Mechanism and nanosize products of the sol-gel reaction using diphenylsilanediol and 3-methacryloxypropyltrimethoxysilane as precursors.
ABSTRACT We use a first-principles calculation and small-angle neutron scattering (SANS) to investigate the mechanism and the nanosize products of the sol-gel reaction with diphenylsilanediol (DPD) and 3-methacryloxypropyltrimethoxysilane (MEMO) precursors in synthesizing a hybrid waveguide material. It is predicted that switching between a DPD hydroxyl and a MEMO methoxy with a reaction rate of 6.8 x 10(-6) s(-1) at 300 K is the fastest process for the first reaction step, thus generating diphenylmethoxysilanol (DPM) and 3-methacryloxypropyldimethoxysilanol (MEDO) as products. However, we determine that this reaction pathway could be modified by the presence of the H2O released from a catalyst such as Ba(OH)2.H2O. Next, switching between the DPM hydroxyl and the MEDO methoxy is followed to generate diphenyldimethoxysilane (DPDM) and 3-methacryloxypropylmethoxysilanediol (MEMDO). However, condensation between a MEMDO hydroxyl and a DPDM methoxy is found to be most favorable for the third reaction step, which generates the DPDM-MEMDO dimer and CH3OH molecule as products. In a similar fashion, a DPDM methoxy of the DPDM-MEMDO dimer can condense with a MEMDO hydroxyl of the second DPDM-MEMDO dimer to increase the chain, but its reaction rate of 2.8 x 10(-11) s(-1) is predicted to be about 5 times smaller than that between a DPDM methoxy and a MEMDO hydroxyl. This implies that the reaction rate for the larger nanostructures becomes smaller. Additionally, our SANS measurements determine that the final products from our sol-gel reaction are on the nanometer scale, at sizes from 1.76 to 2.36 nm.
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ABSTRACT: The interest in organic-inorganic hybrids as materials for optics and photonics started more than 25 years ago and since then has known a continuous and strong growth. The high versatility of sol-gel processing offers a wide range of possibilities to design tailor-made materials in terms of structure, texture, functionality, properties and shape modelling. From the first hybrid material with optical functional properties that has been obtained by incorporation of an organic dye in a silica matrix, the research in the field has quickly evolved towards more sophisticated systems, such as multifunctional and/or multicomponent materials, nanoscale and self-assembled hybrids and devices for integrated optics. In the present critical review, we have focused our attention on three main research areas: passive and active optical hybrid sol-gel materials, and integrated optics. This is far from exhaustive but enough to give an overview of the huge potential of these materials in photonics and optics (254 references).Chemical Society Reviews 02/2011; 40(2):886-906. · 24.89 Impact Factor
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Mechanism and Nanosize Products of the Sol-Gel Reaction Using Diphenylsilanediol and
3-Methacryloxypropyltrimethoxysilane as Precursors
Se Yun Kim,†Saji Augustine,†Young Joo Eo,†Byeong Soo Bae,†Seong Ihl Woo,‡and
Jeung Ku Kang*,†
Department of Materials Science and Engineering and Department of Chemical and Biomolecular Engineering
and Center for Ultramicrochemical Process Systems, KAIST, Daejon 305-701, Republic of Korea
ReceiVed: December 23, 2004; In Final Form: March 14, 2005
We use a first-principles calculation and small-angle neutron scattering (SANS) to investigate the mechanism
and the nanosize products of the sol-gel reaction with diphenylsilanediol (DPD) and 3-methacryloxypro-
pyltrimethoxysilane (MEMO) precursors in synthesizing a hybrid waveguide material. It is predicted that
switching between a DPD hydroxyl and a MEMO methoxy with a reaction rate of 6.8 × 10-6s-1at 300 K
is the fastest process for the first reaction step, thus generating diphenylmethoxysilanol (DPM) and
3-methacryloxypropyldimethoxysilanol (MEDO) as products. However, we determine that this reaction pathway
could be modified by the presence of the H2O released from a catalyst such as Ba(OH)2‚H2O. Next, switching
between the DPM hydroxyl and the MEDO methoxy is followed to generate diphenyldimethoxysilane (DPDM)
and 3-methacryloxypropylmethoxysilanediol (MEMDO). However, condensation between a MEMDO hydroxyl
and a DPDM methoxy is found to be most favorable for the third reaction step, which generates the DPDM-
MEMDO dimer and CH3OH molecule as products. In a similar fashion, a DPDM methoxy of the DPDM-
MEMDO dimer can condense with a MEMDO hydroxyl of the second DPDM-MEMDO dimer to increase
the chain, but its reaction rate of 2.8 × 10-11s-1is predicted to be about 5 times smaller than that between
a DPDM methoxy and a MEMDO hydroxyl. This implies that the reaction rate for the larger nanostructures
becomes smaller. Additionally, our SANS measurements determine that the final products from our sol-gel
reaction are on the nanometer scale, at sizes from 1.76 to 2.36 nm.
There is great interest in designing the ideal waveguide
materials for optical applications. Conventional waveguides are
made of inorganic glasses1,2and organic polymers.3,4However,
there still remain considerable challenges to reducing optical
losses and giving them good adhesion on various substrates,
high thermal stability, and low processing temperatures. Re-
cently, hybrid materials5-7with both organic and inorganic
components have been proposed as good candidates for satisfy-
ing the ideal properties and processing conditions simulta-
neously. In addition, the synthesis of the hybrid materials with
nanometer-scale sizes is also important in making fine nano-
structures and in getting homogeneous properties within a
Both hydrolytic and non-hydrolytic processes have been
proposed to produce the hybrid materials. However, hybrid
materials synthesized by hydrolytic processes have more optical
losses than those synthesized by non-hydrolytic processes.8-11
This is because hydrolytic processes12,13generally generate more
undesired hydroxyls that are sensitive to vibrational excitations.
The non-hydrolytic synthesis of hybrid materials consists of a
two-step process.14In the first step, the precursors are linked
to produce siloxane (-Si-O-Si-) bridges, and the second step
involves cross linking between organic side chains of the
inorganic structures in a 3D network by UV light. However,
depending on the nature of the interface between organic and
inorganic elements in hybrid materials, they are classified into
two different groups.15,16One group has their organic and
inorganic components linked through van der Waals or hydrogen
bonding, and the other group has its network formed through
covalent or ionic bonding.
Buestrich et al.7and Houbertz et al.17,18synthesized a hybrid
material from the sol-gel reactions19using diphenylsilanediol
(DPD) and 3-methacryloxypropyltrimethoxysilane (MEMO)
precursors. A combination of DPD, having OH groups for non-
hydrolytic processes, with MEMO, enabling the resulting
materials to be patterned by UV, provides a good precursor
system. Their IR study7shows that there is no SiO-H stretching
mode (3600-3200 cm-1) in their synthesized hybrid material
that results in low scattering losses (0.3 dB/cm at 1320 nm and
0.6 dB/cm at 1550 nm). It has also good adhesion on a silicon
substrate and a relatively low processing temperature of 150
°C. However, detailed thermodynamic and kinetic parameters
for intermediate nanostructures likely to be produced from the
reactions using DPD and MEMO precursors are not reported.
Also, there still remains considerable ambiguity as to whether
the final products from the sol-gel reaction could be on the
Here, we use a first-principles calculation20-22and small-
angle neutron scattering (SANS) methods to determine the
mechanisms and intermediate nanostructures for the sol-gel
processes using DPD and MEMO precursors. First, we inves-
tigate condensation reactions between two DPDs, between one
* Corresponding author. E-mail: firstname.lastname@example.org. Tel: +82-42-
869-3338. Fax: +82-42-869-3310.
†Department of Materials Science and Engineering.
‡Department of Chemical and Biomolecular Engineering and Center for
Ultramicrochemical Process Systems.
J. Phys. Chem. B 2005, 109, 9397-9403
10.1021/jp044151q CCC: $30.25© 2005 American Chemical Society
Published on Web 04/12/2005
DPD and one MEMO, and between two MEMOs. Also, the
switching reaction between one DPD hydroxyl and one MEMO
methoxy is explored, which generates diphenylmethoxysilanol
(DPM) and 3-methacryloxypropyldimethoxysilanol (MEDO) as
products. Then, the catalytic effects by the H2O released from
a catalyst such as Ba(OH)2‚H2O are additionally investigated
because they could play an important role in reducing the
reaction barrier as reported in the previous studies.23-25More-
over, the reactions by DPM and MEDO molecules are explored.
Next, we investigate the reactions by the diphenyldimethox-
ysilane (DPDM) and 3-methacryloxypropylmethoxysilanediol
(MEMDO) molecules as well as the reactions by the dimer
(DPDM-MEMDO) molecules. Last, the sizes of the final
products are determined on the basis of experimental SANS
measurements and theoretical predictions.
These results are reported and discussed in section III. Section
II provides some details about the calculations, and section IV
summarizes our results.
II. Computational Details
All calculations are performed using the self-consistent
B3LYP20and KMLYP21density functional theories and using
the QCISD22level of theory. The B3LYP was shown to predict
accurate geometries and thermochemical data compared to other
generalized gradient approximations (GGAs)26-30and local
density approximations (LDAs),31,32whereas KMLYP was
proven to be more accurate in predicting transition-state
barriers21,33,34than other DFT methods.
The electronic wave function is expanded using the 6-31G
valence double-?35basis set and the 6-311+G(d,p) valence
diffuse triple-? plus polarization35basis set. In this study, the
full optimizations of all geometry parameters for reactants,
transition states, and products are performed at the B3LYP/6-
31G level of theory, and then the enthalpies of reactions and
transition-state barriers are obtained using the KMLYP/6-
311+G(d,p) single-point energies determined at the B3LYP
geometries. All calculations are performed with Gaussian 03.36
We use DPD and MEMO precursors to make siloxane
bridges. DPD consists of one silicon atom, two hydroxyls, and
two phenyl groups. The valence orbitals for the silicon atom
are sp3-hybridized, and they bond to two phenyl and two
hydroxyl fragments as shown in Figure 1a. Our predicted DPD
Si-OH bond length of 1.64 Å is found to be in close agreement
with the experimental values of 1.63-1.64 Å.37,38In addition,
the DPD Si-phenyl bond length of 1.89 Å is calculated to be
consistent with the QCISD value of 1.90 Å. The MEMO
molecule has one 3-methacryloxypropyl group and three meth-
oxy groups around the silicon atom as described in Figure 1b.
In all possible sol-gel processes using the DPD and MEMO
precursor system, the Si-OH fragments of the DPD and the
Si-OCH3fragments of the MEMO are active groups that form
a siloxane bridge (-Si-O-Si-), thereby releasing H2O or CH3-
OH as a byproduct.
III. Results and Discussion
A. Reaction Mechanism and Resulting Nanostructures.
By computing the partition functions for the reactants and the
transition state, the reaction rate kTSTof the canonical rate
equation39is determined by
where Γ(T) is the thermal tunneling coefficient,21kB is the
Boltzmann constant, h is the Plank constant, QTSis the partition
function for the transition state, QA and QB are the partition
functions for reactants A and B, and ∆E0is the barrier height.
We determine the sequence of the reactions by comparing the
calculated reaction rates.
There are three plausible cases for condensation in the first
step of the sequence, which include reactions between (1) two
DPDs, (2) one DPD and one MEMO, and (3) two MEMOs.
The first case begins by the initial attack of one DPD hydroxyl
on another DPD hydroxyl and then proceeds through the four-
centered transition state as shown in Figure 2a. The predicted
barrier is 14.9 kcal/mol. Additionally, we find that the 14.9 kcal/
mol barrier is also consistent with the 14.4 kcal/mol barrier
determined for the KMLYP/6-31G geometries and the 13.9 kcal/
mol barrier determined for the KMLYP/6-311+G(d,p) geom-
etries. These results indicate that the KMLYP/6-311+G(d,p)//
B3LYP/6-31G energies are proper for an accurate prediction
of the transition-state barriers. The second condensation case
can occur through two different reaction pathways. The first
type is the attack of a DPD hydroxyl on a MEMO methoxy
(Figure 2b), whereas the second one is the attack of a MEMO
methoxy on a DPD hydroxyl (Figure 2c). However, the first
type having the smaller barrier of 15.1 kcal/mol is found to be
more favorable. Figure 2d describes the condensation reaction
between two MEMOs, but its barrier of 69.3 kcal/mol is
predicted to be relatively high compared to those of the two
other cases. Recently, Buestrich and co-workers7proposed one
plausible reaction mechanism for the sol-gel reaction under
the DPD and MEMO precursor system. According to their
mechanism, first one DPD hydroxyl should condense with one
MEMO methoxy to form the DPD-MEMO product having one
siloxane bridge, and CH3OH should also be generated as a
byproduct. Then, the CH3OH would condense with the DPD
side in the DPD-MEMO to substitute its hydroxyl with the
methoxy group. In this respect, the DPD side could remove its
hydroxyl group. However, there could be an alternative pathway
to remove hydroxyl groups in the DPD. If switching between a
DPD hydroxyl and a MEMO methoxy could consecutively take
place twice, then the DPD could remove its two hydroxyl
groups. Moreover, our calculations indicate that switching
between a DPD hydroxyl and a MEMO methoxy is most likely
to occur in the first reaction step because the switching reaction
is predicted to have a rate of 6.80 × 10-6s-1at 300 K, which
is much larger than the rates of 3.77 × 10-9and 1.81 × 10-10
Figure 1. Reactants and the catalyst: (a) DPD (diphenylsilanediol) and (b) MEMO (3-methacryloxypropyltrimethoxysilane) and (c) Ba(OH)2‚
9398 J. Phys. Chem. B, Vol. 109, No. 19, 2005
Kim et al.
s-1for condensations, respectively, between two DPDs and
between one DPD and one MEMO. For the switching reaction,
the concerted attacks of a DPD hydroxyl on a MEMO silicon
and of a MEMO methoxy on a DPD silicon are determined to
generate DPM (DPD hydroxyl substituted with one methoxy)
and MEDO (MEMO methoxy substituted with one hydroxyl)
as products. For the transition state, the DPD Si-OH bond (1.95
Å) is significantly broken while a MEMO Si-OH bond is being
formed as shown in Figure 2e. Simultaneously, the Si-OCH3
bond begins to form on the DPD side, whereas the MEMO Si-
OCH3bond length of 2.10 Å indicates a considerable amount
of the bond being broken.
We also explore the reactions by the byproducts of H2O and
CH3OH molecules resulting from condensations between two
molecules of DPD and MEMO. There exist four types of these
reactions. The first type is condensation between one DPD
hydroxyl and one CH3OH (Figure 3a), generating H2O and DPM
as products with a barrier of 19.8 kcal/mol. The condensation
reaction between the MEMO methoxy and the H2O as shown
in Figure 3b is the second type, and it is determined to produce
CH3OH and MEDO with a barrier of 19.0 kcal/mol. However,
its endothermic enthalpy of 0.6 kcal/mol indicates that it is
thermodynamically unfavorable to occur. Figure 4 describes the
products from the reactions of CH3OH or H2O with the terminal
CdC and CdO bonds in the MEMO, but it is found that the
barriers of 35.3 to 91.4 kcal/mol by CH3OH (the third type)
and the barriers of 40.4 to 141.7 kcal/mol by H2O (the fourth
type) are considerably high as compared to those for the first
and second types. The largest reaction rate of 6.45 × 10-12s-1
is obtained from the first type among these four types. However,
this reaction rate is found to be much smaller than 6.80 × 10-6
s-1for the switching reaction between one DPD hydroxyl and
one MEMO methoxy. In this respect, the most plausible products
from the first reaction step are considered to be DPM and
MEDO molecules resulting from the fastest switching process.
There are 10 possible cases for condensation in the second
reaction step, which includes the reactions between two DPM
hydroxyls, between one DPM hydroxyl and one DPM methoxy,
between two DPM methoxys, between one DPM hydroxyl and
one MEDO hydroxyl, between one DPM methoxy and one
MEDO hydroxyl, between one DPM hydroxyl and one MEDO
methoxy, between one DPM methoxy and one MEDO methoxy,
between two MEDO hydroxyls, between one MEDO hydroxyl
and one MEDO methoxy, and between two MEDO methoxys.
All calculated transition barriers and enthalpies are summarized
in Table 1. Among these reactions, the attack of a DPM
hydroxyl on a MEDO methoxy, whose transition state structure
is shown in Figure 5a, is found to have the largest rate of 1.12
× 10-8s-1. However, the switching reaction between a DPM
hydroxyl and a MEDO methoxy, generating DPDM and
MEMDO as products, is determined to have a much larger rate
of 4.61 × 10-7s-1than those for condensation. In this respect,
the switching reaction is determined to be the fastest process
in the second reaction step. The transition state for the switching
reaction involves several simultaneous bond breakings and
formations: (1) a DPM Si-OH bond being broken with an
elongated bond length of 1.82 Å, (2) a Si-OH bond of 1.84 Å
formed between a DPM hydroxyl and a MEDO silicon, (3) a
Si-OCH3bond being broken with the elongated 1.94 Å on the
MEDO side, and (4) a Si-OCH3of 2.03 Å formed on the DPM
side as shown in Figure 5b.
DPDM and MEMDO are the products from the switching
reaction in the second reaction step. There exist three possible
condensation reactions by these molecules. The first one is the
attack of a MEMDO hydroxyl on a DPDM methoxy, which is
shown in Figure 6a. The transition barrier is 14.0 kcal/mol. Parts
b and c of Figure 6 show condensation reactions between two
MEMDO hydroxyls and between one MEMDO hydroxyl and
one MEMDO methoxy, respectively, with barriers of 18.7 and
19.8 kcal/mol. We find that condensation between one DPDM
methoxy and one MEMDO hydroxyl has the largest reaction
rate of 1.22 × 10-10s-1, thus in the third reaction step the
process to produce the DPDM-MEMDO dimer as seen in
Figure 7f is considered to be the most probable. The dimer can
be also grown by condensation between a methoxy of a dimer
DPDM site and a hydroxyl of a dimer MEMDO site, but its
reaction is determined to have a smaller rate of 2.76 × 10-11
s-1than that for the case between a DPDM methoxy and a
Figure 2. B3LYP/6-31G transition-state geometries for condensations between (a) a DPD hydroxyl on a DPD hydroxyl, (b) a DPD hydroxyl on
a MEMO methoxy, (c) a MEMO methoxy on a DPD hydroxyl, and (d) a MEMO methoxy on a MEMO methoxy as well as for the switching
reaction between (e) a DPD hydroxyl and a MEMO methoxy. Ph and MCP indicate phenyl and methacryloxypropyl groups, respectively. Bond
lengths are in angstroms.
Figure 3. B3LYP/6-31G transition-state geometries for condensations
between (a) a DPD and a CH3OH and (b) a MEMO and a H2O. Ph
and MCP indicate phenyl and methacryloxypropyl groups, respectively.
Bond lengths are in angstroms.
Mechanism/Nanosize Products of Sol-Gel Reaction
J. Phys. Chem. B, Vol. 109, No. 19, 2005 9399
MEMDO hydroxyl. Meanwhile, this rate is found to be slightly
larger than the 1.17 × 10-11s-1for condensation between a
hydroxyl of a dimer MEMDO site and a methoxy of a dimer
MEMDO site. Here, it should be noted that the reaction rate on
the larger nanostructures become smaller. This might be due to
the increased geometry hindrance. Additionally, our SANS
(small-scattering neutron scattering) measurements determine
that the final products from the sol-gel reaction are mostly on
the nanometer scale at sizes from 1.76 to 2.36 nm, which
compare to the value of 2.01 nm determined on the elongated
product by three dimers. The experimental size of the products
is determined from the Rg(radius of gyration) of the products
measured by SANS (small-angle neutron scattering). The
products of the reaction between DPD (50 mol %) and MEMO
(50 mol %) are solvated in 10 wt % acetone-d6to obtain accurate
SANS results. Under this condition, Rgof the products is 0.68
nm. When the structure of the particle is spherical, the radius
of the particle is R ) ?5/3Rg,, whereas the length of the
rodlike particle is L ) ?12Rg. Consequently, the experimen-
tally predicted size of the products is in the range of 1.76-
B. Effects of H2O on the Sol-Gel Reaction. H2O can be
released from the catalyst. The enthalpy of H2O dissociation
from the catalyst of Ba(OH)2‚H2O is calculated to be endother-
mic by 3.8 kcal/mol, whereas those from Sr(OH)2‚H2O and Ca-
(OH)2‚H2O are endothermic by 3.2 and 2.5 kcal/mol, respec-
tively. Figure 8a describes a concerted H2O attack on hydroxyl
fragments of two DPDs by forming a six-centered structure that
involves one H atom transfer from the H2O to the DPD
hydroxyl, bond dissociation of a Si-O bond to generate one
H2O molecule, one H atom transfer from the DPD OH to the
attacking H2O, and formation of a Si-O-Si bond. It is to be
noted that H2O participates in this reaction as a catalyst donating
one hydrogen atom to the DPD hydroxyl and accepting one
hydrogen atom from the other DPD hydroxyl simultaneously.
The predicted barrier for this reaction is 16.9 kcal/mol with
respect to an intermediate water complex formed from two DPD
molecules, and this reaction is determined to be exothermic by
3.3 kcal/mol. Consequently, its predicted barrier and exothermic
enthalpy indicate that the presence of H2O included in the
catalyst can significantly reduce the condensation rate between
two DPDs. Before making a transition-state structure, H2O and
two DPD molecules form a water-mediated structure with a high
exothermic enthalpy of 20.0 kcal/mol due to three hydrogen
bonds between one hydrogen atom of the H2O and the oxygen
atom of a DPD hydroxyl, between one H atom of another DPD
hydroxyl and the oxygen atom of the H2O molecule, and
between the hydrogen atom of one DPD and the oxygen atom
of another DPD.
A concerted attack of the DPD on the MEMO in the presence
of the H2O is also described in Figure 8b, where the transition
state is a six-centered ring structure. This step consists of several
competing reactions. These are (1) one H atom transfer from
the H2O to the O atom of a Si-OCH3group in MEMO, (2)
Si-OCH3bond dissociation, (3) one H atom transfer from the
DPD hydroxyl to the H2O, and (4) Si-O-Si bond formation.
We obtain a transition-state barrier of 12.5 kcal/mol, and this
process is found to be exothermic by 4.9 kcal/mol. The
decreased barrier indicates that the presence of the catalyst can
increase the condensation rate between one DPD and one
MEMO, which is opposite to the case between two DPDs.
The H2O effect on the switching reaction between DPD and
MEMO is also explored. The transition state of this reaction is
based on a six-centered ring structure as shown Figure 8c. The
hydroxyl group of the catalyst H2O attaches the MEMO silicon
atom to form a Si-OH bond in the MEMO while one H2O
molecule is newly generated by bonding between the remaining
H of the attacking water with a DPD hydroxyl. Simultaneously,
a MEMO methoxy transfers to the DPD to replace the DPD
hydroxyl with the methoxy group. The transition barrier for this
case is 30.5 kcal/mol, which is higher than the cases of DPD
with DPD and DPD with MEMO. Consequently, this result
Figure 4. Products of H2O or CH3OH with a CdC or CdO of MEMO: (a) a CH3OH with a CdC, (b) a CH3OH with a CdO, (c) a H2O with
a CdC, and (d) a H2O with a CdO.
9400 J. Phys. Chem. B, Vol. 109, No. 19, 2005
Kim et al.
indicates that the final nanostructure could be modified by the
presence of H2O. This is because the H2O released from the
catalyst plays an important role in changing reaction pathways
to modify the resulting product.
C. Vibrational Frequencies. We compute the frequencies
of SiOH and SiOCH3fragments in the DPD, MEMO, DPM,
and MEDO molecules at the B3LYP/6-31G level of theory using
a scaling factor of 0.961.40The previous study34showed that
the scaling factor for B3LYP frequencies to match with
experimental values is closer to the scaling factor of unity than
for the case with KMLYP. In this respect, we report only the
vibrational frequencies obtained through the B3LYP calcula-
tions. The predicted frequency for a DPD Si-OH stretching
mode is 648 cm-1, whereas the frequency for a DPD SiO-H
stretching mode is in the range of 3623-3621 cm-1, which is
consistent with experimental IR peaks7,17,18of 3600-3200 cm-1.
We also determine the frequencies of 561 and 1044 cm-1,
respectively, for MEMO Si-OCH3and SiO-CH3stretching
modes. In addition, a C-H stretching mode of the MEMO
SiOCH3is found to be in the range of 2921-2911 cm-1. All
frequencies in DPM and MEDO are similar to those in DPD
and MEMO, which are shown in Table 2.
IV. Summary and Conclusions
Making the hybrid materials from the sol-gel reaction with
nanometer-scale sizes is one interesting issue today. This is
because the nanosizes of the resulting hybrid materials are useful
for making fine nanostructures of optical waveguides and getting
homogeneous properties within a waveguide. A first-principles
calculation and experimental SANS measurement methods have
been used to investigate the mechanism and the nanosize
products of the sol-gel reaction using DPD and MEMO
precursors. To determine the size of the products resulting from
the sol-gel reaction, we have first explored the reaction
mechanism by comparing the rates of all possible reactions at
each step. It has been found that the switching reaction between
a DPD hydroxyl and a MEMO methoxy having a kinetic
reaction rate of 6.80 × 10-6s-1at room temperature, which
generates one DPM and one MEDO as products, is the fastest
process in the first reaction step. However, it has been
determined that the reaction pathway can be modified by the
presence of a catalyst such as Ba(OH)2‚H2O because the H2O
released from the catalyst changes the reaction barrier. In the
second reaction step, we have also found that the switching
TABLE 1: KMLYP/6-311+G(d,p)//B3LYP/6-31G Transition-State Barriers and Enthalpies for the Sol-Gel Processes Using
DPD and MEMO Precursorsa
DPD OH + DPD OH f DPD-DPD + H2O
DPD OH + MEMO OCH3f DPD-MEMO + CH3OHb
DPD OH + MEMO OCH3f DPD-MEMO + CH3OHc
MEMO OCH3+ MEMO OCH3f MEMO-MEMO + (CH3)2O
DPD OH + MEMO OCH3f DPM + MEDO6.9
DPM OH + DPM OH f DPM-DPM(I) + H2O
DPM OH + DPM OCH3f DPM-DPM(II) + CH3OHd
DPM OH + DPM OCH3f DPM-DPM(II) + CH3OHe
DPM OCH3+ DPM OCH3f DPM-DPM (III) + (CH3)2O
DPM OH + MEDO OH f DPM-MEDO(I) + H2Of
DPM OH + MEDO OH f DPM-MEDO(I) + H2Og
DPM OCH3+ MEDO OH f DPM-MEDO(II) + CH3OHh
DPM OCH3+ MEDO OH f DPM-MEDO(II) + CH3OHi
DPM OH + MEDO OCH3f DPM-MEDO(III) + CH3OHj
DPM OH + MEDO OCH3f DPM-MEDO(III) + CH3OHk
DPM OCH3+ MEDO OCH3f DPM-MEDO(IV) + (CH3)2Ol
DPM OCH3+ MEDO OCH3f DPM-MEDO(IV) + (CH3)2Om
MEDO OH + MEDO OH f MEDO-MEDO(I) + H2O
MEDO OH + MEDO OCH3f MEDO-MEDO(II) + CH3OHn
MEDO OH + MEDO OCH3f MEDO-MEDO(II) + CH3OHo
MEDO OCH3+ MEDO OCH3f MEDO-MEDO(III) + (CH3)2O
DPM OH + MEDO OCH3f DPDM +MEMDO8.5
DPDM OCH3+ MEMDO OH f DPDM-MEMDO + CH3OH
MEMDO OH + MEMDO OH f MEMDO-MEMDO(I) + H2O
MEMDO OH + MEMDO OCH3f MEMDO-MEMDO(II) + CH3OH
aEnergies are in units of kcal/mol.bDPD OH attack.cMEMO OCH3attack.dDPM OH attack.eDPM OCH3attack.fDPM OH attack.gMEDO
OH attack.hMEDO OH attack.iDPM OCH3attack.jDPM OH attack.kMEDO OCH3attack.lDPM OCH3attack.mMEDO OCH3attack.nMEDO
OH attack.oMEDO OCH3attack.
Figure 5. B3LYP/6-31G transition-state geometries for (a) condensa-
tion between a DPM hydroxyl and a MEDO methoxy and (b) switching
between a DPM hydroxyl and a MEDO methoxy. Ph and MCP indicate
phenyl and methacryloxypropyl groups, respectively. Bond lengths are
Mechanism/Nanosize Products of Sol-Gel Reaction
J. Phys. Chem. B, Vol. 109, No. 19, 2005 9401
reaction between the DPM hydroxyl and the MEDO methoxy
forming DPDM and MEMDO is the most favorable. This
switching has a reaction rate of 4.61 × 10-7s-1. Next, the
condensation between a methoxy of DPDM and a hydroxyl of
MEMDO having a reaction rate of 1.22 × 10-10s-1has been
determined to be most favorable in the third reaction step, which
generates the DPDM-MEMDO dimer. In a similar fashion,
additional growth has been found to occur by condensation
between a methoxy of the dimer in the DPDM site and a
hydroxyl of the dimer in the MEMDO site, which has a reaction
Figure 6. B3LYP/6-31G transition-state geometries for condensations between (a) a DPDM methoxy and a MEMDO hydroxyl, (b) two MEMDO
hydroxyls, and (c) a MEMDO hydroxyl and a MEMDO methoxy. Ph and MCP indicate phenyl and methacryloxypropyl groups, respectively. Bond
lengths are in angstroms.
Figure 7. Products for condensations between (a) two DPM hydroxyls, (b) a DPM hydroxyl and a DPM methoxy, (c) two DPM methoxys (or two
DPD hydroxyls), (d) one DPM hydroxyl and one MEDO hydroxyl, (e) one DPM methoxy and one MEDO hydroxyl (or one DPD hydroxyl and
one MEMO methoxy), (f) one DPM hydroxyl and one MEDO methoxy (or one DPDM methoxy and MEMDO hydroxyl), (g) one DPM methoxy
and one MEDO methoxy, (h) two MEDO hydroxyls (or two MEMO methoxys), (i) one MEDO hydroxyl and one MEDO methoxy, (j) two MEDO
methoxys (or two MEMDO hydroxyls), and (k) one MEMDO hydroxyl and one MEMDO methoxy.
Figure 8. Catalytic effects of H2O on transition states for (a) condensation between two DPDs, (b) condensation between a DPD hydroxyl and a
MEMO methoxy, and (c) switching between a DPD hydroxyl and a MEMO methoxy. Ph and MCP indicate the phenyl group and methacryloxypropyl
group, respectively. Bond lengths are in angstroms, where the B3LYP/6-31G geometries are used.
9402 J. Phys. Chem. B, Vol. 109, No. 19, 2005
Kim et al.
rate of 2.76 × 10-11s-1. In addition, our SANS measurements
have shown that the final products for the sol-gel reaction are
on the nanometer scale at sizes of 1.76-2.36 nm, which
compare to the size of 2.01 nm for the elongated structure by
three dimers. This result implies that the reaction rate on the
larger nanostructures become negligibly small. Additionally, the
computed frequencies of SiOH and SiOCH3of DPD, MEMO,
DPM, and MEDO have been discussed.
Materials and Process Simulation Center (MSC) at the California
Institute of Technology and Korea Advanced Institute of Science
and Technology have been supported by grants from NSF-MRI
and ARO-DURIP and by a SUR grant from IBM. We also
appreciate the financial support from the interdisciplinary
research program and the Center for Ultramicrochemical Process
Systems (CUPS) sponsored by KOSEF (2005).
The computational resources at the
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TABLE 2: Computed Frequencies of Various Stretching
Modes for SiOH and SiOCH3of DPD, MEMO, DPM, and
MEDO at the B3LYP/6-31G Level with a Scaling Factor of
Si-OCH3 SiO-CH3 SiOC-H3
aFrequencies in units of cm-1.bExperimental values taken from
Mechanism/Nanosize Products of Sol-Gel Reaction
J. Phys. Chem. B, Vol. 109, No. 19, 2005 9403