The structural and dynamic properties of 1-bromodecane in urea inclusion compounds investigated by solid-state 1H, 13C and 2H NMR spectroscopy

Article abstract of DOI:10.1002/mrc.2785

For asymmetric guest molecules in urea, the end-groups of two adjacent guest molecules may arrange in three different ways: head-head, head-tail and tail-tail. Solid-state ¹H and ¹³C NMR spectroscopy is used to study the structural properties of 1-bromodecane in urea. It is found that the end groups of the guest molecules are randomly arranged. The dynamic characteristics of 1-bromodecane in urea inclusion compounds are probed by variable-temperature solid-state ²H NMR spectroscopy (line shapes, spin-spin relaxation: T₂, spin-lattice relaxation: T_1Z and T_1Q) between 120 K and room temperature. The comparison between the simulation and experimental data shows that the dynamic properties of the guest molecules can be described in a quantitative way using a non-degenerate three-site jump process in the low-temperature phase and a degenerate three-site jump in the high-temperature phase, in combination with the small-angle wobbling motion. The kinetic parameters can be derived from the simulation. Copyright

Full text of DOI:10.1002/mrc.2785

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Research Article
Received: 18 April 2011
Revised: 17 May 2011
Accepted: 24 May 2011
Published online in Wiley Online Library: 12 July 2011
(wileyonlinelibrary.com) DOI 10.1002/mrc.2785
The structural and dynamic properties
of 1-bromodecane in urea inclusion
compounds investigated by solid-state 1H, 13C
and 2H NMR spectroscopy
Xiaorong Yang∗† and Klaus Mu¨ller
For asymmetric guest molecules in urea, the end-groups of two adjacent guest molecules may arrange in three different ways:
head–head, head–tail and tail–tail. Solid-state 1H and 13C NMR spectroscopy is used to study the structural properties of
1-bromodecaneinurea.Itisfoundthattheendgroupsoftheguestmoleculesarerandomlyarranged.Thedynamiccharacteristics
2
of 1-bromodecane in urea inclusion compounds are probed by variable-temperature solid-state H NMR spectroscopy (line
shapes, spin–spin relaxation: T2, spin-lattice relaxation: T1Z and T1Q) between 120 K and room temperature. The comparison
between the simulation and experimental data shows that the dynamic properties of the guest molecules can be described in
a quantitative way using a non-degenerate three-site jump process in the low-temperature phase and a degenerate three-site
jump in the high-temperature phase, in combination with the small-angle wobbling motion. The kinetic parameters can be
c
derived from the simulation. Copyright ꢀ 2011 John Wiley & Sons, Ltd.
Keywords: solid-state NMR; 1H; 13C and 2H; structure and dynamics; 1-bromodecane; urea inclusion compounds
Introduction
orthorhombic structure which induces the spatial restriction to
the guest molecules. Thus, the guest molecules lose the rotational
freedom. A dynamic discontinuity happens at the solid–solid
phase transition for the UICs with n-alkane.[13] However, this is not
true for UICs with α, ω-dibromoalkanes, a dynamic discontinuity
was postponed to a higher temperature than the phase transition
temperature.[27,37] In order to get more insight into how the
local structure and the dynamic properties of guest molecules
in channels are affected by the nature of the guest molecules,
in this work the asymmetric guest molecule 1-bromodecane in
urea is investigated. Dynamic 2H NMR spectroscopy, including
variable temperature 2H NMR line shape, spin–spin (T2) and
spin–lattice (T1Z and T1Q) relaxation studies, is used to explore
the dynamic properties of 1-bromodecane, selectively deuterated
at the brominated methylene group. The kinetic parameters are
derived quantitatively by analyzing the experimental data on
the basis of suitable motional models. In addition, the structural
order and conformation of the guest molecules are probed by
performing 1H MAS and 13C CP/MAS NMR measurements on the
Inclusion compounds can be formed by various types of organic
or inorganic host compounds. Urea represents a prominent
host component. Urea molecules construct linear, parallel one-
dimensional channels directed along the c-axis of the crystal
stabilized via hydrogen bonds. The diameter of these channels
is ca 5.5–5.8 Å.[1] With suitable guest molecules, for instance,
n-alkane and their derivatives,[2] these channels are stable and
exhibit a hexagonal structure at room temperature. The structural
and dynamic properties of the guest molecules trapped within
these channels have been studied extensively.[3–9]
Functional groups play an important role for the organization
of molecules in supermolecular chemistry. The understanding
of functional group interactions and the molecular dynamics
can provide useful information for practical applications. Urea
inclusioncompounds(UICs)areconsideredasidealmodelsystems
to study the interaction between functional groups of the guest
molecules isolated by the urea channels.[10,11]
Numerous studies about n-alkanes[3,8,12–25] and α, ω-
dibromoalkanes[25–30] in urea indicate that the guest molecules
experience substantial mobility, comprising translational and dif-
fusive motions or jumps about the motional symmetry axis, and
the conformation of the end groups is considered to be in almost
all-trans conformational state.[9,31–33]
The previous calorimetric,[34] X-ray[35,36] studies show that UICs
with the guest molecules experience a solid–solid phase. This
phase transition is considered to be related to the change
of dynamic properties of the guest molecules. In the high
temperature, thehexagonalstructureofureachannelsisstabilized
by the fast rotational motion of the guest molecules. In the low-
temperature phase, the urea channels are distorted into the
1
non-deuterated guest molecules. The data from 13C T1ρ and H
T1ρ experiments provide additional information about the motion
of the guest molecules.
∗
Correspondence to: Xiaorong Yang, Institut fu¨r Physikalische Chemie, Univer-
¨
sitat Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany.
E-mail: r yang2000@yahoo.com
†
Present address: Department of Chemistry, Pennsylvania State University,
University Park, PA 16802, USA.
¨
Institut fu¨r Physikalische Chemie, Universitat Stuttgart, Pfaffenwaldring 55,
D-70569 Stuttgart, Germany
c
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Structural and dynamic properties of 1-bromodecane
Experiment and Data Processing
(90◦x −2τw −67.5−◦ y −2τw −45◦y −τw −45◦y −τr −45◦x −ꢀ−90◦x).[41]
Here, (90◦x − 2τw − 67.5−◦ y − 2τw − 45y◦ − τw − 45◦y) represents the
excitation sequence, whose phase cycle maximizes quadrupolar
order while fully suppressing Zeeman order.[42] A refocusing pulse
was used to generate an echo, resulting in partially relaxed
spectra with minimal baseline distortion and no first-order phase
corrections. The number of scans varied between 512 and 1200
depending on the signal/noise ratio. For the variable temperature
experiments, thetemperaturewascontrolledbyaBrukerBVT2000
temperature control unit. The samples were initially held at each
new temperature for at least 15 min to allow equilibration. The
sample temperature was found to be stable within 1 K.
Materials
All chemicals were obtained from Aldrich Chemicals and used di-
rectly.Selectivelydeuterated1-bromodecane-1,1-d2 wasprepared
as follows:
1-bromodecane-1,1-d2
Ethyl decanoate (CH3(CH2)8COOC2H5) was reduced with LiAlD4
using standard procedures. The obtained decanol-1,1-d2 was
transformed into its bromide by an exchange reaction with NaBr in
an aqueous solution of sulfuric acid. The product 1-bromodecane-
1,1-d2 was purified by distillation.
Data processing and simulations
Data processing of the experimental (from the CXP 300 NMR
spectrometer) and theoretical 2H NMR signals were done on
a SUN workstation by using NMR1 and Sybyl/Triad software
packages (Tripos, St. Louis, MO, USA). The data from 1H MAS
and 13C CP/MAS NMR measurements were handled by utilizing
the MestRec software.[43]
Urea inclusion compounds
The UICs were prepared by slowly cooling warm solutions of
1-bromodecane with urea in methanol. The white needles were
filtered, washed with 2,2,4-trimethylpentane and dried.
The programs employed during the simulations of the
corresponding line shape and relaxation experiments are very
general, and consider various types of molecular motions of the
systems under investigation.[13,37] The theoretical line shapes and
relaxation times were obtained by numerical diagonalization of
the corresponding relaxation matrices by using standard software
packages.[37,44]
Differential scanning calorimetry
Calorimetric studies were accomplished with a differential scan-
ning calorimeter Netzsch DSC 204 (Selb/Germany) under nitrogen
flow at heating rates of 10 ◦C/min.
For the line shape simulations, a slow-motional approach
was used.[44,45] This approach assumes δ-pulses and explicitly
accounts for the interval τe in the quadrupolar echo experiment,
responsible for spin–spin relaxation. The theoretical description
of the spin–lattice relaxation data (T1Z, T1Q) is based on the
Redfieldapproach.[46] Here,partiallyrelaxedspectrawereobtained
after multiplication of motionally averaged 2H NMR spectra (fast
exchange limit) with theoretical damping factors that account for
orientation-dependent spin–lattice relaxation during the interval
τr of the inversion recovery[45] and BBJB experiments.[41]
NMR studies
13C CP/MAS, 1H MAS and 2H NMR experiments were performed on
a Bruker CXP 300 (Rheinstetten/Germany) spectrometer operating
at a static magnetic field of 7.04 T interfaced to a Tecmag
spectrometer control system. The resonance frequencies were
75.47 MHz (13C), 300.13 MHz (1H) and 46.07 MHz (2H). A double
tuned Bruker 4-mm MAS probe was used for the 13C CP/MAS,
T1ρ (
13C) and T1ρ (1H) experiments. π/2 pulse lengths were
4 µs. Contact times were 1.5 ms, and recycle delays of 6 s were
set between successive scans. A sample rotation frequency of
5 kHz was used, and the number of scans was between 512
and 1024. Standard spin-lock pulse sequences were used for
the determination of 13C and 1H T1ρ relaxation times.[38,39] 1H
MAS spectra were measured with a Bruker CXP 2.5 mm double
resonance MAS probe with a π/2 pulse length of 4 µs at a sample
rotationfrequencyof34 kHz. Typically, 32scanswereaccumulated
with recycle delays of 2 s. 13C chemical shifts were referenced to
the external standard adamantane. This value was then expressed
relative to TMS (δ = 0 ppm). Likewise, 1H chemical shift values
were given relative to TMS.
The quadrupole echo sequence (π/2)x − τe − (π/2)y − τe with
a π/2 pulse of 2.0 µs and a pulse spacing of τe = 20 µs was used
to obtain the experimental 2H NMR spectra. Spin–spin relaxation
times (T2) were determined with the same pulse sequence by the
variation of τe. A modified inversion recovery sequence, π − τr −
(π/2)x − ꢀ − (π/2)y − ꢀ, combined with the quadrupolar echo
sequence for signal detection and variable relaxation intervals τr,
was employed for the determination of T1Z (relaxation time for
Zeeman order). Here, instead of the inversion π pulse, a composite
pulse, given by [π/2]φ[π/2]φ π/2[π/2]φ with appropriate phase
cycling (φ = 0, π/2, π and 3π/2) was used.[40] Recycle delays
were set to be at least five times the spin–lattice relaxation
time T1Z. Relaxation times for quadrupolar order, T1Q, were
measured by the broadband Jeener–Broekaert sequence (BBJB)
Results and Discussions
2
Variable temperature H NMR spectroscopy was carried out be-
tween 120 K and room temperature on UICs with 1-bromodecane
selectively deuterated at the end CH2Br group (Fig. 1). Solid-state
1H MAS and 13C CP MAS NMR studies were performed for the
inclusion systems of non-deuterated guest species. Differential
scanning calorimetry shows that the phase transition arising from
the distortion of the urea channel for 1-bromodecane/urea occurs
at ca 133 K. This phase transition temperature is lower than that
of 1,10-dibromodecane/urea (141 K)[26,27,35] but higher than the
value of n-decane/UICs which has a phase transition temperature
of 111 K,[34,35] shown in Table 4. Obviously, the solid–solid phase
transition is shifted to higher temperature with the increase of Br
substituent number. This implies that the change in urea crystal
symmetry from the orthorhombic to the hexagonal form, estab-
lished by X-ray diffraction,[34,35,47] is affected by the Br substituent,
most probably due to an increase in intermolecular interactions
between the guest and host as well as guest and guest species.
1H MAS and 13C CP/MAS NMR measurements
Figure 2 depicts 1H MAS (top trace) and 13C CP/MAS (bottom
trace) NMR spectra of 1-bromodecane/urea. For the CH3 group of
c
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X. Yang and K. Mu¨ller
Table 1. NMR parameters from 13C and 1H NMR studies on
1-bromodecane in solution and UICs
δ13C(ppm)
δ1H(ppm)
C
10H21Br
CT: 1.5 ms
Solutiona
Ureab C10H21Br Solutionc Uread
C-1 + Br
C-2
33.8
33.0
36.5
35.0
32.2
33.5
32.9
25.6
15.6
15.3
H-1+Br
H-2
3.4
1.8
1.5
1.3
0.9
3.5
1.5
1.5
1.5
1.1
C-3,4
C-(5–7)
C-8
28.9–28.3
29.5–29.4
32.0
H-3
H-(4–9)
H-10
C-9
22.7
· · ·
C-10 CH3
14.1
· · ·
C-10 BrH2C
14.1
a 13C chemical shifts of 1-bromodecane in CDCl3.[58]
b 13C chemical shifts of 1-bromodecane in their UICs derived in this
work from 13C CP/MAS studies.
c 1H chemical shifts of 1-bromodecane in CDCl3.
d 1H chemical shifts of 1-bromodecane in their UICs derived in this work
from 1H MAS experiments.
Figure 1. (a) Molecular structure for 1-bromodecane-1,1-d2 and
(b) possible motions of the guest molecules in urea and schematic drawing
of three-site jump motion model.
· · ·
resonanceat15.6 ppmreflectsCH3 H3Cend-groupenvironment
· · ·
whiletheresonanceat15.3 ppmresultsfromtheCH3 BrH2Cend-
group environment. The intensity ratio of the two resonances is
found to be 1 : 1, which indicates that for a given CH3 (or CH2Br)
group, there is an equal chance of having either CH3 or CH2Br as a
neighbour – neither alignment is more preferable over the other.
Cross-polarization was used for 13C NMR spectra recording to
improve the signal/noise ratio. In general, the decay of the 13C
signalasafunctionofcontacttimeisdeterminedby1HT1ρ [38,39] To
.
· · ·
· · ·
assess whether the CH3 H3C and CH3 BrH2C peak intensities
(the ratio of 1 : 1) relate to the populations of the different sites,
13C CP MAS NMR spectra are recorded as a function of contact
time ranging from 0.5 to 10 ms. The ratio of two resonances (15.6
and 15.3 ppm) remains 1 : 1 for all contact times, which clarifies
that the intensities of two end CH3 group peaks respond to the
[11,49]
· · ·
· · ·
population of CH3 H3C and CH3 BrH2C arrangements.
Likewise, for the CH2Br end-group, two different environments
· · ·
· · ·
formed by CH2Br H3C and CH2Br BrH2C arrangements should
co-exist. Thus, it is expected that two resonances corresponding to
these two arrangements should be observed. However, only one
distinguishable resonance is recorded. This finding indicates that
the substituent is the dominant influence on the chemical shift
values of the carbon directly bonded to Br atom, compared with
· · ·
· · ·
CH2Br H3C and CH2Br BrH2C arrangements. The C–Br bond
(1.96 Å[4]) is longer than the C–H bond (1.09 Å[50]), which results in
CH3 andCH2BrendgroupsnottoapproachthecarbonoftheCH2Br
ascloselyasfortheCH3 group. And/orthesensitivityoftheBratom
feeling the change of the environment is less than the H atom due
to Br atom and that this slightly environmental change could not
be transmitted to the carbon of CH2Br. Thus, the carbon nucleus of
the end CH2Br group cannot feel the alternation of the electronic
Figure 2. 1H MAS (top trace) and 13C CP/MAS (bottom trace) NMR spectra
of 1-bromodecane/urea.
1-bromodecane two resonances (15.6 and 15.3 ppm) were
observed in the 13C spectrum, whereas the resonances from
other carbons within the molecule appear as single peaks. For
asymmetric guest molecules X(CH2)nY in UICs, the terminal
groups of guest molecules can be aligned in three different
ways, i.e. head-to-head, tail-to-tail and head-to-tail[10,11] two
resonances of the terminal groups reflect different terminal group
arrangements. A previous report assigned the 13C resonance
of 1-fluorotetradecane/UICs,[48] similarly, it is concluded that the
· · ·
· · ·
environments resulting from CH2Br H3C and CH2Br BrH2C
arrangements.Therefore,onlyoneresonance(36.5 ppm)forCH2Br
end-group can be resolved.
According to previous studies of UICs containing n-alkanes
with a terminal functional group,[5,48] the assignments of 1H MAS
and 13C CP/MAS resonances are listed in Table 1 along with the
chemical shifts of the corresponding guest molecules in solution.
It is found that, in general, the packing effect is responsible for
the downfield shift of the 13C resonances of the guest molecules if
c
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Structural and dynamic properties of 1-bromodecane
Table 2. T1ρ(13C) and T1ρ(1H) data for the C10H21Br in urea
T
1ρ(13C) (ms)
C
10H21Br
T1ρ(1H)
(CTa : 1.5 m)
294 K
300 K
310 K
320 K
(ms)b
C-1
196
195
190
182
200
252
249
298
193
198
195
186
194
254
279
227
200
232
195
190
225
257
262
209
268
292
258
232
337
322
425
298
13
13
14
14
14
13
8.8
9.0
C-2
C-3,4
C-(5–7)
C-8
C-9
· · ·
C-10 CH3
· · ·
C-10 BrH2C
a Contact time.
b Value at 294 K.
Figure 3. Variable temperature 2H NMR line shapes of 1-bromodecane-
1,1-d2 in urea (left: experiment, right: simulation).
compared with the signals in solution.[5] Moreover, CH2Br carbon
resonance shows a downfield shift of about 2.5 ppm, suggesting
that 1-bromodecane in urea adopts an all-trans conformation
while there is a mutual exchange between the gauche and trans
conformation in solution.[9,31–33,35,36,51]
Because three- and six-site models cannot be distinguished, all
simulations were done with the three-site jump model. For the
three-site jump model (Fig. 1), the angle θ between the rotation
axis and the C–D bond is set to 90◦ and the azimuthal angle ꢀφ
among the jump sites is −120◦, 0◦ and 120◦, respectively. The
sites p2 and p3 have identical fractional populations, which are
differentfrom thepopulationof theuniquesitep1 withaminimum
potential, that is p2 = p3 = p1, the summation of the populations
of all the three sites is unity (p1 + p2 + p3 = 1).
1H MAS spectrum of 1-bromodecane in urea exhibits much low
resolution, only two resonances are observed, corresponding to
the protons of CH2Br and the others within the molecules. The
nearly same 1H T1ρ values are obtained for most of hydrogens
in 1-bromodecane. These findings suggest that there is the
strong 1H dipolar coupling in guest molecules. The CH3 group
1
has a small H T1ρ value which reflects the end methyl rotation
along the chain,[13,35,52] as listed in Table 2. Table 2 also gives the
temperature-dependent 13C T1ρ data. In general, the 13C T1ρ data
of the carbons within the chain increase with rising temperature,
indicating that the motions of guest molecules undergoing are
on the high-temperature branch of the relaxation curve. 13C T1ρ
value decreases towards the center of the chain, which might
reflect that overall guest fluctuation motion, being sensitive to the
change of the position between the guests and hosts, dominates
13C relaxation in the rotating frame. It will be shown below that
the rotation of the guest molecules along the channel axis is very
The simulated line shapes are presented in Fig. 3 as well. A
best-fit reproduction of the experimental spectrum at 120 K is
obtained for the static case with a quadrupole coupling constant
of 165 kHz and an asymmetry parameter η = 0. In order to get the
good reproduction of the experimental spectra, the quadrupole
coupling constant was slightly reduced at elevated temperatures.
Above the phase transition, a value of 145 kHz is used. This
can be attributed to the occurrence of a fluctuation motion. If
the guest molecules undergo a rotational motion in the rapid
motional regime (with symmetry ≥C3), the 2H NMR spectrum
regains the axially symmetric Pake powder pattern line shape but
its spectral width is half of the spectrum in the slow motional
regime.[13] However, the experimental results demonstrate that
the reduction of the spectral width is more than a factor of 1/2,
the additional reduction is attributed to small-angle fluctuations
perpendicular to the long molecular axis.
fast, thus is not expected to affect 13C T1ρ
.
Line shapes and spin–spin relaxation times
The temperature-dependent 2H NMR line shapes, shown in
Fig. 3, were recorded between 120 K and room temperature
for 1-bromodecane/UICs. The spectrum at 120 K is typical for
a static powder pattern, with a 116-kHz splitting between the two
perpendicular singularities. Upon sample heating, characteristic
variations of the line shapes are observed, indicating that dynamic
processes occur with rate constants in the order of the quadrupole
coupling constant. Above 160 K, the line shapes are found to be
independent of temperature, suggesting that the motion enters
the fast exchange limit. Thus, the splitting of the perpendicular
singularities remains constant with a value of 52 kHz.
To account for the dynamic properties of the guest molecules,
various types of motional models were examined. We tried
numerous theoretical simulations which showed that a three-
site jump model combined with an overall chain small angle
fluctuation can be used to interpret the motion of the guest
molecules investigated. Due to the C6 symmetry of the urea
channel, six-site jump model is more realistic for the guest motion.
If one motional model is thought to be suitable for a system,
it should be possible to reproduce the line shapes as well as the
pulse spacing dependence of the line shapes and the spin–spin
(T2) relaxation times. The experimental T2 values (overall powder
values) for the 1-bromodecane/urea are plotted in Fig. 4. Figure 5
exhibits two representative series of partially relaxed 2H NMR
spectra, at 130 and 150 K, as a function of the pulse intervals
in the quadrupole echo sequence along with the calculated
counterparts derived from the assumed three-site jump model.
The theoretical partially relaxed 2H NMR spectra are obtained with
the same parameters (population, quadrupole coupling constant
2
and correlation times) as used in the simulations of the H NMR
line shapes in Fig. 3. Clearly, the calculated partially relaxed spectra
agree very well with the experimental results.
It is also noted that the spectra at 130 K display the properties
of two superimposed spectra which may reflect two different
c
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