A Computational Investigation of the Different Intermediates during

J. Phys. Chem. B 2006, 110, 24311-24317

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A Computational Investigation of the Different Intermediates during Organoalkoxysilane Hydrolysis Samuel A. French,*,‡ Alexey A. Sokol, and C. Richard A. Catlow DaVy Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London, W1S 4BS, UK

Andreas Kornherr,† Gerhard E. Nauer,† and Gerhard Zifferer Institute of Physical Chemistry, UniVersity of Vienna, Wa¨hringer Strasse 42, 1090 Wien, Austria ReceiVed: June 6, 2006; In Final Form: August 18, 2006

Using a combination of atomistic molecular dynamics (MD) simulations and density functional theory (DFT) calculations, the four steps of hydrolysis of aminopropyl-, thiolpropyl-, and butyltrimethoxysilane have been studied. Large box MD simulations at constant pressure and temperature yield appropriate pair distribution functionsswhich allows us to quantify the number of surrounding water moleculessas well as the density of the systems. These densities serve as input for small box DFT calculations, which allow further geometry optimization and calculation of the electronic structure of the systems. The periodic DFT calculations are compared with gas-phase simulations. In all cases, the first step of hydrolysis is exothermic with the extent depending on the type of silane as well as on the number of hydrogen bonds in the initial stage.

Introduction Polysiloxanes are an important class of industrial polymers and, hence, one of the most studied inorganic polymers.1-3 The Si-O-Si backbone of this class of polymers leads to a variety of intriguing properties, arising from the strength of the Si-O bond. In contrast to most organic polymers, polysiloxanes are useful for high-temperature applications,4 e.g., as heat-transfer agents and high performance elastomers. In addition, the SiOH group of the silane monomer can form strong bonds to oxidic/hydroxidic metal surfaces.5,6 If the silane monomer also contains functionalized organic groups, it can further be used as an adhesion promoter7 providing a link between metallic surfaces and other organic coatings (resins or polymers). Semiinorganic polysiloxanes (or organic/inorganic hybrids, as they are often called) have thus attracted much attention both from scientists and engineers as their physical and chemical properties depend on the extent, as well as the chemical nature, of the organic R group(s). Thus, a tailoring of desired properties via knowledge-based materials design is possible; e.g., coatings based on this hybrid material offer a relatively high hardness due to their inorganic components and, at the same time, a high flexibility due to their organic polymer components.4 The present study is motivated by the increasing use of such semi-inorganic polysiloxane layers for corrosion protection of metal surfaces8-10 by industry and the associated need for an improved understanding of its formation. The synthesis of these polymers normally follows a sol-gel route11 consisting of the hydrolysis12 of organoalkoxysilanes into (organo-)silanols followed by the condensation of these metastable silanols13 into polysiloxanes. In our previous studies,14-16 the physical pro* To whom correspondence should be addressed. E-mail: frencsa@ matthey.com; Fax: +44(0)20 7629 3569; Tel: +44(0)20 7409 2992. † Center of Competence in Applied Electrochemistry GmbH, Wiener Neustadt, Austria. ‡ Current address: Johnson Matthey Technology Centre, Sonning Common, RG4 9NH.

cesses during the adsorption and docking of silanols at metal oxide surfaces and the influence of the tail group R on the polycondensation of trihydroxysilanols13 were investigated. Here we focus on the first stage, i.e., the hydrolysis of organotrialkoxysilanes into organotrihydroxysilanols. In both hydrolysis and polycondensation processes, several different unstable intermediates are involved, which are extremely difficult to investigate directly via experimental methods. Therefore, the only possible way of studying these complex processes in detail is to use atomistic computer simulations to model each step in silico. In principle, the hydrolysis process of organoalkoxysilanes can be divided into the following three steps which, of course, in reality cannot be strictly separated, but take place simultaneously with different molecules undergoing different steps at the same time (Me standing for the methyl, CH3 group):

R-Si(OMe)3 + 3H2O f R-Si(OMe)2(OH) + 2H2O + MeOH (1) R-Si(OMe)2(OH) + 2H2O + MeOH f R-Si(OMe)(OH)2 + H2O + 2MeOH (2) R-Si(OMe)(OH)2 + H2O + 2MeOH f R-Si(OH)3 + 3MeOH (3) The initial system consists of organotrimethoxysilanes with three water molecules per silane molecule to enable full hydrolysis. In reality, some amount of acid or base is always added to act as a catalyst to accelerate hydrolysis.17 However, in our study we are only interested in the energy and structure of the different intermediates depending on the organic R group and their interaction with the water and methanol molecules; therefore, any catalyst is omitted. Accordingly, in contrast to experimental measurements, which often deliver inconsistent data of the hydrolysis process and speed12 depending on the catalyst concentration, we are able to study an in silico system free of such influences.

10.1021/jp063503l CCC: $33.50 © 2006 American Chemical Society Published on Web 11/10/2006

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Figure 1. A graphical description of our sequential method of using different techniques to probe larger time and length scales.

Each of the systems in eqs1-3 serves as an appropriate starting configuration for a subsequent computational investigation both via MD (molecular dynamics) simulations as well as DFT (density functional theory) calculations. The combination of these two computational methods offers the ability to gain a detailed insight into the hydrolysis process and its intermediates on an atomic scale. As in our former studies, three organosilanes with different tail groups (polar and nonpolar) were investigated: aminopropyltrimethoxysilane, thiolpropyltrimethoxysilane, and butyltrimethoxysilane. Again, the polarity of the organic tail group plays an important role, influencing the structure and morphology of the different silanes and their interaction with the chemical surrounding (i.e., other silane molecules, water, and methanol). Numerical Method In this study we have combined simulations using MD and DFT to gain information about the interaction of silanes with solvent and how hydrolysis may occur. Using MD, we can explore time scales inaccessible to DFT, whereas the use of DFT allows us to ascertain the electronic structure and hence the bonding patterns of the solvated silanes. At each step of the hydrolysis process, we have performed an MD calculation on a box containing 60 silanes and 180 solvent molecules, from which we can calculate, for example, pair distribution functions, relative energies, and densities. Following these calculations, we have then performed an MD simulation on a smaller box containing 5 silanes and 15 solvent molecules with a density taken from that of the large system. The final configuration from this system is then taken to perform periodic DFT calculations, again for all three silanes at all four steps of hydrolysis. From these simulations we have then cut individual silanes and performed gas-phase DFT calculations. This process allows us to compare the silanes in different states over different time and length scales as summarized in Figure 1. More detail of each of the methods used follows. Constant temperature and constant pressure (NPT ensemble) MD simulations of the different configurations were performed using the Anderson thermostat and barostat18 with a time step of 1 fs as implemented in Discover, which is the MD engine of

Materials Studio 3.0 (from Accelrys). The external pressure was set to zerosa frequently used approximation considering that the internal pressure exceeds the external pressure by about 3-4 orders of magnitude. Cutoff based electrostatic Coulomb interactions as well as van der Waals terms (employing a Lennard-Jones 9-6 function) with a cutoff radius of 12.5 Å were applied for the calculation of energies and forces. In addition, a neutral charge group approach has been employed so that for the actual computation, molecules are prevented from being artificially split when one of the atoms is inside and another is outside the atom-based cutoff. The applicability of this group-based cutoff was checked by comparison with preliminary calculations using the (much more time-consuming) Ewald summation yielding discrepancies smaller than 3-5%. Interactions between atoms and molecules were accounted for using the COMPASS19-23 force field, which is highly optimized for the simulation of condensed phases thus delivering very accurate values for the simulated densities. This was also proven by Pereira et al.24 modeling water as well as alcohols and silica alkoxids by using two different MD codes (Discover and DL_Poly): the simulated values compared very well with experiment thus proving the validity of the method as well as that of the force field used (a precursor of COMPASS for the Discover runs). All initial configurations were independently constructed by a modified Markov25 process (with bond conformational probabilities chosen to account for intramolecular and intermolecular interactions between the molecules), using the Amorphous Cell tool in Materials Studio 3.0. The number of molecules per simulation box was 60 silanes plus the corresponding number of water and/or methanol molecules (180). The overall simulation time was always 200 ps with averages taken over the last 10 ps for structure analysis (pair distribution functions) and statistics collected over the last 100 ps for the computation of the density. These densities served as the input for the setup of the smaller systems (5 silanes plus 15 solvent molecules) for subsequent DFT calculations, after equilibration via MD. Because of the high computational demand of electronic structure calculations, the final DFT step was only possible for one selected config-

Intermediates during Organoalkoxysilane Hydrolysis uration for each type of silane. To check that our selected initial configurations are a good representation, we compared the classical potential energy E of 10 independently generated and optimized systems. For all three silanes, we find no significant dependence of E on the specific configuration. Thus, one of the 10 structures equilibrated with these small MD simulations served as input to further DFT geometry optimization calculations. All the DFT investigations were performed using the linear combination of atomic orbitals approximation with a double numerical basis set augmented by polarization functions (with a 5.5 Å cutoff) as implemented in the DMol3 code.26 The DNP basis set has been shown to perform very well in the reproduction of binding energies and structures close to the basis set limit, which is due to their numerical form that represents the true radial shape of the atomic and ionic functions of interest. Both gas phase and periodic boundary conditions calculations employed the gradient corrected PBE exchange-correlation functional.27 PBE is a high quality general purpose functional, not fitted to a specific system and so performs well across the board. It has also proved to be quite suitable for the description of hydrogen bonded systems, which are of key interest here. Due to the large size of the simulation box and the molecular structure of the system, all electronic calculations were carried out at the Γ (k ) 0) point only. The “medium” accuracy convergence criteria were used throughout for both electronic structure and atomic optimization calculations, which guarantees the energy per bond, bond lengths and angles to converge to ca. 1 kcal/mol, 0.01 Å, and 1°, respectively. Results and Discussion Throughout our investigation, the interplay between different methods has been extensive, allowing us to maximize our understanding of these systems. We now detail the results obtained from each of the methods described above through each step of hydrolysis concentrating on structure and how the different R groups impact on the configurational geometries present in the system. The energy profile of the hydrolysis mechanism and the influence of the tail groups are considered. In the following sections, we will describe MD simulations, periodic DFT calculations, and then gas-phase calculations for all steps from organotrimethoxysilanes to organotrihydroxysilanes. Note, within all these calculations there is competition between the solvents driving force to produce the minimum energy structural motif of tetrahedral hydrogen bond networks for H2O and linear for MeOH. Large Box MD Simulations. Starting our analysis with the initial stage (i.e. before hydrolysis starts, eq 1) the most obvious feature of the equilibrated silane structures is water clustering around polar tail groups. Figure 2 shows the atomic pair distribution function g(r) of the tail and head of organoalkoxysilanes with the surrounding solvent. Thus, the pure water shell around the Si-atom of the Si(OMe)3 group (i.e., the Owater-Si distance r) as well as around the NH2 (aminopropyltrimethoxysilane), SH (thiolpropyltrimethoxysilane) or CH3 (butyltrimethoxysilane) group (i.e., Owater-N, Owater-S, Owater-C distance r, respectively) is analyzed. As already mentioned, the average over the last 10 ps of a 200 ps NPT run is taken to decrease the scatter in the presented data. For aminopropyltrimethoxysilane, a pronounced first peak around 0.3 nm in the g(r) function for the tail (Figure 2a) can be seen while for thiolpropyltrimethoxysilane (Figure 2c) this peak is much smaller and for butyltrimethoxysilane (Figure 2e)

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Figure 2. Pair distribution functions g(r) for a specific atom (see text) of the tail (left) and the Si atom of the head (right) of three different organotrimethoxysilanes with the O atom of water: aminopropyltrimethoxysilane (a) and (b), thiolpropyltrimethoxysilane (c) and (d), butyltrimethoxysilane (e) and (f).

it is not developed at all. Obviously, the polar amino group strongly attracts water molecules and thus significantly increases the local water concentration, whereas thiol (being much less polar than amino) only somewhat attracts water molecules. Accordingly, the nonpolar methyl group shows no increase of the local water concentration, which, in fact, is significantly decreased around the carbohydrate tail. This feature is also reflected in the absolute number of water molecules n around the tail within a certain distance r, which can be calculated according to:

n)

∫0r4πr2g(r)

ntotal dr V

(4)

with ntotal being the total number of water molecules in the simulation box with volume V. Choosing 0.5 nm for r (which is slightly larger than the peak maximum for all silanes), n is calculated as 3.4, 1.7, and 1.1 for aminopropyltrimethoxysilane, thiolpropyltrimethoxysilane, and butyltrimethoxysilane, respectively. In contrast to the tail, the head of all organotrimethoxysilanes is the same, a Si(OMe)3 group. Accordingly, all the pair distribution functions (Figure 2b,d,f) look quite similar showing featureless g(r) functions with no peaks but a slowly converging graph for larger distances r. Water clustering around the polar amino group can also be observed for organodimethoxyhydroxy- and organo-methoxydihydroxysilanes; i.e., organosilanes after one or two hydrolysis steps. In Figure 3, the pair distribution functions g(r) of the tail of these organosilanes with water are presented. Again, a similar behavior to that in Figure 2 is found with only aminopropylsilane showing a distinct peak around 0.3 nm. The absolute numbers of water molecules, n, around the tail calculated via eq 4 are summarized in Table 1. The absolute numbers are, of course, smaller when compared to the initial system as the absolute number of water molecules per simulation box ntotal is decreased to 120 and 60 for

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Figure 3. Pair distribution functions g(r) for a specific atom of the tail of organodimethoxyhydroxysilanes (left) and organomethoxydihydroxysilanes (right) with the O atom of water: aminopropylsilane (a) and (b), thiolpropylsilane (c) and (d), butylsilane (e) and (f).

TABLE 1. Absolute Number of Water Molecules (n) within a Distance 0.5 nm (r) of the Tail of Organosilanes aminopropyldimethoxyhydroxysilane aminopropylmethoxydihydroxysilane thiolpropyldimethoxyhydroxysilane thiolpropylmethoxydihydroxysilane butyldimethoxyhydroxysilane butylmethoxydihydroxysilane

2.3 1.6 1.2 0.8 0.7 0.4

organodimethoxyhydroxy- and organomethoxydihydroxysilanes, respectively. Nevertheless, the gradation between the aminopropyl, thiolpropyl, and butyl group for organo-dimethoxyhydroxysilanes (3.3:1.7:1) is nearly the same as for organotrimethoxysilanes (3.1:1.5:1) with the decrease of n directly reflecting the reduction in water concentration, by a third. For organomethoxydihydroxysilanes, the gradation of the local water concentration around polar and nonpolar tails is slightly different: 4:2:1. Both polar tails (amino and thiol) show an enhanced tendency toward water clustering compared to the butyl tail. Apparently, the decrease in n due to lower water concentrations (two out of three water molecules are replaced by methanol; i.e., a reduction of 66.7%) is to some extent compensated (see also Figure 3b, where the first peak of the g(r) graph increases). In comparison with the right part of Figure 2, the pair distribution functions of the headgroup with water molecules (not shown) looks quite similar, except for organomethoxydihydroxysilanes where a steeper rise at 0.3 nm is observed. Upon full hydrolysis, all methoxy groups of the organotrimethoxysilane have been replaced by hydroxy groups while at the same time the surrounding solvent is pure methanol. In Figure 4, the g(r) functions of the tail as well as of the head of the organotrihydroxysilanes with methanol are shown. Again, clustering of the less polar solvent (methanol) around the most polar tail (aminopropyl) is observable but the difference between the thiolpropyl and the butyl tail is now quite small: the absolute numbers of the methanol molecules within a distance of 0.5 nm around the different tails read 3.5, 2.4, and

Figure 4. Pair distribution functions g(r) for a specific atom of the tail (left) and the Si atom of the head (right) of three different organotrihydroxysilanes with the O atom of methanol: aminopropyltrihydroxysilane (a) and (b), thiolpropyltrihydroxysilane (c) and (d), butyltrihydroxysilane (e) and (f).

2.0, which results in a ratio of 1.75:1.2:1. The lower polarity of methanol compared to water is reflected in less pronounced clustering around the tails. The head of the organotrihydroxysilanes shows no significant clustering but, in accordance with the g(r) function of the Si(OMe)(OH)2 head (not shown), there is a very steep initial rise at approximately 0.3 nm. This steep rise is different to Figure 2 (right column) but can be traced back to the less steric hindrance of the Si atom of the headgroup as all methoxy groups are substituted by smaller hydroxy groups thus allowing for a smaller distance between solvent molecules and the Si atom. DFT Periodic Calculations. As already mentioned, small box MD simulated structures were used to provide as unbiased as possible a starting point for the periodic DFT calculations, which focus on the structural implications of solvating the monomers in a water/methanol mixture. The importance of the degree of interaction between the tail group and the hydrogen bond network of water and the significance of steric hindrance caused by the solvent in defining the geometry of the monomers is first described, then in the next section this feature is contrasted with the corresponding gas-phase configurations. Finally, the energetics of all the hydrolysis steps is summarized. In the initial state (prehydrolysis), there is an appreciable hydrogen-bonding network with the water clustered together regardless of which tail group is considered. The polarity of the tail, however, does impact on the hydrogen-bonding network of the system; for example, there are more hydrogen bonds in the case of the amino tail, as shown in Figure 5a. The ability of the heteroatom to participate in saturating the coordination of water is higher for the amino compared to the other species. In fact, there are more than twice as many fourcoordinate water molecules in the amino case as compared to thiol and butyl, whereas thiol has fewer two-coordinate water molecules than butyl. Compared to the scorpionlike (as shown in Figure 1, bottom left-hand side) structure of the organosilanes in the gas phase,

Intermediates during Organoalkoxysilane Hydrolysis

Figure 5. Number of hydrogen bonds (a) and relative energies (b) of different hydrolysis stages. Blue refers to aminopropyl-, orange to thiolpropyl-, and black to butylsilane; periodic systems (lines) and gas phase (dotted lines).

as described in our previous studies and below, when the solvent water is included in our model and the silane headgroups are all large methoxy groups, all molecules remain in a linear conformation, as they are inhibited from folding up by the solvent, leading to too great a penalty to rearrange. In all cases, there is a drive toward the formation of channels of water through the silanes. In our simulations, we see the channels traverse the whole cell, therefore forming a periodically repeating water channel throughout the material. The formation of solvent rich regions is most pronounced for the butyl tail as the hydrophobic hydrocarbon tails group together and repel water forming one-dimensional channels through the system. In the case of the amino group, a two-dimensional system of channels spaced by silanes is formed with a dense system of hydrogen bonds. The thiol forms a thin three-dimensional network with pronounced tetrahedral motifs, but neither the sulfur nor the hydrogen bonded to it plays any role in the network. From Figure 5b it is clear that the first step of releasing a methyl group and replacing it with a hydroxyl stabilizes all three systems. It is apparent, though, that the hydrogen-bonding network of the amino group means that there is only a 2.5 kJ/ mol change in energy compared with 32 kJ/mol for the thiol tail group. There is also an increase in the number of hydrogen bonds for all systems, which is due to the increased degrees of freedom in the system, where a bulky methyl is freed to move as a methanol while the hydroxyl group is better able to find the optimum position for hydrogen bonding. The introduction of the hydroxyl leads to the formation of the scorpion configuration of one R-Si(MeOH)2OH in the case of the amino tail, whereas the other two cases show linear monomers. As mentioned previously, methanol is most stable when forming chains, which we observe, with the silane hydroxyl headgroups taking part in all cases, but this is magnified in the case of the amino group where nitrogen and attached hydrogens

J. Phys. Chem. B, Vol. 110, No. 48, 2006 24315 also participate. Interestingly, there is a large increase in the number of hydrogen bonds in the case of the thiol, which is largely due to the participation of sulphur and the hydrogen bonded to it in the network after the first hydrolysis step. The next step of hydrolysis leads to 10 methanol and 5 water molecules in each cell. With the increase in size of the solvent molecules, all R-SiMeOH(OH)2 are linear with their OH groups interacting with both water and methanol. Both the butyl and amino tails show maximum stability at this stage of hydrolysis, which corresponds to their largest number of hydrogen bonds. The last step of hydrolysis is endothermic in all cases although the energy difference is never more than 2 kJ mol-1. In the case of aminopropyl- and butylsilanes this corresponds to a drop in the number of hydrogen bonds. As the periodic models correspond to the upper bound on the solvent concentration, now we turn our attention to the limit of isolated species to separate solvent effects from the intrinsic properties of the organosilanes. DFT Gas-Phase Calculations. Typical silane conformations found in periodic calculations were used as initial structures in gas-phase molecular calculations. In each case, silane and solvent (water, or methanol in subsequent calculations) molecules are considered in isolation. Butylsilane has a nonpolar tail, which weakly interacts with the head group resulting in minor folding of the molecule, which is anticipated for large molecules in a vacuum. In contrast, in the case of polar tail groups, the most stable conformations are always those where there is a strong intramolecular interaction between the head and the tail. Especially for aminopropyltrihydroxysilane, the high polarity of the amino group results in the pronounced “scorpion” configuration shown in Figure 1, bottom left hand side. Before the start of hydrolysis, the interaction between the tail amino group and the methoxy group of the head is by a hydrogen bond (NH‚‚‚OCH3). This bond is the weakest of such interactions obtained in our calculations for amino tail groups, as is evidenced by its length of 2.47 Å. As expected, the scorpion configuration is also found to be the global minimum structure of the thiolpropyltrihydroxysilane although with a large SH‚‚‚OMe separation distance (3.02 Å), which points to a very weak interaction. For both amino and thiol tail groups, upon hydrolysis, the tail-to-head interaction changes dramatically with the interaction mode switching to the proton of the hydroxyl group becoming coordinated to an anion of the tail as discussed below. Considering the silane molecules in isolation and comparing with results of periodic calculations allows us to separate the intra- and intermolecular transformations, which these molecules undergo upon hydrolysis. The strong interaction of polar tails with other molecules noted above considerably affects the overall feasibility of hydrolysis. Hydrolysis of each Si-OMe bridge in butylsilane is a mildly exothermic process yielding between 2 and 9 kJ/mol. Molecular stabilization on the first hydrolysis step in amino- and thiolpropylsilanes is an order of magnitude stronger (cf. 36 and 27 kJ/mol, respectively). The subsequent steps, however, are similar between all three compounds, the reason for which can be easily understood from the structural configurations that are formed. Upon hydrolysis of the first Si-O-Me bridge, the most stable aminopropyldimethoxyhydroxysilane molecule is formed with the one NH‚‚‚OMe hydrogen bond replaced by two hydrogen bonds, a weak NH‚‚‚OH bond (also 2.47 Å long) and a medium OH‚‚‚N (2.07 Å) bond. Similar trends are found for thiolpropyldimethoxyhydroxysilane with the formation of an OH‚‚‚S bond of 2.36 Å, however, a weak secondary SH‚‚‚OMe bond

24316 J. Phys. Chem. B, Vol. 110, No. 48, 2006 of 3.11 Å length is formed, which is not the case for the aminopropyldimethoxyhydroxysilane. No hydrogen bond is formed at this stage in butyldimethoxyhydroxysilane. Significant hydrogen bond strengthening occurs on hydrolysis of the second Si-O-Me bridge with the OH‚‚‚N bond length decreasing from 2.07 to 1.76 Å. In contrast, this stage of hydrolysis barely changes the bonding scorpion structure of thiolpropylmethoxydihydroxysilane with 2.38 and 3.06 Å bond distances. The formation of the strongly self-interacting scorpion coil is complete for the aminopropyltrihydroxysilane molecule with the OH‚‚‚N hydrogen bond contracting to 1.74 Å. In case of the thiolpropylsilane the final stage of hydrolysis removes the only available stabilizing SH‚‚‚OMe interaction, which results in the formation of a final medium strength OH‚‚‚S bond (2.37 Å). The Mulliken Population Analysis can be compared for each step of hydrolysis and can be shown to rationalize the gas-phase energy profile in Figure 5b. There is very little change in the charge on the tail carbon when the tail is the butyl group at any stage of hydrolysis (-0.231, -0.231, -0.247, -0.232), whereas in contrast, the nitrogen Mulliken charge shows a large decrease upon the first step of hydrolysis and then shows a minimum before rising slightly at the last step (-0.454, -0.519, -0.520, -0.515). The sulfur decreases slightly at the first step and then rises at the last step (-0.364, -0.405, -0.407, -0.375). Therefore, in the gas-phase we see that the energy profile maps onto the Mulliken Population Analysis with the saturated nature of the butyl leading to its almost flat energy profile, which contrasts with the amino tail group, where the ability to form more hydrogen-bonds leads to the large exothermic step before the profile flattens out as N becomes saturated. Energetics of Hydrolysis. In all cases, we can see in Figure 5b that irrespective of the tail group the initial step in hydrolysis is exothermic. However, it is clear that the tail group greatly affects the energy of hydrolysis. When the amine is present, the difference in energy between each state is much lower as the hydrogen bonding capability of the amine is nearly saturated from its participation in the hydrogen-bonding network (see Figure 5a). In contrast for the thiolpropyl and butyl groups, the increase in the relative number of hydrogen bonds is more pronounced when there are hydroxyls on the silane after partial hydrolysis rather than methoxy species, which leads to greater stability with silane-silane interactions and silane-solvent interactions. Although there is not a one-to-one correspondence, it is clear that there are many more hydrogen bonds formed in stage 2 (1 Me + 2H2O) for the thiolpropyl- and butylsilane models than in that of the aminopropylsilane, which corresponds with a much greater increase in stability for those systems with a single hydroxyl (stage 2) than fully methoxylated (stage 1, 0 Me + 3H2O). For the aminopropylsilane, we find that there is little difference in energy (