydrolysis of tetramethoxysilane in methanol [18]. A very weak broad peak centered at 590 cm 1 appears mid-way through the reaction and is assigned to a siloxane SiOSi stretch. Many condensed structures (linear, branched, rings) can be formed due to the large number of active sites available per silicon atom. The Raman peaks of the individual molecules are not resolved but result in a very broad peak. The vibrational assignment of fully hydrolyzed tetramethoxysilane, silicic acid, has been controversial [18,28] due to its instability and the close proximity of vibrations of dimers and siloxane networks. However, in a study of the hydrolysis of tetraethoxysilane [17] silicic acid was assigned to a Raman peak at 783 cm 1. Recently, aqueous silicic acid and its deuterated isomer were produced from amorphous silica at low enough concentration and pH (16.6 mM, pH 2) to minimize any reaction while the Raman spectrum was measured [29]. The sole peak at 787 cm 1 was identified as the symmetric SiO stretching mode of silicic acid. In our reaction both the frequency and the late appearance of the peak at 786 cm 1 further corroborates its assignment to silicic acid. A temporal progression of three peaks in the Raman spectra acquired during the hydrolysis of dimethyldimethoxysilane (DMDMOS) in water (Fig. 3), led to the assignments of the symmetric SiO/SiC stretch for aqueous dimethyldimethoxysilane (622 cm 1), dimethylmethoxyhydroxysilane (635 cm 1), and dimethyldihydroxysilane (649 cm 1). Additional peaks in the 650–800 cm 1 region appeared at the same time as dimethyldimethoxysilane and nearly match weak peaks assigned in the Raman spectrum of neat (unsolvated) dimethyldimethoxysilane [26,30]. Condensation was

M.D. Bennett et al. / Journal of Molecular Structure 1023 (2012) 204–211

209

Fig. 5. Structures of methylmethoxysilanes and their hydroxysis intermediates calculated by DFT/B3LYP/6-31G for (A1) tetramethoxysilane (TMOS), (A2) trimethoxyhydroxysilane, (A3) dimethoxydihydroxysilane, (A4) methoxytrihydroxysilane, (A5) tetrahydroxysilane, (B1) methyltrimethoxysilane (MTMOS), (B2) methyldimethoxyhydroxysilane, (B3) methylmethoxydihydroxysilane, (B4) methyltrihydroxysilane, (C1) dimethyldimethoxysilane (DMDMOS), (C2) dimethylmethoxyhydroxysilane, (C3) dimethyldihydroxysilane, (D1) trimethylmethoxysilane (TMMOS), (D2) trimethylhydroxysilane. Mol files for these structures are included in supplementary information.

observed due to the presence of a peak at 546 cm 1 assigned to the SiOSi stretch. As trimethylmethoxysilane (TMMOS) was hydrolyzed in water (Fig. 4) a single intense peak in the 600–700 cm 1 region was observed to slowly gain intensity throughout the course of the reaction. The most intense peak, observed at 619 cm 1, is assigned to the SiO/SiC symmetric stretch of trimethylsilanol rather than solvated trimethylmethoxysilane. The symmetric SiO/SiC stretch of trimethylmethoxysilane liquid is 604 cm 1 and would be expected to increase by only approximately 5 cm 1, to 609 cm 1, upon dissolving in water. The observed peaks match the Raman spectrum of trimethylsilanol [31]. The absence of any peaks due to trimethylmethoxysilane can be explained by its low aqueous solubility which is predicted to be approximately 300 ppm, low enough to

be difficult to detect by Raman spectroscopy, Condensation to hexamethyldisiloxane is expected to yield a strong Raman peak due to the symmetric SiOSi stretch [27] at 525 cm 1. No peak attributable to a siloxane bond was detected in the seven day period that the reaction was monitored. Condensation may be very slow or the solubility of hexamethyldisiloxane in water may be too low to be detectable by Raman spectroscopy. 3.2. DFT calculations of structures and frequencies The theoretical structures and frequencies of the four methylmethoxysilanes and their hydrolysis products were calculated using the DFT/B3LYP/6-31G method. The calculated structures of the four methoxysilanes (trimethylmethoxysilane, dimethyldimethoxysi-

210

M.D. Bennett et al. / Journal of Molecular Structure 1023 (2012) 204–211

Table 1 Calculated Si–O and Si–C bond lengths in hydrolyzed methoxysilanes using the DFT/ B3LYP/6-31G methoda. Bond lengths (Å) Si–O(CH3) (MeO)4 n=0 n=1 n=2 n=3 n=4

n Si(OH)n

Me(MeO)3 n=0

n Si(OH)n

n=1 n=2 n=3

a

Si–C

Methyltrimethoxysilane and derivatives 1.649 1.651 1.661 1.651 1.656 1.660 1.654 1.658 1.670 1.657b 1.668

1.866

1.862 1.860 1.862

Me2(MeO)2 n=0 n=1 n=2

n Si(OH)n

Dimethyldimethoxysilane and derivatives 1.876b 1.667b 1.667 1.674 1.874b 1.673b 1.873b

Me3(MeO)1 n=0

n Si(OH)n

Trimethylmethoxysilane and derivatives 1.677 1.880 1.890b 1.681 1.879b 1.890

n=1

b

Si–O(H)

Tetramethoxysilane and derivatives 1.643b 1.642b 1.651 1.643b 1.650b 1.642 1.651b 1.650b

Structures as .mol files are available as the supplementary files. Averaged bond length reported, differences of 0.002 Å or less.

lane, methyltrimethoxysilane, and tetramethoxysilane) and the four fully hydrolyzed products (trimethylhydroxysilane, dimethyldihydroxysilane, methyltrihydroxysilane, and silicic acid)

have already been published [20,21] as well as the intermediates in the hydrolysis of TMOS. Our calculated structures agreed to within 0.003 Å for bond lengths and 5° for angles. We have calculated structures for the hydrolysis intermediates of DMDMOS and TMMOS which are shown in Fig. 5, and for completeness, the structures of all of the molecules in this study are included. Just as was noted by Ignatyev et al. [21], the methoxy Si–O(C) bond lengths increased by about 0.01 Å with each additional methyl group bonded to silicon (Table 1). The Si–O(H) bonds are approximately 0.01 Å longer than the methoxy Si–O(C) bonds in the same molecule and the Si–O(H) bond lengths also increase by 0.01 Å with each additional methyl group as the SiC bond lengths increase by the same amount. Hydrolyzing a methoxy group to hydroxy has little effect on the Si–O bond lengths; methoxy Si–O(C) bonds remain about 0.01 Å shorter than Si–O(H). Expansion of the Si–X and Si–C bond lengths has also been observed in the series SiFnMe4 n as n decreases [32,33] where it was attributed to negative hyperconjugation. Lengthening a bond usually weakens it so, lower stretching frequencies of SiO/SiC are expected for methoxysilanes with greater methyl groups. The equilibrium structures of the series of methylmethoxysilanes and hydroxysilane intermediates were used to calculate forces and harmonic vibrational frequencies. Since the symmetric SiO/SiC stretch was the only frequency identified in the experimental solution mixtures only results for that frequency will be described here. Our calculated frequencies for the SiO/SiC symmetric stretch were about 15–25 cm 1 lower than those we observed by Raman spectroscopy. The calculated structures are for isolated molecules so small differences in calculated frequencies due to solvation are possible. The Raman frequencies of the aqueous solvated methylmethoxysilanes appear approximately 5 cm 1 higher than for the neat liquids (Table 2), When we included the water solvent in the calculation of the four methoxysilanes (as a uniform dielectric, PCM model) the frequencies

Table 2 Raman and DFT calculations of the SiO/SiC symmetric stretch of methylmethoxysilanes and methylmethoxyhydroxysilanes (silanols). Experimental–Raman (cm Liquid (MeO)4 n=0 n=1 n=2 n=3 n=4

n Si(OH)n

Me(MeO)3 n=0 n=1 n=2 n=3

a b c d e f g h i j k

n Si(OH)n

Me2(MeO)2 n=0 n=1 n=2

n Si(OH)n

Me3(MeO)1 n=0 n=1

n Si(OH)n

1

)

MeOH/H2O Soln. (Previous)

Tetramethoxysilane and derivatives 642b 645c 675c 695c 720c 787f Methyltrimethoxysilane and derivatives 628g

Dimethyldimethoxysilane and derivatives 617h

Trimethylmethoxysilane and derivatives 604i 617k

Theoretical (cm

1

)

Aqueous soln. (This work)

DFT/B3LYP/6-31G (This work)a

DFT/B3LYP (Previous)

647 673 696 730 786

656 680 706 737 786

634d 650e 671e 700e 745e

633 652 671 695

635 650 677 691

621d

622 634 649

613 634 646

635e

n.o. 619

602 613

603j 606e

Scale factor – 1.031 (empirically derived only for SiO/SiC symmetric stretch). Refs. [12,24]. Tetramethoxysilane in methanol, Ref. [18]. Basis set 6-31G; force constant scale factors: SiC – 1.092, SiO – 1.030; Ref. [21]. Basis set aug-cc-pVDZ; force constant scale factors SiC – 1.052, SiO – 1.064; Ref. [21]. Ref. [27]. Ref. [10]. Refs. [11,12]. Ref. [24]. Basis set 6-31G; force constant scale factors: SiC – 1.092, SiO – 1.030; Ref. [20]. Ref. [29].

675e 617d

M.D. Bennett et al. / Journal of Molecular Structure 1023 (2012) 204–211

increased by only 1–4 cm 1. While it is possible that solvent effects on the calculated frequencies could be more pronounced for the hydoxysilane intermediates, modeling the effects of hydrogen bonding to a water solvent would be a more complicated calculation which we did not pursue. Differences between the observed and calculated frequencies certainly also arise from neglecting anharmonicity which usually results in overestimated frequencies calculated. Scaling factors to adjust harmonic frequencies for comparison with observed fundamental vibrations are commonly employed. Scaling factors optimized to adjust all the frequencies in a molecule are available in the literature (0.9614 for DFT/B3LYP/6-31G [34]). This method has been in use for more than a decade and has proven to be efficient for most molecules. However, in the case of the symmetric SiO/SiC stretching frequencies, the calculated frequencies underestimate the experimental ones so that application of the standard scaling factor for this specific vibration would actually increase the differences between them to about 30–50 cm 1 for the series of molecules studied here. In fact, Ignatyev et al. [21] noted that, except for the SiO stretching frequencies which were lower than experimental, their calculations of methylmethoxysilanes with a more complete basis set (B3LYP/aug-cc-pVDZ) resulted in a fairly good fit to all the frequencies in the molecules without any type of scaling. This suggests that the SiO vibration deviates from harmonic forces with a steeper slope to the potential energy surface than usually expected. An empirical scale factor which minimizes the average deviation between the experimental and calculated frequencies was derived specifically for the SiO/SiC symmetric stretch vibration using the series of molecules studies here. A factor of 1.031 or a 3.1% increase in the calculated SiO/SiC symmetric stretch frequencies gave an average deviation of 5 cm 1 from the experimental frequencies. In Table 2 we compare our scaled calculated frequencies of the SiO/ SiC symmetric stretch to those we observed by Raman spectroscopy in aqueous solutions. The scaled frequencies match well the trends seen in the experimental frequencies. The methoxysilanes with fewer methyl groups have higher vibrational frequencies, presumably due to the shorter SiO and SiC bond lengths. The replacement of a methoxy group by hydroxy in each step of the hydrolysis reaction resulted in an increase of the SiO/SiC stretching frequency by 15–30 cm 1 due a combination of effects. The longer SiO(H) bond tends to reduce the frequency while the lower reduced mass increases the frequency. The reduced mass has the greater effect and results in a subsequent increase in the SiO/SiC symmetric stretch frequency with each addition of hydroxide and reduction of methoxy groups in the molecule. The vibrational assignments were also compared to recently calculated DFT frequencies of methylmethoxysilanes and some of their hydrolysis products (Table 2). Scaling factors for force constants (SQMFF theory [20,21]) involving the SiC and SiO bonds were greater than one and thus compensated for the underestimation of frequencies involving those bonds. Both sets of theoretical SiO/SiC symmetric stretch frequencies adequately fit the experimental frequencies for the 14 molecules although our set of scaled frequencies does somewhat better with tetramethoxysilane and its hydrolysis products. 4. Conclusions Hydrolysis followed by condensation of a series of methylmethoxysilanes occurs slowly in water when the pH is near 7 and the ionic strength of the solution is very low. For the first time, all intermediates in the sequential hydrolysis steps were separately detectable by Raman spectroscopy over a period of hours. The

211

SiO/SiC symmetric stretch peak wavenumber increased by approximately 25 cm 1 with each hydrolysis step, which enabled the identification of specific intermediates in the hydrolysis of methylmethoxysilanes. DFT structures and frequencies of all intermediates calculated with the B3LYP/6-31G method provided a good model for the frequencies of the SiO/SiC symmetric stretching vibrations. Acknowledgement The project was supported by a Dow Corning Corporation Research Grant #30819. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.molstruc.2012. 06.008. These data include MOL files and InChiKeys of the most important compounds described in this article. References [1] J.N. Cha, K. Shimizu, Y. Zhou, S.C. Christiansen, B.F. Chmelka, G.D. Stucky, D.E. Morse, Proc. Natl. Acad. Sci. USA 96 (1999) 361–365. [2] Y. Zhou, K. Shimizu, J.N. Cha, G.D. Stucky, D.E. Morse, Angew. Chem. Int. Ed. 38 (1999) 780–782. [3] A.R. Bassindale, K.F. Brandstadt, T.H. Lane, P.G. Taylor, J. Inorg. Biochem. 96 (2003) 401–406. [4] K.M. Delak, N. Sahai, Chem. Mater. 17 (2005) 3221–3227. [5] R.K. Iler, The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties, John Wiley & Sons, New York, 1979. [6] K.A. Smith, J. Org. Chem. 51 (1986) 3827–3830. [7] H. El Rassy, A.C. Pierre, J. Non-Cryst. Solids 351 (2005) 1603–1610. [8] E. Andideh, M. Lerner, G. Palmrose, S. El-Mansy, T. Scherban, G. Xu, J. Blaine, J. Vac. Sci. Technol., B 22 (2004) 196–201. [9] M.S. Afifi, T.A. Mohamed, Al-Azhar Bull. Sci. 7 (1996) 1237–1250. [10] T. Tanaka, Bull. Chem. Soc. Jpn. 33 (1960) 446–449. [11] T. Tanaka, Bull. Chem. Soc. Jpn. 34 (1961) 1752–1756. [12] T. Tenisheva, A. Lazarev, R. Uspenskaya, J. Mol. Struct. 37 (1977) 173–186. [13] B. Riegel, S. Plittersdorf, W. Keifer, N. Hüsing, U. Schubert, J. Mol. Struct. 410– 411 (1997) 157–160. [14] C.J. Brinker, G.W. Scherer, Sol–Gel Science, Academic Press, Boston, 1990. [15] M.S. Afifi, T.A. Mohamed, Al-Azhar Bull. Sci. 8 (1997) 63–71. [16] C.A. Capozzi, R.A. Condrate, L.D. Pye, R.P. Hapanowicz, Mater. Lett. 15 (1992) 233–241. [17] M.E. Mullins, B.C. Cornilsen, A.A. Kline, L.M. Sokolov, S. Surapanini, in: L.D. Pye, W.C. LaCourse, H.J. Stevens (Eds.), The Physics of Non-Crystalline Solids, The Society of Glass Technology, Washington, 1992, pp. 499–503. [18] I. Artaki, M. Bradley, T.W. Zerda, J. Jonas, J. Phys. Chem. 89 (1985) 4399–4404. [19] J.L. Lippert, S.B. Melpolder, L.W. Kelts, J. Non-Crystal. Solids 104 (1988) 139– 147. [20] M. Montejo, F. Partal Ureña, F. Márquez, I.S. Ignatyev, J.J. López González, J. Mol. Struct. 744–747 (2005) 331–338. [21] I.S. Ignatyev, M. Montejo, F. Partal Ureña, T. Sundius, J.J. López González, Vib. Spectrosc. 40 (2006) 1–9. [22] P. Pulay, G. Fogarasi, G. Pongor, J.E. Boggs, A. Vargha, J. Am. Chem. Soc. 105 (1983) 7037–7047. [23] T. Sundius, J. Mol. Struct. 218 (1990) 321–326. [24] US EPA. Estimation Programs Interface Suite™ for MicrosoftÒ Windows, Vol. 3.11. United States Environmental Protection Agency, Washington, DC, USA. , 2003 (accessed 10.06.03). [25] R.K. Iler, The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties, John Wiley & Sons, New York, 1979. 172–177. [26] N. Kozlova, V. Bozov, I. Kovalev, M. Voronkov, Izvest. Akad. Latv. SSR Ser. 5 (1971) 604–616. [27] A.L. Smith, The Analytical Chemistry of Silicones, John Wiley & Sons, New York, 1991. pp. 308–309. [28] M.S. Bradley, J.H. Krech, J. Maurer, J. Phys. Chem. 94 (1990) 5402–5405. [29] G.J. McIntosh, P.J. Swedlund, T. Söhnel, Phys. Chem. Chem. Phys. 13 (2011) 2314–2322. [30] T.F. Tenisheva, A.N. Lazarev, Zh. Prikl. Spektrosk. 43 (1985) 91–99. [31] J. Rouvier, V. Tabacik, G. Fleury, Spectrochim. Acta A 29A (1973) 229–242. [32] A.E. Reed, P.v.R. Schleyer, J. Am. Chem. Soc. 109 (1987) 7362–7373. [33] A.E. Reed, P.v.R. Schleyer, J. Am. Chem. Soc. 112 (1990) 1434–1445. [34] A.P. Scott, L. Random, J. Phys. Chem. 100 (1996) 16502–16513.