Deuterium nuclear magnetic resonance (NMR) powder spectra,
deuterium spin−lattice relaxation times
(T
1)
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 T
1 data for the
1,4-di-tert-butylbenzene-d
18/thiourea (DTBB-d
18/TU)
inclusion compound display two distinct mimina. Internal methyl
rotation
modulates T
1 in the higher temperature region,
while tert-butyl reorientation affects
T
1 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-d
18/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-d
4 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 T
1 data
for
the DTBB-d
4/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-d
6-tert-butylbenzene in
thiourea (ITBB-d
6/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-d
6/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-d
3-tert-butylbenzene/thiourea clathrate (ETBB-d
3/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 T
1'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.