The guest ordering and dynamics in urea inclusion compounds studied by solid-state 1H and 13C MAS NMR spectroscopy

Journal of Molecular Structure 1006 (2011) 113–120

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The guest ordering and dynamics in urea inclusion compounds studied by solid-state 1H and 13C MAS NMR spectroscopy Xiaorong Yang ⇑, Klaus Müller Institut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany

a r t i c l e

i n f o

Article history: Received 6 June 2011 Received in revised form 28 August 2011 Accepted 29 August 2011 Available online 6 September 2011 Keywords: 13 C NMR 1 H NMR Urea inclusion compounds Guest–host systems

a b s t r a c t Urea inclusion compounds with different guest species were studied by 13C CP MAS and 1H MAS NMR spectroscopy. It is possible to arrange the asymmetric guest species in three different ways: head–head, head–tail and tail–tail. 13C CP MAS NMR studies indicate that the preference arrangement is determined by the interaction strength of the end functional groups. 13C relaxation experiments are used to study the dynamic properties of urea inclusion compounds. 13C relaxation studies on urea inclusion compounds with n-alkane or decanoic acid show that the 13C T1 and 13C T1q values exhibit the position dependence towards the center of the chain, indicating internal chain mobility. The analysis of variable-temperature 13 C T1q experiments on urea inclusion compounds with hexadecane and pentadecane, for the first time, suggests that chain fluctuations and lateral motion of n-alkane guests may contribute to the 13C T1q relaxation. Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved.

1. Introduction By extensive hydrogen-bonds the urea molecules are connected to form an array of linear, hexagonal channels with inner diameter of ca. 5.25 Å that can accommodate guest molecules with suitable shape and size [1,2]. The guest molecules in urea channels, imposed spatial constraint, can experience a variety of dynamic processes, such as rapid rotation along the long molecular axis, fast torsional libration at the chain ends and rapid methyl-group rotation [3–7]. A wide range of techniques have been applied to investigate the dynamic properties of urea inclusion compounds, including solidstate NMR spectroscopy [3–6], incoherent quasi-elastic neutron scattering [8], molecular dynamics simulation [9] and X-ray diffraction [10]. The vast majority of these investigations have probed the dynamic properties of the guest molecules [3–7,11], and some studies about the dynamics of the urea molecules have also been done [12]. Among these different techniques for studying molecular motions, dynamic NMR spectroscopy is well established, which give access to different motional time-scales [3–7,11]. Dynamic 2H NMR studies for urea inclusion compounds containing alkane guests have revealed the guest molecules rotating around the channel long axis in the ps-ns timescale range at the room temperature [5,6,13]. 2H NMR studies on monocarboxylic acids/urea show ⇑ Corresponding author. Present address: Department of Chemistry, Pennsylvania State University, University Park, PA 16803, USA. Tel.: +1 814 867 2975; fax: +1 814 865 3228. E-mail address: [email protected] (X. Yang).

that the monocarboxylic monomers or dimers experience a fast uniform rotation diffusion about the c-axis on the 2H NMR timescale and wobble slowly at a limited angle about a perpendicular arc [3] In this work, spin–lattice relaxation time in the rotating frame, T1q (13C), provides a way to investigate the slower motions in the low to mid-kilohertz frequency range [14,15]. On the other hand, T1q (1H) provides information about spatially sensitive proton spin diffusion [14,16,17]. In organic chemical reactions and the assemblies of supermolecules, the functional groups play an important role. The study of guest molecules isolated by urea channels has the potential to provide the interaction strength between functional groups, which can be used to optimize the reactions. Hence, this work is dedicated to the understanding of the effect of the end functional group on the local structure and dynamics of the guest molecules in nano-channels of the urea. Urea inclusion compounds with n-alkane (n-hexadecane, decane and pentadecane) or n-alkane of different di- or mono-substituting end functional groups (ACOOH, AF) are investigated by 1H MAS and 13C CP MAS NMR spectra and spin lattice relaxation experiments (1H T1q, 13C T1q and 13C T1). By the present solid state NMR studies, new information about the dynamics and ordering of the guest molecules in urea inclusion compounds, which indirectly also affect the guest–guest and the guest–host interactions in these systems, can be provided. In addition, for the first time, variable temperature spin–lattice relaxation experiments in the rotating frame (13C T1q) are performed on samples with the guests of hexadecane and pentadecane in urea or urea-d4. By analyzing the experimental data, dynamic properties of the guest molecules in the microsecond time scale are derived. The

0022-2860/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2011.08.056

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dynamic parameters are compared with the available data in previous 2H NMR studies [3–7].

3. Results and discussion 3.1.

2. Experimental 2.1. Sample preparation All chemicals for the chemical synthesis were obtained from Aldrich Chemicals, and were used without further purification. 2.1.1. Urea-d4 Urea-d4 was prepared by slowly cooling a warm solution of urea in methanol-d1 (the molar ratio of urea to methanol-d1 = 1:4), followed by filtration of the solution and drying of the product. This procedure was repeated once to ensure an almost complete exchange of the urea protons by deuterons. 2.1.2. Preparation of urea inclusion compounds The urea inclusion compounds were prepared by slowly cooling warm solutions of the guest molecules n-decane, n-pentadecane, n-hexadecane, 1-fluorodecane, decanoic acid, dodecanedioic acid, sebacic acid with urea or urea-d4 in methanol or methanol-d1, respectively. The white needles were filtered, washed with 2,2,4trimethylpentane, and dried. 2.2. Methods 2.2.1. DSC (differential scanning calorimeter) measurement Calorimetric studies were performed with a differential scanning calorimeter Netzsch DSC 204 (Selb/Germany) under nitrogen flow at heating rates of 10 °C/min. 2.2.2. High resolution solid state 1H MAS and 13C CP MAS NMR measurement 13 C CP/MAS and 1H MAS NMR experiments were performed on Bruker CXP 300 (Rheinstetten/Germany) spectrometer operating at static magnetic fields of 7.04 T. The resonance frequencies were 75.47 (13C), 300.13 MHz (1H) for the CXP 300 spectrometer. A double tuned Bruker 4 mm MAS probe was used for the 13C CP/MAS, T1 (13C), T1q (13C), and T1q (1H) experiments. p/2 pulse lengths were 4 ls. Contact times varied between 500 ls and 6 ms, as indicated in the text, and recycle delays of 6 s were used between successive scans. A sample rotation frequency of 5 kHz was used, and the number of scans was between 512 and 1024 depending on the signal/noise ratio. 13C T1 data were obtained with cross-polarization (CP) excitation as proposed by Torchia [18]. Standard spin-lock pulse sequences were used for the determination of 13C and 1H T1q relaxation times [19,20]. The 13C were spin locked in a field of 50 kHz. 1 H MAS spectra were measured with a Bruker CXP 2.5 mm double resonance MAS probe (at 300.13 MHz) with a p/2 pulse length of 4 ls. Typically, 32 or 64 scans were accumulated with recycle delays of 2 s or 4 s and a sample rotation frequency of 34 kHz was used. The sample temperature was controlled by a Bruker BVT 2000 unit. The samples were initially held at each new temperature for at least 15 min to allow full equilibration. In general, the accuracy of temperature was found to be within ±1 K. The data from 1H MAS and 13C CP/MAS NMR measurements were handled by utilizing the MestRec software [21]. 13C chemical shifts were referenced to the external standard adamantane. This value was then expressed relative to TMS (d = 0 ppm). Likewise, 1H chemical shift values are given relative to TMS (d = 0 ppm).

13

C CP MAS and 1H MAS NMR spectra

Fig. 1 shows high resolution 13C NMR spectra of urea inclusion compounds with different guest molecules. According to previous studies [22–24], the assignments of these five urea inclusion compounds were done. They are listed in Tables 1–3 along with chemical shifts of these guest molecules in solution. In the 13C NMR spectra of these UICs, high resolution is achieved, suggesting that the guest molecules are of high mobility. Because the guest molecules are isolated by the urea channels, dipolar coupling between the guests is reduced. For 1-fluorodecane/urea inclusion compounds, the CH2F group gives rise to two broadened resonances at 85.2 and 83.2 ppm due to dipolar coupling between the F nucleus and the directly bonded carbon [24] (the 13C NMR spectrum is recorded with only proton decoupling), as illustrated in Fig. 1b. For urea inclusion compounds with asymmetric guest molecules, two adjacent guest molecules can arrange in three different ways: head–head, head–tail and tail–tail [25,26]. The relative ratio of these arrangements depends on the interaction strength of the end functional groups [26,27]. Thus, in 1-fluorodecane, for the CH2F group with two different arrangements, CH2F  H3C and CH2F  FH2C, four resonances might be observed. However, only two broadened resonances arising from the dipolar coupling between the F atom and the directly bonded carbon are registered, suggesting that the electronic environment of the carbon in the CH2F group is not affected by the different end arrangements. Compared with dipolar coupling, the influence of the different end arrangements on the 13C chemical shift in the CH2F group is too weak to be observed. Likewise, the CH3 group exists in two different arrangements, CH3  FH2C and CH3  H3C. Again, due to the dipolar coupling between the F atom and its neighbour carbons [24], one resonance of the CH3 arising

Fig. 1. 13C CP MAS NMR spectra of urea inclusion compounds with guest molecules: (a) n-decane, (b) 1-fluorodecane, (c) decanoic acid, (d) dodecanedioic acid, (e) sebacic acid.

X. Yang, K. Müller / Journal of Molecular Structure 1006 (2011) 113–120 Table 1 NMR parameters from C10H22/urea a

a

13

C CP MAS and 1H MAS NMR studies on n-decane/urea.

d (ppm)

CT : 1.5 ms

Solution

C1 C2 C3 C4 C5

14.2 22.9 32.2 29.6 29.9

b

Urea

T1q(H) (ms)

T1q(C) (ms)

T1(C) (s)

4.1 9.7 9.2 10 10

247 301 255 271 223

1.8 3.8 4.3 4.8 4.6

c

15.1 25.2 34.8 33.1 33.3

Contact time. C chemical shifts of n-decane in CDCl3 [23]. C chemical shifts of n-decane in their UICs in this work.

b 13 c 13

Table 2 NMR parameters from 13C CP MAS and 1H MAS NMR studies on 1-fluorodecane/urea.

a

C10H21F/urea CTa: 6 ms

d (ppm) Ureab

C1 + F C1 + F C2 C3 C4–7 C8 C9 C10

85.2 83.2 32.5 27.8 32.9 34.5 24.8 14.8

T1q(H) (ms)

T1q(C) (ms)

T1(C) (s)

10

6.7

8.5

9.5 11 11 9.3

6.0 6.7 7.2 6.2

6.8 8.5 8.5 4.6

Contact time. C chemical shifts of 1-fluorodecane in their UICs in this work.

b 13

Table 3 NMR parameters from 13C CP MAS and 1H MAS NMR studies on sebacic acid, dodecanedioic acid, and decanoic acid in urea. CTa: 1 ms

d (ppm)

T1q(H) (ms)

T1q(C) (ms)

T1(C) (s)

1.3 1.5 1.6 1.5

514 187 194 175

5.6 2.3 2.1 2.4

HOOCC10H20COOH/urea (CTa: 0.3 ms) COOH 174 182 C2 33.6 35.6 C3 24.5 26.3 C4 28.8 31.8 C5–6 28.7 31.4

0.8 1.1 1.1 1.1 1.1

292 177 194 161 177

2.3 3.4 3.1 2.9 3.3

CH3C8H16COOH/urea (CTa: COOH 181 C2 34.3 C3 24.8 C4 29.2 C5 29.4 C6 29.5 C7 29.4 C8 32.0 C9 22.8 C10 14.1

3.5 4.0 4.1 4.2 4.3 4.1 4.0 4.3 4.1 4.2

408 139 164 216 189 157 162 196 232 259

5.7 2.5 2.3 2.9 2.3 2.6 2.9 3.1 3.4 4.0

Solutionb HOOCC8H16COOH/urea COOH C2 C3 C4–5

Ureac 182 35.2 26.5 30.9

1 ms) 182 36.1 26.1 31.9 32.3 33.5 32.9 35.4 25.1 15.6

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In contrast to two peaks of the end group in other asymmetric guest molecules [24–26], the resonances of the CH3 or COOH end-group of decanoic acid display a single peak. As reported previously [28–30], the aliphatic monocarboxylic acids form exclusively head-to-head dimers in urea channels. Thus both CH3 and COOH end-groups have only one arrangement, i.e. CH3  CH3 and COOH  COOH which correspond to two single peaks recorded at 15.6 and 182 ppm, respectively. Spectra of dodecanedioic acids/ urea, Fig. 1d, and sebacic acids/urea, Fig. 1e, are similar. The two guest molecules have the COOH groups at the both ends and thus are expected to form infinite one-dimensional intra-channel hydrogen-bonded chains of guests [28]. 13 C NMR spectra of pentadecane and hexadecane in urea and urea-d4 are shown in Fig. 2. The assignments of the peaks and the 13C NMR line widths are summarized in Table 4 along with the chemical shifts of the guest species in solution. The comparison of the 13C NMR line widths in the UICs from urea and urea-d4 shows a larger value for the guest species in urea-d4 (see Table 4). Because the 13C NMR spectra are recorded with only proton decoupling, 13CA2H dipolar coupling between the guest species and the host matrix causes the additional line broadening of the guest species in urea-d4. However, the 13C NMR line width of the urea carbonyl group is found to become smaller upon urea deuteration, which is attributed to interference effects [31,32] between the 1H decoupling r.f. field and the urea reorientation motions, i.e., 180° flips around the C@O and the CAN bonds [33,34]. This is supported by a broadening for the carbonyl 13C NMR signal with an increase of the 1H decoupler power. Similar observations were found for other UICs [35,36]. When the signal intensity is recorded as a function of contact time for cross-polarization, the optimum contact time with maximal signal intensity is found to vary considerably with guest species and deuteration of the urea matrix. Thus, for hexadecane/ urea, a maximum 13C CP signal was observed at contact times of 0.5 ms and 4 ms for urea and urea-d4, respectively. As a consequence of the loss of the urea protons, a longer contact time is required for the deuterated urea clathrates, suggesting that the urea protons contribute to magnetization transfer during cross-polarization as well. Similar trends for the 13C signal intensity during cross-polarization were also observed recently for other urea inclusion compounds [35,36]. It is surprising that this is not found for pentadecane/urea. The contact time of 0.5 ms is found to be suitable for the pentadecane guest molecules in both urea and uread4, which can be correlated to the 1H T1q values, as shown below in Tables 5 and 6. A ratio of about 7 of 1H T1q in urea-d4 to that in urea is obtained for hexadecane while a value of about 2.5 is derived for pentadecane. Fig. 3 exhibits 1H MAS NMR spectra recorded at a sample spinning rate of 34 kHz for pentadecane and hexadecane in urea and urea-d4. In general, very low resolved 1H NMR spectra are observed, which indicates that there is strong 1H dipolar coupling

a

Contact time. C chemical shifts of sebacic acid, dodecanedioic acid, and decanoic acid in CDCl3 [23]. c 13 C chemical shifts of sebacic acid, dodecanedioic acid, and decanoic acid in their UICs in this work. b 13

from the CH3  FH2C arrangement is pronouncedly broadened, flanking the other resonance of the CH3 at 14.8 ppm which reflects the CH3  H3C arrangement [24]. For the decanoic acid/urea inclusion compound, a surprisingly high resolved 13C NMR spectrum is registered (see Fig. 1c). The chemical shift of every carbon in the chain can be distinguished.

Fig. 2. 13C CP MAS NMR spectra of n-pentadecane in: (a) urea, (b) urea-d4, and n-hexadecane in: (c) urea, (d) urea-d4.

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Table 4 NMR parameters from 13C CP MAS and 1H MAS NMR studies on n-pentadecane and nhexadecane in urea and urea-d4. C15H32

a

C1 C2 C3 C4 C5–7 C@O C16H34 C1 C2 C3 C4 C5–8 C@O

Dm1/2(13C)c (Hz)

d13C (ppm) b

Solution

Urea

Urea-d4

Urea

Urea-d4

14.2 22.8 32.1 29.5 29.9

14.7 25.1 34.5 33.1 33.5 164

14.5 24.9 34.3 32.9 33.4 164

15.4 17.3 18.2 19.3 26.3 108

17.6 21.5 22.1 23.1 27.2 69.8

15.2 25.4 34.8 33.5 33.8 164

15.2 25.4 34.8 33.5 33.8 164

15.4 16.2 17.7 24.5 25.9 115

13.3 17.7 19.3 43.3 43.3 62.1

14.1 22.8 32.0 29.5 29.9

Fig. 3. 1H MAS NMR spectra of n-pentadecane in: (a) urea, (b) urea-d4, and n-hexadecane in: (c) urea, (d) urea-d4.

a 13

C chemical shifts of pentadecane and hexadecane in CDCl3 [23]. C chemical shifts of pentadecane and hexadecane in their UICs in this work. 13 C NMR line width.

b 13 c

in these UICs. Fig. 3b and d also showed that there are urea peaks at 7 ppm even in urea-d4 inclusion compounds, which suggests that the urea host lattice is not completely deuterated. The comparison of the chemical shifts of the guest species in urea and in solution shows that the chemical shifts of the guest species in urea exhibit a downfield shift. As reported previously [37,38], these chemical shift alterations can be traced back to conformational changes. On the basis of these observations and the

results from 2H NMR, Raman, IR and X-ray investigations [3–6,22,37,38], it was concluded that the guest species in urea adopt a nearly all-trans conformational state while in solution a fast equilibrium between gauche and trans conformers exists. For the methyl group in n-alkane and in decanoic acid, these effects are less pronounced, which can be attributed to the presence of a small fraction of gauche conformers and/or fast methyl rotation established from IR and 2H NMR spectroscopy [3–6]. The 13C chemical shift alteration of the dodecanedioic acid carbonyl group is much larger than that of the decanoic acid carbonyl group, which might be related to the particular situation that decanoic acid exists in head-to-head dimers with the two facing COOH groups [28–30]. These dimers can be regarded as one guest molecule, and thus the acid alkyl chains of decanoic acid/urea have conformations similar to those of the pure n-alkane chain. This was also reported previously [3,29,39,40]. However, the dodecanedioic

Table 5 T1q(C), T1q(H) and T1(C) values as a function of temperature for n-pentadecane in urea and urea-d4. C15H32/urea (CTa: 0.5 ms)

C1 C2 C3 C4 C5–7 C15H32/urea-d4 (CTa: 0.5 ms) C1 C2 C3 C4 C5–7 a b

T1q(C) (ms)

T1q(H) (ms)b

T1(C) (s)b

294 K

300 K

305 K

310 K

315 K

320 K

204 189 150 176 139

136 142 135 136 126

169 189 138 146 129

212 230 172 173 153

360 246 175 187 166

404 260 207 202 179

8.2 6.3 6.0 5.8 5.9

2.7 4.9 5.7 5.9 6.0

346 328 289 367 298

240 325 231 237 205

178 242 198 205 186

262 291 222 240 204

482 597 355 386 311

574 628 489 556 404

14 16 16 16 16

4.5 9.5 11 12 13

Contact time. Value at 294 K.

Table 6 T1q(C), T1q(H) and T1(C) values as a function of temperature for n-hexadecane in urea and urea-d4. C16H34/urea (CTa: 0.5 ms)

C1 C2 C3 C4 C5–8 C16H34/urea-d4 (CTa: 4 ms) C1 C2 C3 C4 C5–8 a b

Contact time. Value at 294 K.

T1q(C) (ms) 294 K

300 K

305 K

310 K

315 K

320 K

262 357 235 261 184

248 279 217 200 161

240 251 204 189 161

131 154 137 125 110

188 201 172 161 138

255 248 185 178 164

746 687 479 595 284

729 668 381 494 280

661 547 362 407 277

649 507 351 363 269

572 474 310 314 263

686 545 327 355 317

T1q(H) (ms)b

T1(C) (s)b

2.4 2.7 2.6 2.4 2.4

2.6 6.2 8.7 8.9 9.1

8.1 14 16 17 18

2.3 5.4 7.0 7.5 9.2

325 K

– 652 347 402 331

X. Yang, K. Müller / Journal of Molecular Structure 1006 (2011) 113–120

acid forms one-dimensional infinite intra-channel chains by hydrogen-bonds [28]. It might well be that in the latter case a stronger hydrogen bond occurs which gives rise to the larger change of the chemical shift value. 3.2. T1(C), T1q(C) and T1q(H) relaxation data The dynamic properties of the guest components are assessed by 13C T1q, 13C T1 and 1H T1q measurements. The derived data are summarized in Tables 1–3, 5 and 6. In general each proton of the guest molecule has the same 1H T1q value (apart from the COOH group due to the large steric hindrance and/or dipolar coupling of the COOH group, and the methyl group which has a smaller value because of the rotation of the methyl group) due to the strong proton spin diffusion. For 1-fluorodecane/urea, the 13C T1q values of all carbons within the chains are identical and rather small (see Table 2), which can be related to the fast overall chain rotation of the guest molecules. 13 C T1 values of all carbons within the chains are much longer than 13 C T1q values. The methyl group in the CH3  CH3 arrangement has a smaller 13C T1 value compared to other carbons in the chains, which is attributed to the fast methyl group rotation as observed for n-alkane/urea inclusion compounds. Again, a similar 13C T1 value is recorded for all other carbons within the chains. Obviously, there is no efficient relaxation process present which affects 13C T1 relaxation. For decanoic acid, dodecanedioic acid and sebacic acid, the 13C T1q and 13C T1 values of the carboxyl group are relatively large (see Table 3), which can be attributed to the formation of hydrogen bonds between the guests and guests and the reduced internal conformational mobility. The head-to-head arrangement is a tight unit as a result of strong hydrogen bonding and the conformation of the chain in the proximity of the molecular head is quite rigid [41]. The 13C T1q and 13C T1 values of the decanoic acid are positiondependent, the 13C T1q and 13C T1 values increase towards the two ends of the dimers. Although 2H NMR studies on monocarboxylic acids/urea show that the monocarboxylic monomers or dimers experience a fast uniform rotation diffusion about the c-axis on the 2H NMR time-scale [3], it is also shown that alkanoic acid chains wobble slowly at a limited angle about a perpendicular arc [3]. This slower and limited wobbling motion might explain the 13C T1q and 13C T1 values of the decanoic acid, which is consistent with the position-dependent behavior of 13C T1q and 13C T1 values. The position-dependent 13C T1q and 13C T1 values of the decanoic acid might also reflect the presence of some internal chain mobility. This is supported by 2H NMR studies on single-crystals of monocarboxylic acids/urea compounds with n = 8, 12 and 16 at ambient temperature with the long axis of the crystal perpendicular to the static magnetic field. For the spectra of the three compounds doublets for methyl and bulk methylene deuterons were observed and for the longer chain guests separate signals for penultimate methylene groups were resolved as well [3]. From the data analysis it was shown that at room temperature a small gauche population occurs near the methyl end of the alkanoic acid chains. Unlike the position dependence of 13C T1q and 13C T1 values of decanoic acid, for dodecanedioic acid and sebacic acid the 13C T1q and 13C T1 values at different positions in the chain are almost identical (with the exception of carboxyl carbon due to larger spatial hindrance of two facing COOH groups), which implies that there is no internal chain mobility for these two UICs as for monocarboxylic acids/urea. The dicarboxylic acids in urea have two COOH groups at both chain ends, and thus form infinite H-bonded chains [28]. Hence the guest chain may experience fast and

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uniform motion and this motion is too fast to affect 13C T1q and 13 C T1 relaxation. For hexadecane/urea and pentadecane/urea it is found that the 13 C T1q values slightly vary with chain position, as reflected by a T1q decrease towards the center of the chain (see Tables 5 and 6). This finding indicates that besides the rapid rotation of the overall chains established by 2H NMR studies [3–6], the guest molecules also possess internal chain mobility, for instance, torsional libration about the penultimate CAC bond, methyl rotation about its local threefold symmetry axis C3 and gauche-trans isomerization which were examined by 2H NMR studies [7]. Urea deuteration is characterized by an increase for both the 13C T1q and 1H T1q values due to reduced effective hetero- and homonuclear dipolar interactions between guest and host components. The 13C T1 values are much longer than the 13C T1q values, and they both exhibit distinct position dependence. Thus, the 13C T1q values decrease towards the center of the chain while 13C T1 values increase towards the center of the chain. The position-dependent 13 C T1 values might reflect internal chain mobility as well. Upon urea deuteration, the 13C T1 values of the pentadecane/urea increase due to reduced effective hetero- and homonuclear dipolar interactions between guest and host components, however the 13 C T1 values of the hexadecane/urea are almost unaffected by urea deuteration. The 13C T1 and 1H T1q of the end methyl groups for both samples show smaller values, as a consequence of the high mobility of end methyl which was proved by 2H NMR studies [3–6]. Moreover, from the variable temperature 13C T1q data (see Tables 5 and 6, Fig. 5) a minimum is found at 300 K for pentadecane/urea and 310 K for hexadecane/urea. The decay curves of T1q (the intensity as a function of the pulse space in spin-lock sequence) are given in Fig. 4 for C4 at 300 K and C3 at 305 K in hexadecane/urea. A semi-quantitative analysis of these 13C T1q

Fig. 4. The intensities of (a) C4 peak at 300 K and (b) C3 peak at 305 K in C16H34/urea as a function of the pulse space in spin-lock sequence.

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relaxation data was done by using the well-known expressions [42] for T1q in the presence of isotropic motions and a dominant heteronuclear dipolar interaction

1 13

T 1q ð CÞ

¼

A2 ½4  Jðx1;C Þ þ Jðx0;H  x0;C Þ þ 3  Jðx0;C Þ 2 þ 6  Jðx0;H þ x0;C Þ þ 6  Jðx0;H Þ

with J n ðxi Þ ¼

sc 1 þ x2i s2c

;

xi ¼ cBi

ð1Þ ð2Þ

Here A2 is a factor which contains the gyromagnetic ratios of the interacting nuclei, and the average internuclear distance as h1/r3i. x0,C and x0,H are the Larmor frequencies of the 13C and 1H nuclei, respectively, and x1,C is the nutation frequency of the radio frequency field B1(13C). By fitting the 13C T1q minimum with Eq. (1), factor A of the motion corresponding to the minimum were calculated. Substituting the derived factor A back into Eq. (1), a set of correlation times as a function of temperature are obtained by fitting the experimental 13C T1q curve, as summarized in Tables 7 and 8. It is found from Tables 7 and 8 that the correlation times of hexadecane are slightly longer than those of pentadecane at the same temperature, which implies that the pentadecane guest molecules have a higher mobility than hexadecane guests. This is related to the interactions between the guests and hosts, because steric repulsion between host and guest atoms dominates, longer chains have more atoms in contact with the urea channel. These findings are in agreement with the results from the 2H NMR studies, as reflected by an

approximately linear dependence of activation energies on increasing chain length for alkane guest rotation about the long axis [3,36]. The intermolecular interactions between guests and interactions between the guest molecules and the wall of the urea channels also result in chain-length dependence of the guest diffusion behavior in n-alkane/urea inclusion compounds as studies by pulsed field-gradient spin-echo NMR spectroscopy. It is reported that the guests are diffusing and have the slow and fast diffusion components which are averaged out with the increase of the diffusion time. The diffusion coefficients decrease as the carbon number increases and the diffusion coefficient of n-alkane molecules in the inner part of a urea channel is much smaller than that of the guest molecules in the outer parts [43]. On the basis of these results it is concluded that for pentadecane and hexadecane 13C T1q relaxation does not reflect the motions being responsible for 2H T1Z relaxation [6], but is governed by another motional process which occurs on a slower time-scale. It can be seen that the correlation times of ca. 106 s derived from the 13 C T1q data analysis are much longer than those reported previously from the quantitative 2H T1Z analysis – correlation time of ca. 1010 s for rapid and almost unrestricted rotation of the alkyl chains around the channel long axis [6]. That is, the motion dominating the 13C T1q relaxation is much slower than that for 2H T1Z relaxation. Chain fluctuations of the n-alkane guests may dominate 13C T1q relaxation. This assumption is supported by the position-dependent behavior of 13C T1q values. Lateral motion is sensitive to the variation of the distance, position and interaction between the guest and host, thus it can also provide some contribution to the 13C T1q relaxation, as reflected by an increase of 13C T1q values on urea deuteration.

Fig. 5. T1q values as a function of the temperature: (a) C15H32/urea, (b) C15H32/urea-d4, (c) C16H34/urea, (d) C16H34/urea-d4.

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X. Yang, K. Müller / Journal of Molecular Structure 1006 (2011) 113–120 Table 7 The correlation times as a function of temperature for n-pentadecane in urea and urea-d4. Correlation time sc (s) 294 K

300 K

305 K

310 K

315 K

320 K

C15H32/urea C1 C2 C3 C4 C5–7

Ea (kJ/mol)

6.4  106 5.3  106 3.8  106 5.2  106 3.8  106

2.5  106 2.4  106 2.5  106 2.5  106 2.4  106

1.3  106 1.2  106 2.2  106 1.7  106 1.9  106

8.9  107 8.5  107 1.2  106 1.1  106 1.3  106

4.8  107 7.7  107 1.1  106 1  106 1.1  106

4.2  107 7.1  107 9.2  107 9.5  107 1  106

85.9 ± 6.9 63.8 ± 10.4 47.0 ± 4.5 54.4 ± 7.9 43.9 ± 3.5

C15H32/urea-d4 C1 C2 C3 C4 C5–7

8.8  106 5.5  106 6.1  106 8  106 6.9  106

5.6  106 5.4  106 4.3  106 4.3  106 3.8  106

2.5  106 2.4  106 2.4  106 2.4  106 2.4  106

9.6  107 1.3  106 1.5  106 1.4  106 1.6  106

4.7  107 5.2  107 7.5  107 7  107 8.1  107

3.9  107 4.9  107 5.3  107 4.7  107 6  107

106.9 ± 8.6 87.9 ± 11.2 80.3 ± 4.8 90.4 ± 2.5 77.8 ± 2.9

Table 8 The correlation times as a function of temperature for n-hexadecane in urea and urea-d4. Correlation time sc (s) 294 K

300 K

305 K

310 K

315 K

320 K

C16H34/urea C1 C2 C3 C4 C5–7

9.3  106 1.1  105 7.5  106 9.5  106 7.4  106

8.7  106 8.1  106 6.9  106 6.9  106 6.2  106

8.4  106 7.1  106 6.4  106 6.4  106 6.1  106

2.4  106 2.4  106 2.4  106 2.4  106 2.4  106

9.8  107 1.1  106 1.2  106 1.2  106 1.2  106

6.7  107 8.5  107 1  106 1  106 9.4  107

C16H34/urea-d4 C1 C2 C3 C4 C5–7

5.2  106 6.1  106 6.7  106 8.5  106 3.7  106

5  106 5.8  106 4.8  106 6.8  106 3.5  106

4.3  106 4.2  106 4.3  106 5.1  106 3.4  106

4.2  106 3.5  106 4.1  106 4.2  106 3  106

2.4  106 2.4  106 2.4  106 2.4  106 2.4  106

1.3  106 1.4  106 1.7  106 1.5  106 1.3  106

On the other hand, there is evidence that the motions of long alkyl chains are quite different in the middle of the chain and at its ends. In particular, NMR [44], Raman [45], and molecular dynamics [46] results have shown that in n-alkane UICs gauche defects may be localized only at the ends of the chains, in good agreement with the smaller chemical shift alteration of end carbons of the guest species in urea and in solution (see above). Despite the strong confinement of the chains by the urea channel, the end CD3 groups and the CD2 groups near the chain ends have more space to undergo reorientation, as proved, for instance, for nonadecane/urea by the separate 2H NMR splittings of the a and b segments. All the other bulk CD2 groups are in the trans conformation, and their CAD bonds are perpendicular to the channel axis. Moreover, since the conformational defects are in fast exchange, they might provide some contributions to the factor A in Eq. (1), thus give rise to the position-dependent 13C T1q values of urea inclusion compounds with hexadecane and pentadecane. Recently a comprehensive 13C NMR relaxation study for a series of n-alkane/UICs was published [47]. It was shown that the 13C T1 values are almost independent of the particular chain length, and vary with the chain position. Both results were attributed to the overall and internal chain mobility, as also verified, for instance, by variable temperature 2H NMR studies on n-alkane/UICs [4–6]. Similar observations as described above are obtained from the present work. As mentioned above, the decanoic acid head-to-head dimer has some similarities with the n-alkane/urea inclusion compounds [3,29,39,40]. Head–head dimers of the aliphatic monocarboxylic acids in urea channel can be regarded as one guest molecule, and the acid alkyl chains of decanoic acids/urea have conformations similar to those of the pure n-alkane chain [3,29,39,40]. Therefore, the position-dependent behavior of the 13C T1q and 13C T1 values

325 K

Ea (kJ/mol) 92.5 ± 17.5 88.4 ± 12.2 72.7 ± 12.8 78.1 ± 10.8 68.8 ± 13.3

1  106 1.3  106 1.2  106 1.2  106

42.5 ± 10.5 51.2 ± 5.7 44.5 ± 4.7 55.9 ± 4.7 33.8 ± 6.4

for decanoic acid is also similar to those of the n-alkane (see Tables 1, 5 and 6). The present results can be further compared with a former 13C NMR study on the urea inclusion compounds with the long-chain analogues CnH2nBr2 (n = 10, 11) [36]. UICs with dibromoalkanes CnH2nBr2 (n = 10, 11) or n-alkanes are characterized by hexagonal channels in which the guests adopt a nearly all-trans conformational state [3–6,22,36–38]. The guests undergo almost unhindered rotational motions around their long axes established from 2H NMR studies [6,36]. While the variable temperature 13C T1q data for the dibromoalkane guest species showed that the underlying motions were on the low temperature side of the T1q relaxation curve and transitional motion was used to explain the 13C T1q data [36], likewise, the present studies indicate that lateral motion could also contribute to the 13C T1q relaxation of n-alkane chains in urea inclusion compounds. Another 13C NMR relaxation study was also reported on the urea inclusion compounds with the guest molecules 1,6-dibromodecane [35], where 13C T1 relaxation reflects the same motion being responsible for 2H T1Z and T1Q relaxation, i.e. the interconversion between two gauche conformers of the guest molecules while 13 C T1q relaxation is governed by another motional process which occurs on a slower time-scale  overall fluctuations of the dibromohexane guests with an activation energy of ca. 11 kJ/mol. This activation energy is much smaller than the present value of ca. 60 kJ/mol (see Tables 7 and 8, obtained from the Arrhenius behavior of the correlation times as a function of temperature). The correlation time of ca. 109 s is derived from the analysis of the 13 C T1q data for 1,6-dibromohexane/urea, three orders of magnitude smaller than those for UICs with hexadecane and pentadecane (ca. 106 s for overall fluctuation). These may be related to the shorter length of 1,6-dibromohexane guest chains and/or specially

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structural properties of 1,6-dibromohexane/urea (monoclinic crystal structure, commensurability of the 1,6-dibromohexane guest and the urea host substructure and the gauche conformation of the end group for the guest [48,49]. It is well-known that the guest molecules with the size smaller than, or equal to the cross-dimensions of urea channel can be trapped into urea channels. In some cases, urea channel may distort to trap slightly larger molecules. However, this distortion of urea channel requires the higher host:guest ratio to enclose the guests, as predicted by the topologic models [50]. Thus, the difference of host:guest ratio and/or the distortion of urea channel may also result in some dynamic changes of the guests, as observed between UICs with dibromoalkane, carboxylic acids and UICs with n-alkane [3,5,6,13,27,35,36]. 4. Conclusions Urea inclusion compounds with different guest species were studied by 13C CP MAS and 1H MAS NMR spectroscopy. It is shown that for the asymmetric guest species, the arrangement of the end groups of two adjacent guest molecules may take the mode of head–head, head–tail or tail–tail, depending on the interaction strength of the end functional groups. The interaction between the guests and hosts is confirmed by 13C CP/MAS NMR experiments on n-alkanes in urea and urea-d4. 13 C relaxation studies on urea inclusion compounds with n-alkane show that the 13C T1 and 13C T1q values exhibit the distinct position dependence, signifying internal chain mobility. The 13C T1q values decrease towards the center of the chain while 13C T1 values increase towards the center of the chain. However, the 13C T1 and 13C T1q values of decanoic acid increase towards the two ends of the dimers. From the semi-quantitative 13C T1q analysis of urea inclusion compounds with hexadecane and pentadecane, correlation times of ca. 106 s are obtained. It is argued that chain fluctuations and lateral motion of n-alkane guests may contribute to the 13C T1q relaxation, although a final proof is still missing.

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Acknowledgements

[41]

Financial support by the Deutsche Forschungsgemeinschaft and the Graduiertenkolleg No. 448 ‘‘Modern Methods of Magnetic Resonance in Materials Science’’ is gratefully acknowledged.

[42]

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