13C-NMR Observed Conformations and Motions of Neat Liquid and

Chapter 16

Downloaded by PURDUE UNIVERSITY on August 5, 2013 | http://pubs.acs.org Publication Date (Web): October 14, 2011 | doi: 10.1021/bk-2011-1077.ch016

13C-NMR

Observed Conformations and Motions of Neat Liquid and Crystalline n-Hexatriacontane and as a Guest in the Narrow Channels of Its Inclusion Compound Formed with α-Cyclodextrin M. A. Hunt,1 S. Villar-Rodil,2 M. A. Gomez-Fatou,3 I. D. Shin,4 F. C. Schilling,5 and A. E. Tonelli*,6 1Oak

Ridge National Laboratory, Oak Ridge, TN 37831 Institute of Coal (INCAR), CSIC, Oviedo, Spain 3Department of Polymer Physics and Engineering, Institute of Science and Technology of Polymers (ICTP), C.S.I.C., Madrid, Spain 4College of Pharmacy & Health Sciences, Campbell University, Buies Creek, NC 27506 537433 South Ocotillo Canyon Dr., Tucson, AZ 85739 6Fiber & Polymer Science, North Carolina State University, Campus Box 8301, Raleigh, NC 27695-8301 *E-mail: [email protected] 2National

A non-covalently bonded inclusion compound (IC) was formed between a 36 carbon guest n-alkane, hexatriacontane (HTC), and the host α-cyclodextrin (α-CD) and observed by solid-state 13C-NMR, as were neat HTC in both its liquid melt and in its crystallline solid. Based on the number and frequencies of observed 13C resonances, HTC in its neat crystals is restricted to the fully extended all trans conformation, while in the melt HTC chains are experiencing rapid inter-conversions between all possible conformations containing trans and gauche bonds. The spin-lattice relax-ation times, T1(13C), observed for interior CH2 carbons in crystalline HTC are ~500 s and for the molten liquid are 1-3 s. In the crystal HTC chains experience virtually no ~100 MHz motions, while molten HTC chains are efficiently moving at both this and much higher frequencies, leading to

© 2011 American Chemical Society In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


an over two orders of magnitude decrease in HTC spin-lattice relaxation times. While the narrow channels (~0.5 nm) of its α-CD-IC largely restrict the HTC chains to the all-trans conformation, the T1(13C) relaxation times of its interior CH2 carbons range from 1-4 s at temperatures from -30 to 85 °C. In other words, even though the conformations of HTC chains in the narrow α-CD-IC channels are severely restricted compared to those of neat molten HTC chains, they are also experiencing efficient ~100 MHz motions that lead to virtually identical T1(13C)s in both environments. Here we attempt to identify similarities and differences between the types, length-scales, and cooperativities of the motions experienced by HTC chains in the neat melt and in the narrow crystalline channels of its α-CD-IC.

Introduction The conformations and motions experienced by polymer chains in various environments are intimately related to the physical behaviors manifested by the materials made from them. Because NMR spectroscopy can be used to simultaneously probe both the conformations and motions of polymers, their connections to polymer properties can be drawn by NMR examination of samples whose constituent chains reside in distinct and well defined environments, such as those in mobile liquid and rigid crystalline polymers. In addition, through formation of non-covalently bonded inclusion compounds (ICs) between guest polymer chains and certain small molecule hosts, polymer chains can be separated, extended, and confined to occupy the narrow channels of the resultant IC or clathrate. This is illustrated in Figure 1 with the IC-host α-cyclodextrin (α-CD), a cyclic oligosaccharide containing 6 glucose units and resulting from the enzymatic degradation of starch. The diameter of the α-CD cavity and the resultant channel in its columnar polymer-α-CD-IC crystal is very narrow at ~0.5 nm, similar to the inter-chain separation found in bulk polymer crystals. The extensions and geometrical constraints experienced by individual chains are similar in both of these solid-state environments. However, what may distinguish them are differences between the motional constraints imposed by regularly and closely packed chains in the bulk crystal and the host channels in α-CD-ICs, which are necessarily highly cooperative in the first case and may not be in the second. Using 13C-NMR, here we probe and compare the conformations and motions of the 36 carbon n-alkane hexatriacontane (HTC), in its bulk crystal, in the melt, and as a guest in the narrow channels of its IC formed with α-CD. The observed resonance frequencies, δ(13C), are used to obtain conformational information, while spin-lattice relaxation times, T1(13C), are measured to characterize the natures of the ~100 MHz motions experienced by HTC in each of these environments. 266 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Figure 1. Structure of α-CD and a schematic of a polymer-α-CD-IC.

Experimental Section Materials α-CD was purchased from Cerestar in powder form. Hexatriacontane (HTC, n-C36H74) and a xylene mixture were obtained from Aldrich and used without further purification. Formation of the HTC-α-CD-IC was described previously (1) and is schematically illustrated in Figure 2. Approximately 62 mg of HTC was placed in the bottom of a test tube. Then 1 g of α-CD was carefully placed in the test tube above the HTC. Approximately 6 mL of water was poured slowly down the side of the test tube. The test tube was sealed with a rubber septum and placed in an oil bath at 90 °C. The HTC melted and formed a thin layer on top of the water, while the α-CD dissolved in the water. The HTC-α-CD-IC precipitate began to form immediately. After three days at 90 °C, the mixture was slowly cooled to room temperature by turning off the bath heater. The precipitate was then washed with 100 mL of xylenes at 100 °C and water at room temperature and allowed to dry overnight at 50 °C.

Methods A Bruker AVANCE 500 MHz NMR Spectrometer with an Oxford Narrow Bore Magnet was used to measure 13C T1s of molten HTC using the inversion recovery pulse sequence. The carbon frequency was 125.77 MHz and DMSO-d6 was used for spin-locking at approximately 85 °C. Chemical shifts were referenced to DMSO assuming it resonates at 39.51 ppm vs TMS.

267 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Figure 2. Formation of HTC-α-CD-IC. High-resolution solid-state 13C-NMR experiments were performed at ICTP (Madrid, Spain) on a Bruker AvanceTM 400 spectrometer (Bruker Analytik GmbH Karlsrube, Germany) equipped with a Bruker UltrashieldTM 9.4T (13C frequency of 100.62 MHz), 8.9 cm vertical-bore super-conducting magnet. Cross-polarization and magic angle spinning (CP-MAS) NMR spectra were acquired wth a standard Bruker broad band MAS probe, spinning at 10 KHz. In all cases, high-power proton decoupling was used. The spectra were acquired with 1 ms CP contact time, 5 s recycle delay, and 1920 transients. The NMR spectra were processed and analyzed with the software package XWIN-NMRTM by Bruker. All freeinduction decays were subjected to standard Fourier transformation with 10 Hz line broadening and phasing. The chemical shifts were externally referenced to adamantane. Solid-state spin-lattice relaxation times, T1(13C), were measured for crystalline HTC and the HTC-α-CD-IC by inversion recovery experiments using the Torchia pulse sequence (2). Typically 800 transients were used for each point and 20-30 points were used for each measurement, covering the decay of the signal until it reached 30-40% of its maximum value. The NMR signal intensities of carbon nuclei observed for different time delays were used to calculate the spin-lattice relaxation times by fitting the signal intensity data to a single or double exponential decay function. The criterion for an acceptable fit was set at 95% of residuals within the range ± 0.03.

Results and Discussion HTC Conformations Figure 3 presents the 125.8 MHz 13C- NMR spectrum of neat liquid HTC measured at 85 °C, where 5 distinct resonances are observed. Their assignment 268 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


may be made by reference to Figure 4, which outlines the substituent effects (3) experienced by the HTC nuclei. The most intense resonance, at 28.75 ppm vs TMS, is produced by the internal methylene carbons C5-32, while the much smaller resonance slightly upfield at 28.37 ppm belongs to the C4,33 nuclei. Even though each of the C4-33 carbons have 2α, 2β, and 2γ carbon substituents, carbons C4 and C33 experience increased shielding from one of their γ-substituents (3), C1 or C36, respectively, because the C2−C3 and C34−C35 bonds are expected (4, 5) to have a slightly higher gauche population than the other internal C−C bonds.

Figure 3. 125.8 MHz 13C- NMR spectrum of neat liquid HTC measured at 85 °C (ppm vs TMS). It should be noted that the chemical shift scale above assumes that δ(DMSO) vs TMS observed at 85 °C remains at 39.51ppm, the value observed at room temperature. The small resonance ~2.5 ppm downfield from that of the internal C5-32 carbons belongs to C3,34, which experience only a single shielding γ-substituent. Finally the two small resonances at 21.60 and 12.81 ppm belong to the C2,35 methylene and C1,36 methyl carbons, respectively, which have 2α,1β,1γ and 1α,1β,1γ substituents.

C1,36, C2,35, C3,34, and C4-33 have α,β,γ; 2α,β,γ; 2α,2β,γ; and 2α,2β,2γ substituents, respectively. α,β-substituents deshield ~ 9-10 ppm, while γ-substituents produce a maximum shielding of ~ -5 ppm (3) (See Figure 4). 269 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Figure 4. HTC conformations and 13C nuclear shielding. 13C nuclei separated by three bonds in the shielding (~ -5 ppm) gauche (left) and non-shielding (0 ppm) trans (right) conformations.

Figure 5. 100.6 MHz CP-MAS/DD 13C- NMR spectra of neat crystalline HTC at 25 °C and HTC-α-CD-IC at 25 and 85 °C. 270 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Figure 5 provides a comparison of the expanded CP-MAS spectra of neat crystalline HTC recorded at 25 and HTC-α-CD-IC recorded at 25 and 85 °C, showing only HTC resonances. All resonances observed in neat crystalline HTC (Figure 5) appear downfield from their positions in the liquid spectrum (Figure 3). The downfield shifts are either ~ 2 or ~ 4 ppm, depending on whether or not a particular carbon has a single (C1,2,3,34,35,36) or a pair (C4-33) of γ-substituents, reflecting the fact that the internal C−C bonds in liquid HTC are ~40% gauche± (4) which provides a shielding of (1 or 2)x0.4x(-5 ppm) ~ -(2 or 4 ppm). The only notable differences between the CP-MAS spectra of HTC in its neat crystal and in its α-CD-IC is the splitting of the C2,35 methylene resonances at 24.8 ppm in crystalline HTC into two resonances at ~24.8 and ~22.8 ppm and the small resonance at 30.5-31.0 ppm, some -2 to -2.5 ppm upfield from the main resonance of the internal methylenes at ~33 ppm in the spectrum of HTC-α-CD-IC. We assign this latter peak to C5,32, because they are γ to C2,35. In neat crystalline HTC, all internal C−C bonds are trans, so the two resonances observed for the C2,35 and C5,32 methylenes of HTC residing in the narrow α-CD-IC channels strongly suggest two conformational populations for the C3−C4 and C33−C34 bonds: i) rigidly trans (24.8 and 33 ppm) and ii) trans and gauche± conformations inter-converting rapidly (22.8 and 30.5-31 ppm) on the ~100 MHz frequency scale, like the internal C−C bonds in liquid HTC. This explains the two distinct conformational environments and resultant resonance frequencies for the C2,5,32,35 carbons. It is important to note that the HTC methyl carbon region is absent in all three expanded spectra in Figure 5, but a single methyl resonance at ~15 ppm appears in each full spectrum. This is consistent with C2−C3 and C34−C35 bonds that are rigidly trans, similar to all remaining C−C bonds between C4 and C33. Apparently, only the C3−C4 and C33−C34 bonds are exhibiting significant contents of rapidly inter-converting gauche± and trans conformations, even though the C2−C3 and C34−C35 bonds might also have been expected to, since they are even closer to the HTC termini.

HTC Dynamics The spin-lattice relaxation times, T1( 13C), observed for molten HTC are presented in Table 1, and those measured for neat crystalline HTC and HTC residing in the narrow channels of its α-CD-IC are compared in Table 2. The T1(13C)s observed for interior CH2 carbons in neat crystalline HTC are ~500 s and for the molten liquid HTC are 1-2 s, which are consistent with previous reports (6, 7). The interior CH2 carbons of HTC chains in the narrow channels (~0.5 nm) of its α-CD-IC exhibit T1(13C) relaxation times of 1-4 sec. over the temperature range -30 to 85 °C. In the crystal HTC chains experience virtually no ~100 MHz motions, while molten HTC chains are efficiently moving at this frequency, leading to more than a two orders of magnitude decrease in HTC spin-lattice relaxation times. Even though the conformations of HTC chains in the narrow α-CD-IC channels are severely restricted compared to those of neat molten HTC chains, they are also experiencing efficient ~100 MHz motions that lead to virtually identical T1(13C)s in both environments. 271 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

Table 1. Spin-lattice relaxation times, T1( 13C), for liquid HTC at 85 °C


HTC Carbon = δ(13C), ppm

T1(

13C),

C1,36 = 12.81

3.4

C2,35 = 21.60

2.6

C4,33 = 28.36

1.7

C5-32 = 28.75

1.2

C3,34 = 30.95

2.1

s

Table 2. Spin-lattice relaxation times, T1( 13C), observed for HTC (C5-32) in the neat liquid and crystal and in the channels of its α-CD-IC T1(13C), s

Environment

Conformation

IC (-30 °C)

1.2

α-CD-IC-Channel

nearly all trans

IC (25 °C)

1,3.6

“

“

“

“

IC (85 °C)

2.6

“

“

“

“

Neat (25 °C)

505

Bulk crystal

all trans

Neat (85 °C)

2.1

Bulk liquid

random coil

Sample

Thus, we are faced with identifying similarities and differences between the types, length-scales, and cooperativities of HTC chain motions in the very different environments provided by its neat melt and the narrow crystalline channels of its α-CD-IC, as indicated in Figure 6.

Figure 6. Comparison of conformations and environments of HTC chains in their neat randomly-coiling liquid and confined to the narrow channels of their α-CD-IC. In Table 3 the spin-lattice relaxation times of neat α-CD and for HTC and α-CD observed in their IC are presented. First, it is obvious that in the cage 272 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


crystalline structure of neat α-CD, the constituent carbons in each glucose ring evidence T1(13C)s an order of magnitude longer than those observed in the columnar crystal structure of the HTC-α-CD-IC. It is also clear that the T1(13C)s of the ring carbons (C1-5) in α-CD are also ~ an order of magnitude longer than those of the guest HTC chains residing in the columnar IC channels formed by the stacks of host α-CD. As a result, we cannot attribute the very short T1(13C)s observed for the included guest HTC chains to facile ~100 MHz motions of the host α-CD protons. The near coincidence of T1(13C)s observed for HTC in the melt and in the narrow channels of its α-CD-IC must be a consequence solely of the ability of HTC chains to undergo facile ~100 MHz motions in both of these very disparate environments. Lyerla et al. (7) examined the 13C spin-lattice relaxation times for a series of liquid n-alkanes and a single branched alkane, 2-methylnonadecane. From observations made at 15.08 MHz, they were able to determine the corresponding effective correlation times, τeff, for rotation of their C−H bond vectors,

where NH is the number of protons directly attached to a carbon nucleus and rCH is the length of the C−H bond. They noted that i) for a particular carbon (C1, C2, C3, etc.) τeff progressively increases as the chain length increases, ii) τeffs increase from the chain ends toward the interior of each n-alkane, and iii) the ratio of correlation times for the terminal methyl groups to those of the internal methylene carbons also increases as the length of the n-alkane increases. This suggested that the rotational motion of C−H bond vectors in hydrocarbons can be analyzed as a sum of rigid body rotation of the entire n-alkane, with a rate of τ0-1, plus internal motion due to rotations about individual C−C bonds i, with a rate τi-1. The methyl branch in 2-methylnonadecane caused its C1-5 carbons to have resonances distinct from those of the C15-19 carbons at the unsubstituted terminus. However, the T1(13C) observed and the correlation time derived for C5 were the same as those observed and derived for the C6-15 carbons in 2-methylnonadecane and the C5-16 carbons in the linear isomer eicosane. As a consequence, Lyerla et al. (7) concluded that “the segmental motion, which determines the correlation time of a carbon in a (liquid) long chain n-alkane, involves ca.5-6 carbons on each side of a given carbon.” From this we can at least suggest that the efficient ~100 MHz motions responsible for the short T1(13C)s observed for interior HTC carbons in the melt involve somewhat cooperative inter-conversions between the conformations of 10-12 carbon atom chain segments. Clearly distinguishable from these longer-range cooperative motions are the much faster, more localized, and probably less correlated t ↔ g± conformational inter-conversions occurring about the -C−C- bonds in n-alkanes Cn (n ≥ 4) at frequencies much greater than 100 MHz. These result in the rapid establishment of equilibrium bond conformational populations that are reflected in the resonance frequencies observed in liquid n-alkanes, which are described above by the conformationally sensitive γ-gauche effect, and are ineffective in the spin-relaxation of their carbon nuclei. 273 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

Table 3. Solid-State 13C NMR spin-lattice relaxation times for neat α-CD and HTC and α-CD in HTC-α-CD-IC T1, s ← C1

C4

C2,C3,C5

C6

200, 13

190, 12

190, 12

103, 7

HTC/IC (25 °C)

37

27

21

4, 0.4

1, 3.6

HTC/IC (85 °C)

21

17

13

0.6

2.6

Sample Cage α-CD (25°C)


→

α-CD

CH2(HTC)

HTC (crystal at 25 °C)

505

HTC (melt at 85 °C)

1.2

Turning now to the ~100 MHz motions of HTC chains isolated in the narrow channels of its α-CD-IC, two things are clear from the observed splitting of the C2,35 methylene resonance at 24.8 ppm in crystalline HTC into two resonances at ~24.8 and ~22.8 ppm in HTC-α-CD-IC (also C5,32 at ~33 and ~31 ppm) and the two distinct T1s observed for the internal carbons at 25 °C (See Table 3). First, the short T1s observed for the internal carbons of HTC included in the α-CD-IC channels are not dominated by rigid-body rotation of all-trans HTC chains in the host α-CD-IC channels. Second, roughly half the C3−C4 and C33−C34 bonds are experiencing t ↔ g± inter-conversions that are much too rapid to cause spin-lattice relaxation of HTC carbon nuclei at the ~100 MHz frequency employed here to observe them, while the other half are rigidly trans. Consistent with the distinct T1s observed for terminal and interior carbons in crystalline n-alkanes (6), we observe T1s of 1.8, 30, and 505 s for C1,36, C2,35, and C3-34, respectively, in neat crystalline HTC and 3.3 and 1.3 s for the C1,36, and C2-35 HTC carbons in the α-CD-IC. When compared to the single T1s observed for all n-alkane carbons in their high temperature rotator phases (6), it is clear that neither in the neat crystal or α-CD-IC are the short HTC T1s a result of rigid body rotation. In addition, the two T1s observed at 25 °C for the internal HTC carbons in its α-CD-IC suggest that the HTC chains are moving under the constraints of two distinct channel environments. In Figure 7 note that the difference in diameters of the head and tail portions of α-CDs results in a gradual undulation of their columnar channels that is similar to stacked “hour-glasses”. Thus, the motions of those portions of HTC chains in the head-to-head channel regions would not be as constrained as those in the tail-to-tail regions, possibly providing sufficient room for facile ~100 MHz motions important to spin-lattice relaxation. These are likely the regions where C3−C4 and C33−C34 bonds experience the much more rapid t ↔ g± conformational inter-conversions that lead to resonance frequencies for some C2 and C35 carbons that are shifted upfield by conformationally averaged γ-gauche shielding from those in a rigid trans arrangement with C5 and C32. To assist our analysis and discussion of the ~100 MHz motions that efficiently relax the HTC carbon nuclei included in the narrow channels of the HTC-α-CD-IC, 274 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


we briefly summarize a fully atomistic molecular dynamics (MD) modeling of 30 repeat unit (60 carbon) polyethylene (PE-60) chain confined to a single α-CD- IC channel, and conducted by Pozuelo et al. (8). Both rigid and flexible α-CD-IC channels were considered during 1ns MD tra-jectories run at 500K. At the end of both trajectories, each -CH2–CH2- bond had a trans (0 ± 7-11°) population of over 99%. These small torsional fluctuations occured much more rapidly than ~100 MHz, and could not produce efficient T1(13C) relaxation of the HTC carbons when confined in the narrow α-CD-IC channels. In addition, virtually no rigidbody rotational or translational motions were observed for the included PE-60 chain, consistent with the previously mentioned observation of distinct T1s for the terminal and interior HTC carbons in its α-CD-IC.

Figure 7. The columnar structure of polymer-α-CD-ICs. The black areas indicate the channels occupied by the included guest polymer chains. Though an order of magnitude less frequently observed than in a single free PE-60 chain, t ↔ g± conformational inter-conversions were observed about ~ 5 and 0 of the PE-60 bonds when confined to flexible and rigid α-CD-IC channels, respectively. Extrapolation of these PE-60-α-CD-IC MD simulation results, suggests that nearly each of the 59 PE-60 -C–C- bonds [(5 bonds/ns)x 10ns = 50 bonds] would undergo a t ↔ g± conformational inter-conversion over a longer 10ns trajectory, with a corresponding frequency of ~ 100 MHz, while maintaining an overall nearly all trans average PE conformation. This might explain the short T1s observed for the predominantly all trans HTC chains in the HTC-α-CD-IC, but do the motions of these highly constrained conformational inter-conversions also explain the similar short T1s observed for HTC in the melt? 275 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


According to the analysis of the spin-lattice relaxations observed in liquid n-alkanes by Lyerla et al. (7), apparently not, because “the segmental motion, which determines the correlation time of a carbon in a (liquid) long chain n-alkane, involves ca.5-6 carbons on each side of a given carbon.” While the ~100 MHz chain motions effective in relaxing the carbon nuclei in liquid HTC and in its α-CD-IC are both conformational in origin, they likely differ both in their length-scales and cooperativities. In the α-CD-IC, t ↔ g± transitions are likely restricted to shorter-range ttt → g±tg∓ (kink) and possibly ttttt → g±tttg∓ (jog) transitions in the somewhat flexible α-CD channels (9), and, because t → g± transitions require surmounting a higher intrachain barrier than g± → t transitions and g± conformations are rare there, they are also likely more cooperative. Liquid HTC chains are not, however, constrained by α-CD-IC channels, but must only avoid segmental overlaps during and after their conformational transitions, and so their ~100 MHz motions are likely less cooperative and apparently longer in range. Before concluding, it should be noted that the conformations and motions of n-alkane guests included in the narrow channels of their inclusion compounds or clathrates made with other hosts, such as urea (U), cyclotriphosphazenes (TPP), perhydrotriphenylene (PHTP), etc. generally appear to be distinct (10–13) from those discussed here for HTC in its α-CD-IC. This may be due to the unique covalently-bonded cyclic nature of host CDs (See Figure 1). The constraints experienced by guest polymers, that are threaded through host CDs and that are eventually completely included in the channels formed by the uniaxially stacked CDs when they crystallize to form a polymer-CD-IC, are expected to be more severe than those produced by the channels formed from U, TPP, and PHTP hosts, whose constraining channel walls lack the strength of the covalent bonding in individual CDs.

Summary and Conclusions In the rigid crystals of HTC there are apparently almost no ~100 MHz motions able to relax the HTC 13C nuclei. In the randomly-coiling liquid there are relatively long-range conformational motions, that are able to efficiently relax the HTC 13C nuclei. Though predominantly restricted to the nearly fully extended all trans conformation in the narrow (~5Å) α-CD-IC channels, apparently here there are also facile ~100 MHz conformational (t ↔ g) motions occurring that very efficiently relax the HTC 13C nuclei. However, in the α-CD-IC channels the motions of guest HTC chains are more cooperative in nature and of a shorter-range than in the neat liquid.

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277 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

13C-NMR Observed Conformations and Motions of Neat Liquid and

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