Solid-state NMR study of guest molecule dynamics in 4-alkyl-tert-butylbenzene/thiourea inclusion compounds

Article abstract of DOI:10.1021/jp972128h

Deuterium nuclear magnetic resonance (NMR) powder spectra, deuterium spin-lattice relaxation times (T₁) and ¹³C CP/MAS NMR spectroscopy are used to investigate guest motion in 4-alkyl-tert-butylbenzene/thiourea inclusion compounds (alkyl = tert-butyl, isopropyl, and ethyl). Differential scanning calorimetry data indicate no solid-solid phase transitions for any of the three inclusion compounds in the temperature range -100 to +200°C. Carbon-13 CP/MAS dipolar dephasing experiments indicate that the phenyl ring of all three guests reorient rapidly about the C₁-C₄ axis at room temperature. Deuterium T₁ data for the 1,4-di-tert-butylbenzene-d₁₈/thiourea (DTBB-d₁₈/TU) inclusion compound display two distinct mimina. Internal methyl rotation modulates T₁ in the higher temperature region, while tert-bulyl reorientation affects T₁ at lower temperatures. Activation energies of 12.0 (±0.5) kJ/mol and 11.3 (±0.4) kJ/mol, respectively, were determined. Low-temperature ²H spectra of the DTBB-d₁₈/TU inclusion compound provide insight into the conformation of the methyl protons within the tert-butyl group. Deuterium NMR spectra indicate that the phenyl ring of the guest DTBB-d₄ in thiourea reorients between three positions in the host hexagonal channel. Distortions of the thiourea channel at lower temperatures affect the populations of the three sites. Deuterium T₁ data for the DTBB-d₄/TU inclusion compound allows a comparison of the rate of phenyl ring reorientation with that of tert-butyl motion and shows that the two motions within the same molecule are not correlated. The deuterium NMR spectra of the guest 4-isopropyl-d₆-tert-butylbenzene in thiourea (ITBB-d₆/TU) can be simulated using a model where six-site exchange of the isopropyl group modulates the line shape while the methyl rotation remains fast (k > 10⁸ s^-1). At the temperatures investigated, ²H spin-lattice relaxation times for ITBB-d₆/ TU are being influenced by internal methyl rotation within the isopropyl group. An activation energy of 13.1 (±0.5) kJ/mol was calculated. Similarly, the changes in the ²H spectra of the 4-ethyl-d₃-tert-butylbenzene/ thiourea clathrate (ETBB-d₃/TU) indicate that the ethyl group also reorients between six sites in the host channel, superimposed by fast methyl rotation, which remains rapid (> 10⁸ s^-1) on a lowering of temperature. Again ²H T₁'s are being influenced by internal methyl rotation (E_a = 11.6 (±0.5) kJ/mol) over the temperature range investigated. Finally, the rate of methyl rotation within each of the three functional groups is correlated with the strength of intramolecular interactions within the respective alkyl groups.

Full text of DOI:10.1021/jp972128h

J. Phys. Chem. B 1997, 101, 9087-9097
9087
Solid-State NMR Study of Guest Molecule Dynamics in 4-Alkyl-tert-butylbenzene/Thiourea
Inclusion Compounds
Paul S. Sidhu,† Glenn H. Penner,*,† Kenneth R. Jeffrey,‡ Baiyi Zhao,† Zi Lin Wang,† and
Irene Goh†
Guelph-Waterloo Centre for Graduate Work in Chemistry, Department of Chemistry and Biochemistry,
UniVersity of Guelph, Guelph, Ontario, Canada N1G 2W1, and Guelph-Waterloo Program for
Graduate Work in Physics, Department of Physics, UniVersity of Guelph, Guelph, Ontario, Canada N1G 2W1
ReceiVed: July 1, 1997X
Deuterium nuclear magnetic resonance (NMR) powder spectra, deuterium spin-lattice relaxation times (T1)
and 13C CP/MAS NMR spectroscopy are used to investigate guest motion in 4-alkyl-tert-butylbenzene/thiourea
inclusion compounds (alkyl ) tert-butyl, isopropyl, and ethyl). Differential scanning calorimetry data indicate
no solid-solid phase transitions for any of the three inclusion compounds in the temperature range -100 to
+200 °C. Carbon-13 CP/MAS dipolar dephasing experiments indicate that the phenyl ring of all three guests
reorient rapidly about the C1-C4 axis at room temperature. Deuterium T1 data for the 1,4-di-tert-butylbenzene-
d18/thiourea (DTBB-d18/TU) inclusion compound display two distinct mimina. Internal methyl rotation
modulates T1 in the higher temperature region, while tert-butyl reorientation affects T1 at lower temperatures.
Activation energies of 12.0 ((0.5) kJ/mol and 11.3 ((0.4) kJ/mol, respectively, were determined. Low-
2
temperature H spectra of the DTBB-d18/TU inclusion compound provide insight into the conformation of
the methyl protons within the tert-butyl group. Deuterium NMR spectra indicate that the phenyl ring of the
guest DTBB-d4 in thiourea reorients between three positions in the host hexagonal channel. Distortions of
the thiourea channel at lower temperatures affect the populations of the three sites. Deuterium T1 data for
the DTBB-d4/TU inclusion compound allows a comparison of the rate of phenyl ring reorientation with that
of tert-butyl motion and shows that the two motions within the same molecule are not correlated. The deuterium
NMR spectra of the guest 4-isopropyl-d6-tert-butylbenzene in thiourea (ITBB-d6/TU) can be simulated using
a model where six-site exchange of the isopropyl group modulates the line shape while the methyl rotation
2
remains fast (k > 108 s-1). At the temperatures investigated, H spin-lattice relaxation times for ITBB-d6/
TU are being influenced by internal methyl rotation within the isopropyl group. An activation energy of
13.1 ((0.5) kJ/mol was calculated. Similarly, the changes in the 2H spectra of the 4-ethyl-d3-tert-butylbenzene/
thiourea clathrate (ETBB-d3/TU) indicate that the ethyl group also reorients between six sites in the host
channel, superimposed by fast methyl rotation, which remains rapid (>108 s-1) on a lowering of temperature.
Again 2H T1’s are being influenced by internal methyl rotation (Ea ) 11.6 ((0.5) kJ/mol) over the temperature
range investigated. Finally, the rate of methyl rotation within each of the three functional groups is correlated
with the strength of intramolecular interactions within the respective alkyl groups.
Introduction
clathrates are often unable to locate the atomic positions of the
guest molecule, but rather show a smeared-out electron density
resulting from fast reorientational motion of the guest.10
Deuterium NMR studies of the guest molecular dynamics in
thiourea inclusion compounds of cyclohexane,11 ferrocene,12 and
1,4-di-tert-butylbenzene13 have recently been reported. We have
chosen to continue studies of the 1,4-di-tert-butylbenene/thiourea
(DTBB/TU) inclusion compound (1),in addition to examining
Thiourea is known to form channel inclusion compounds with
a wide variety of organic guest molecules, such as cyclohexane
derivatives, branched-chain paraffins, and some alkyl-substituted
aromatic compounds.1,2 In the presence of guest molecules, the
thiourea lattice usually adopts a rhombohedral structure.3-5
X-ray diffraction studies have shown that the thiourea molecules
form a hexagonal channel, of approximately 7.0 Å diameter, in
which guests of appropriate size reside.6,7 The lattice is held
together by hydrogen-bonded thiourea molecules. Calorimetric
studies of thiourea clathrates often show phase transitions, which
are associated with a change in the channel’s size or shape.8
The thiourea host has a repeating structure in that the thiourea
molecules point directly into the center of the channel at every
third level, and it is usually at this point where the guest
molecule is located.9
A prerequisite for the understanding of the stability of these
inclusion compounds is a knowledge of the dynamic behavior
of the guest molecule. X-ray diffraction studies of thiourea
1
† Guelph-Waterloo Centre for Graduate Work in Chemistry.
‡ Guelph-Waterloo Program for Graduate Work in Physics.
X Abstract published in AdVance ACS Abstracts, October 1, 1997.
the 4-isopropyl-tert-butylbenzene/thiourea (ITBB/TU) inclusion
compound deuterated exclusively on the isopropyl methyl
S1089-5647(97)02128-7 CCC: $14.00 © 1997 American Chemical Society

9088 J. Phys. Chem. B, Vol. 101, No. 44, 1997
Sidhu et al.
groups, and the 4-ethyl-tert-butylbenzene/thiourea (ETBB/TU)
clathrate deuterated on the ethyl CH3 group. These studies will
provide insight into the effect of steric interactions (both host-
guest and intramolecular) on the dynamics of these three alkyl
groups.
The dynamics of guest molecules in clathrate compounds is
complex because these molecules are often only weakly held
within the structure. As well as internal rotational motion, the
guest may undergo reorientation and translation within the lattice
formed by the host molecules. In this particular study, the
following molecular reorientations must be considered: (1)
motion of the methyl groups about the CH3-C bonds; (2)
motion of the tert-butyl, isopropyl, or ethyl group(s); (3) motion
of the phenyl ring within the host lattice. Another important
question to be addressed is whether the rates of molecular
reorientation are primarily determined by intramolecular or
guest-host interactions.
In the intermediate dynamic range, when the reorientational
2
rates are on the order of ø (∼105 s-1), the H powder spectra
obtained by Fourier transforming the quadrupolar echo signal
are severely distorted due to the very large T2 anisotropy
associated with the dynamic process.14 In this motional regime
there is a wide variation in T2 with orientation of the axis of
motion with respect to the direction of the applied magnetic
field. As a consequence, crystallites with different orientations
will exhibit different T2’s and will contribute in varying degrees
to the echo signal and the subsequent spectrum. The deuterium
spectra in the intermediate rate regime can be used as a method
of determining both the type and rate of the molecular motion.
Provided a suitable model for the motion is available, the spectra
may be simulated.15
A further complicating factor in considering the motion of
guest molecules is the possibility that the host cavity does not
have the symmetry of the guest molecule. This can result in
unequal populations of the sites taken up by the exchanging
deuterium nucleus, leading to asymmetry in the powder
spectrum.16
Enclathrated molecules may also undergo a librational motion,
which can be modeled as precession of the principal symmetry
axis on a cone, which may have a circular or elliptical shape.17
This type of motion has a very small activation energy and is
usually only encountered in the fast rate limit. Although the
motion is best described as diffusion within a cone where the
possible angles, ψ, are governed by a distribution function g(ψ),
a simpler model may be employed in which the molecule is
allowed to precess on the surface of a cone of constant half-
angle ψ.17
The spin-lattice relaxation time, T1, characterizes the return
of the bulk magnetization to its equilibrium state after a
perturbing rf pulse. Relaxation is stimulated by time dependent
nuclear spin interactions and is most efficient when these
fluctuations occur at the resonance frequency of the nucleus.
For deuterons, the primary source of these fluctuating fields is
the time dependent part of the quadrupolar interaction.
When the reorientational motion can be described by an
exponential correlation function and the temperature dependence
of the correlation time follows Arrhenius behavior, the rate of
relaxation may be fit to the following equations:18
Theoretical Background
Solid-state deuterium NMR spectroscopy is one of the
principal techniques used to study guest motion in inclusion
compounds. In deuterium NMR spectroscopy, the spectrum is
dominated by the interaction between the deuterium quadrupole
moment, eQ, and the electric field gradient, whose principal
component is eqzz. The quadrupolar coupling constant, ø, is a
measure of the strength of the quadrupolar interaction and is
given by
e2qzzQ
ø )
(1)
h
The asymmetry parameter of the electric field gradient tensor
is defined as the difference between the two smaller principal
components of the diagonalized electric field gradient tensor,
eqxx and eqyy, with respect to the largest component, eqzz.
eqyy - eqxx
η )
(2)
eqzz
In aliphatic compounds, the electric field gradient is usually
axially symmetric along the C-D bond direction. For static
methyl deuterons, η is usually zero and ø generally lies between
165 and 175 kHz. In that case, the spectrum from a deuterium
nucleus located in a static C-D bond consists of a doublet with
spacing
τc
4τc
1 + 4ω0 τc2
1
T1
) K
+
(4)
(5)
2
2
2
(
)
1 + ω0 τc
Ea
τ ) τ exp
( )
c
∞
RT
3e2qzzQ
∆V )
(3 cos2 R - 1)
(3)
τc is the correlation time for the motion (τc k-1), τ∞ is its
value at infinite temperature, and ω0 is the Larmor precession
frequency. In addition, Ea is the activation energy for the motion
influencing T1, and K depends on the strength of the dominant
nuclear spin interaction (in this case ø2) and the exact nature of
the reorientational motion.17 The form of the expression in
parentheses in eq 4 assumes an exponential correlation function.
In this case, if a plot of T1 vs inverse temperature is made,
one often sees a V-shaped curve, with a T1 minimum at ω0τc ≈
1. There is, in general, considerable information to be extracted
from this T1 plot. The slope of the curve on either side of the
T1 minimum is proportional to the activation energy for the
motion influencing T1. The T1 intercept gives a value for τ∞,
while the depth of the minimum can be used to determine a
value for the effective quadrupolar coupling constant, ø. Note
that knowledge of τ∞ and Ea allows calculation of the motional
rate at any temperature, through eq 5. In addition, 2H spectral
line shape changes are usually observed at temperatures cor-
4h
where R is the angle between the C-D bond and the static B0
field. In powdered samples, the line shape is determined by
the random distribution of the C-D bond orientations with
respect to the magnetic field, yielding a Pake doublet with a
characteristic splitting between peaks of 3ø/4 and shoulders
separated by 3ø/2.
Molecular motion has two general effects on the deuterium
powder spectrum. The motion averages the effective quadru-
polar coupling constant to a value smaller than the static one.
When the rate of reorientation is much less than ø, the motion
has no influence, but when the rates are greater than ø, the
spectra are determined by the motionally averaged quadrupole
interaction. The motionally averaged spectrum may not neces-
sarily have the axial symmetry of the static case. When the
motion has less than 3-fold symmetry, the axial symmetry is
lost.

4-Alkyl-tert-butylbenzene/Thiourea Inclusion Compounds
J. Phys. Chem. B, Vol. 101, No. 44, 1997 9089
responding to the low-temperature (ascending) side of the T1
vs 1/T plot, for the particular motion influencing both the spectra
and T1’s.
interfaced to an IBM PC. The computer was programmed to
provide automatic temperature regulation for the sample and
data acquisition. The sample probe was designed to study
powder samples over a temperature range of 100-400 K. The
sample chamber was surrounded by a thick-walled copper
container. Cold N2 gas was passed through channels that had
been milled into the copper vessel, to cool the sample chamber.
The temperature of the chamber was electronically regulated
to within (0.1 °C over the course of a measurement using a
copper-constantan thermocouple to monitor the temperature and
a heater would nonconductively on the copper vessel. A second
separate thermocouple located near the sample itself was used
to record the actual sample temperature, which was known to
within (0.5 °C.
The deuterium T1 data were acquired using an inversion
recovery pulse sequence modified for quadrupolar nuclei: (π)x-
τ-(π/2)(x-τQ-(π/2)y-τQ-acquire.22 Typically, 12 values of
τ were used to determine T1 at each temperature. The pulse
spacing, τQ, for the T1 determination was 40 µs. The time
between repetitions of the pulse sequence was always greater
than 10T1. The data were fit using a nonlinear least-squares
algorithm employing an exponential fitting function to obtain
the relevant relaxation time parameters. The intensity at each
pulse spacing was determined from the echo integral. The T1
values were reproducible to within 5% at a given temperature.
Quadrupolar echo spectra were simulated by standard meth-
ods using the program MXQET.15 Simulated spectra were
visually matched to experimental line shapes. The T1 data were
least-squares fit to the BPP spectral density function23 to obtain
the activation energy, Ea, the correlation time at infinite
temperature, τ∞, and the effective quadrupolar coupling constant,
øeff (eqs 4 and 5).
Usually, solid-state 13C NMR is characterized by very broad
resonances, but in some cases, the signals may be narrowed
experimentally to a degree where chemical shifts can be
resolved.19 The dipole-dipole interactions and chemical shift
anisotropy usually affect the NMR response of solid samples,
but are averaged to zero in the liquid phase due to isotropic
reorientation. In solids, these interactions can be removed by
the combination of magic angle spinning (MAS) and high-power
dipolar decoupling. In addition, cross polarization (CP) from
1H to 13C greatly increases the signal strength.
The nonquaternary suppression (NQS) pulse sequence is used
to attenuate 13C signals from carbons that have directly bonded
protons. Experimentally, a brief delay, without proton decou-
pling, is inserted between the contact pulse and the free induction
decay, during the normal cross polarization pulse sequence.20
A very important exception to this rule occurs when the carbon
under study undergoes rapid molecular motion. Some methyl
13C resonances are difficult to attenuate entirely, because rapid
methyl rotation reduces the magnitude of proton-carbon dipolar
coupling. This can also apply to rapidly rotating CH or CH2
groups. Therefore, this pulse sequence can qualitatively deter-
mine whether or not a CH group or a CH2 group is undergoing
rapid reorientation in the solid state.
Experimental Section
Deuterated samples of 1,4-di-tert-butylbenzene (DTBB) were
prepared by Friedel-Crafts alkylation of benzene or benzene-d6
with tert-butyl-d9 chloride or tert-butylchloride and ferric
chloride. Products were recrystallized from ethanol and were
shown to be pure by high-resolution NMR and melting point.
The DTBB/TU clathrate was prepared by mixing equal weights
of thiourea and deuterated DTBB in enough methanol to dissolve
both solids at room temperature. After slow evaporation the
crystals were harvested and washed with pentane in order to
remove excess DTBB. Elemental analysis showed that the
DTBB/thiourea clathrate crystallized in a 1:6 complex.
4-Isopropyl-d6-tert-butylbenzene was prepared by first react-
ing 4-tert-butylphenylmagnesium bromide with acetone-d6 to
give 4-tert-butyl-R,R-dimethyl-d6 benzyl alcohol. The alcohol
was then reduced by lithium in a liquid ammonia/tetrahydrofuran
mixture at -78 °C (dry ice/acetone bath) to give nearly pure
product. The product was not further purified but was
selectively separated by adding the resultant liquid to 10 mL
of a saturated thiourea/methanol solution. The crystalline
inclusion compound was separated by filtration and air dried.
The product was shown to be pure by solution NMR.
13C CP/MAS NMR spectra were recorded on a Bruker ASX
200 spectrometer at a frequency of 50.3 MHz. The 13C π/2
pulses were 4.0 µs, and cross polarization times of 3 ms were
used. Typically 100-150 transients were accumulated, and the
recycle delay was 5 s. The samples were spun at 5 kHz, using
a 7 mm Bruker MAS probe. The chemical shifts were
referenced to the low-frequency signal of adamantane (29.5
ppm), and following acquisition, an exponential apodization of
10 Hz was applied before the Fourier transform.
Heat capacity measurements were performed on a Dupont
Model 2910 differential scanning calorimeter. Approximately
10 mg of sample was hermetically sealed in an aluminum pan.
Liquid nitrogen was used to cool the sample below room
temperature, and a ramp (heating) rate of 10 K/min was
employed. The reference sample was an empty aluminum pan,
and indium metal was used as the standard for temperature
calibration.
4-Ethyl-d3-tert-butylbenzene was prepared by the reaction of
4-tert-butylbenzaldehyde and the CD3MgI Grignard reagent,
with subsequent reduction of the alcohol by the aforementioned
method. The product was shown to be pure by solution NMR.
It should be noted that thiourea inclusion compounds could not
be made with tert-butylbenzene or 4-methyl-tert-butylbenzene
as guests.
The deuterium spectra were recorded on a home-built, phase
coherent pulsed NMR spectrometer operating at 44.7 MHz,
using the quadrupolar echo pulse sequence: (π/2)(x-τQ-(π/
2)y-τQ-acquire.21 The length of the π/2 pulse was ap-
proximately 3.5 µs. At each temperature, the echo signals were
collected for a τQ value of 40 µs. The echo signals were Fourier
transformed to obtain the deuterium NMR spectra. The
spectrometer was used with a 50 mm bore, 6.8 T (Bruker/
Nalorac) superconducting magnet. The nuclear signal was
digitized in a Nicolet Explorer digital oscilloscope, which was
Results and Discussion
1,4-Di-tert-butylbenzene/Thiourea. The 13C CP/MAS spec-
trum of 1,4-di-tert-butylbenzene in the thiourea inclusion
compound, at room temperature, is shown in Figure 1. The
signals in the spectrum were assigned to the carbons of the guest
based on 13C chemical shift correlation tables.24 Note that there
is only one signal for all six methyl carbons, which indicates
that tert-butyl rotation is fast on the 13C spectral time scale at
room temperature and that the two tert-butyl groups occupy
equivalent crystallographic sites. In addition, there is only one
signal for all four unsubstituted CH atoms of the benzene ring,
again implying magnetic equivalence, likely due to molecular
motion. This is confirmed by the NQS experiment. The
resonance corresponding to the CH phenyl carbons is not
eliminated by dipolar dephasing, indicating that the phenyl ring
is rotating rapidly at room temperature. The rapid reorientation

9090 J. Phys. Chem. B, Vol. 101, No. 44, 1997
Sidhu et al.
Figure 1. 13C CP/MAS powder spectrum of the 1,4-di-tert-butylbenzene/thiourea inclusion compound, at room temperature, with signal assignments.
is confirmed by 2H data for the same compound, which indicate
that the motion of the phenyl ring is proceeding at a rate greater
than 107 s-1 down to 120 K.13
reorientation slows first upon lowering of the sample temper-
ature.13 Both motions are in the extreme narrowing limit at
room temperature (ω0τc , 1), so the high-temperature minimum
is due to changes in the rate of methyl rotation, while tert-butyl
rotation is influencing T1 in the low-temperature region. In
addition, the ratio of relaxation times at the two T1 minima
provides further proof of this model. It can be shown that, if
methyl rotation slows first as the temperature decreases, then
the ratio of T1 at the two minima should be 1.5. If tert-butyl
rotation slows first, then the above ratio would be 9.0.25 The
measured ratio is approximately 6 ms/5 ms ) 1.2. Thus, the
first case is correct.
The 13C CP/MAS spectrum for pure, unenclathrated solid 1,4-
di-tert-butylbenzene, at room temperature, is depicted in Figure
2A. With the exception of a few spinning sidebands in the 40-
80 ppm range, the spectrum is similar to that of 1,4-di-tert-
butylbenzene in thiourea, showing that the resonances corre-
spond to the same carbons as those of the DTBB guest in the
thiourea clathrate. One exception is the signal corresponding
to the CH ring carbons, which is split into two equal intensity
peaks, suggesting that the symmetry of the phenyl ring is
reduced due to a combination of crystal packing and lack of
phenyl ring motion. Carbons 2 and 3 of the ring give one signal,
as do carbons 5 and 6. However, carbon 2 is now magnetically
inequivalent to carbon 6. The lack of phenyl ring motion is
confirmed by the nonquaternary suppression pulse sequence,
as the doublet described above disappears (Figure 2B). This
indicates a strong heteronuclear dipolar coupling, which is not
averaged by fast motion. Once more, these results agree with
2H NMR data, which indicate that the phenyl ring is static at
all temperatures below the melting point (349 K).13
By fitting the high temperature T1 data to eqs 4 and 5, an
activation energy of 12.0 ( 0.5 kJ/mol was determined for
methyl rotation, and τ∞ was (1.4 ( 0.5) × 10-12 s. A similar
analysis of the low-temperature region of the T1 curve yields
an activation energy for tert-butyl rotation of 11.3 ( 0.4 kJ/
mol, while the correlation time at infinite temperature is 2.0 (
0.7 × 10-14 s. Note that these two values allow calculation of
the motional rate at any temperature, according to eq 5. The
near equivalence of activation energies (11.3 and 12.0 kJ/mol)
does not necessarily mean that rate of each motion should show
the same temperature dependence, because the rate also depends
on τ∞. Indeed, τ∞ is different by approximately 2 orders of
magnitude, which then dictates that methyl rotation slows down
first as the temperature is lowered. A fit of the relaxation curve
yields an effective static quadrupolar coupling constant, ø, of
166.7 ( 0.8 kHz, which is typical for methyl deuterons.
From differential scanning calorimetry data, there are no
phase transitions for the 1,4-di-tert-butylbenzene/thiourea inclu-
sion compound, in the temperature range -100 °C (173 K) to
+200 °C (473 K), other than decomposition of the inclusion
compound at 178 °C and melting of the thiourea host at
approximately 185 °C.
The 2H T1 data for 1,4-di-tert-butylbenzene-d18 in the thiourea
inclusion compound is shown in Figure 3. Two distinct minima
are apparent, each corresponding to one of the two motions of
the tert-butyl group. From simulations of the 2H spectral data,
it can be clearly shown that methyl rotation slows first as the
temperature decreases. The deuterium NMR spectra for DTBB-
d18/TU cannot be simulated using a model where tert-butyl
In pure, solid 1,4-di-tert-butylbenzene, the tert-butyl rotation
is hindered by relatively strong intermolecular interactions.26
This results in tert-butyl dynamics that are almost equivalent,
as far as motional rates are concerned, to the internal rotation
of the methyl groups (the rate of tert-butyl reorientation starts
to decrease from 1 × 107 s-1 at temperatures immediately below
ambient temperature).13 In the DTBB/thiourea inclusion com-

4-Alkyl-tert-butylbenzene/Thiourea Inclusion Compounds
J. Phys. Chem. B, Vol. 101, No. 44, 1997 9091
Figure 2. (A) Normal 13C CP/MAS spectrum of pure, unenclathrated 1,4-di-tert-butylbenzene. (B) 13C CP/MAS spectrum with dipolar dephasing.
SCHEME 1
the distortion is a constriction in one direction, which continu-
ously increases as the temperature is lowered (see Scheme 1),
Figure 3. 2H spin-lattice relaxation time data, as a function of
as was also observed for the thiourea/cyclohexane inclusion
temperature, along with best fit curves for (A) 1,4-di-tert-butyl-d18-
compound,11,27 the adamantane/thiourea clathrate,28 and the
benzene/thiourea inclusion compound (squares), (B) 4-isopropyl-d6-
tert-butylbenzene/thiourea inclusion compound (triangles), and (C)
n-C16H34/urea adduct.29 In our case, the distortion causes the
4-ethyl-d3-tert-butylbenzene/thiourea inclusion compound (circles).
fractional population of site A to decrease, relative to sites B
and C, which remain equally populated. Simulations in which
the relative population of site A was larger that sites B and C
gave spectra with too large an effective quadrupolar coupling
constant.13
pound, the guest molecules are less tightly held by the thiourea
host lattice. The consequence of this fact is that, as determined
from deuterium spin-lattice relaxation time measurements and
deuterium NMR spectra,13 the tert-butyl rotation rate does not
slow below 1 × 106 s-1, even down to 77 K. The methyl
rotation dynamics are almost the same in both compounds,
because methyl rotation is dominated by intramolecular interac-
tions within the tert-butyl group, which broadly speaking, do
not change from the pure solid to the thiourea inclusion
compound. On the other hand, intermolecular interactions
between neighboring molecules, which influence tert-butyl
rotation, are markedly different from the neat solid (stronger)
to the thiourea clathrate (weaker).
With the previously described model and an exchange rate
of 108 s-1, the experimental spectra from 392.0 K (PA ) 0.312,
PB ) 0.344, PC ) 0.344) to 172.3 K (0.047, 0.476, 0.476) could
be successfully simulated. As the temperature is lowered from
392 K, the asymmetry parameter η gradually increases toward
1.0 at 270.3 K. Then η decreases to essentially zero at 185.6
K. Below this temperature η increases again to about 0.4 at
2
121.7 K. The four H spectra below 172.3 K could not be
simulated using a model where one of the three sites is less
populated than the other two.13
2
The H NMR spectra of 1,4-di-tert-butylbenzene-d4 in the
Intuitively, the simplest model for the four lowest temperature
spectra would be to allow the population of site A to decrease
to zero and consider a two-site exchange in which the sites are
allowed to have different populations, reflecting the introduction
thiourea inclusion compound, in the higher temperature region,
were previously simulated using a model where the phenyl ring
takes up three sites in the hexagonal thiourea channel.13 As
the temperature is lowered below room temperature, the channel
distorts from hexagonal symmetry. The easiest way to describe
2
of a more complex distortion. The H spectra below 172.3 K
were successfully simulated using this model and keeping the

9092 J. Phys. Chem. B, Vol. 101, No. 44, 1997
Sidhu et al.
Figure 5. Deuterium spin-lattice relaxation times for 1,4-di-tert-
butylbenzene-d4 in the thiourea inclusion compound, as a function of
inverse temperature.
steric argument that if they pointed toward the phenyl ring, they
would be in close contact with the ortho protons of the benzene
ring. The correct conformation for the tert-butyl group at 77
K is shown below (2).
Figure 4. 2H NMR powder spectrum of 1,4-di-tert-butylbenzene-d18
in thiourea, at 77 K (B), along with simulations with the axial methyl
proton point toward the phenyl ring (A) and pointing away from it
(C).
TABLE 1: Fractional Populations of the Three Sites Taken
Up by the Phenyl Ring in the Thiourea Channel, of the
1,4-Di-tert-butylbenzene/Thiourea Inclusion Compound,
2
Internal Rotation versus Molecular Rotation. Previous
experimental and theoretical work on alkylbenzenes yields
liquid- and gas-phase barriers to internal rotation for ethyl,30
isopropyl,31 and tert-butyl32 groups of 4-8, 1-9, and 2-5 kJ/
mol, respectively. Evidently, these barriers are not sufficiently
high that the alkyl groups can be considered as locked into one
position with respect to the phenyl ring. The question then arises
as to whether the alkyl groups are rotating together with the
rest of the molecule inside the thiourea channel. Our previous
work on 1,4-di-tert-butylbenzene in thiourea13 showed clearly
that both the tert-butyl groups and the phenyl ring were rotating
rapidly, even down to very low temperatures (below 100 K). If
two parts of a molecule can rotate about a common axis, similar
activation energies do not necessarily mean that the two groups
are rotating together. The correlation times, τc (or rotational
rates, k), may still be different, indicating that the two groups
are not rotating together, if the τ∞ (or k∞) values are different
(see eq 5).
In the previous work on the 1,4-di-tert-butylbenzene-d4/
thiourea inclusion compound,13 deuterium spin-lattice relax-
ation time measurements showed a T1-temperature profile that
did not reach a T1 minimum, even down to 110 K. Due to
instrumental limitations, it was not possible at the time to go
below this temperature. This precluded a detailed T1 fit, to
determine Ea and τ∞, for the three-site exchange of the phenyl
ring in the thiourea lattice. In this work (see Figure 5), we
have acquired T1 data for DTBB-d4/TU down to 88 K, and the
presence of a definite minimum is apparent. We found an
activation energy for the three-site reorientation of 8.4 ( 0.5
kJ/mol and a correlation time at infinite temperature of (1.1 (
0.4) × 10-13 s. Using these two values, along with eq 5, it is
possible to find the rate of the three-site exchange of the phenyl
ring, as a function of temperature. From the earlier fit of the
low-temperature T1 data for 1,4-di-tert-butylbenzene-d18/thiourea
2
Corresponding to Scheme 1, Based on H Spectra
temp (K)
PA
PB
PC
392.0
357.5
329.7
295.7
270.3
249.7
224.0
203.7
185.6
172.3
158.6
138.4
121.7
101.9
0.312
0.306
0.298
0.280
0.256
0.240
0.204
0.156
0.092
0.047
0
0.344
0.347
0.351
0.360
0.372
0.380
0.398
0.422
0.454
0.476
0.420
0.300
0.280
0.220
0.344
0.347
0.351
0.360
0.372
0.380
0.398
0.422
0.454
0.476
0.580
0.700
0.720
0.780
0
0
0
rate at 108 s-1. The fractional populations of the three sites are
recorded in Table 1.
Finally, the 2H spectrum of the 1,4-di-tert-butylbenzene-d18/
thiourea inclusion compound, at 77 K, pictured in Figure 4B,
provides insight into the conformation of the methyl deuterons.
2
At this temperature, methyl rotation has stopped on the H
spectral time scale (k < 1 × 103 s-1), while tert-butyl rotation
is still in the intermediate exchange region (k′ ) 2 × 106 s-1).
There are two components to the experimental spectrum. The
inner, narrower component corresponds to the 12 methyl C-D
bonds that point off to the side, making an angle of ap-
proximately 70.5° with respect to the rotation axis. The outer
component corresponds to the six C-D bonds that make an
angle of 0° with respect to the tert-butyl rotation axis (the “axial”
deuterons). Whether these C-D bonds point toward or away
from the phenyl ring is of some question. However, simulations
indicate that these C-D bonds point away from the phenyl ring
(Figure 4C), not toward it (Figure 4A). This agrees with the

4-Alkyl-tert-butylbenzene/Thiourea Inclusion Compounds
J. Phys. Chem. B, Vol. 101, No. 44, 1997 9093
(Ea ) 11.3 kJ/mol and τ∞ ) 2.0 × 10-14 s), corresponding to
tert-butyl reorientation, it is possible to compare the rate of the
phenyl ring rotation with the rate of rotation of the tert-butyl
group, as a function of temperature. Equations 6 and 7 are the
result of fitting the T1 curves for DTBB-d4/TU and DTBB-d18,
respectively.
and guest considered below. A third motion is present, which
is a librational motion of the molecule, this libration spectrally
manifesting itself as a slight reduction in the effective quadru-
polar coupling constant.
As the temperature is changed from 273 to 15 K, the effective
asymmetry parameter gradually increases from zero at 273 K
to ∼0.5 at 153 K. This implies that the relative populations of
the sites taken up by the isopropyl deuterons are changing as
the temperature is lowered. The asymmetry cannot be within
the three sites taken up by the methyl deuterons, because of
the C3 symmetry of the methyl group. It is the populations of
the six sites of the isopropyl group which are becoming unequal,
due to the previously mentioned distortion of the thiourea
channel, from hexagonal symmetry, which gradually increases
as the temperature decreases.11,27,28 In addition, the gradual
coalescence of the central dip of the spectra, as the temperature
decreases, suggests that a motional rate is decreasing on the 2H
NMR time scale.
From our studies on 1,4-di-tert-butylbenzene in thiourea,13
and various other thiourea inclusion compounds, it is understood
that the host channel distorts continuously with a decrease in
temperature by decreasing the length of one of the three
hexagonal axes (see Scheme 2). This distortion results in a
decrease in the populations of sites 2, 3, 5, and 6, while sites 1
and 4 would correspondingly increase in population, because
when the isopropyl group is positioned in sites 1 and 4, there is
no methyl group pointing directly into the distorted (compressed)
axis. In the other four sites, one of the two methyl groups points
directly into the distorted axis, where there is less space (more
steric hindrance). Since the distortion is likely symmetric, one
would expect sites 2, 3, 5, and 6 to decrease in population
equally, while sites 1 and 4 would have equal, but larger,
populations. The seven spectra were successfully simulated
using this model. At 293 K the asymmetry parameter is zero,
implying that all populations are equal and that the host channel
has hexagonal symmetry.
τc(ring) ) 1.1 × 10-13 exp[(8400) J mol-1/RT] (6)
τc(butyl) ) 2.0 × 10-14 exp[(11300) J mol-1/RT] (7)
From these equations it is obvious that the two motions have
the same rate at only one temperature, 205 ( 10 K.
Using standard bond angles, bond lengths, and elementary
trigonometry, we estimate that the phenyl hydrogens of DTBB
project approximately 2.15 Å from the molecular axis of rotation
(which passes through C1 and C4 of the phenyl ring). In
contrast, the tert-butyl hydrogens are about 2.34 Å from the
same axis. This may help to explain why the activation energy
for tert-butyl reorientation is higher than that for phenyl ring
rotation. The tert-butyl protons are closer to the hexagonal
thiourea channel and are thus likely to interact more strongly
with the host, as compared to the phenyl ring protons of DTBB.
4-Isopropyl-tert-butylbenzene/Thiourea. The room-tem-
perature 13C CP/MAS spectrum of 4-isopropyl-tert-butylbenzene
in thiourea is presented in Figure 6A. The resonances were
assigned based on 13C chemical shift correlation calculations.24
The two ipso carbons are now magnetically inequivalent because
two different substituents are present, as are carbons 2 and 3,
in comparison to 1,4-di-tert-butylbenzene. The signals due to
the isopropyl carbon and the tert-butyl methyl carbons overlap.
The fact that only one signal is seen for the isopropyl methyls
and the tert-butyl methyls (each) suggests that they are each
exchanging rapidly and occupying crystallographically equiva-
lent sites. Similarly, two signals for the four CH phenyl carbons
suggest that the ring reorientation proceeds at rate greater than
103 Hz. This conjecture of rapid rotation is supported by the
NQS experiment. In this spectrum, the CH phenyl carbons
retain most of their intensity, implying a reduction in the
magnitude of heteronuclear CH dipolar coupling, due to rapid
reorientation.
As was mentioned above, the change in the depth of the
central dip of the spectra indicates that a motional rate is
decreasing. We attempted to simulate the deuterium powder
spectra with three different models. In the first model, we held
the rate of the six-site reorientation at 108 s-1, while allowing
the rate of methyl rotation to gradually decrease from the fast
rate limit. Secondly, we allowed both motions to decrease from
108 s-1 in concert, as the temperature drops from ambient
temperatures. Neither of these two models were able to simulate
From differential scanning calorimetry data, no phase transi-
tions were observed for the 4-isopropyl-tert-butylbenzene/
thiourea inclusion compound, between -100 °C (173 K) and
200 °C (473 K), other than decomposition of the clathrate at
167 °C and melting of the thiourea host at 183 °C.
2
the experimental H powder spectra. An alternative choice
available to us was to hold the rate of methyl rotation at 108
s-1, while decreasing the rate of the six-site exchange. As can
be seen in Figure 7, the match between the two sets of spectra
is very good. The rate of the six-site reorientation is indicated
with each of the seven spectra. A least-squares fit of the rates
to the Arrhenius equation (ln k ) ln A - Ea/RT) gave an
activation energy for this motion of 8.4 ( 1.8 kJ/mol and a ln
To investigate the motion of the isopropyl group in the
thiourea channel, 2H NMR powder spectra of ITBB-d6/TU were
obtained between 273 and 153 K (Figure 7). The spectrum at
the highest temperature, which has a quadrupolar splitting of
12.0 ( 0.5 kHz, and an asymmetry parameter of approximately
zero, is characteristic of both fast (>108 s-1) methyl rotation
and fast isopropyl reorientation. The nature of the isopropyl
exchange is uncertain. The projected angle between the two
methyl groups is very close to 120°, so the motion must be at
least 3-fold, in agreement with the axially symmetric spectrum
observed at room temperature. However, one must also consider
the effect of the thiourea host channel on the motion. The 6-fold
symmetric hexagonal channel implies that the isopropyl group
rotates through six sites. The reduction in quadrupolar splitting
would be the same as a three-site jump. The six positions of
the isopropyl group in the thiourea channel are depicted in
Scheme 2. The isopropyl proton is not shown for clarity, but
is at an angle of 120° to both methyl groups. Since this proton
is so much smaller than the methyl groups, we will consider its
influence to be negligible in the steric interactions between host
2
A value of 18.5 ( 0.5. It should be noted that all H spectra
acquired at temperatures below 153 K were featureless “bumps”
that provided no definite information concerning rates or types
of molecular reorientation.
The 2H spin-lattice relaxation time data for 4-isopropyl-d6-
tert-butylbenzene in thiourea are presented in Figure 3. It can
be seen that a well-defined minimum is present. In the
temperature range in which we performed our measurements,
the rate of the six-site exchange (of the isopropyl group) was
2
always less than 107 s-1 (as determined from H spectra), and
as T1 is generally modulated by motion in the rate 108 to 1012
s-1, the minimum is likely due to changes in the rate of methyl
rotation. Indeed, when one compares the rate of six-site

9094 J. Phys. Chem. B, Vol. 101, No. 44, 1997
Sidhu et al.
Figure 6. 13C CP/MAS spectra of (A) the 4-isopropyl-tert-butylbenzene/thiourea inclusion compound and (B) the 4-ethyl-tert-butylbenzene/
thiourea inclusion compound.
exchange of the isopropyl group (from 2H spectra) with the rate
of motion calculated from the T1 curve, the two rates are
different by approximately 3 orders of magnitude (see Figure
8). Thus the motion influencing T1 in the observed minimum
cannot possibly be the six-site exchange and therefore is
assigned to methyl rotation. We attribute the decrease in T1 at

4-Alkyl-tert-butylbenzene/Thiourea Inclusion Compounds
J. Phys. Chem. B, Vol. 101, No. 44, 1997 9095
Figure 7. 2H NMR powder spectra of 4-isopropyl-d6-tert-butylbenzene in the thiourea inclusion compound along with best fit simulations, at the
temperatures indicated. The rate of six-site exchange of the isopropyl group within the host channel is listed with each pair of spectra.
SCHEME 2
Figure 8. Plot of reorientational rate, as a function of temperature,
for the rate of six-site exchange of the isopropyl group (in ITBB/TU)
and six-site exchange of the ethyl group (in ETBB/TU), both calculated
from 2H NMR spectra. In addition, we have included the rate of internal
low temperatures to rotation of the phenyl ring. At these low
methyl rotation, determined from deuterium spin-lattice relaxation
temperatures both the isopropyl group and the two methyl
groups are rotating too slowly (k < 106 s-1), but the phenyl
ring rotation is reorienting at a rate comparable to the spec-
trometer frequency (k ≈ 107 s-1). The deuterons are most likely
times, for all three inclusion compounds. Filled triangles: six-site,
ITBB/TU. Filled circles: six-site, ETBB/TU. Empty circles: methyl
rotation, DTBB/TU. Empty triangles: methyl rotation, ITBB/TU.
Empty squares: methyl rotation, ETBB/TU.
2
relaxing via a H-1H dipole-dipole mechanism. When the
of 14.2 kJ/mol for pure isopropylbenzene, as determined from
proton spin-lattice relaxation time measurements,33 14 kJ/mol
for pure 1,4-diisopropylbenzene34 and 13 kJ/mol for 1,3,5-
triisopropylbenzene.35 In pure isopropylbenzene the rate of
isopropyl group reorientation is slow on the 1H inverse Larmor
frequency time scale. This is likely because of the lack of
symmetry of the isopropyl group, as compared to the C3
symmetry of the tert-butyl groups of 1,4-di-tert-butylbenzene
(the three-site exchange of the tert-butyl group in pure DTBB
phenyl ring and a methyl group are coplanar, the dipolar
coupling is several kilohertz. The small deviation from the fitted
curve at higher temperatures is most probably due to changes
in the rate of six-site exchange of the isopropyl group, which
dominates the 2H T1 time scale only at temperatures above room
temperature.
By fitting the T1 data (Figure 3, triangles) of the central
minimum, we determined the correlation time at infinite
temperature to be (4.9 ( 2.0) × 10-13 s, and the activation
energy for methyl rotation to be 13.1 ( 0.5 kJ/mol. This
compares to an activation energy, for internal methyl rotation,
2
is fast on the H spectral time scale at room temperature13).
However, in the case of the ITBB/thiourea inclusion compound,

9096 J. Phys. Chem. B, Vol. 101, No. 44, 1997
Sidhu et al.
Figure 9. 2H NMR powder spectra of the 4-ethyl-d3-tert-butylbenzene/thiourea inclusion compound, deuterated on the CH3 ethyl group, along
with best fit simulations and the rate of six-site exchange of the ethyl group at each temperature.
there is a six-site symmetry imposed by the hexagonal host
channel. This, together with the relatively weak guest-host
interaction, results in isopropyl group reorientation that reaches
the fast rate limit at ambient temperature.
the drop in temperature, representing, again, a compression along
one of the axes of the hexagonal channel. The spectra also
indicate that the six-site reorientation slows down first, while
methyl rotation proceeds at a rate greater than 108 s-1, even
down to 172 K. As with ITBB-d6/TU, we were unable to
simulate the 2H powder spectra using any other feasible model
of reorientation. From the deuterium spectra, we determined
the rate of the six-site exchange as a function of temperature
(see Figure 8). From a least-squares fit to the six data points,
we determined an activation energy for the six-site exchange
of 9.0 ( 0.1 kJ/mol and a value of 18.1 ( 0.1 for ln A. This
compares to 8.4 ( 1.8 kJ/mol for the six-site exchange of the
isopropyl group in ITBB/TU and 11.3 ( 0.4 kJ/mol for the
three-site exchange of the tert-butyl group in DTBB/TU. Again,
as was the case with ITBB/TU, all 2H spectra acquired at
temperatures below 172 K were featureless and provided little
information about the reorientation of the guest’s ethyl group.
The narrow central peak at higher temperatures and the broad
shoulders observed at lower temperatures are due to a small
amount of liquid ethylbenzene-d3 in the sample.
In Figure 3, a temperature profile of the deuterium spin-
lattice relaxation time for the ETBB-d3/TU inclusion compound,
deuterated on the ethyl CH3 group, is presented. In the central
region there is a broad minimum, which, as in the case of ITBB/
TU-d6, we attribute to the methyl group rotation.
By fitting the T1 data, an activation energy for this motion
of 11.6 ( 0.5 kJ/mol was found. The constant K (see eq 4) is
3.1 × 1010 s-2, while the correlation time at infinite temperature
is approximately (2.0 ( 0.8) × 10-13 s. This compares to an
activation energy, for internal methyl rotation, of 13.2 kJ/mol
in pure ethylbenzene, 15 kJ/mol in 1,3-diethylbenzene, and 12
kJ/mol in 1,2-diethylbenzene.35 As in the case of ITBB/TU
4-Ethyl-tert-butylbenzene/Thiourea. The 13C CP/MAS
room-temperature spectrum of 4-ethyl-tert-butylbenzene/thiourea
is presented in Figure 6B. The resonances were assigned based
on 13C chemical shift correlation calculations.24 The signals in
the low-field region are similar to those of 4-isopropyl-tert-
butylbenzene/thiourea, as carbons 2 and 3 (along with carbons
1 and 4) are inequivalent. In the dipolar dephasing experiment,
carbons 2, 3, and 7 retain most of their intensity, indicating
that both the phenyl ring and the ethyl CH2 group are reorienting
at a rate greater than 103 Hz at room temperature.
From differential scanning calorimetry data, no phase transi-
tions were found for the 4-ethyl-tert-butylbenzene/thiourea
inclusion, other than decomposition of the inclusion compound
at 117 °C and melting of the thiourea at 181 °C.
Deuterium powder NMR spectra of the 4-ethyl-d3-tert-
butylbenzene/thiourea (ETBB-d3/TU) inclusion compound, along
with simulations, appear in Figure 9. The spectrum at 272.6 K
is a Pake doublet, with an effective quadrupolar coupling
constant øeff ) 34.3 ( 0.5 kHz. This value is indicative of fast
(k > 108 s-1) methyl rotation and fast six-site exchange of the
ethyl group through six positions in the hexagonal thiourea
channel. At this temperature, the populations of the six sites
are equal, suggesting that the host channel is not appreciably
2
distorted. As the temperature is decreased, the H spectrum
coalesces and forms a featureless line shape at 172.3 K. The
shape of the spectrum does not then change appreciably on going
down in temperature to 85 K. The simulations indicate that
the populations of two of the six sites continually decrease with

4-Alkyl-tert-butylbenzene/Thiourea Inclusion Compounds
J. Phys. Chem. B, Vol. 101, No. 44, 1997 9097
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(see Figure 3, triangles), there are downward deviations from
the fitted curve at both high and low temperatures (see Figure
3, circles).
It is interesting to compare the rate of internal methyl rotation
for each of the three inclusion compounds studied here. The
rate of this motion, as a function of inverse temperature,
calculated from 2H spin-lattice relaxation time measurements,
is shown in Figure 8. Not only does it confirm that, for ITBB/
TU and ETBB/TU, the six-site exchange does not determine
T1 in the temperature range investigated it also provides
information about steric interactions within the three functional
groups. Intuitively, one would expect intramolecular interac-
tions within the alkyl group to be weakest within an ethyl group,
intermediate for an isopropyl group, and strongest for a tert-
butyl functionality. At any given temperature, the rate of methyl
rotation is fastest for ETBB/TU, intermediate for ITBB/TU, and
slowest for DTBB/TU. Therefore, the rate of methyl rotation
and the strength of these steric interactions correlate very well.
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Matter; Blu¨mich, Ed.; Springer-Verlag: New York, 1994.
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Summary
We have shown that the guest molecules 1,4-di-tert-butyl-
benzene, 4-isopropyl-tert-butylbenzene, and 4-ethyl-tert-butyl-
benzene, when enclathrated in the channels of thiourea, execute
a number of motional modes. The slowest mode is the
reorientation of the alkyl groups. As a consequence of their
lower symmetry, the rotation of the isopropyl and ethyl groups
is slower than that of the tert-butyl group. The methyl
substituents that make up these groups rotate faster, and their
rates follow the sequence kethyl > kisopropyl > kt-butyl. It is further
shown that in the 1,4-di-tert-butylbenzene inclusion compound
the ring and the tert-butyl group are not reorienting at the same
rate and that the activation energy for ring rotation is smaller
than that for tert-butyl rotation.
Acknowledgment. We would like to thank NSERC Canada
for financial support. We are grateful to Sandy Smith for
assistance with the differential scanning calorimeter and Patrik
Nechala for a synthesis of ETBB.
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