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

Article abstract of DOI:10.1016/j.molstruc.2011.08.056

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 T_1ρ values exhibit the position dependence towards the center of the chain, indicating internal chain mobility. The analysis of variable-temperature ¹³C T_1ρ 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 T_1ρ relaxation.

Full text of DOI:10.1016/j.molstruc.2011.08.056

Journal of Molecular Structure 1006 (2011) 113–120
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
The guest ordering and dynamics in urea inclusion compounds studied by
1
13
solid-state H and C 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
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,
Article history:
Received 6 June 2011
Received in revised form 28 August 2011
Accepted 29 August 2011
Available online 6 September 2011
13
head–tail and tail–tail. C CP MAS NMR studies indicate that the preference arrangement is determined
13
by the interaction strength of the end functional groups. C relaxation experiments are used to study the
1
3
dynamic properties of urea inclusion compounds. C relaxation studies on urea inclusion compounds
with n-alkane or decanoic acid show that the 1 C T
3
13
1
1q
and C T values exhibit the position dependence
Keywords:
towards the center of the chain, indicating internal chain mobility. The analysis of variable-temperature
1
3
C NMR
H NMR
1
3
1
1q
C T experiments on urea inclusion compounds with hexadecane and pentadecane, for the first time,
13
suggests that chain fluctuations and lateral motion of n-alkane guests may contribute to the C T
1q
Urea inclusion compounds
Guest–host systems
relaxation.
Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved.
1
. Introduction
that the monocarboxylic monomers or dimers experience a fast
uniform rotation diffusion about the c-axis on the H NMR time-
2
By extensive hydrogen-bonds the urea molecules are connected
scale and wobble slowly at a limited angle about a perpendicular
arc [3] In this work, spin–lattice relaxation time in the rotating
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, im-
posed spatial constraint, can experience a variety of dynamic pro-
cesses, such as rapid rotation along the long molecular axis, fast
torsional libration at the chain ends and rapid methyl-group rota-
tion [3–7].
1
3
1q
frame, T ( C), provides a way to investigate the slower motions
in the low to mid-kilohertz frequency range [14,15]. On the other
1
1q
hand, T ( H) provides information about spatially sensitive pro-
ton spin diffusion [14,16,17].
In organic chemical reactions and the assemblies of supermole-
cules, the functional groups play an important role. The study of
guest molecules isolated by urea channels has the potential to pro-
vide 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-chan-
nels of the urea. Urea inclusion compounds with n-alkane (n-hexa-
decane, decane and pentadecane) or n-alkane of different di- or
mono-substituting end functional groups (ACOOH, AF) are investi-
A wide range of techniques have been applied to investigate the
dynamic properties of urea inclusion compounds, including solid-
state NMR spectroscopy [3–6], incoherent quasi-elastic neutron
scattering [8], molecular dynamics simulation [9] and X-ray dif-
fraction [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 molecu-
lar motions, dynamic NMR spectroscopy is well established, which
1
13
gated by H MAS and C CP MAS NMR spectra and spin lattice
2
1
13
13
give access to different motional time-scales [3–7,11]. Dynamic H
relaxation experiments ( H T
1
q
,
1q 1
C T and C T ). By the present
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 temper-
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 experi-
2
ature [5,6,13]. H NMR studies on monocarboxylic acids/urea show
1
3
1q
ments in the rotating frame ( C T ) are performed on samples
⇑
Corresponding author. Present address: Department of Chemistry, Pennsylvania
State University, University Park, PA 16803, USA. Tel.: +1 814 867 2975; fax: +1 814
with the guests of hexadecane and pentadecane in urea or urea-d
4
.
8
65 3228.
E-mail address: r_yang2000@yahoo.com (X. Yang).
By analyzing the experimental data, dynamic properties of the
guest molecules in the microsecond time scale are derived. The
0
022-2860/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.08.056

1
14
X. Yang, K. Müller / Journal of Molecular Structure 1006 (2011) 113–120
dynamic parameters are compared with the available data in pre-
vious H NMR studies [3–7].
3. Results and discussion
2
1
3
1
3
.1. C CP MAS and H MAS NMR spectra
2
. Experimental
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 com-
pounds were done. They are listed in Tables 1–3 along with chem-
2
.1. Sample preparation
All chemicals for the chemical synthesis were obtained from Al-
13
ical shifts of these guest molecules in solution. In the C NMR
drich Chemicals, and were used without further purification.
spectra of these UICs, high resolution is achieved, suggesting that
the guest molecules are of high mobility. Because the guest mole-
cules are isolated by the urea channels, dipolar coupling between
the guests is reduced.
2
.1.1. Urea-d
Urea-d was prepared by slowly cooling a warm solution of urea
in methanol-d (the molar ratio of urea to methanol-d = 1:4), fol-
4
4
2
For 1-fluorodecane/urea inclusion compounds, the CH F group
1
1
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
lowed by filtration of the solution and drying of the product. This
procedure was repeated once to ensure an almost complete ex-
change of the urea protons by deuterons.
1
3
carbon [24] (the C 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].
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-d in methanol or methanol-d ,
4 1
respectively. The white needles were filtered, washed with 2,2,4-
trimethylpentane, and dried.
Thus, in 1-fluorodecane, for the CH
arrangements, CH C and CH
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 envi-
2
F group with two different
C, four resonances might
2
Fꢀ ꢀ ꢀH
3
2
Fꢀ ꢀ ꢀFH
2
ronment of the carbon in the CH
ferent end arrangements. Compared with dipolar coupling, the
2
F group is not affected by the dif-
2
2
.2. Methods
1
3
influence of the different end arrangements on the C chemical
shift in the CH F group is too weak to be observed. Likewise, the
group exists in two different arrangements, CH C and
ꢀ ꢀ ꢀFH
ꢀ ꢀ ꢀH C. Again, due to the dipolar coupling between the F atom
arising
.2.1. DSC (differential scanning calorimeter) measurement
Calorimetric studies were performed with a differential scan-
2
CH
CH
3
3
2
ning calorimeter Netzsch DSC 204 (Selb/Germany) under nitrogen
flow at heating rates of 10 °C/min.
3
3
and its neighbour carbons [24], one resonance of the CH
3
1
13
2
.2.2. High resolution solid state H MAS and C CP MAS NMR
measurement
1
3
1
C CP/MAS and H 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
1
3
1
7
5.47 ( C), 300.13 MHz ( H) for the CXP 300 spectrometer. A dou-
13
ble tuned Bruker 4 mm MAS probe was used for the C CP/MAS, T
1
1
3
13
1
(
C), T
1
q
(
C), and T
1q
( H) 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 sig-
1
3
nal/noise ratio. C T
1
data were obtained with cross-polarization
(
CP) excitation as proposed by Torchia [18]. Standard spin-lock
13
1
pulse sequences were used for the determination of C and
relaxation times [19,20]. The 13C were spin locked in a field
of 50 kHz.
1H MAS spectra were measured with a Bruker CXP 2.5 mm dou-
ble resonance MAS probe (at 300.13 MHz) with a /2 pulse length
of 4 s. Typically, 32 or 64 scans were accumulated with recycle
H
T
1q
p
l
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 accu-
1
racy of temperature was found to be within ±1 K. The data from H
1
3
MAS and C CP/MAS NMR measurements were handled by utiliz-
13
ing the MestRec software [21]. C chemical shifts were referenced
to the external standard adamantane. This value was then ex-
pressed relative to TMS (d = 0 ppm). Likewise, H chemical shift
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.
1
(
values are given relative to TMS (d = 0 ppm).

X. Yang, K. Müller / Journal of Molecular Structure 1006 (2011) 113–120
115
Table 1
In contrast to two peaks of the end group in other asymmetric
guest molecules [24–26], the resonances of the CH or COOH
NMR parameters from 13C CP MAS and 1H MAS NMR studies on n-decane/urea.
3
C
10
H
22/urea
d (ppm)
T
1
q
(H) (ms)
T
1q
(C) (ms)
T1(C) (s)
end-group of decanoic acid display a single peak. As reported pre-
viously [28–30], the aliphatic monocarboxylic acids form exclu-
a
Solutionb
Ureac
CT : 1.5 ms
sively head-to-head dimers in urea channels. Thus both CH
COOH end-groups have only one arrangement, i.e. CH
ꢀ ꢀ ꢀCH
3
and
and
C1
C2
C3
C4
C5
14.2
22.9
32.2
29.6
29.9
15.1
25.2
34.8
33.1
33.3
4.1
9.7
9.2
10
247
301
255
271
223
1.8
3.8
4.3
4.8
4.6
3
3
COOHꢀ ꢀ ꢀ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].
10
a
b
c
Contact time.
13
C chemical shifts of n-decane in CDCl
C chemical shifts of n-decane in their UICs in this work.
3
[23].
13
1
3
C NMR spectra of pentadecane and hexadecane in urea and
urea-d are shown in Fig. 2. The assignments of the peaks and
4
Table 2
NMR parameters from 13C CP MAS and 1H MAS NMR studies on 1-fluorodecane/urea.
13
the C NMR line widths are summarized in Table 4 along with
the chemical shifts of the guest species in solution. The comparison
1
3
C
10
H
21F/urea
d (ppm)
T
1
q
(H) (ms)
T
1
q
(C) (ms)
T1(C) (s)
of the C NMR line widths in the UICs from urea and urea-d
shows a larger value for the guest species in urea-d (see Table
4). Because the C NMR spectra are recorded with only proton
4
a
Ureab
CT : 6 ms
4
1
3
C1 + F
C1 + F
C2
C3
C4–7
C8
85.2
83.2
32.5
27.8
32.9
34.5
24.8
14.8
13
2
decoupling, CA H dipolar coupling between the guest species
10
6.7
8.5
and the host matrix causes the additional line broadening of the
1
3
4
guest species in urea-d . However, the C NMR line width of the
9.5
11
11
6.0
6.7
7.2
6.2
6.8
8.5
8.5
4.6
urea carbonyl group is found to become smaller upon urea deuter-
ation, which is attributed to interference effects [31,32] between
C9
C10
9.3
1
the H decoupling r.f. field and the urea reorientation motions,
a
Contact time.
i.e., 180° flips around the C@O and the CAN bonds [33,34]. This
b
13
13
C chemical shifts of 1-fluorodecane in their UICs in this work.
is supported by a broadening for the carbonyl C NMR signal with
1
an increase of the H decoupler power. Similar observations were
found for other UICs [35,36].
Table 3
When the signal intensity is recorded as a function of contact
time for cross-polarization, the optimum contact time with maxi-
mal signal intensity is found to vary considerably with guest spe-
cies and deuteration of the urea matrix. Thus, for hexadecane/
NMR parameters from 13C CP MAS and 1H MAS NMR studies on sebacic acid,
dodecanedioic acid, and decanoic acid in urea.
a
CT : 1 ms
d (ppm)
Solutionb
T
1q
(H) (ms)
T
1q
(C) (ms)
T1(C) (s)
Ureac
13
urea, a maximum C CP signal was observed at contact times of
0
4
.5 ms and 4 ms for urea and urea-d , respectively. As a conse-
8
HOOCC H
16COOH/urea
COOH
C2
C3
182
1.3
1.5
1.6
1.5
514
187
194
175
5.6
2.3
2.1
2.4
quence of the loss of the urea protons, a longer contact time is re-
quired for the deuterated urea clathrates, suggesting that the urea
protons contribute to magnetization transfer during cross-polari-
zation as well. Similar trends for the C signal intensity during
cross-polarization were also observed recently for other urea inclu-
sion 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 suit-
35.2
26.5
30.9
C4–5
1
3
a
HOOCC10
COOH
C2
C3
C4
H20COOH/urea (CT : 0.3 ms)
174
182
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
33.6
24.5
28.8
28.7
35.6
26.3
31.8
31.4
able for the pentadecane guest molecules in both urea and urea-
C5–6
1
d
4
, which can be correlated to the H T
1
q
values, as shown below
a
1
CH
3
C
8
H16COOH/urea (CT : 1 ms)
1q 4
in Tables 5 and 6. A ratio of about 7 of H T in urea-d to that
COOH
C2
C3
C4
C5
C6
C7
C8
C9
181
182
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
in urea is obtained for hexadecane while a value of about 2.5 is de-
rived for pentadecane.
34.3
24.8
29.2
29.4
29.5
29.4
32.0
22.8
14.1
36.1
26.1
31.9
32.3
33.5
32.9
35.4
25.1
15.6
1
Fig. 3 exhibits H MAS NMR spectra recorded at a sample spin-
ning rate of 34 kHz for pentadecane and hexadecane in urea and
1
urea-d
4
. In general, very low resolved H NMR spectra are ob-
1
served, which indicates that there is strong H dipolar coupling
C10
a
Contact time.
b
13
C chemical shifts of sebacic acid, dodecanedioic acid, and decanoic acid in
3
[23].
C chemical shifts of sebacic acid, dodecanedioic acid, and decanoic acid in
CDCl
c
13
their UICs in this work.
from the CH
flanking the other resonance of the CH
the CH C arrangement [24].
ꢀ ꢀ ꢀH
For the decanoic acid/urea inclusion compound, a surprisingly
high resolved C NMR spectrum is registered (see Fig. 1c). The
chemical shift of every carbon in the chain can be distinguished.
3
ꢀ ꢀ ꢀFH
2
C arrangement is pronouncedly broadened,
3
at 14.8 ppm which reflects
3
3
1
3
Fig. 2. 13C CP MAS NMR spectra of n-pentadecane in: (a) urea, (b) urea-d
n-hexadecane in: (c) urea, (d) urea-d
4
, and
4
.

1
16
X. Yang, K. Müller / Journal of Molecular Structure 1006 (2011) 113–120
Table 4
NMR parameters from 13C CP MAS and 1H MAS NMR studies on n-pentadecane and n-
hexadecane in urea and urea-d
4
.
c
C
15
H
32
d
13C (ppm)
D
m
1/2(13C) (Hz)
Solutiona
Ureab
Urea-d
Urea
Urea-d
4
4
C1
C2
C3
C4
C5–7
C@O
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
Fig. 3. 1H MAS NMR spectra of n-pentadecane in: (a) urea, (b) urea-d
n-hexadecane in: (c) urea, (d) urea-d4.
, and
4
16 34
C H
C1
C2
C3
C4
C5–8
C@O
14.1
22.8
32.0
29.5
29.9
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
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
a
b
c
13
3
C chemical shifts of pentadecane and hexadecane in CDCl [23].
13
13
C chemical shifts of pentadecane and hexadecane in their UICs in this work.
C NMR line width.
2
established from IR and H NMR spectroscopy [3–6].
1
3
in these UICs. Fig. 3b and d also showed that there are urea peaks at
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
The C chemical shift alteration of the dodecanedioic acid car-
bonyl group is much larger than that of the decanoic acid carbonyl
group, which might be related to the particular situation that deca-
noic acid exists in head-to-head dimers with the two facing COOH
groups [28–30]. These dimers can be regarded as one guest mole-
cule, and thus the acid alkyl chains of decanoic acid/urea have con-
formations similar to those of the pure n-alkane chain. This was
also reported previously [3,29,39,40]. However, the dodecanedioic
7
[
37,38], these chemical shift alterations can be traced back to con-
formational changes. On the basis of these observations and the
Table 5
T
1q
(C), T
1q
(H) and T
1
(C) values as a function of temperature for n-pentadecane in urea and urea-d
4
.
a
(H) (ms)b
T
1(C) (s)b
C
15
H
32/urea (CT : 0.5 ms)
T
1
q
(C) (ms)
T
1q
2
94 K
300 K
305 K
310 K
315 K
320 K
C1
C2
C3
C4
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
C5–7
a
C
15
H
32/urea-d
4
(CT : 0.5 ms)
C1
C2
C3
C4
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
C5–7
a
Contact time.
Value at 294 K.
b
Table 6
(C), T
T
1
q
1q
(H) and T
1
(C) values as a function of temperature for n-hexadecane in urea and urea-d
4
.
a
(H) (ms)b
1(C) (s)b
C
16
H
34/urea (CT : 0.5 ms)
T
1
q
(C) (ms)
T
1q
T
2
94 K
300 K
305 K
310 K
315 K
320 K
325 K
C1
C2
C3
C4
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
2.4
2.7
2.6
2.4
2.4
2.6
6.2
8.7
8.9
9.1
C5–8
a
C
16
H
34/urea-d
4
(CT : 4 ms)
C1
C2
C3
C4
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
–
8.1
14
16
17
18
2.3
5.4
7.0
7.5
9.2
652
347
402
331
C5–8
a
Contact time.
Value at 294 K.
b

X. Yang, K. Müller / Journal of Molecular Structure 1006 (2011) 113–120
117
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.
uniform motion and this motion is too fast to affect 13C T
1q
and
1
3
C T
For hexadecane/urea and pentadecane/urea it is found that the
C T values slightly vary with chain position, as reflected by a
decrease towards the center of the chain (see Tables 5 and 6).
1
relaxation.
1
3
1q
T
1q
3.2. T
1
(C), T
1q
(C) and T
1q
(H) relaxation data
This finding indicates that besides the rapid rotation of the overall
chains established by H NMR studies [3–6], the guest molecules
2
The dynamic properties of the guest components are assessed
also possess internal chain mobility, for instance, torsional libra-
tion about the penultimate CAC bond, methyl rotation about its lo-
by 13C T
,
13
1
1q
1 1q
C T and H T measurements. The derived data are
summarized in Tables 1–3, 5 and 6. In general each proton of the
cal threefold symmetry axis C
which were examined by H NMR studies [7]. Urea deuteration is
characterized by an increase for both the C T and H T values
1q 1q
due to reduced effective hetero- and homonuclear dipolar interac-
tions between guest and host components.
3
and gauche-trans isomerization
guest molecule has the same 1H T
value (apart from the COOH
2
1q
1
3
1
group due to the large steric hindrance and/or dipolar coupling
of the COOH group, and the methyl group which has a smaller va-
lue because of the rotation of the methyl group) due to the strong
proton spin diffusion.
1
3
values are much longer than the 13C T
values, and
1 1q
The C T
they both exhibit distinct position dependence. Thus, the C T
values decrease towards the center of the chain while 13C T
values
increase towards the center of the chain. The position-dependent
For 1-fluorodecane/urea, the 13C T
values of all carbons within
13
1
q
1q
the chains are identical and rather small (see Table 2), which can
be related to the fast overall chain rotation of the guest molecules.
1
1
1
3
3
13
C T
1
values of all carbons within the chains are much longer than
values. The methyl group in the CH arrangement has
ꢀ ꢀ ꢀCH
value compared to other carbons in the chains,
which is attributed to the fast methyl group rotation as observed
C T
1
values might reflect internal chain mobility as well. Upon
urea deuteration, the C T values of the pentadecane/urea in-
13
C T
1
q
3
3
1
1
3
a smaller C T
1
crease due to reduced effective hetero- and homonuclear dipolar
interactions between guest and host components, however the
1
3
13
for n-alkane/urea inclusion compounds. Again, a similar C T
1
va-
1
C T values of the hexadecane/urea are almost unaffected by urea
1
3
1
lue is recorded for all other carbons within the chains. Obviously,
1 1q
deuteration. The C T and H T of the end methyl groups for
1
3
there is no efficient relaxation process present which affects
C
both samples show smaller values, as a consequence of the high
mobility of end methyl which was proved by H NMR studies
2
T
1
relaxation.
For decanoic acid, dodecanedioic acid and sebacic acid, the 13C
[3–6].
1
3
Moreover, from the variable temperature 13C T
T
1
q
and C T
1
values of the carboxyl group are relatively large
1q
data (see Ta-
bles 5 and 6, Fig. 5) a minimum is found at 300 K for pentadec-
ane/urea and 310 K for hexadecane/urea. The decay curves of T
(
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
1q
(the intensity as a function of the pulse space in spin-lock se-
quence) are given in Fig. 4 for C4 at 300 K and C3 at 305 K in hexa-
13
decane/urea.
A
semi-quantitative analysis of these
C T
1q
[
41].
The 13C T
dependent, the C T
and 13C T
values of the decanoic acid are position-
1q 1
1
3
13
1q 1
and C T values increase towards the two
2
ends of the dimers. Although H NMR studies on monocarboxylic
acids/urea show that the monocarboxylic monomers or dimers
experience a fast uniform rotation diffusion about the c-axis on
2
the H 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
1
3
13
1q 1
the C T and C T values of the decanoic acid, which is consis-
13
13
1q 1
tent with the position-dependent behavior of C T and C T
values.
The position-dependent 13C T and 13C T
values of the deca-
1q 1
noic 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 perpen-
dicular 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 T
and 13C T
values of
1q 1
1
3
decanoic acid, for dodecanedioic acid and sebacic acid the C T
1q
1
3
1
and C T values at different positions in the chain are almost
identical (with the exception of carboxyl carbon due to larger spa-
tial hindrance of two facing COOH groups), which implies that
there is no internal chain mobility for these two UICs as for mono-
carboxylic 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
Fig. 4. The intensities of (a) C4 peak at 300 K and (b) C3 peak at 305 K in
34/urea as a function of the pulse space in spin-lock sequence.
16
C H

1
18
X. Yang, K. Müller / Journal of Molecular Structure 1006 (2011) 113–120
relaxation data was done by using the well-known expressions
42] for T in the presence of isotropic motions and a dominant
heteronuclear dipolar interaction
approximately linear dependence of activation energies on increas-
ing chain length for alkane guest rotation about the long axis
[3,36]. The intermolecular interactions between guests and inter-
actions between the guest molecules and the wall of the urea chan-
nels 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 diffu-
sion 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].
[
1q
2
1
A
¼
½4 ꢀ Jðx
1;CÞ þ Jðx
0;H
ꢁ
x
0;CÞ þ 3 ꢀ Jð
x
0;C
Þ
1
3
2
T
1
q
ð CÞ
þ 6 ꢀ Jð
x
0;H
þ
x
0;CÞ þ 6 ꢀ Jð
x
0;HÞꢂ
ð1Þ
ð2Þ
s
c
with Jnð
x
i
Þ ¼ 1
;
x
i
¼
cB
i
xi2
2
þ
s
c
Here A2 is a factor which contains the gyromagnetic ratios of the
3
interacting nuclei, and the average internuclear distance as h1/r i.
13
1
x
0,C and
x
0,H are the Larmor frequencies of the C and H nuclei,
On the basis of these results it is concluded that for pentadecane
1
3
respectively, and
x
1,C is the nutation frequency of the radio fre-
1q
and hexadecane C T relaxation does not reflect the motions
1
3
2
quency field B
1
(
C).
being responsible for H T1Z relaxation [6], but is governed by an-
1
3
By fitting the C T
1q
minimum with Eq. (1), factor A of the mo-
other motional process which occurs on a slower time-scale. It can
ꢁ6
tion 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 experimen-
be seen that the correlation times of ca. 10 s derived from the
1
3
C T
from the quantitative H T1Z analysis – correlation time of ca. 10
for rapid and almost unrestricted rotation of the alkyl chains around
1q
data analysis are much longer than those reported previously
2
ꢁ10
s
1
3
1q
tal C T curve, as summarized in Tables 7 and 8. It is found from
13
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 mobil-
ity than hexadecane guests. This is related to the interactions be-
tween 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
the channel long axis [6]. That is, the motion dominating the C T
1q
relaxation is much slower than that for H T1Z relaxation. Chain fluc-
tuations of the n-alkane guests may dominate C T
2
1
3
1q
relaxation.
This assumption is supported by the position-dependent behavior
1
3
of C T
1q
values. Lateral motion is sensitive to the variation of the
distance, position and interaction between the guest and host, thus
1
3
it can also provide some contribution to the C T
1q
relaxation, as re-
13
flected by an increase of C T values on urea deuteration.
1q
1q 4 4
Fig. 5. T values as a function of the temperature: (a) C15H32/urea, (b) C15H32/urea-d , (c) C16H34/urea, (d) C16H34/urea-d .

X. Yang, K. Müller / Journal of Molecular Structure 1006 (2011) 113–120
119
Table 7
The correlation times as a function of temperature for n-pentadecane in urea and urea-d
4
.
Correlation time
94 K
c
s (s)
2
300 K
305 K
310 K
315 K
320 K
Ea (kJ/mol)
85.9 ± 6.9
63.8 ± 10.4
47.0 ± 4.5
54.4 ± 7.9
43.9 ± 3.5
C
C
15
H32/urea
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
6
6
6
6
6
ꢁ6
ꢁ6
ꢁ6
ꢁ6
ꢁ6
ꢁ6
ꢁ6
ꢁ6
ꢁ6
ꢁ6
ꢁ7
ꢁ7
ꢁ6
ꢁ6
ꢁ6
ꢁ7
ꢁ7
ꢁ6
ꢁ7
ꢁ7
ꢁ7
ꢁ7
C1
C2
C3
C4
6.4 ꢃ 10
5.3 ꢃ 10
3.8 ꢃ 10
5.2 ꢃ 10
3.8 ꢃ 10
2.5 ꢃ 10
2.4 ꢃ 10
2.5 ꢃ 10
2.5 ꢃ 10
2.4 ꢃ 10
1.3 ꢃ 10
1.2 ꢃ 10
2.2 ꢃ 10
1.7 ꢃ 10
1.9 ꢃ 10
8.9 ꢃ 10
8.5 ꢃ 10
1.2 ꢃ 10
1.1 ꢃ 10
1.3 ꢃ 10
4.8 ꢃ 10
7.7 ꢃ 10
1.1 ꢃ 10
4.2 ꢃ 10
7.1 ꢃ 10
9.2 ꢃ 10
9.5 ꢃ 10
ꢁ6
1 ꢃ 10
ꢁ6
ꢁ6
C5–7
1.1 ꢃ 10
1 ꢃ 10
15
H
32/urea-d
4
ꢁ
ꢁ
ꢁ
6
6
6
ꢁ6
ꢁ6
ꢁ6
ꢁ6
ꢁ6
ꢁ6
ꢁ6
ꢁ6
ꢁ6
ꢁ6
ꢁ7
ꢁ6
ꢁ6
ꢁ6
ꢁ6
ꢁ7
ꢁ7
ꢁ7
ꢁ7
C1
C2
C3
C4
8.8 ꢃ 10
5.5 ꢃ 10
6.1 ꢃ 10
5.6 ꢃ 10
5.4 ꢃ 10
4.3 ꢃ 10
4.3 ꢃ 10
3.8 ꢃ 10
2.5 ꢃ 10
2.4 ꢃ 10
2.4 ꢃ 10
2.4 ꢃ 10
2.4 ꢃ 10
9.6 ꢃ 10
1.3 ꢃ 10
1.5 ꢃ 10
1.4 ꢃ 10
1.6 ꢃ 10
4.7 ꢃ 10
5.2 ꢃ 10
7.5 ꢃ 10
3.9 ꢃ 10
4.9 ꢃ 10
5.3 ꢃ 10
4.7 ꢃ 10
106.9 ± 8.6
87.9 ± 11.2
80.3 ± 4.8
90.4 ± 2.5
77.8 ± 2.9
ꢁ7
ꢁ7
ꢁ7
ꢁ
6
ꢁ7
8 ꢃ 10
7 ꢃ 10
ꢁ
6
ꢁ7
ꢁ7
C5–7
6.9 ꢃ 10
8.1 ꢃ 10
6 ꢃ 10
Table 8
The correlation times as a function of temperature for n-hexadecane in urea and urea-d
4
.
Correlation time
94 K
s
c
(s)
2
300 K
305 K
310 K
315 K
320 K
325 K
Ea (kJ/mol)
C
C
16
H34/urea
9.3 ꢃ 10ꢁ
6
5
6
6
6
8.7 ꢃ 10
8.1 ꢃ 10
6.9 ꢃ 10
6.9 ꢃ 10
6.2 ꢃ 10
ꢁ6
ꢁ6
ꢁ6
ꢁ6
ꢁ6
8.4 ꢃ 10
7.1 ꢃ 10
6.4 ꢃ 10
6.4 ꢃ 10
6.1 ꢃ 10
ꢁ6
ꢁ6
ꢁ6
ꢁ6
ꢁ6
2.4 ꢃ 10
2.4 ꢃ 10
2.4 ꢃ 10
2.4 ꢃ 10
2.4 ꢃ 10
ꢁ6
ꢁ6
ꢁ6
ꢁ6
ꢁ6
9.8 ꢃ 10
1.1 ꢃ 10
1.2 ꢃ 10
1.2 ꢃ 10
1.2 ꢃ 10
ꢁ7
ꢁ6
ꢁ6
ꢁ6
ꢁ6
6.7 ꢃ 10
8.5 ꢃ 10
ꢁ7
ꢁ7
92.5 ± 17.5
88.4 ± 12.2
72.7 ± 12.8
78.1 ± 10.8
68.8 ± 13.3
C1
C2
C3
C4
ꢁ
1.1 ꢃ 10
ꢁ
ꢁ6
ꢁ6
7.5 ꢃ 10
1 ꢃ 10
1 ꢃ 10
ꢁ
9.5 ꢃ 10
7.4 ꢃ 10ꢁ
9.4 ꢃ 10
ꢁ7
C5–7
16
H
34/urea-d
4
5.2 ꢃ 10ꢁ
6
6
6
6
6
5 ꢃ 10
ꢁ6
4.3 ꢃ 10
4.2 ꢃ 10
4.3 ꢃ 10
5.1 ꢃ 10
3.4 ꢃ 10
ꢁ6
ꢁ6
ꢁ6
ꢁ6
ꢁ6
4.2 ꢃ 10
3.5 ꢃ 10
4.1 ꢃ 10
4.2 ꢃ 10
ꢁ6
ꢁ6
ꢁ6
ꢁ6
2.4 ꢃ 10
2.4 ꢃ 10
2.4 ꢃ 10
2.4 ꢃ 10
2.4 ꢃ 10
ꢁ6
ꢁ6
ꢁ6
ꢁ6
ꢁ6
1.3 ꢃ 10
1.4 ꢃ 10
1.7 ꢃ 10
1.5 ꢃ 10
1.3 ꢃ 10
ꢁ6
ꢁ6
ꢁ6
ꢁ6
ꢁ6
42.5 ± 10.5
51.2 ± 5.7
44.5 ± 4.7
55.9 ± 4.7
33.8 ± 6.4
C1
C2
C3
C4
ꢁ
ꢁ6
ꢁ6
6.1 ꢃ 10
5.8 ꢃ 10
4.8 ꢃ 10
6.8 ꢃ 10
3.5 ꢃ 10
1 ꢃ 10
ꢁ
ꢁ6
ꢁ6
ꢁ6
ꢁ6
ꢁ6
ꢁ6
6.7 ꢃ 10
1.3 ꢃ 10
1.2 ꢃ 10
1.2 ꢃ 10
ꢁ
8.5 ꢃ 10
3.7 ꢃ 10ꢁ
3 ꢃ 10
ꢁ6
C5–7
On the other hand, there is evidence that the motions of long al-
kyl chains are quite different in the middle of the chain and at its
ends. In particular, NMR [44], Raman [45], and molecular dynamics
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
[
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 con-
analogues C
n 2
H2nBr (n = 10, 11) [36]. UICs with dibromoalkanes
n 2
C H2nBr
(n = 10, 11) or n-alkanes are characterized by hexagonal
channels in which the guests adopt a nearly all-trans conforma-
finement of the chains by the urea channel, the end CD
the CD groups near the chain ends have more space to undergo
reorientation, as proved, for instance, for nonadecane/urea by the
3
groups and
tional state [3–6,22,36–38]. The guests undergo almost unhindered
2
2
rotational motions around their long axes established from
NMR studies [6,36]. While the variable temperature 13C T
for the dibromoalkane guest species showed that the underlying
motions were on the low temperature side of the T relaxation
H
1
q
data
2
separate H NMR splittings of the
a and b segments. All the other
bulk CD groups are in the trans conformation, and their CAD
2
1q
1
3
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
curve and transitional motion was used to explain the C T
1q
data
[36], likewise, the present studies indicate that lateral motion
1
3
1q
could also contribute to the C T relaxation of n-alkane chains
1
3
position-dependent C T
1q
values of urea inclusion compounds
in urea inclusion compounds.
with hexadecane and pentadecane.
Another 13C NMR relaxation study was also reported on the
Recently a comprehensive 13C NMR relaxation study for a series
urea inclusion compounds with the guest molecules 1,6-dibromo-
1
3
13
of n-alkane/UICs was published [47]. It was shown that the C T
1
1
decane [35], where C T relaxation reflects the same motion
2
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.
being responsible for H T1Z and T1Q relaxation, i.e. the interconver-
sion between two gauche conformers of the guest molecules while
1
3
1q
C T relaxation is governed by another motional process which
occurs on a slower time-scale ꢁ overall fluctuations of the dibro-
mohexane 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 behav-
ior of the correlation times as a function of temperature). The
As mentioned above, the decanoic acid head-to-head dimer has
some similarities with the n-alkane/urea inclusion compounds
ꢁ9
[
3,29,39,40]. Head–head dimers of the aliphatic monocarboxylic
correlation time of ca. 10 s is derived from the analysis of the
1
3
acids in urea channel can be regarded as one guest molecule, and
the acid alkyl chains of decanoic acids/urea have conformations
1q
C T data for 1,6-dibromohexane/urea, three orders of magni-
tude smaller than those for UICs with hexadecane and pentadecane
ꢁ6
similar to those of the pure n-alkane chain [3,29,39,40]. Therefore,
(ca. 10 s for overall fluctuation). These may be related to the
shorter length of 1,6-dibromohexane guest chains and/or specially
the position-dependent behavior of the 13C T
and C T values
13
1q 1

1
20
X. Yang, K. Müller / Journal of Molecular Structure 1006 (2011) 113–120
structural properties of 1,6-dibromohexane/urea (monoclinic crys-
tal 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].
[4] H.L. Casal, D.G. Cameron, E.C. Kelusky, J. Chem. Phys. 80 (1984) 1407.
[
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[
5] M.S. Greenfield, R.L. Vold, R.R. Vold, Mol. Phys. 66 (1989) 269.
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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 dis-
tort 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 differ-
ence of host:guest ratio and/or the distortion of urea channel may
also result in some dynamic changes of the guests, as observed be-
tween UICs with dibromoalkane, carboxylic acids and UICs with
n-alkane [3,5,6,13,27,35,36].
[
9] M.C. Menziani, P.G. de Benedetti, U. Segre, M. Brustolon, Mol. Phys. 92 (1997)
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Compounds, Logos Verlag, Berlin, 2007.
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[
[13] M.S. Greenfield, R.L. Vold, R.R. Vold, J. Chem. Phys. 83 (1985) 1440.
[
[
14] F.A. Bovey, P.A. Mirau, NMR of Polymers, Academic Press, New York, 1996.
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[16] K. Schmidt-Rohr, H.W. Spiess, Multidimensional Solid-state NMR and
Polymers, Academic Press, New York, 1994.
[
[
17] V. McBrierty, D. Douglass, T. Kwei, Macromolecules 11 (1978) 1265.
18] D.A. Torchia, J. Magn. Reson. 30 (1978) 613.
4
. Conclusions
[19] E.O. Stejskal, J. Schaefer, M.D. Sefcik, R.A. McKay, Macromolecules 14 (1981)
75.
2
[
[
20] J. Schaefer, E.O. Stejskal, R. Buchdahl, Macromolecules 10 (2) (1977) 384.
21] J.C. Cobas, F.J. Sardina, Concept Magn. Reson. A 19A (2003) 80.
Urea inclusion compounds with different guest species were
1
3
1
studied by C CP MAS and H 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
[22] F. Imashiro, T. Maeda, T. Nakai, A. Saika, T. Terao, J. Phys. Chem. 90 (1986)
498.
[
5
23] AIST, Integrated Spectral Database System of Organic Compounds (data are
provided by the National Institute of Advanced Industrial Science and
Technology (Japan)), <http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/direct_
frame_top.cgi>.
strength of the end functional groups. The interaction between
the guests and hosts is confirmed by 13C CP/MAS NMR experiments
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[
on n-alkanes in urea and urea-d
4
.
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1
3
C relaxation studies on urea inclusion compounds with n-al-
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3
13
kane show that the C T
1 1q
and C T values exhibit the distinct
[
[
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1
3
position dependence, signifying internal chain mobility. The
C
1
3
T
1
q
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1
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1
3
values increase towards the center of the chain. However, the
C
[
[
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1
3
T
1
and C T
ends of the dimers. From the semi-quantitative C T
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1q
values of decanoic acid increase towards the two
13
1
q
analysis
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[
[
[
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ꢁ6
correlation times of ca. 10 s are obtained. It is argued that chain
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1
3
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1q
to the C T relaxation, although a final proof is still missing.
[
(
Acknowledgements
(
[
Financial support by the Deutsche Forschungsgemeinschaft and
the Graduiertenkolleg No. 448 ‘‘Modern Methods of Magnetic Res-
onance in Materials Science’’ is gratefully acknowledged.
1
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[
[
[
[
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