Probing the Nature and Local Structure of Phosphonic Acid Groups

ARTICLE pubs.acs.org/JPCC

Probing the Nature and Local Structure of Phosphonic Acid Groups Functionalized in Mesoporous Silica SBA-15 Yu-Chi Pan,† Hui-Hsu Gavin Tsai,† Jyh-Chiang Jiang,‡ Chia-Chun Kao,† Tsai-Lung Sung,† Po-Jui Chiu,† Diganta Saikia,† Jen-Hsuan Chang,† and Hsien-Ming Kao*,† † ‡

Department of Chemistry, National Central University, Chung-Li, Taiwan 32054, R.O.C. Department of Chemical Engineering, National Taiwan University of Science and Technology, #43, Sec. 4, Keelung Rd., Taipei, 106, Taiwan, R.O.C.

bS Supporting Information ABSTRACT: Well-ordered mesoporous silicas SBA-15 functionalized with variable contents of phosphonic acid groups (up to 25 mol % based on silica) have successfully synthesized via cocondensation of tetraethoxysilane (TEOS) and diethylphosphatoethyltriethoxysilane (PETES) using triblock copolymer Pluronic P123 as the structure-directing agent under acidic conditions. The status and local structures of the phosphonic functional groups are investigated by extensive multinuclear solid-state NMR studies. Solid-state 13C and 31P NMR results reveal that phosphonic ester moieties are obtained for the as-synthesized samples and for the samples subjected to template removal by concentrated H2SO4. The generation of phosphonic acid groups can be accomplished by dealkylation reaction via treating the templateextracted samples with concentrated HCl. Two distinct local environments for the phosphorus sites of phosphonic acid groups have been observed at 32 and 22 ppm in the 31P magic angle spinning (MAS) NMR spectra. The relative ratio between these two species is not sensitive to the loading of phosphonic acid groups incorporated, but it strongly depends on the moisture present in the materials. The PO3H2 groups forming the hydrogen bonds with the nearby Q3 SiOH are the major species responsible for the 22 ppm peak based on the results of 1H f 31P f 29Si double cross-polarization NMR experiments and density functional theory calculations (DFT). Of particular interest is that 29Si{31P} rotational echo double resonance (REDOR) NMR experiments are utilized to measure 31P29Si distances between the phosphorus site in the functional groups and the silicon sites in the silica framework. A 29Si31P distance of 5.0 Å is obtained for the phosphorus site in the functional groups to the silicon site of the Q3 species for the as-synthesized sample. A reasonable fitting to the REDOR data for the acidified sample can also be achievable by assuming the presence of different structural units, whose 31P29Si distance information is referred from the DFT results. The combination of REDOR and 1H f 31P f 29Si double cross-polarization NMR measurements and the DFT calculations allow one to gain deeper insights into the local environments of the organic groups functionalized in mesoporous silica materials.

’ INTRODUCTION Since the pure mesoporous silica materials have no intrinsic catalytic activity, extensive efforts have been devoted to functionalize them with appropriate organic functional groups in order to tune their surface properties for designated applications in various research fields.110 The properties of organic-functionalized mesoporous silica materials strongly depend on the nature, loading, and framework distribution of organic functional groups and their spatial proximities. To ensure a better control of the loading and homogeneous distribution of the organic groups, the materials are often prepared by the one-pot direct-synthesis route, namely, via cocondensation of tetraethoxysilane (TEOS) and an organosilane containing the desired organic functionality. Of particular interest is that mesoporous silica materials functionalized with a variety of acidic functional groups, such as sulfonic,9 carboxylic,1113 and phosphonic acid groups,1417 r 2011 American Chemical Society

have been synthesized and applied in a number of acid-catalyzed reactions. Moreover, these acidic organic functionalized materials also exhibit high enough proton conductivity to make them promising candidates as electrolytes for fuel cell systems.1820 The organic functional groups that incorporated in mesoporous silica materials via one-pot synthesis route are often assumed to be uniformly distributed without direct spectroscopic evidence. It is essential to gain detailed insights into the local structure and framework distribution of the organic groups and their interactions with the pore surface via suitable characterization techniques. To address this issue, solid-state NMR methods can make significant contribution. For example, we recently demonstrated Received: June 26, 2011 Revised: December 19, 2011 Published: December 19, 2011 1658

dx.doi.org/10.1021/jp206017j | J. Phys. Chem. C 2012, 116, 1658–1669

The Journal of Physical Chemistry C that the carboxylic acid groups can interact with the surface silanol groups (SiOH) in mesoporous silica SBA-1, as revealed by 29Si{1H} 2D heteronuclear correlation (HETCOR) NMR.12 Further, NMR techniques based on the heteronuclear dipolar interaction, for example, rotational echo double resonance (REDOR) NMR,21,22 allow the characterization of bonded or nonbonded interatomic distances independent of interaction details and thus in principle can directly provide the information about the framework distribution and locations of organic functional groups in mesoporous silicas. REDOR NMR has been used to determine interatomic distances in a variety of materials.2328 However, a significant drawback to REDOR NMR is the need for isotopic labeling to increase the concentration of NMR-active nuclei such as 13C and 15N; both spins are of low natural abundances and thus are not very NMR friendly for direct NMR distance measurements. This requirement is particularly relevant to organic functional groups used for the synthesis of organic functionalized mesoporous silica materials since the isotopic labeled organosilane precursors are often not commercially available. As a result, there is no study to exploit NMR methods that allow distances between spins on the site of functional groups and in the silica framework to be determined, although mesoporous silica materials functionalized with a variety of organic groups have been extensively synthesized for applications. One of the aims of the present work is to explore the feasibility of using double-resonance NMR methods designed for measuring internuclear distances to determine the local structures and interactions between the organic functional groups and the various silicon species in mesoporous silicas. Unlike other common rare nuclei such as 13C and 15N, we have chosen the phosphonic functional groups, in view of the sensitivity (large γ and 100% natural abundance) of the 31P nucleus as a sensitive NMR probe. Bonhomme and co-workers have made significant efforts on the setup of 31P29Si dipolar- and J-based NMR experiments on phosphosilicate phases and mesoporous silica based materials.29 We are aware that phosphorus-containing organosilane has been successfully used to functionalize mesoporous silicas with phosphonic acid moiety.1417 Since the phosphonic acid moiety does not require any isotopic labeling, it can in principle serve as an excellent site to probe its distances with the various silicon species present in the mesopore surface. Our goal is to compare the NMR results with those from density functional theory (DFT) calculations30 involving small motifs and to explore the limits and applicability of the doubleresonance NMR methods. Such distance information is of great importance and can be used to evaluate if there are any interactions between organic functional groups and the mesopore surface. This is the first report that provides 31P29Si distances of organic functionalized mesoporous silicas by 29Si{31P} REDOR NMR, which are valuable for understanding the framework distribution and compositions in organic functionalized mesoporous silica materials.

’ EXPERIMENTAL SECTION Sample Preparation. Mesoporous silicas SBA-15 containing variable loadings (1025 mol % based on silicon) of phosphonic acid groups were prepared by following the previously published procedures.14 The materials were prepared via cocondensation of diethylphosphatoethyltriethoxysilane (PETES, Gelest) and TEOS under acidic conditions. In a typical synthesis, 4 g of

ARTICLE

Pluronic P123 was dissolved in 160 mL of aqueous solution of HCl. Then, the premixed TEOS and PETES in a various molar ratio were added into this solution. The resulting mixture was stirred at room temperature for 3 h to obtain a clear solution. The solution was then heated to 60 °C and 75.4 mg of NaF was added. The synthesis composition is 0.04 F:(1 x) TEOS:x PETES:0.02 P123:0.12 HCl:220 H2O, where x denotes the molar percentage ratio of PETES/(PETES + TEOS) ranging from 5 to 25%. After aging under stirring at 60 °C for 48 h, the resulting powder was filtrated and dried at 50 °C. The surfactant was removed by treating the as-synthesized samples with H2SO4.11 The diethyl ester group of PETES was acidified by treating 0.5 g of template-extracted sample with 50 mL of HCl (12 M) at 100 °C for 6 h to give phosphonic acid group (PO(OH)2). The samples were denoted as P-x-y, where x represents the molar percentage ratio of PETES/(TEOS + PETES) in the initial mixture and y can be either as, re, or ac, corresponding to the sample at various stages, namely the as-synthesized (as), sample subjected to template removal (re), followed by acidification with HCl (ac). These samples were in “hydrated” states because they were stored in vials for 1 week before NMR measurements. Characterization Methods. Powder X-ray diffraction (XRD) patterns were collected on Wiggler-17A beamline (λ = 0.133 320 nm) at the National Synchrotron Radiation Research Center in Taiwan. N2 adsorptiondesorption isotherms were measured at 77 K on a Micromeritics ASAP 2020 analyzer. The sample was degassed at 180 °C for several hours before measurements. Specific surface areas were calculated by using the BrunauerEmmettTeller (BET) method in the relative pressure range of P/P0 = 0.050.3. The pore size distribution was obtained from the analysis of the adsorption branch of the isotherm by the BarrettJoynerHalenda (BJH) method. Pore volumes were obtained from the volumes of N2 adsorbed at P/P0 = 0.95 or in the vicinity. The transmission electron microscopic (TEM) images were obtained by a JEOL JEM 2100 microscope with an acceleration voltage of 200 kV. To prepare samples for TEM measurements, a small amount of the material was first suspended in absolute alcohol by sonication in an ultrasonic water bath. A drop of this suspension was then placed onto a carbon-coated copper grid, followed by drying at room temperature. Multinuclear NMR Characterization. Solid-state 31P and 29Si magic angle spinning (MAS) NMR and 13C CPMAS (crosspolarization magic angle spinning) spectra were recorded on a Varian Infinityplus-500 NMR spectrometer, equipped with Chemagnetics T3 probes. The Larmor frequencies for 31P, 13 C, and 29Si nuclei are 201.0, 125.0, and 99.7 MHz, respectively. Single-pulse experiments with a π/6 pulse of 2 μs, a spinning speed of 5 kHz, and a recycle delay of 300 s were used to acquire the quantitative 29Si MAS NMR spectra. Quantitative 31P MAS NMR spectra were acquired with a π/4 pulse of 3 μs and a recycle delay of 100 s. The use of a longer recycle delay did not change the relative peak intensities in the 31P MAS NMR spectra. Chemical shifts were externally referenced to tetramethylsilane (TMS) for 13 C and 29 Si, and to 85% H 3 PO4(aq) for 31 P at 0.0 ppm. 1 H f 31P f 29Si Double Cross-Polarization NMR. The pulse sequence for 1H f 31P f 29Si double CP NMR is shown in Scheme 1.29 Considering the shorter T1 of 1H spins (compared to 31P spins), the 31P magnetization is first transferred from the 1 H spins during the 1H f 31P contact time (CT1) via a variable 1659

dx.doi.org/10.1021/jp206017j |J. Phys. Chem. C 2012, 116, 1658–1669

The Journal of Physical Chemistry C Scheme 1. Pulse Sequence for 1H f 31P f 29Si Double CPMAS NMR Experimentsa

The first CP period of CT1 is for 1H f 31P contact time and the second CP period of CT2 is for 31P f 29Si contact time. A variable amplitude CP (ramp) is applied on the 31P channel during the first contact time and on the 29Si channel during the second contact time. a

amplitude CP (ramp)31 in order to broaden the Hartmann Hahn matching profile. Afterward, the 29Si NMR signals are detected via the transfer of the 31P magnetization during the 31P f 29Si contact time (CT2) under proton decoupling conditions. 29 Si{31P} REDOR NMR. The REDOR technique was originally proposed by Gullion and Schaefer to recouple the heteronuclear dipolar coupling between an isolated IS spin pair.21,22 Although this interaction, which we seek to measure, is removed by MAS, it is suitably recoupled by the application of rotor-synchronized π pulses on the dipolar coupled I spins, applied twice per rotor period, with a single S spin π pulse applied halfway during the sequence. The 29Si{31P} REDOR NMR spectra were acquired by using a standard REDOR pulse sequence, in which XY-8 compensated phase cycling was used on the 31P channel to minimize off-resonance effects and the effect of rf field power fluctuations during the REDOR measurements. The intensities of the 29Si echoes, measured with and without applying a series of π pulses on the 31P channel at half-integral multiples of the rotor cycles, are denoted as Sf and S0, respectively. The expected difference signal, normalized to the full echo intensity, i.e., the REDOR fraction, is calculated by an average over all possible internuclear vector orientations21,22 ΔS 1 Z 2π Z π ¼ 1 dα sin β dβ cosðΔΦn Þ ð1Þ 0 S0 n 4π 0 where ΔS is the difference in intensity between S0 and Sf. In this expression, ΔΦ is the dipolar dephasing angle given by the product of the dipolar coupling constant D and the dipolar evolution time (NTr, defined as the number of rotor cycles (2n) multiplied by the rotor period): pffiffiffi ð2Þ ΔΦn ¼ 4 2NTr D sin β cos β sin α where β and α are the polar and azimuthal angles of the internuclear vector in the rotor frame. The ratio ΔS/S0 gives a dephasing curve which can be fit by simulation to give the dipolar coupling and hence the distance between the two nuclei. For the 29 Si{31P} REDOR NMR experiments, the heteronuclear 31 P29Si dipolar couplings were determined from weighted nonlinear least-squares fitting of the data in Mathematica32 using the analytical functions reported by Mueller.33 For all the REDOR calculations the spins were assumed to be at fixed

ARTICLE

positions within a rigid lattice. The low natural abundance of 29Si (4.7%) in the framework, combined with the weak homonuclear couplings, means that the 31P29Si dipolar interactions should approximate clusters of isolated spin pairs for which theoretical REDOR behavior can be calculated. 2D 31P31P Homonuclear NMR Experiments. The 2D 31 P31P exchange NMR experiments were performed with a NOESY-type sequence with three 90° pulses. After the initial excitation, an additional π/2 pulse is incorporated after the evolution time (t1) to store the 31P magnetization along the z-axis, followed by a mixing time (tmix), during which proton spin diffusion can occur. Magnetization is exchanged only between homonuclear dipolar-coupled phosphorus species, and their separation can be probed by varying the mixing time. The 31 P31P exchange sequence produces homonuclear correlated spectra giving rise to off-diagonal intensities at positions where nuclei undergo chemical exchange or spin diffusion during the mixing time tmix. 2D 31P radio frequency driven dipolar recoupling (RFDR) experiments34 were also carried out at a spinning speed of 10 kHz. The 2D 31P RFDR NMR experiment is initiated by cross polarization via 1H spins followed by an evolution period, t1. A pair of π/2 pulses flanking the longitudinal mixing period tmix prepares the spin polarizations for exchange and returns them to the transverse plane for further evolution during t2. Rotorsynchronized π pulses (one per rotor period) were applied during the mixing period to induce exchange via homonuclear dipoledipole couplings which are otherwise eliminated by MAS alone. 2D 31P31P Single Quantum-Double Quantum (SQ-DQ) MAS NMR. We have tried to use POST-C7 recoupling sequence35 to reintroduce the possible 31P31P homonuclear dipolar interaction under MAS conditions. The 31P magnetization is first transferred from the 1H spins during the contact time via a variable-amplitude CP process. To achieve an efficient broadband excitation and reconversion of DQ coherences, various excitation and reconversion periods in the range of 0.71.4 ms were used under conditions of an rf field strength (ωrf) of 58.8 kHz for 31P at a spinning speed of 8.4 kHz (ωr) to meet ωrf = 7ωr. Density Functional Theory Calculations. To provide the structural information for the simulation of the REDOR NMR results, several motifs such as T3Q3Q4, T3Q4Q3 (without and with hydrogen bonds) and T3T3 were constructed and energy minimized using density functional theory (DFT). Each peripheral Si atom of these two motifs was capped and saturated with hydrogen atoms. DFT calculations within the suite of Gaussian 03 programs30 were performed by using the B3LYP density functional, Becke’s three-parameter exchange functional,36 and LeeYangParr gradient-corrected correlation37 functional with moderate-sized 6-31G(d,p) basis set.38 The 31P, 29Si, and 13C NMR chemical shifts were calculated at the B3LYP/6-311+(2d,p)//B3LYP/6-31G(d,p) level for the motifs proposed.

’ RESULTS AND DISCUSSION Structural Characterization. In an earlier study,14 only SBA-

15 functionalized with 10 mol % of PETES was reported. It has been recognized that the content of organic groups is a key factor that determines many important properties of the mesoporous silica materials, such as adsorption capacity for metal ions, 1660


The Journal of Physical Chemistry C

ARTICLE

enhanced hydrothermal stability, and surface reactivity and hydrophobicity. 4,5 Therefore, there is a need to explore the achievable loadings of phosphonic acid groups in the SBA-15 materials that still retain their ordered structures. Figure 1 shows the XRD patterns of P-x-y as a function of PETES contents and different sample treatments. All the XRD patterns are similar and exhibit an intense (100) peak, associated with two small weak (110) and (200) diffraction peaks, characteristic of p6mm symmetry. The d100 spacing and a0 unit cell parameters are summarized in Table 1. These parameters changed slightly for the template-extracted sample before and after acidification, suggesting the good stability of the materials. The cell parameter a0 decreases with increasing the PETES contents. This implies that PETES may slightly disturb the formation of the micelles.

Figure 2 displays the nitrogen adsorptiondesorption isotherms of P-x-re (x = 1025). All the isotherms are of type IV isotherms with clear H1-type hysteresis loops at high relative pressure, according to the IUPAC classifications, which is typically observed for SBA-15 type materials. The results indicated the presence of large mesopores with narrow pore size distributions. A significant shrinkage in mesopores was observed as the PETES contents were increased. As seen in Table 1, a slight decrease in the surface area and pore volume was observed when the template-extracted samples (P-x-re) were further treated with HCl (i.e., acidification) to become acidified samples (P-x-ac). The textural properties of the materials studied are listed in Table 1. The microporosity is small as compared to the pore volume for all the samples. Basically, these structural properties are consistent with the reported values in the literature.14 The TEM images (Figure 3) of the P-20-ac sample recorded in the (100) and (110) directions showed a well-ordered 2D hexagonal (p6mm) mesostructure. From the bright-dark contrast in the TEM image, the pore size (67 nm) of P-20-ac is in good agreement with the results of nitrogen sorption measurements (Table 1).

Figure 1. Powder XRD patterns of P-x-re as a function of PETES content, where x = (a) 10, (b) 15, (c) 20, and (d) 25. The XRD patterns of P-10-ac and P-20-ac (after acidification with HCl) are shown in the insets.

Figure 2. Nitrogen adsorptiondesorption isotherms of P-x-re as a function of PETES content, where x = (a) 10, (b) 15, (c) 20, and (d) 25. The black and red portions represent the adsorption and desorption branches of the isotherms.

Table 1. Structural and Textural Properties of P-x-y after Template Extraction (re) and Acidification (ac) sample

d100 (nm)

a0 (nm)a

ABET (m2/g)b

Vp (cm3/g)c

micropore (cm3/g)d

pore size (nm)

P-10-re P-10-ac

11.2 11.2

13.0 13.0

515 472

1.24 1.09

0 0.010

7.1 7.1

P-15-re

11.2

13.0

485

1.00

0.021

7.4

P-15-ac

11.2

13.0

483

0.99

0.019

7.3

P-20-re

12.3

14.2

451

0.57

0.022

7.5

P-20-ac

12.3

14.2

380

0.52

0.019

7.2

P-25-re

15.4

17.8

497

0.40

0.051

3.6

P-25-ac

15.7

18.1

507

0.42

0.087

3.5

√ Lattice parameters a0 were calculated based on the formula a0 = 2d100/ 3. b ABET: BET surface area. c Vp: pore volume. d The micropore properties were obtained from the t-plot analysis. a

1661



ARTICLE

Figure 4. 13C CPMAS NMR spectra of (a) P-20-re and (b) P-20-ac. The spectra were acquired at a spinning speed of 6 kHz and a recycle delay of 3 s. Figure 3. TEM images of P-20-ac: (a) in the direction parallel to the pore axis and (b) in the direction perpendicular to the pore axis. 13

C CPMAS NMR is often used to confirm the presence of organic functional moieties and the removal of surfactant species from the templateextracted samples. In the present case, it is also a powerful tool to determine whether the ethoxy groups on the 31P site of the phosphonate groups are hydrolyzed to become the desired phosphonic acid functional groups before and after acidified treatment. As shown in Figure 4a, there are additional peaks at 62.2 and 15.4 ppm due to the ethoxy groups on the 31P site observed for the template-extracted sample before acidification (i.e., P-x-re). The peaks at 3.3 and 17.3 ppm are ascribed to the α and β carbons to the silicon in the SiCH2CH2P moiety, respectively. After treatment with HCl, the disappearance of the peaks at 62.2 and 15.4 ppm confirms that all the ethoxy groups have been successfully converted to hydroxyl groups as expected (Figure 4b). The 31P MAS NMR spectrum (Figure 5a) of the as-synthesized P-20-as shows a single resonance peak at 33 ppm, in agreement with previous NMR studies.14 This indicates that the ethoxy groups of the phosphonate functional groups are not hydrolyzed at the as-synthesized state. After template removal or acidification (Figure 5, b and c), the original 31P peak at 33 ppm slightly shifts to 32 ppm and an additional peak at 22 ppm was observed. The observed 31P NMR peaks clearly indicate that there are two distinct local environments of the 31P nuclei after treatment of template removal and acidification. The peak Nature and Status of Phosphonate Groups.

shift from 33 to 32 ppm is due to the transformation of phosphonate (PO(OEt)2) groups to phosphonic acid groups (PO(OH)2 or PO(OH)(OEt)). The peak at 22 ppm could be due to the product of side reactions between PO3H2 groups and nearby silanol (SiOH) groups (Scheme 2a).14,15 Alternately, this peak was attributed to the phosphonic acid groups that are engaged in hydrogen bonding in an earlier report.39 The relative intensities between the peaks at 32 and 22 ppm are 0.71:0.29 and 0.65:0.35 for P-20-re and P-20-ac, respectively, suggesting an increase in the intensity of the latter 31P species after treatment with HCl. Additional experiments were performed to clarify the nature of phosphorus-containing species for the 31P peak at 22 ppm. Since it was proposed to be associated with the phosphonic acid groups that are engaged in hydrogen bonding in an earlier report,39 the 31 P MAS NMR spectra for the P-20-ac sample with “partial dehydration” and “full dehydration” were recorded. The sample with full dehydration was prepared by heating the sample under a vacuum line at 110 °C for at least 24 h, while the partial dehydrated sample was thermally treated for a shorter period of 2 h. As seen in Figure 6, the intensity of the peak at 22 ppm is higher than that of its rehydrated analogue when the sample was partially dehydrated and became dominated after full dehydration. The appearance of spinning sidebands, due to the chemical shift anisotropy of the 31P spin, indicates the dehydration has been achieved. If the 22 ppm is associated with water molecules present in the sample, its intensity in theory should be decreased after sample dehydration. On the contrary, an increase in the relative intensity of the 22 ppm to the 32 ppm was observed after 1662



Figure 5. 31P MAS NMR spectra of P-20-y, where y = (a) as, (b) re, and (c) ac, acquired with a recycle delay of 100 s and at a spinning speed of 10 kHz. Asterisks denote spinning sidebands.

Scheme 2. Two Possible Pathways for the Conversion of the Q3 Species to the Q4 Species after Solvent Extraction, Followed by Acidification

dehydration. Close examination of the spectra revealed the presence of an additional species centered at ∼7 ppm. It may be due to the species B having hydrogen bonds with the Q2 species instead of the Q3 species. After rehydration, the relative intensity of the 22 ppm to the 32 ppm changed back to its hydrated form, suggesting the behavior is reversible. The rehydration was performed on the sample in contact with the moisture from a saturated NaCl solution for several days. By contrast, the 13C CPMAS NMR spectra are insensitive to the moisture within the sample. This observation suggests that the 22 ppm is due to the phosphonic acid groups that are engaged in hydrogen bonding with the silanol groups (SiOH) present in the sample. When the hydrated sample was subjected to

ARTICLE

Figure 6. 31P MAS NMR spectra of P-20-ac at different stages: (a) hydrated, (b) partially dehydrated, and (c) fully dehydrated, acquired with a recycle delay of 100 s and at a spinning speed of 10 kHz. Asterisks denote spinning sidebands.

dehydration, the water molecules surrounded around the phosphonic acid groups were removed, and most likely the phosphonic acid groups have more chances to contact with nearby Q3 to form hydrogen bonds. The same experiments were performed on the sample at different stages, namely P-20-as and P-20-re. Since P-20-as only contains PO(OEt)2 groups, the peak at 22 ppm was barely observable after the sample was fully dehydrated. On the other hand, the intensity of the peak at 22 ppm was slightly increased for the dehydrated P-20-re sample, since a few of PO(OEt)2 groups have become PO(OH)(OEt) groups, which has the ability to form hydrogen bonds or to react with the silanol groups (SiOH) on the mesopore surface. Framework Compositions. The presence and loading levels of the functional groups and the degree of silica condensation, one of the important parameters to determine the stability of the silica framework, in the samples under various treatments are better probed by 29Si MAS NMR spectroscopy. Five major signals at 92, 101, and 110 ppm, corresponding to Q2 (Si(OSi)2(OH)2), Q3 (Si(OSi)3(OH)), and Q4 (Si(OSi)4) species, and at 66 and 55 ppm, corresponding to T3 (RSi(OSi)3, where R refers to organic functional group) and T2 (RSi(OSi)2(OH)) sites, were observed, as shown in Figure 7, and their relative intensities are summarized in Table 2. The observation of T groups indicates the presence of organosilane groups in the material. The intensity of T groups increases with the concentration of PETES in the initial composition, indicating that the incorporated functional groups in the mesoporous silica materials is proportional to the PETES contents added into the synthesis mixture. For the samples after the surfactant removal, there is a significant change in the relative intensities between T2 and T3, and between Q3 and 1663



ARTICLE

Figure 8. 1H f 31P f 29Si double cross-polarization NMR spectra of P-20-ac as a function of the 31P f 29Si contact time (CT2): (a) 5 ms and (b) 20 ms, which were obtained with 65 000 and 42 000 transients, respectively. The spectra were acquired with a recycle delay of 5 s and at a spinning speed of 5 kHz.

Figure 7. 29Si MAS NMR spectra of (A) P-10-y and (B) P-20-y, where y = (a) as, (b) re, and (c) ac. The dashed lines are the components used for spectral deconvolution.

Table 2. Relative Intensities of Tm and Qn Groups Obtained from the 29Si MAS NMR Spectra for the Various Samples T1 (%)

T2 (%)

T3 (%)

Q2 (%)

Q3 (%)

Q4 (%)

Tm/(Tm + Qn) (%)

x

y

10

as

3.1

4.0

4.7

3.1

28.5

56.6

11.7

re

4.7

6.5

1.2

22.5

65.1

11.2 11.7

20

ac

0.2

11.7

3.2

22.3

62.7

as

3.0

7.1

12.3

3.8

28.8

45.1

22.3

re

6.8

14.7

4.6

27.9

46.0

21.5

ac

0.4

19.2

1.2

21.8

57.4

19.6

Q4 groups. This implies that the solvent extraction process induces further condensation of silanol groups. Based on distinct T3 and T2 signals in the 29Si MAS NMR spectra of all samples, the relative integrated intensities of Qn and Tm NMR signals allows the quantitative assessment of the incorporation degree of the organic moiety. From these normalized peak areas, the ratios of Tm/(Tm + Qn) are in close agreement with those expected based on the composition of the initial mixture (Table 2). For the template-extracted sample treated with HCl, it is clear that higher Q4/Q3 values are obtained, accompanied by a significant decrease in the intensity of Q2 species. The exact relative concentrations of T3 and Q species are essential for accurate REDOR data analysis. The measured values for the total Tm species to Q3 and Q4 are 1:1.30 and 1:2.02 for P-20-as and 1:1.11 and 1:2.93 for P-20-ac, respectively. There is 11.4% increase in Q4 when the template-extracted P-20-re sample is treated with HCl to form the acidified P-20-ac sample. Two possible pathways

for such an increase are proposed in Scheme 2. One is the formation of cyclic POSi (Q4) resulting from the side reaction of PO3H2 groups and nearby Q2 and/or Q3 groups, and the other is the further condensation of Q2 and Q3 silanols, as illustrated in parts a and b of Scheme 2, respectively. To distinguish these pathways, a control experiment was performed to synthesize pure silica SBA-15 without the phosphonic functional groups but subjected to the same treatment for template removal and acidification. As shown in Figure S1 (Supporting Information), there is a slight increase in the Q4/Q3 ratio for the pure silica SBA-15 after acidification, suggesting the contribution of the increase in Q4 due to the second pathway is not negligible. On the basis of the observation of the additional 31P NMR peak at 22 ppm, we can conclude that both pathways contributed to the increase in the intensity of the Q4 species after treatment with solvent extraction and HCl. Proximity between 31P and 29Si Spins. To further clarify the exact nature of the species at 22 ppm, 1H f 31P f 29Si double cross-polarization NMR experiments were performed on P-20-ac to probe the proximity between 31P and 29Si spins as a function of the 31P f 29Si contact time (CT2), and the corresponding NMR spectra are shown in Figure 8. As shown in Figure 8, two major peaks at 66 and 110 ppm, corresponding to the T3 and Q4 species, were observed with a short contact time of 5 ms. Some shoulders around 55 and 100 ppm, corresponding to the T2 and Q3 species, were also observable when the 31P f 29Si contact time was prolonged. As expected, the P atoms of PETES are in close proximity to the Tm species since PETES itself has the T sites which contain P atoms. The distance between 31P and 29Si for PETES is around 4.5 Å according to the DFT calculations. Since the Qn species are also observed, one should expect that the distances for the P atoms in PETES to the Si atoms in the Qn sites are not too far away than 4.5 Å. Since each T site can be surrounded by two or three Qn species, the observed Qn signals are more intense as compared to that of Tm species. The poor resolution for the Qn double CP NMR signals cannot allow us to distinguish which Qn species is closer to the T site. Nevertheless, the double CP NMR results confirm that the P atoms of PETES are in close proximity to the Qn species. Although a 2D 31P f 29 Si HETCOR (heteronuclear correlation) experiment could resolve this issue, it is not experimentally feasible because each 1 H f 31P f 29Si double CPMAS NMR spectrum takes an extensive spectrometer time. 1664



ARTICLE

Scheme 3. Motifs Proposed for the Calculations of 31P, 29Si, and 13C NMR Chemical Shiftsa

a

The PSi distance information is for REDOR NMR analysis only (see Figure 12).

Table 3. Calculated 31P, 29Si, and 13C NMR Chemical Shifts (in ppm) for the Motifs Proposed in Scheme 3 δ (29Si)b species

δ(31P)a

δ(13C)b T3

Q

SiC

CP

A (free)

32.0

106.95

65.59

8.96

25.39

B (H-bonded)

24.2

107.39

74.88

9.39

25.59

C (cyclic POSi)

38.1

108.83

65.33

9.01

26.63

D (POP)

36.9; 36.4

73.66; 72.85

9.77; 9.66

26.30; 27.85

a

The 31P chemical shift for the free PO3H2 species is set to 32 ppm in order to be consistent with the experimental observation. b Both 29Si and 13C chemical shifts are referenced to TMS (0 ppm).

Possibility of POP Species for the 22 ppm Peak. Four possible local environments for the P atoms in PETES are proposed in Scheme 3, namely, species A for free PO3H2 without hydrogen bonds, species B for PO3H2 with hydrogen bonds with SiOH, species C for the formation of cyclic POSi species, and species D for the formation of POP species. These motifs were used for the calculations of NMR chemical shifts (for 31P, 29Si, and 13C) and the results are listed in Table 3. As seen in Table 3, the calculated 31P chemical shift of the species B is 24.2 ppm, which is very close to the experimental observation at 22 ppm. The other species C and D, on the other hand, exhibited a downfield shift relative to the species A. The predicted NMR chemical shifts suggest that the species B (hydrogen bonded) is the most possible species responsible for the peak at 22 ppm observed experimentally. The formation of POP species (species D) could be possible if the PETES loadings are high (for example, 20%). In theory, one should expect more POP species to be formed as the PETES loading goes up. This can be easily checked by looking at the 31P MAS NMR spectra as a function of the PETES loading (5, 10, and 20%). As shown in Figure S2 in the Supporting Information and in Figure 5c, all the acidified samples exhibited similar spectral features. The intensity ratio for the 22 ppm peak to the 32 ppm peak is 40:60 for both P-5-ac and

P-10-ac (Figure S2), which is slightly higher than that of P-20-ac (35:65, see Figure 5c). Even though P-5-ac and P-10-ac have a PETES loading diluted with a factor of 4 and 2, respectively, as compared to P-20-ac, the intensity of the peak at 22 ppm did not exhibit a quantitative manner. This observation suggests that the 22 ppm peak is unlikely due to the formation of the POP species (species D in Scheme 3). Another straightforward experiment to verify the presence of the POP species for the 22 ppm peak is 31P double quantum dipolar based NMR. Ideally, only the 22 ppm peak can be present in such a SQ-DQ correlation spectrum if it corresponds to a POP bond. We have utilized POST-C7 recoupling sequence35 to reintroduce the possible 31P31P homonuclear dipolar interaction under MAS conditions with excitation and reconversion periods in the range of 0.71.4 ms. However, no signal was observed for the first slice (t1 = 1 μs) with various excitation and reconversion periods, as shown in Figure S3 in the Supporting Information. Therefore, the possibility of the formation of the POP species can be excluded based on the combined results of 31P MAS and DQ dipolar based NMR experiments and DFT calculations. 31 P29Si Distances from DFT Structures. The DFT results will be the guideline to simulate the 31P29Si REDOR NMR data. 1665



Figure 9. Optimized structures of DFT calulations of (a) T3Q3Q4 and (b) T3Q4Q3 motifs without hydrogen bond formation between T3 and various Q species.

Thus, DFT calculations were performed on the two proposed framework compositions, T3Q3Q4 and T3Q4Q3 motifs, in order to gain the distance information about the phosphonate functional groups and various Si units. The optimized structures of these two motifs without hydrogen bond formation are shown in Figure 9. It shows that the distance for the P atom to the Si atom of T3 is 4.5 Å. The distances of the P atom on T3 species to the Si atoms on Q3 and Q4 are 5.509 and 7.134 Å, respectively, for the case of T3Q3Q4. On the other hand, the distances of the P atom on T3 species to the Si atoms on Q4 and Q3 are 5.682 and 8.351 Å, respectively, for the case of T3Q4Q3, which make this motif unlikely because the latter distance is too far to have significant 29Si{31P} REDOR fractions for the Q3 species. Therefore, the T3Q4Q3 motif can be excluded if significant 29 Si{31P} REDOR fractions for the Q3 species were observable. 29 Si{31P} REDOR NMR for the As-Synthesized P-20-as Sample. NMR methods for measuring internuclear distances are based on the determination of the dipolar coupling between two spins. In the absence of any motion, this dipolar coupling is inversely proportional to the cube of the distance between the spins. However, the dipolar coupling, even between nearby spins, is often very small on the time scale of many motional processes (e.g., a 31 P29 Si distance of 5 Å corresponds to a dipolar coupling of 77.4 Hz). Thus, any residual motion at or faster than this time scale (generally more than 110 ms) will reduce or, for isotropic motion, average the measured dipolar coupling constant to zero, resulting in a longer than expected distance.

ARTICLE

Extensive efforts have been devoted to analyze REDOR NMR data for the samples with 10% and 20% PETES loadings at different status. We have chosen P-20-y (y = as and ac), instead of P-10-y, for detailed REDOR analysis. The reasons are provided as follows. First, the PETES loading of P-10-y is diluted by a factor of 2 as compared to P-20-y. As a result, the REDOR effects are smaller. Moreover, the REDOR effects for the Tm species suffer less S/N ratios because the intensity of Tm is much lower in the case of P-10-y. Second, on the basis of 29Si MAS NMR results (Table 2), the ratios of the total Tm species to the Q3 and Q4 species are 1:1.11 and 1:2.93 for P-20-ac, which are beneficial to the REDOR simulation by considering a simple model system. On the other hand, the ratios of the total Tm species to the Q3 and Q4 species are close to 1:1.87 and 1:5.27 for P-10-ac. This suggests that a large portion of Q 3 and Q4 is not directly surrounded by the Tm species, whose NMR intensities are not affected by REDOR. The unaffected Q3 and Q4 should be considered as background signals and thus a dilution factor, which is difficult to be determined without a sophisticated model system, must be included for the REDOR analysis. Therefore, it imposes further limitation for the accuracy of the REDOR analysis. The situation becomes worse in the case of the fully dehydrated P-10-ac sample because its Qn species are less intense due to a poor CP transfer from the 1H spins. The poor S/N ratios cannot allow an accurate analysis. Besides, the resolution of the Q3 and Q4 for fully dehydrated P-10-ac sample is not as good as that of the hydrated one. Third, both P-10-ac and P-20-ac exhibited similar local environments for the 31P spins, as revealed by their 31P MAS NMR spectra (Figure 5 and Figure S2 in the Supporting Information). Moreover, the possibility of the formation of the POP species has been excluded. Therefore, detailed REDOR analyses were performed on P-20-as and P-20-ac in order to gain more insights into the framework distribution and local environments of the phosphonic functional groups incorporated in SBA-15. 29 Si{31P} REDOR NMR experiments of P-20-as were first performed at room temperature since there is only one distinct 31 P local environment of this sample. Representative 29Si{31P} REDOR NMR spectra are shown in Figure 10. The distance information obtained from the DFT structures can be used as guidelines for fitting the experimental REDOR NMR data. The reduction in the intensity of the 29Si echo (ΔS/S0) (or REDOR fraction) extracted from spectra as a function of dephasing time is shown in Figure 11. For the case of T species, a 1:1 molar ratio of P:Si is used because each PETES group contains one Si T site and one P site. However, the best fit can only be obtained by using a PSi distance of 4.5 Å (Figure 11a), in accordance with the DFT results, accompanied by a scaling factor of 0.55 to take into account of molecular averaging. It is expected that there are some molecular motions due to the carbon chain of PETES. For the case of Q3 species, on the other hand, the best fit to the REDOR data collected at room temperature was obtained with a dipolar coupling constant of 55 Hz which, assuming no motional averaging, corresponds to a SiP distance of 5.6 Å. This value can be considered as the maximum PSi distance for the T species from PETES to the nearby Q3 silanol species. If the molecular averaging is considered as the case for T species, a much shorter PSi distance of 5.0 Å was obtained (blue line in Figure 11a). Since the T3:Q3 ratio is 1:1.30, a fraction of 30% Q3 should be located in the second sphere of T3, which assumes a PSi distance of around 8 Å according to the DFT calculation results. From the analysis of REDOR data, it can be concluded that the PSi 1666



Figure 10. 29Si{31P} REDOR NMR spectra of P-20-as: control (S0, black) and dephasing (Sf, red) with a dephasing time of 10 ms. The observed 29Si NMR signals were enhanced by the transfer of 1H magnetization via cross polarization. The NMR spectra were acquired at a spinning speed of 5 kHz.

distance between the T species and the Q3 species is in the range of 5.05.6 Å, which is in relatively good agreement with the DFT calculation results for the T3Q3Q4 motif (Figure 9a). Similarly, the REDOR NMR data for the Q4 species can be fitted based on the fact that the T3:Q4 ratio is 1:2.02. As shown in Figure 11c,d, each PETES group should have two Q4 species as the first sphere of neighbors and a fraction of only 2% Q4 as the second sphere of neighbors. Therefore, a factor of 2 is needed to account for the observed REDOR fractions. The distance of 6.1 Å for the P site in PETES to the Si atom in Q4 can give a reasonable fitting, as shown in Figure 11c. The above REDOR NMR analysis is based on the isolated 31 P29Si spin pair. It is often assumed that the one-pot synthesis route ensures a uniform distribution of organic functional groups in mesoporous silica materials. Therefore, a homogeneous distribution of PETES groups was assumed for the fitting of the REDOR NMR data for the first place. In the unfortunate case, two PETES groups could be close to each other and any significant 31P31P homonuclear coupling will degrade the accuracy of REDOR analysis. Since the peak at 22 ppm results from the initial peak at 32 ppm, a simple 31P31P 2D exchange NMR experiment was performed on P-20-ac to check this possibility. As shown in Figure S4 (Supporting Information), the 32 and 22 ppm peaks have no significant cross peaks even at a long mixing time of 800 ms. This observation confirms that the PETES group is distant from each other. The 2D 31P RFDR NMR spectrum (Figure S5 in the Supporting Information) consists of only the signals along the diagonal, which do not provide information about connectivities. The absence of cross peaks indicates that the 31P spins in the species for the peaks at 32 and 22 ppm are not strongly dipolar coupled to each other. In comparison to our previous study,40 since SBA-15 has a larger pore size than that of SBA-1, we expect that PP distance may be over 10 Å. In the present work, therefore, homonuclear dipolar interactions are ignored because the 31P31P and 29Si29Si couplings are expected to be much less than the sample spinning speeds employed (5 kHz).

ARTICLE

Figure 11. 29Si{31P} REDOR fractions of (a) T3, (b) Q3, and (c) Q4 species for P-20-as as a function of the dephasing time obtained at room temperature. For part a, the simulated curves (red and blue) are the data calculated using the distance of 4.5 Å multiplied without and with a factor of 0.55 (due to the effects of molecular motion), respectively. For part b, the simulated curves were calculated as follows: ΔS/S0 = 1.0 [ f(5.6 Å) + 0.3 f(8.0 Å)] (red) and ΔS/S0 = 0.55 [f(5.0 Å) + 0.3 f(8.0 Å)] (blue), where f(r Å) is the REDOR fractions caused by a PSi distance of r Å. For part c, the simulated curve (blue) was calculated from ΔS/S0 = 0.55 [2 f(6.1 Å) + 0.02 f(8.0 Å)]. (d) structural information obtained from the REDOR NMR results.

One typical way to reduce the molecular motion is to perform the experiments at low temperatures, where the translational motion of the molecules will be frozen out. Upon cooling the sample from room temperature to 100 °C, similar REDOR fractions were, however, observed, for the same dephasing time. This indicates that there is residual motion at or faster than this REDOR time scale, which cannot be frozen out even at 100 °C. Any remaining motion (e.g., librations) of the functional moiety on the surface will reduce the measured dipolar coupling. Since a difference in the measured distance is observed between PO3H2 and its own Si site (T site), the error due to any residual motion or small librational motions is believed to be significant. It is not possible at this moment to totally exclude the possibility that small librational motions may result in a measured distance that is slightly longer than the actual distance. Therefore, in the present case it may be appropriate to consider the present 29Si{31P} REDOR NMR experiment as a method for determining the maximum internuclear distance between spins. REDOR Effects for P-20-ac. As two distinct 31P resonances, i.e., 32 and 22 ppm, were observed for P-20-ac, the analysis of REDOR data became more complicated. We have performed a whole series of simulations to examine the effects of other coupled PSi systems on the PSi distance as measured by REDOR NMR. In all these simulations, we have used the structural information obtained from the motifs A, B, and C proposed in Scheme 3 using a similar methodology for the case of P-20-as. The ratio of species A to the sum of species B and C is 65:35, as determined from the 31P MAS NMR spectrum (Figure 5c). The ratio between species B and C is determined by fitting the 1667



ARTICLE

calculations are valuable for understanding the framework distribution and compositions in organic functionalized mesoporous silica materials.

’ ASSOCIATED CONTENT

bS

29

Si NMR for pure silica SBA15 (Figure S1), P MAS NMR spectra, SQ-DQ, 2D exchange and RFDR NMR spectra (Figures S2S5). This material is available free of charge via the Internet at http://pubs.acs.org. Supporting Information. 31

’ AUTHOR INFORMATION Corresponding Author

*Fax: +886-3-4227664. E-mail: [email protected].

’ ACKNOWLEDGMENT The financial support of this work by the National Science Council of Taiwan is gratefully acknowledged. ’ REFERENCES

Figure 12. 29Si{31P} REDOR fractions of (a) T3, (b) Q3, and (c) Q4 species for P-20-ac as a function of the dephasing time obtained at room temperature. The simulated curves (red) were calculated based on the equation listed on the right. The PSi distances are obtained from the structures of species A (free PO3H2), B (H-bonded PO3H2), and C (cyclic POSi) illustrated in Scheme 3. For part b, the species C is not considered for the simulation and the contribution of the second sphere Q species was ignored. A similar methodology for the case of P-20-as is applied: a dynamic scaling factor of 0.55 is used for the species A, whereas no mobility (i.e., a dynamic scaling factor of 1) for the species B and C was assumed. The details for the simulation are listed in the equations shown on the right.

REDOR data for the T3 species. As seen in Figure 12, the fitting for the REDOR data of the T3 species is good if the ratio of species B to C is 20:15. Using this ratio and the distance information provided in Scheme 3 for species B and C, the REDOR data for the Q3 and Q4 species can be fit relatively well.

’ CONCLUSIONS A combination of solid-state NMR experiments and DFT calculations was employed to probe the local structure and framework distribution of phosphonic acid functionalized mesoporous silica SBA-15 materials. The NMR distance measurements involving an isolated spin pair such as 31P29Si allowed useful distance information to be obtained. A PSi distance of 5.0 Å was determined for the P site in the phosphonate functional groups to the Q3 species in the silica framework from the 29 Si{31P} REDOR NMR data of the as-synthesized sample. After template removal and acid treatment, the phosphonic acid groups having hydrogen bonds with the nearby Q3 species are the major species responsible for the 31P peak observed at 22 ppm. The combined results of REDOR and 1H f 31P f 29Si double cross-polarization NMR measurements and the DFT

(1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Burkett, S. L.; Sims, S. D.; Mann, S. Chem. Commun. 1996, 1367. (3) Macquarrie, D. J. Chem. Commun. 1996, 1961. (4) Stein, A.; Melde, B. J.; Schroden, R. C. Adv. Mater. 2000, 12, 1403. (5) Sayari, A.; Hamoudi, S. Chem. Mater. 2001, 13, 3151. (6) Lim, M. H.; Blanford, C. F.; Stein, A. J. Am. Chem. Soc. 1997, 119, 4090. (7) Lim, M. H.; Stein, A. Chem. Mater. 1999, 11, 3285. (8) Asefa, T.; Kruk, M.; MacLachlan, M. J.; Coombs, N.; Grondey, H.; Jaroniec, M.; Ozin, G. A. Adv. Funct. Mater. 2001, 11, 447. (9) Margolese, D.; Melero, J. A.; Christiansen, S. C.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 2000, 12, 2448. (10) Feng, X. D.; Fryxell, G. E.; Wang, L.-Q. ; Kim, A. Y.; Liu, J.; Kemner, K. M. Science 1997, 276, 923. (11) Yang, C.-M.; Zibrowius, B.; Sch€uth, F. Chem. Commun. 2003, 1772. (12) Tsai, H.-H. G.; Jheng, G.-L. ; Kao, H.-M. J. Am. Chem. Soc. 2008, 130, 11566. (13) Tsai, C.-T. ; Pan, Y.-C.; Ting, C.-C.; Vetrivel, S.; Chiang, A. S. T.; Fey, G. T. K.; Kao, H.-M. Chem. Commun. 2009, 5018. (14) Corriu, R. J. P.; Datas, L.; Guari, Y.; Mehdi, A.; Reye, C.; Thieuleux, C. Chem. Commun. 2001, 763. (15) Yang, Q.; Yang, J.; Liu, J.; Li, Y.; Li, C. Chem. Mater. 2005, 17, 3019. (16) Wang, P.; Zhao, L.; Wu, R.; Zhong, H.; Zou, H.; Yang, J.; Yang, Q. J. Phys. Chem. C 2009, 113, 1359. (17) Mauder, D.; Akcakayiran, D.; Lesnichin, S. B.; Findenegg, G. H.; Shenderovich, I. G. J. Phys. Chem. C 2009, 113, 19185. (18) Jin, Y. G.; Qiao, S. Z.; Xu, Z. P.; Yan, Z. M.; Huang, Y. N.; da Costa, J. C. D.; Lu, G. Q. J. Mater. Chem. 2009, 19, 2363. (19) McKeen, J. C.; Yan, Y. S.; Davis, M. E. Chem. Mater. 2008, 20, 5122. (20) Marschall, R.; Bannat, I.; Caro, J.; Wark, M. Microporous Mesoporous Mater. 2007, 99, 190. (21) Gullion, T.; Schaefer, J. J. Magn. Reson. 1989, 81, 196. (22) Gullion, T.; Schaefer, J. In Advances in Magnetic Resonance; Warren, W. S., Ed.; Academic Press: New York, 1989; Vol. 13, p 57. (23) Shir, I. B.; Kababya, S.; Amitay-Rosen, T.; Balazs, Y. S.; Schmidt, A. J. Phys. Chem. B 2010, 114, 5989. (24) (a) Wickham, J. R.; York, S. S.; Rocher, N. M.; Rice, C. V. J. Phys. Chem. B 2006, 110, 4538. (b) van W€ullen, L.; K€oster, T. K.-J.; Wiemh€ofer, H.-D. ; Kaskhedikar, N. Chem. Mater. 2008, 20, 7399. 1668



ARTICLE

(25) (a) Huang, W.; Varani, G.; Drobny, G. P. J. Am. Chem. Soc. 2010, 132, 17643. (b) Mani, R.; Tang, M.; Wu, X.; Buffy, J. J.; Waring, A. J.; Sherman, M. A.; Hong, M. Biochemistry 2006, 45, 8341. (26) (a) Kao, H.-M. ; O’Connor, R. D.; Mehta, A. K.; Huang, H.; Poliks, B.; Wooley, K. L.; Schaefer, J. Macromolecules 2001, 34, 544. (b) Kao, H.-M. ; Stefannescu, A. D.; Wooley, K. L.; Schaefer, J. Macromolecules 2000, 33, 6214. (c) Kao, H.-M. ; Liu, H.; Grey, C. P.; Jiang, J.-C.; Lin, S.-H. J. Phys. Chem. B 2000, 104, 4923. (d) Huo, H.; Peng, L.; Grey, C. P. J. Phys. Chem. C 2011, 115, 2030. (27) Ganapathy, S.; Fournier, M.; Paul, J. F.; Delevoye, L.; Guelton, M.; Amoureux, J. P. J. Am. Chem. Soc. 2002, 124, 7821. (28) (a) Zhang, L.; de Araujo, C. C.; Eckert, H. J. Phys. Chem. B 2007, 111, 10402. (b) Zhang, L.; Eckert, H. J. Phys. Chem. B 2006, 110, 8946. (29) (a) Coelho, C.; Babonneau, F.; Bonhomme-Coury, L.; Maquet, J.; Laurent, G.; Bonhomme, C. J. Sol-Gel Sci. Technol. 2006, 40, 181. (b) Babonneau, F.; Baccile, N.; Laurent, G.; Maquet, J.; Aza€is, T.; Gervais, C.; Bonhomme, C. C. R. Chim. 2010, 13, 58. (c) Coelho, C.; Aza€is, T.; Bonhomme, C.; Bonhomme-Coury, L.; Boissiere, C.; Laurent, G.; Massiot, D. C. R. Chim. 2008, 11, 387. (d) Lejeune, C.; Coelho, C.; Bonhomme-Coury, L.; Aza€is, T.; Maquet, J.; Bonhomme, C. Solid State Nucl. Magn. Reson. 2005, 27, 242. (e) Coelho, C.; Bonhomme-Coury, L.; Laurent, G.; Bonhomme, C. Inorg. Chem. 2007, 46, 1379. (f) Coelho, C.; Aza€is, T.; Bonhomme-Coury, L.; Maquet, J.; Bonhomme, C. C. R. Chim. 2006, 9, 472. (30) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, Jr. T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision D.02; Gaussian Inc.: Wallingford, CT, 2004. (31) Peersen, O. B.; Wu, X.; Kustanovich, I.; Smith, S. O. J. Magn. Reson. Ser. A 1993, 104, 334. (32) Wolfram, S. Mathematica. A System for Doing Mathematics by Computer; Addison-Wesley: Reading, MA, 1991. (33) (a) Mueller, K. T.; Jarvie, T. P.; Aurentz, D. J.; Roberts, B. W. Chem. Phys. Lett. 1995, 242, 535. (b) Mueller, K. T. J. Magn. Reson. Ser. A 1995, 113, 81. (34) Bennet, A. E.; Ok, J. H.; Griffin, R. G.; Vega, S. J. Chem. Phys. 1992, 96, 8624. (35) Hohwy, M.; Jakobsen, H. J.; Eden, M.; Levitt, M. H.; Nielsen, N. C. J. Chem. Phys. 1998, 108, 2686. (36) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (37) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (38) Petersson, G. A.; Al-Laham, M. A. J. Chem. Phys. 1991, 94, 6081. (39) Aliev, A.; Ou, D. L.; Ormsby, B.; Sullivan, A. C. J. Mater. Chem. 2000, 10, 2758. (40) Kao, H.-M.; Chiu, P.-J. ; Jheng, G.-L.; Kao, C.-C.; Tsai, C.-T.; Tsai, H.-H. G.; Chou, Y.-K. New J. Chem. 2009, 33, 2199.

1669


Probing the Nature and Local Structure of Phosphonic Acid Groups

Recommend Documents