Solid-State NMR Studies of the Formation of Monomers and Dimers in

Nov 29, 2007 - mixture of SA and the water-washed TiNT is shown to induce slow physical ... acid mixed with titanate nanotube9,10 (TiNT) thermal annea...
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J. Phys. Chem. C 2007, 111, 18615-18623

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Solid-State NMR Studies of the Formation of Monomers and Dimers in Stearic Acid Confined in Titanate Nanotubes Xiao-Ping Tang,*,† Gregory Mogilevsky,‡ Harsha Kulkarni,‡ and Yue Wu‡ Department of Physics and Astronomy, UniVersity of LouisVille, LouisVille, Kentucky 40292, and Department of Physics & Astronomy, UniVersity of North Carolina, Chapel Hill, North Carolina 27510 ReceiVed: August 9, 2007; In Final Form: October 8, 2007

This work employs two-dimensional solid-state NMR methods to uncover the structure and molecular arrangement of physically trapped stearic acid (SA) inside a titanate nanotube (TiNT). Thermal annealing the mixture of SA and the water-washed TiNT is shown to induce slow physical trapping of SA into the TiNT. In contrast to that the solid-state bulk exhibits only one carbonyl 13C peak of NMR and consists solely of dimer, the physically trapped SA exhibits two carbonyl 13C peaks with different chemical shifts that are assigned respectively to dimer and monomer both by the 13C homonuclear double-quantum measurement and by the 13C chemical-shift-tensor measurement. The trapped SA monomer and dimer are shown to grow simultaneously during thermal annealing with a constant number ratio between them at ∼1:1. The 13C homonuclear double-quantum and the 1H-13C HETCOR spectra indicate that the trapped SA monomer and dimer do not form separated clusters but are neighbors to each other. As such, the trapped SA in the TiNT undertakes a novel molecular arrangement alternating with dimer and monomer. The carbonyl 13C chemicalshift-tensor data and the hydroxyl 1H isotropic-chemical-shift data suggest that the hydrogen bond of the dimer is stronger under nanoconfinement than in the solid bulk SA. The observed novel molecular arrangement for the fatty acid and the correlated variation of the carbonyl 13C and the hydroxyl 1H chemical shift between the different molecular arrangements are of theoretical interest.

Introduction Confinement inside a nanopore can significantly change the interaction between molecules. The emerging novel molecular structure and arrangement and molecular dynamics are of fundamental interest and application value.1-7 Because the carboxyl group is critically important in biology for engaging in a remarkable variety of chemical bonding and the fatty acids are among the most common and energy-related biomolecules, inevitably there is interest in understanding the fatty acids under nanoconfinement, particularly when considering that they experience similar constraints in biological systems such as intracellular fatty acids. Reference 8 showed that for palmitic acid mixed with titanate nanotube9,10 (TiNT) thermal annealing near the bulk melting temperature (Tm,bulk) of palmitic acid induced fast chemisorption of palmitic acid on the nanotube surface followed by slow physical trapping of palmitic acid into the nanopores in the TiNT matrix. The physically trapped palmitic acid exhibited8 marked elevation of the melting temperature compared to the neat bulk palmitic acid. It also displayed8 a much longer carbonyl 13C nuclear spin-lattice relaxation time11 (T1), revealing the confinement effect on the molecular motion. In contrast to solid neat bulk palmitic acid, which exhibits a single carbonyl 13C peak of NMR, the trapped palmitic acid displayed two carbonyl 13C peaks with the intensity ratio at ∼2:1. The larger peak was downfield-shifted and the smaller peak was upfield-shifted with respect to the single peak of the solid neat bulk palmitic acid.8 It was tentatively suggested8 * Corresponding author. Telephone: (502) 852-0917. Fax: (502) 8520742. E-mail: [email protected]. † University of Louisville. ‡ University of North Carolina.

that the two carbonyl 13C peaks correspond to two components of palmitic acid: one confined in the nanopores within the nanotube and the other in the interstitial nanopores between the nanotubes. However, the two components exhibited identical 13C T , identical 1H T , and identically elevated melting 1 1 temperatures, together pointing to spatial proximity between them instead of spatial separation. Hence, further clarification is needed for the spatial relationship between the two trappedfatty-acid components. In addition, the molecular structure and molecular arrangement of each component remain to be determined. Unfortunately, many techniques frequently used for characterizing nanomaterials such as transmission electron microscopy are unsuitable to measure the molecular structure or the intermolecular connectivity of solid fatty acids because of the required experimental conditions of these techniques and particularly because of the technical limitation of these techniques when confronting confined fatty acids in the nanopore enveloped by a thick wall. By contrast, NMR is capable of probing both the intramolecular structure and the intermolecular connectivity11-13 and is routinely used to study carboxylic acids.8,14-22 Thus, the current work exploits the unique capability of the two-dimensional (2D) solid-state NMR12,13 to investigate the intermolecular connectivity in each component of the trapped fatty acid and the spatial relationship between the two components. The marked difference of the carbonyl 13C chemical shift between the two components is particularly useful such that their distinct peaks conveniently serve as their corresponding fingerprints in various 2D NMR spectra to obtain other informative NMR parameters. For neat bulk saturated monocarboxylic acids in the solid state, molecules pair off as dimers23-26 by forming strong

10.1021/jp076410s CCC: $37.00 © 2007 American Chemical Society Published on Web 11/29/2007

18616 J. Phys. Chem. C, Vol. 111, No. 50, 2007 hydrogen bonds of O‚‚‚H-O between the carbonyl group (Cd O) of one molecule and the hydroxyl group (H-O-C) of the other molecule. In each dimer the two carboxyl groups from the paired molecules together form two hydrogen bonds antiparallel to each other.23,24 Given that the prominent effects induced by nanoconfinement most likely occur in and near the carboxyl, in the current work the first step is to determine whether the trapped fatty acid in the nanopore retains the dimeric structure and at what fraction. To this aim the carbonyl 13C homonuclear double quantum13,27 and the carbonyl 13C chemical-shift tensor12,13,28,29 are measured to differentiate the trapped dimeric stearic acid (SA) and the trapped monomeric SA. The double quantum directly measures13,27 the spatial connectivity between the carbonyl carbon atoms, and the chemical-shift anisotropy is14,15 a sensitive tool probing the protonation state of the carboxyl. To detect any change of the hydrogen bond in the trapped fatty acid dimer compared to the dimer in the solid neat bulk fatty acid, the second step is to measure the 1H-13C heteronuclear-correlation (HETCOR) spectrum11-13,30 between the carbonyl carbon and the hydroxyl proton. Additionally, the HETCOR spectrum can also measure the methylene and methyl protons through correlating to the carbonyl carbon to reveal the nanoconfinement effect on the backbone chain and the tail group of the fatty acid. In all these 2D NMR spectra cross peaks connecting the different components of fatty acid can disclose their spatial relationship. Materials and Methods The titanate nanotube is produced by alkaline hydrothermal synthesis as described in refs 9 and 10. In short, for the current study 4 g of Anatase 32 nm nanocrystals (Alfa Aesar) were mixed with 160 g of NaOH and 400 mL of distilled water. The mixture was placed in a Teflon container and was inserted into a steel autoclave. The autoclave was placed inside an oven and was kept at 140 °C for 72 h. Afterward, the caked solid precipitate was collected, ground, and placed inside a centrifuge cylinder and topped off either with 1.0 M diluted hydrochloric acid and water (“acid-washed TiNT”) or with only fresh distilled water (“water-washed TiNT”). Acid-washed TiNT was used in the previous work of ref 8, and water-washed TiNT was used in the current work. Specifically, for preparing the water-washed TiNT the centrifuge cylinder was stirred up using a Vortexer (Glas Col) and centrifuged, and the top solution was sifted off. This water-wash process was repeated 20-25 times until the pH value of the top water solution reached 5.5-6. The precipitated white powder was dried in an oven at 50 °C and was then ground using a mortar and pestle. The signature X-ray and Raman spectra of the resultant material confirmed that the water-washed TiNT sample contains exclusively TiNTs. The tube dimension and geometry were also characterized using transmission electron microscopy (TEM). Figure 1 displays a TEM micrograph of the water-washed TiNTs, and as shown, the TiNTs are uncapped with the hollow semicylindrical pores surrounded by the tube wall which resembles a scroll. The inner diameter and the outer diameter of the water-washed TiNTs are respectively at about 6 and 12 nm, and with the interlayer distance of the wall at about 0.85 nm. The tube length ranges from several hundred nanometers to several microns. Thus, the ratio between the tube length and the pore diameter is large. Similar to the previous thermal-annealing experiments8 on the mixture of palmitic acid and acid-washed TiNT, the current work found that thermal annealing the mixture of palmitic acid with the water-washed TiNT near the bulk melting temperature (Tm,bulk) of palmitic acid also produced two components of

Tang et al.

Figure 1. TEM micrograph of water-washed titanate nanotubes. The scale bar is 20 nm.

physically trapped palmitic acid. Similar to what were shown in ref 8, the two components exhibit distinct carbonyl 13C chemical shifts but identical hydroxyl 1H T1 values and identical carbonyl 13C T1 values. The hydroxyl 1H T1, the carbonyl 13C T1, and the carbonyl 13C chemical shifts measured here are correspondingly identical to those obtained in ref 8. In the current work experiments are also conducted using a series of fatty acids, specifically including CH3(CH2)8COOH (decanoic acid), CH3(CH2)10COOH (dodecanoic acid), CH3(CH2)12COOH (tetradecanoic acid), CH3(CH2)14COOH (hexadecanoic acid or palmitic acid), and CH3(CH2)16COOH (octadecanoic acid or stearic acid). Thermal annealing the mixtures of each of these fatty acids with water-washed TiNT at the appropriate mass ratio near the Tm,bulk of the respective fatty acid is shown to induce two trapped-fatty-acid components with ∼3.1 ppm of chemical-shift difference between their carbonyl 13C peaks but with identical 1H and 13C T1 values. Therefore, the emergence of two trapped-fatty-acid components is a common feature for the confined medium-chain saturated monocarboxylic acids in the water-washed TiNT and in the acid-washed TiNT. The following discussion focuses on the 2D NMR measurements of the trapped stearic acid in the water-washed TiNT, but similar results are also observed on the other fatty acids. The 1-13C-labeled stearic acid (SA), CH3(CH2)1613COOH, with isotope enrichment >99% was acquired from Cambridge Isotope. The bulk melting temperature is measured as ∼345 K by the in situ simultaneous measurement of the carbonyl 13C cross-polarization13 (CP) spectrum and the non-CP spectrum as described in ref 8. For the current work the water-washed TiNT and the 1-13C-labeled SA at the mass ratio of 1:1 are mixed together and transferred into Pyrex inserts for the NMR measurement under magic angle spinning13 (MAS). At room temperature before thermal annealing the carbonyl 13C CPMAS spectrum of the mixture displays only one narrow peak at 181.9 ppm, obviously associated with the bulk SA given that not only the chemical shift but also the measured hydroxyl 1H T1 and the carbonyl 13C T1 are respectively identical to those measured for the solid neat bulk SA. The sample of the mixture is slowly

Monomers and Dimers in SA Confined in TiNT

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Figure 2. Carbonyl 13C CPMAS spectra (referenced to TMS) acquired at the spinning speed of 5 kHz and with the CP contact time at 3 ms for the mixture of 1-13C labeled SA and water-washed TiNT at the mass ratio of 1:1. The bottom six spectra are acquired during continuous thermal annealing at 340 K after the indicated time durations. The sample is subsequently heated to 345 K and then cooled to 300 K, and the top two spectra are respectively acquired at these temperatures.

Figure 3. Carbonyl 13C CPMAS spectra acquired at 300 K to determine the 13C T1. Each spectrum is acquired after the 13C Zeeman order decays for the indicated time period. The 181.9-ppm peak decays much faster and corresponds to the 13C T1 of ∼320 s, which is identical to that of the solid neat bulk SA. The 182.7- and 179.6-ppm peaks correspond to a 13C T1 of ∼1800 s.

heated to 340 K and is kept at 340 K for over 200 min. During warm-up, a weak broad peak emerges accompanied by slight decrease of the bulk peak. The hydroxyl 1H T1 and the carbonyl 13C T corresponding to the broad peak are measured as only 1 ∼1/100 of those of the solid neat bulk SA. For the same reason given in ref 8 the broad peak is assigned to the chemisorbed SA on the nanotube surface. During thermal annealing at 340 K a series of 13C CPMAS spectra are consecutively acquired to in situ monitor the transfer of SA into the TiNT and are plotted in Figure 2 versus the annealing time. As shown, even during slow warm-up a fraction of the bulk SA starts to transfer into two sharp peaks appearing at 182.7 and 179.6 ppm. Thus, in situ monitoring under continuous thermal annealing is a better way to study the thermal annealing process than the approach adopted in ref 8 by repeating the warm-anneal-cool-measure cycle. As shown in Figure 2, during the lengthy annealing the intensities of the 182.7- and 179.6-ppm peaks grow simultaneously and slowly accompanied by gradual decrease of the 181.9-ppm peak of the bulk SA. After annealing over 200 min the 182.7-ppm peak and the 179.6-ppm peak stop growing and the 181.9-ppm bulk peak stops decreasing (Figure 2). The sample is then heated to 345 K. As shown in Figure 2, at 345 K the 181.9-ppm bulk peak disappears from the CPMAS spectrum, indicating the melting of the solid bulk SA, whereas the other peaks remain unchanged, implying elevation of the melting temperature for the corresponding SA components. When the sample is subsequently cooled to 300 K, the 181.9ppm peak of the solid bulk SA reappears (Figure 2), indicating that the molten bulk SA freezes again. As shown in Figure 3, at 300 K the carbonyl 13C T1 is ∼1800 s for both the 182.7ppm peak and the 179.6-ppm peak, whereas it is ∼320 s for the 181.9-ppm peak. The latter is identical to the carbonyl 13C T1 of the solid neat bulk SA. Similar to the discussion given in ref 8, the 182.7-ppm peak and the 179.6-ppm peak are assigned to the physically trapped SA inside the nanopore. For further comparison with ref 8, another mixture of the water-washed TiNT and SA at a smaller mass ratio of ∼1:0.5 is prepared. Thermal annealing this mixture at 340 K was observed to proceed through similar events except that after

lengthy thermal annealing the 181.9-ppm peak of the bulk SA transferred entirely into the 182.7- and 179.6-ppm peaks. Therefore, the current thermal-annealing experiment on the mixtures of SA with the water-washed TiNT compares well with the previous experiment8 on the mixtures of palmitic acid with the acid-washed TiNT, both revealing that under thermal annealing physical trapping of fatty acid into the nanopore is a slow process and that if the mixture contains excess fatty acid there will remain some bulk fatty acid even after lengthy thermal annealing. Because of the advantage in simultaneously measuring the bulk fatty acid as reference for the trapped fatty acid, the following discussion will focus on the NMR measurement on the sample of the TiNT/SA mixture with the mass ratio at 1:1. As shown above, the thermally annealed sample displays three carbonyl 13C peaks at 179.6, 181.9, and 182.7 ppm; the middle peak is associated with the excess solid bulk SA and the other two peaks are associated with two components of the trapped SA in the nanopore. Finally, it is of note that in Figure 2 the intensity ratio between the 182.7-ppm peak and the 179.6ppm peak is ∼2:1 and the ratio remains nearly constant during thermal annealing at 340 K. This shows that the two trapped SA components grow simultaneously during thermal annealing. This behavior is also observed for the TiNT/SA mixture at the mass ratio of ∼1:0.5 and was observed for the mixtures of the acid-washed TiNT and palmitic acid in the previous study.8 The current NMR measurements are conducted on a 300 MHz Avance console using a 4-mm double-resonance CPMAS13 probe. Samples are sealed in the Pyrex inserts. Because of isolation from the other 13C peaks,8,31 the carbonyl 13C nuclear spin resonance is the chosen probe for the current work and as such the 1-13C-labeled SA is used. Because of the extremely long carbonyl 13C T1 for the solid bulk neat SA and the trapped SA, only CP is a viable method for acquiring the 13C nuclear spin resonance in affordable measurement time. In the current work RAMP-CP32 with varying rf power on the proton channel is applied for all measurements. For all 2D NMR measurements the 13C nuclear spin resonances are directly acquired because MAS is ineffective to remove the strong homonuclear dipolar coupling among the abundant 1H spins even at 15 kHz, the

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Figure 4. (a) RAMP-CP32 pulse sequence with varying rf power on the proton channel and with TPPM-1533 to reduce the 13C-1H dipolar coupling during acquisition. (b) Two-dimensional sequence for measuring the 13C homonuclear double quantum. The BABA sequence27 is applied during the excitation period to generate the 13C homonuclear double quantum and during the reconversion period to transfer the double quantum into single quantum. The double quantum is encoded during t1, and the single quantum is encoded during t2. For proton decoupling CW is applied during the excitation and reconversion periods and during t1 and TTPM-15 is applied during t2. (c) SUPER28 sequence for measuring the 13C chemical-shift anisotropy. The 13C powder pattern is encoded during t1, and the 13C isotropic chemical shift is encoded during t2. Tr is the rotor period. (d) 1H-13C heteronuclear-correlation sequence (HETCOR30). The 1H isotropic chemical shift is encoded during t1, and the 13C isotropic chemical shift is encoded during t2. During t1 the frequency-switched Lee-Goldburg sequence (FSLG)31 is exploited to remove the strong 1H-1H dipolar coupling.

spinning speed limit of the CPMAS probe under use. Figure 4a displays the pulse sequence for acquiring the 13C CPMAS spectrum, and Figure 4b-d displays the 2D pulse sequences. To obtain well-resolved carbonyl 13C peaks, TPPM-1533 is applied during acquisition to optimize heteronuclear dipolar decoupling between the 13C and the abundant 1H nuclear spins. For 2D NMR the indirectly acquired spectrum is resolved for the different SA components by correlating it with the directly acquired and well-resolved carbonyl 13C single-quantum spectrum which presents the three carbonyl 13C peaks, each at the corresponding isotropic chemical shift (δiso). Figure 4b displays the 2D pulse sequence for measuring the 13C homonuclear double quantum (DQ). Because the homonuclear DQ evolving from the through-space dipolar coupling depends on the internuclear distance, by using the 1-13C-labeled SA the carbonyl 13C homonuclear DQ can reveal the spatial connectivity between the carbonyl carbon atoms within the same SA component and between the different SA components. As it is necessary to circumvent the extremely long carbonyl 13C T1, RAMP-CP is applied at the beginning of the pulse sequence to transfer nuclear spin polarization from the 1H to 13C spins.

Tang et al. BABA27 is employed to reintroduce the through-space 13C homonuclear dipolar coupling under MAS. The excitation period and the reconversion period each consist of several supercycles. Each supercycle covers four rotor periods and consists of two differently and appropriately phased BABA pulse segments, to compensate27 the off-resonance/chemical shift and the pulse imperfection. DQ evolves during the excitation period and is selected by the appropriate phase cycling scheme. DQ is encoded during t1, and during the reconversion period DQ transfers back into directly measurable 13C single quantum which is acquired during t2 to generate the well-resolved carbonyl 13C peaks. Figure 4c displays the SUPER sequence28 for acquiring chemical-shift anisotropy13 (CSA) by measuring the powder pattern. Because the carbonyl carbon exhibits a characteristic chemical-shift tensor when engaging in different chemical bonds,14,15,31,34 the carbonyl 13C CSA helps identify the chemical bonding of the carboxyl in the three SA components. Again, RAMP-CP was applied at the beginning of the SUPER sequence for the aforementioned reason. Because the centerband of carbonyl 13C does not overlap the methylene and the methyl 13C peaks, TOSS13 was not necessary and was not used in the current measurement. CSA was also obtained using the 13C CPMAS spectrum acquired at the moderate spinning speeds including 1.7 kHz, which present a sufficient number of spinning sidebands for the CSA analysis. Figure 4d displays the HETCOR30 pulse sequence to correlate the carbonyl 13C δiso with the 1H δiso. The 1H resonance is encoded during t1 while employing FSLG30,35 to reduce the strong 1H-1H dipolar coupling. The connectivity between the carbonyl carbon and the various protons is detected by RAMP-CP, during which the nuclear spin polarization is transferred from the 1H to 13C spins via the 1H-13C dipolar coupling. Because this dipolar coupling is13 sensitive to the 1H-13C distance, a short contact time of CP (τCP) induces correlation peaks connecting the carbonyl carbon with only nearby protons such as the hydroxyl proton or the R-methylene protons in the same molecule. Long τCP induces additional correlation peaks connecting the carbonyl carbon with the other protons in the same molecule and with the protons in the neighboring molecules. Results and Discussion Figure 5 displays the acquired carbonyl 13C homonuclear DQ spectrum13,27 with the spinning speed at 10 kHz and the excitation and reconversion periods,27 each at 1600 µs. The horizontal axis displays the 13C isotropic spectrum showing the three carbonyl 13C peaks. The vertical axis displays the 13C homonuclear DQ spectrum revealing the connectivity between the carbonyl carbon atoms within the same SA component and between different SA components. The 181.9-ppm peak of the bulk SA connects with a strong DQ peak at ∼363.8 ppm. The strong DQ peak obviously originates from the dipolar coupling between the two carbonyl 13C spins in the dimer considering that in the solid bulk SA the two carbonyl carbon atoms of a dimer are connected by the two opposite hydrogen bonds of CdO‚‚‚HsOsC.23,24 For the trapped SA, a strong DQ peak at ∼365.4 ppm connects with the 182.7-ppm peak, whereas no discernible DQ peak at 359.2 ppm connects with the 179.6ppm peak. Thus, the larger component of the trapped SA associated with the 182.7-ppm peak is also dimeric like the solid bulk SA, whereas the smaller component associated with the 179.6-ppm peak is monomeric. As shown later, these assignments are also confirmed by the CSA measurements. Thus, in marked contrast to the solid bulk SA, which consists solely of dimer, the trapped SA consists of not only dimer but also of

Monomers and Dimers in SA Confined in TiNT

Figure 5. Two-dimensional correlation spectrum of the carbonyl 13C homonuclear double quantum versus single quantum acquired at 300 K and at the spinning speed of 10 kHz. The length was 1600 µs for the excitation period and for the reconversion period. The horizontal axis displays the 13C isotropic spectrum with the three carbonyl 13C peaks. The vertical axis displays the 13C homonuclear DQ spectrum. The 181.9-ppm peak of the solid bulk SA connects to a strong DQ peak at ∼363.8 ppm. For the trapped SA a strong DQ peak at ∼365.4 ppm connects to the 182.7-ppm peak, whereas no discernible DQ peak appears at 359.2 ppm connecting to the 179.6-ppm peak. Two weak DQ peaks appear at ∼(179.6 + 182.7) ppm connecting with the 179.6ppm peak and with the 182.7-ppm peak.

monomer. According to the 13C CPMAS spectra shown in Figures 2 and 3, the mass ratio between the trapped dimeric SA and the trapped monomeric SA is ∼2:1. Correspondingly, the number ratio between the trapped dimer and the trapped monomer is approximately 1:1. Two weak cross peaks, corresponding to the DQ at ∼(179.6 + 182.7) ppm which connects with the 179.6-ppm peak and the 182.7-ppm peak, also appear in Figure 5. Because this DQ originates from the dipolar coupling between the carbonyl 13C nuclear spin of the trapped SA dimer and the carbonyl 13C nuclear spin of the trapped SA monomer, the trapped monomer must be close to the trapped dimer and vice versa. This spatial proximity is also shown by the HECTOR spectra shown later in Figure 7. In Figure 5 the absence of even a weak DQ peak at 359.2 ppm, which could evolve from the dipolar coupling between the carbonyl 13C nuclear spins from neighboring monomers, implies that the trapped monomers do not cluster together. Based on the combined results, the trapped dimer and the trapped monomer are neighbors and, because the number ratio between them is ∼1:1, they should be alternately arranged in space. The spatial proximity between the trapped monomer and the trapped dimer is consistent with that the identical 13C T1 and the identical 1H T1 are measured for them, as a result of the averaging effect of spin diffusion which becomes effective at a short distance even under MAS given the long time scale of these T1 values (at least over 10 s). To the contrary, consistent with the expected large distance between the bulk SA and the trapped SA, Figure 5 shows no discernible cross DQ peak between the bulk SA and either the trapped SA monomer or the trapped SA dimer. The large spatial separation is also in accordance with that the 13C T1 is drastically different between the bulk SA and the trapped SA. Finally, it should be noted that the measured cross DQ peaks in Figure 5 are not a result of interconversion between the trapped dimer and monomer because such fast interconversion on a time scale of milliseconds would otherwise smear out the splitting between the 179.6- and 182.7-ppm peaks. In practice, the lower limit of the rate of such interconversion can be determined by the 13C single quantumsingle quantum correlation spectrum under fast MAS. At the spinning speed of 10 kHz, the cross peaks connecting the

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Figure 6. 13C CPMAS spectrum acquired at 300 K and at the spinning speed of 1.7 kHz. The spectrum displays three sets of spinning sidebands, each corresponding to one of the three SA components. (a) The spinning sidebands are plotted partially to give an expanded view, and the centerband peak for each component is marked by an arrow with the respective isotropic chemical shift. (b) The entire spinning sidebands are displayed.

TABLE 1: Principal Parameters of CSA for the Three Carbonyl 13C Peaks Obtained29 from the Corresponding Spinning-Sideband Patterns Displayed in Figure 6 182.7-ppm peak 181.9-ppm peak 179.6-ppm peak

σ11 (ppm)

σ22 (ppm)

σ33 (ppm)

252 238 266

189 198 164

107 110 109

carbonyl 13C single quantum of the trapped dimer with the carbonyl 13C single quantum of the trapped monomer become visible only for the mixing time longer than 2 s. At such a long mixing time spin diffusion is effective even under MAS at the spinning speed of 10 kHz. Hence, if interconversion indeed occurs between the trapped dimer and the trapped monomer, the interconversion rate must be much slower than 0.5 Hz. Therefore, the cross DQ peaks displayed in Figure 5 are certainly not a result of interconversion between the trapped dimer and monomer. The carbonyl 13C CSA were obtained both using the spinningsideband patterns29 of the 13C CPMAS spectrum acquired at the spinning speed of 1.7 kHz and using the 13C SUPER28 spectrum acquired at the spinning speed of 4 kHz. Figure 6a displays an expanded partial view of the sideband patterns, and Figure 6b displays the entire sideband patterns. For each of the three SA components the centerband is located at the corresponding δiso and marked by an arrow. Each centerband is associated with a set of sidebands separated by integral multiples of 1.7 kHz. The sideband pattern of the 182.7-ppm peak and the sideband pattern of the bulk 181.9-ppm peak are similar, whereas they are markedly different from the sideband pattern of the 179.6-ppm peak. For each SA component the principal parameters of the chemical shift tensor, δ11, δ22, and δ33,11-15,31 are obtained by fitting the corresponding spinning-sideband pattern29 and are tabulated in Table 1. Figure 7 displays the carbonyl 13C powder patterns13 obtained from the 2D cross sections of the SUPER spectrum.28 The powder pattern of the solid neat bulk SA (top) and the powder pattern associated with the 181.9-ppm peak (third from top) are identical, again confirming that the 181.9-ppm peak corresponds to the excessive bulk SA. For the trapped SA, the powder pattern associated with the 182.7-ppm peak (second from top) resembles the powder pattern of the solid bulk SA, whereas the powder pattern

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Figure 7. Carbonyl 13C powder patterns extracted from the measured SUPER28 spectra at 300 K. The top one corresponds to the solid neat bulk SA, and the other three correspond to the 182.7-ppm peak, the 181.9-ppm peak of the solid bulk SA, and the 179.6-ppm peak for the SA/TiNT mixture.

TABLE 2: Principal Parameters of CSA for the Three Carbonyl 13C Peaks Obtained by Fitting the Corresponding Powder Patterns Acquired by SUPER28 and Displayed in Figure 7 182.7-ppm peak 181.9-ppm peak 179.6-ppm peak

σ11 (ppm)

σ22 (ppm)

σ33 (ppm)

250 240 267

193 198 167

105 108 104

associated with the 179.6-ppm peak is substantially different. The δ11, δ22, and δ33 parameters are obtained for each SA component by fitting the corresponding powder pattern and are listed in Table 2. The δ11, δ22, and δ33 parameters in Tables 1 and 2 correspondingly compare well with each other, proving the validity of these measurements. Inspecting Tables 1 and 2, the δ33 values are approximately the same between the 179.6ppm peak and the other two peaks. On the contrary, the two in-plane parameters, δ11 and δ22, vary markedly from the 179.6ppm peak to the other two peaks. Particularly, the value of δ11 - δ22 corresponding to the 179.6-ppm peak is almost as twice large as these corresponding to the other two peaks. It has been recognized14,15,31,34 that the carbonyl 13C chemicalshift tensor parameters are sensitive to the bondings engaged by the carboxyl. For the carbonyl carbon the smallest principal parameter of the chemical-shift tensor, δ33, corresponds36 to the principal axis perpendicular to the plane of the CdO and C-O bonds. For the protonated carboxyl with the hydroxyl group (C-O-H) free from bonding, a significant length difference is expected between the double bond of CdO and the single bond of C-O. In this case δ22 corresponds to the in-plane principal axis approximately parallel to the carbonyl bond and δ11 corresponds to the other in-plane principal axis approximately perpendicular to the CdO bond.15,36 For the hydrogen-bonded carboxyl with the hydroxyl group (C-O-H) engaging in the hydrogen bond, the hydroxyl proton of the carboxyl group tends to stretch away, leading to the contraction of the C-O bond. Therefore, similar to deprotonated carboxyl for the hydrogenbonded carboxyl, the length difference between the CdO bond and the C-O bond is also reduced, giving rise to decreased anisotropy of the in-plane electron density. In this case δ11 corresponds15,36 to the in-plane principal axis approximately between the CdO bond and the C-O bond. Most notably, δ11 decreases and δ22 increases in comparison to the protonated carboxyl.14,15 Inspecting Tables 1 and 2, the large difference

between the measured δ11 and δ22 corresponding to the 179.6ppm peak, ∼100 ppm, is characteristic for the protonated carboxyl,14,15,34 confirming the earlier assignment of the 179.6ppm peak to the monomeric SA. The much smaller values of δ11 - δ22, ∼40 ppm for the 181.9-ppm peak and ∼60 ppm for the 182.7-ppm peak, suggest that the carboxyl is hydrogenbonded in their associated SA components. Indeed, it is expected when two SA molecules pair off as a dimer with two strong hydrogen bonds forming between their carboxyl groups. For the doubly hydrogen bonded dimer the length difference between the CdO bond and the C-O bond should be reduced and so should the anisotropy of the in-plane electron density. Thus, the much reduced differences between δ11 and δ22 measured for the 182.7-ppm peak and for the solid bulk SA are again consistent with the earlier assignment of these two peaks to the dimeric SA. The 2D 1H-13C HETCOR30 spectra plotted in Figure 8a, 8b, and 8c were acquired at the spinning speed of 10 kHz and with τCP respectively at 150, 400, and 2000 µs. The horizontal axis displays the 13C isotropic spectrum presenting the three carbonyl 13C peaks. The vertical axis displays the 1H isotropic spectrum. For fatty acids the isotropic chemical shift is above 10 ppm for hydroxyl protons and is close to 0 ppm for methylene and methyl protons.31 In Figure 8a for τCP ) 150 µs, the carbonyl 13C peak of the solid bulk SA with δ iso ≈ 181.9 ppm connects to a strong hydroxyl 1H peak with δiso ≈ 12.1 ppm. The carbonyl 13C peak of the trapped SA dimer with δ iso ≈ 182.7 ppm connects to a strong hydroxyl 1H peak with δiso ≈ 15.1 ppm. The carbonyl 13C peak of the trapped SA monomer with δiso ≈ 179.6 ppm connects to a strong hydroxyl 1H peak with δiso ≈ 10.8 ppm. Figure 8a also shows that all three carbonyl 13C peaks connect to a weak methylene 1H peak at ∼2.4 ppm associated with the R-CH2 group. Because the distance between the carbonyl carbon and the R-methylene proton is similar to the distance between the carbonyl carbon and the hydroxyl proton,23,24 it is not surprising that cross-polarization from the R-methylene proton to the carbonyl carbon also becomes effective even at the short τCP of 150 µs. At τCP ) 400 µs (Figure 8b), for each SA component a strong correlation peak also emerges, connecting the corresponding carbonyl 13C peak to the backbone methylene 1H peaks at δiso ≈ 1.0 ppm. At τCP ) 1200 µs (Figure 8c), for each SA component a weak correlation peak emerges, connecting the corresponding carbonyl 13C peak to the methyl 1H peak of the methyl tail group with δiso ≈ 0.5 ppm. As shown in Figure 8, for the methylene and methyl 1H peaks the isotropic chemical shifts do not exhibit significant differences between the bulk SA, the trapped dimeric SA, and the trapped monomeric SA. By contrast, the isotropic chemical shift of the hydroxyl 1H peak is markedly different between the three SA components. In particular, the carbonyl 13C δiso and the hydroxyl 1H δ iso follow the same trend, specifically increasing from the trapped SA monomer to the bulk SA and to the trapped SA dimer. The HETCOR spectra together suggest that confinement produces only a marginal effect on the backbone chain and on the methyl tail group but produces a significant effect on the carboxyl head group. Cross peaks also appear in Figure 8b,c connecting the trapped dimer to the trapped monomer, again implying that the trapped SA dimer and the trapped SA monomer are neighbors. Because the mass of the trapped monomer is about half the mass of the trapped dimer, the cross peak connecting the carbonyl 13C peak of the trapped dimer (directly acquired) to the hydroxyl 1H peak (indirectly acquired) of the trapped monomer should be twice as large in intensity

Monomers and Dimers in SA Confined in TiNT

J. Phys. Chem. C, Vol. 111, No. 50, 2007 18621

Figure 8. 1H-13C HETCOR30 spectra acquired at 300 K and at the spinning speed of 10 kHz with τCP respectively at (a) 150, (b) 400, and (c) 2000 µs. The horizontal axis (F2) displays the 13C isotropic spectrum showing the three carbonyl 13C peaks. The vertical axis (F1) displays the 1H isotropic spectrum.

Figure 9. 13C spectra for neat bulk stearic acid acquired in the solid state (at 298 K) by CP and acquired in the liquid state (at 346.5 K) by the one-pulse non-CP sequence.

as the cross peak connecting the carbonyl 13C peak of the trapped monomer (directly acquired) to the hydroxyl 1H peak (indirectly acquired) of the trapped dimer. Indeed, the latter is weaker and is not visible at the current scale of display. Again, it should be noted that these cross peaks are not a result of interconversion between the trapped dimer and the trapped monomer because such fast interconversion in a mixing time of hundreds of microseconds would otherwise smear out the splitting between the 179.6- and 182.7-ppm peaks. Figure 8 shows that both the carbonyl 13C peak and the hydroxyl 1H peak of the trapped SA monomer exhibit large upfield shifts of >3 ppm referenced to those of the trapped SA dimer. This invokes comparison with what is shown in Figure 9, that the carbonyl 13C peak experiences an upfield shift of ∼1.3 ppm when the solid neat bulk SA melts into the liquid neat bulk SA. When reversing the process in which the liquid neat bulk SA freezes into the solid neat bulk SA, the downfield shift of the same magnitude is recorded. In comparison to these observations on the neat bulk SA, the bulk SA in the SA/TiNT mixture, associated with the 181.9-ppm peak in the solid state, similarly showed an upfield shift and a downfield shift, respectively, upon melting and freezing. In view of that the solid bulk SA is dimeric with a stable hydrogen bond23,24 and the liquid bulk SA is only partially dimeric because of constant

formation and breakup of the hydrogen bond, the carboxyl group may be considered as partially protonated in the liquid bulk SA but as deprotonated in the solid bulk SA. The carbonyl 13C δiso has been observed37 to increase by approximately several parts per million when carboxylic acids are changed from the protonated state to the deprotonated state. Thus, the increase of the carbonyl 13C δiso from the trapped monomer to the trapped dimer is naturally considered to originate from that the former is protonated and the latter is deprotonated. The HETCOR spectra also showed similar increases for the hydroxyl 1H δiso from the trapped SA monomer to the trapped dimer. Therefore, the current work shows that the hydroxyl 1H δiso and the carbonyl 13C δiso may together serve as probes for the protonation state. In ref 14b, the analysis on a collection of experimental data on the hydrogen-bonded carboxyl suggests that for the carbonyl carbons in the deprotonated state stronger hydrogen bonding gives rise to smaller δ22. Both the solid bulk SA and the trapped SA dimer are in the deprotonated state, and the latter showed slightly smaller δ22 (Tables 1 and 2). Following the empirical rule, the hydrogen bonding in the trapped SA dimer is stronger than that in the solid bulk SA. It is of interest to compare this conclusion with the measured δiso difference between the two SA components. Referenced to those of the solid bulk SA, as shown in Figure 8, the carbonyl 13C peak and the hydroxyl 1H peak of the trapped SA dimer correspondingly exhibit significant downfield shifts. If the hydrogen bonds in the dimer are asymmetric such that the proton is not located in the middle of the hydrogen bond of O‚‚‚H-O, in general a potential surface with double minima emerges for the proton. Under such a circumstance, because the dimer consists of two antiparallel hydrogen bonds, the hydroxyl protons can simultaneously jump in opposite directions along the two hydrogen bonds between the two potential minima. The rate of the simultaneous proton jumping can affect the strength of the hydrogen bonds.20 For the consideration of the NMR measurement, simultaneous proton jumping modulates the 1H-13C dipolar coupling and thus induces20 nuclear spin-lattice relaxation. Compared to the solid bulk SA, the measured carbonyl 13C T1 for the trapped SA is much longer (Figure 3), suggesting that simultaneous proton jumping becomes faster in the trapped SA dimer and thus the energy barrier between the double potential minima is lowered. Following ref 38 the lower energy barrier corresponds to a smaller length difference between the O‚‚‚H and O-H bonds and corresponds to the shorter and stronger hydrogen bond of

18622 J. Phys. Chem. C, Vol. 111, No. 50, 2007 O‚‚‚H. As discussed in ref 39, a shorter hydrogen bond leads to a larger isotropic chemical shift for the hydroxyl proton. Therefore, the HETCOR spectra in Figure 8 pointed to a stronger hydrogen bond for the trapped SA dimer than for the solid bulk SA, which is in agreement with the earlier conclusion drawn from the δ22 data. Nevertheless, care should be taken regarding the qualitative explanation proposed here, which needs confirmation by theoretical calculations. The current work presents an interesting system consisting of the same species of fatty acid with different molecular connections and different hydrogenbonding strengths. It is of interest for theoretical study of the NMR results for the three SA components in terms of the isotropic chemical shifts and the chemical-shift anisotropy for both the carbonyl carbon and the hydroxyl proton. Conclusions In summary, thermal annealing the mixture of SA and the water-washed TiNT is shown to induce slow physical trapping of SA into the nanotube. The physically trapped SA exhibits two distinct carbonyl 13C peaks with the isotropic chemical shifts respectively at 182.7 and 179.6 ppm; in contrast, the solid bulk SA exhibits only one carbonyl 13C peak with the isotropic chemical shift at 181.9 ppm. The spectrum of the carbonyl 13C homonuclear double quantum indicates that the 182.7-ppm peak is associated with the dimeric SA, just like the solid bulk SA, and the 179.6-ppm peak is associated with the monomeric SA. These assignments are also confirmed by the obtained carbonyl 13C chemical-shift anisotropy by fitting the spinning-sideband patterns or the SUPER spectrum. The δ11 and δ22 values of the carbonyl carbon corresponding to the 179.6-ppm peak are characteristic for the protonated carboxyl. By contrast, the δ11 and δ22 values corresponding to the 182.7-ppm peak are similar to those of the solid bulk SA and are characteristic for the deprotonated carboxyl. The appearance of the cross peaks of the 13C homonuclear double quantum connecting the trapped SA monomer to the trapped SA dimer indicates that they form neighbors. This is also confirmed by the 1H-13C HETCOR spectra displaying cross peaks connecting the trapped SA monomer with the trapped SA dimer. On the other hand, the absence of even a weak peak of the 13C homonuclear double quantum connecting the trapped SA monomers implies that the trapped SA monomers are not neighbors to each other. Therefore, the trapped SA undertakes a novel molecular arrangement. Contrasting to the solid bulk SA consisting solely of dimer, the trapped SA consists of not only dimer but also monomer. The trapped SA monomer and the trapped SA dimer are neighbors to each other, instead of forming separate domains. Moreover, the trapped dimer and monomer are shown to grow simultaneously during thermal annealing of the SA/TiNT mixture at 340 K with a constant number ratio between them of approximately 1:1. Therefore, the molecular arrangement of the trapped SA must alternate with dimer and monomer and is possibly ordered. The origin of such a novel molecular arrangement for the fatty acid should be of interest for theoretical study. The 1H isotropic chemical shifts obtained by the 1H-13C HETCOR measurement are similar among the solid bulk SA, the trapped SA dimer, and the trapped SA monomer for the methylene protons and the methyl protons, suggesting marginal nanoconfinement effect on the backbone chain and the methyl tail group. On the contrary, the carbonyl 13C isotropic chemical shift and the hydroxyl 1H isotropic chemical shift showed similar trends of significant increase from the trapped SA monomer to the solid bulk SA and to the trapped SA dimer. The downfield shift of the carbonyl carbon from the trapped SA monomer to

Tang et al. the trapped SA dimer compares well with the previous observations37 of such a downfield shift in carboxylic acids from the protonated state to the deprotonated state. The downfield shift of the hydroxyl proton from the bulk SA to the trapped SA dimer and the measured smaller δ22 value for the latter both suggest that the trapped SA dimer possesses stronger hydrogen bonding than the dimer of the solid bulk SA. The correlated variation of the hydroxyl 1H isotropic chemical shift with the carbonyl 13C isotropic chemical shift and chemical-shift tensor among the different SA components suggests that when joined with the latter the former is useful to enhance the NMR power for studying the fatty acid. Acknowledgment. We thank Drs. A. Kleinhammes and C. Chen for stimulating discussions. Financial support from Research Corporation is gratefully acknowledged. References and Notes (1) Alba-Simionesco, C.; Coasne, B.; Dosseh, G.; Dudziak, G.; Gubbins, K. E.; Radhakrishnan, R;, Sliwinska-Bartkowiak, M. J. Phys.: Condens. Matter 2006, 18, R15. (2) Yashonath, S.; Santikary, P. J. Phys. Chem. 1994, 98, 6368. (3) (a) Hu, H. W.; Carson, G. A.; Granick, S. Phys. ReV. Lett. 1991, 66, 2758. (b) Hu, H. W.; Granick, S. Science 1991, 253, 1339. (4) (a) Warnock, J.; Awschalom, D. D.; Shafer, M. W. Phys. ReV. Lett. 1986, 57, 1753. (b) Awschalom, D. D.; Warnock, J. Phys. ReV. B 1987, 35, 6779. (5) Klein, J.; Kumacheva, E. Science 1995, 269, 816; J. Chem. Phys. 1998, 108, 7010. (6) Tang, X.-P.; Wang, J.-C.; Cary, L.; Kleinhammes, A.; Wu, Y. J. Am. Chem. Soc. 2005, 127, 9255. (7) (a) Miyahara, M.; Gubbins, K. E. J. Chem. Phys. 1997, 106, 2865. (b) Radhakrishnan, R.; Gubbins, K. E.; Sliwinska-Bartkowiak, M. J. Chem. Phys. 2002, 116, 1147. (c) Hung, F. R.; Dudziak, G.; Sliwinska-Bartkowiak, M.; Gubbins, K. E. Mol. Phys. 2004, 102, 223. (8) Tang, X.-P.; Mezick, B. K.; Kulkarni, H.; Wu, Y. J. Phys. Chem. B 2007, 111, 1507. (9) (a) Ma, R. Z.; Bando, Y.; Sasaki, T. Chem. Phys. Lett. 2003, 380, 577. (b) Zhang, S.; Peng, L.-M.; Chen, Q.; Du, G. H.; Dawson, G.; Zhou, W. Z. Phys. ReV. Lett. 2003, 91, 256103. (10) Bavykin, D. V.; Parmon, V. N.; Lapkin, A. A.; Walsh, F. C. J. Mater. Chem. 2004, 14, 3370. (11) Ernst, R. R.; Bodenhausen, G.; Wokaun, A. Principle of Nuclear Magnetic Resonance in One and Two Dimensions; Clarendon Press: Oxford, 1987. (12) Schmidt-Rohr, K.; Spiess, H. W. Multidimensional Solid-State NMR and Polymers; Academic Press: London, 1994. (13) Duer, J. M. J.; et. al., Solid-state NMR Spectroscopy: Principles and Applications; Blackwell Science: Oxford, 2002. (14) (a) Gu, Z.; McDermott, A. J. Am. Chem. Soc. 1993, 115, 4282. (b) Gu, Z.; Zambrano, R.; McDermott, A. J. Am. Chem. Soc. 1994, 116, 6368. (15) Facelli, J. C.; Gu, Z.-T.; McDermott, A. Mol. Phys. 1995, 86, 865. (16) Akita, C.; Kawaguchi, T.; Kaneko, F.; Yamamoto, H. J. Phys. Chem. B 2004, 108, 4862. (17) Ando, S.; Ando, I.; Shoji, A.; Ozaki, T. J. Am. Chem. Soc. 1988, 110, 3380. (18) Imaoka, N.; Takeda, S.; Chihara, H. Bull. Chem. Soc. Jpn. 1988, 61, 1865. (19) Douliez, J.-P. Langmuir 2004, 20, 1543. (20) Wu, W.; Noble, D. L.; Owers-Bradley, J. R.; Horsewill, A. J. J. Magn. Reson. 2005, 175, 210. (21) Sto¨ckli, A.; Meier, B. H.; Kreis, R.; Meyer, R.; Ernst, R. R. J. Chem. Phys. 1990, 93, 1502. (22) Hamilton, J. A. R. R. J. Lipid Res. 1998, 39, 467. Hamilton, J. A. R. R. Proc. Natl. Acad. Sci. U.S.A. 1989, 86, 2663. (23) Moreno, E.; Cordobilla, R.; Calvet, T.; Lahoz, F. J.; Balana, A. I. Acta Crystallogr. 2006, C62, o129. (24) Bond, A. D. New J. Chem. 2004, 28, 104. (25) Asada, E.; Goto, M. Bull. Chem. Soc. Jpn. 1978, 51, 2456. (26) Leiserowitz, L. Acta Crystallogr. 1976, B32, 775. (27) Feike, M.; Demco, D. E.; Graf, R.; Gottwald, J.; Hafner, S.; Spiess, H. W. J. Magn. Reson. A 1996, 122, 214. (28) Liu, S.-F.; Mao, J.-D.; Schmidt-Rohr, K. J. Magn. Reson. 2002, 155, 15. (29) Herzfeld, J.; Berger, A. J. Chem. Phys. 1980, 73, 6021. (30) Van Rossum, B.-J.; Fo¨rster, H.; De Groot, H. J. M. J. Magn. Reson. 1997, 124, 516.

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