Solid-state NMR spectroscopy study of molecular dynamics of pyridine 1-oxide encapsulated in p-tert-butylcalix[4]arene

Article abstract of DOI:10.1021/acs.jpca.0c07414

The mixture of guests 25% pyridine 1-oxide (PNO) and 75% nitrobenzene (NB) encapsulated in a host cavity of p-tert-butylcalix[4]arene (p-tBC) is a suitable system for the study of intermolecular weak interactions, and the preferred orientation of the guest molecules within the host was derived. Variable temperature deuterium nuclear magnetic resonance line shape and spin?lattice relaxation studies were performed on three samples of 25% PNO as a guest (selectively and entirely deuterated) with 75% NB encapsulated in p-tBC as a host system. It is found that the PNO molecular motion supports the two-site jump model and is highly mobile throughout the temperature range from ?130 to 20 °C. PNO reorients about its C₂ molecular symmetry axis followed by reorientation around the compound’s C₄ axis of symmetry of a host. Calculation showed that besides precise molecular jumping, the guest also experiences the vibrational motion with a variation angle dispute about 15°. The correlation times for the molecular guest motion and two-dimensional exchange spectroscopy confirmed a fast exchange limit inside the cavity. These molecular dynamics follow an Arrhenius behavior motion from which the small activation energy is delivered that strongly suggests a relatively weak intermolecular interaction between the host and guest species. PNO has definite dipole orientational motion in the cavity where its aromatic covalent bond structure minimizes contact with the host p-tert-butyl groups. Experimentally found, PNO-d₅ deuterons in the single crystal collapse at 55 and 125° at ambient temperature. The first splitting of 124 kHz belongs to D₃(D₅), the second splitting of 100 kHz belongs to D₂(D₆), and the last splitting of 52 kHz belongs to D₄ of the ring of PNO inside the p-tBC.

Full text of DOI:10.1021/acs.jpca.0c07414

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Solid-State NMR Spectroscopy Study of Molecular Dynamics of
Pyridine 1‑Oxide Encapsulated in p-tert-Butylcalix[4]arene
John Nobleman*
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ABSTRACT: The mixture of guests 25% pyridine 1-oxide (PNO) and 75% nitrobenzene (NB) encapsulated in a
host cavity of p-tert-butylcalix[4]arene (p-tBC) is a suitable system for the study of intermolecular weak interactions,
and the preferred orientation of the guest molecules within the host was derived. Variable temperature deuterium
nuclear magnetic resonance line shape and spin−lattice relaxation studies were performed on three samples of 25%
PNO as a guest (selectively and entirely deuterated) with 75% NB encapsulated in p-tBC as a host system. It is
found that the PNO molecular motion supports the two-site jump model and is highly mobile throughout the
temperature range from −130 to 20 °C. PNO reorients about its C2 molecular symmetry axis followed by
reorientation around the compound’s C4 axis of symmetry of a host. Calculation showed that besides precise
molecular jumping, the guest also experiences the vibrational motion with a variation angle dispute about 15°. The
correlation times for the molecular guest motion and two-dimensional exchange spectroscopy confirmed a fast
exchange limit inside the cavity. These molecular dynamics follow an Arrhenius behavior motion from which the
small activation energy is delivered that strongly suggests a relatively weak intermolecular interaction between the
host and guest species. PNO has definite dipole orientational motion in the cavity where its aromatic covalent bond
structure minimizes contact with the host p-tert-butyl groups. Experimentally found, PNO-d5 deuterons in the single
crystal collapse at 55 and 125° at ambient temperature. The first splitting of 124 kHz belongs to D3(D5), the second splitting of 100
kHz belongs to D2(D6), and the last splitting of 52 kHz belongs to D4 of the ring of PNO inside the p-tBC.
with p-tBC inclusion compounds. Based on all experimental
data, the weak intermolecular interaction, molecular reorienta-
tion, and the exhibited planar reorientation have been approved.
Interestingly, the benzene inclusion compound reorients about
the 6-fold axis followed by its 4-fold axis, the pyridine guest
shows the reorientation about the C2 molecular symmetry axis,
and in pyridine-d5, the ortho-, meta-, and para- deuterons based
on solid-state 2H NMR have been resolved where the splitting of
these deuterons led to the motional averaging of the quadrupolar
coupling constant of 182.4 kHz.
1. INTRODUCTION
Deuterium (2H) solid-state nuclear magnetic resonance (NMR)
is an ideal method for the study of guest dynamics in the
symmetric and asymmetric forms of the host cavity and confirms
the cooperative intermolecular effect. In the solid state, a
quadrupolar coupling constant may arise from the chemically
equivalent and crystallographically inequivalent nuclei. The
molecular reorientation motion vibration changes the electric
2
field gradient tensor where an observed H NMR line shape
shows a change in the spectral line.
p-tert-Butylcalix[4]arene (hereafter abbreviates as p-tBC) has
almost a cylindrical cavity, with an average radius of 5.29 Å and a
In this work, we utilized powder diffraction X-ray diffraction
(XRD) and single-crystal X-ray diffraction defining the crystal
structure and localizing of a guest structure, before using the
variable temperature measurement of 2H solid-state NMR line
height of 6.56 Å, and forms a volume of 183π Å3 (i.e., MV20
=
592.5 3.0 cm3 p-tBC). Its surface area is equal to 125π Å2,
which can confine the guest molecules like benzene, toluene,
pyridine, nitrobenzene, and pyridine 1-oxide as shown in Figure
1A−F. Inside the cavity of p-tBC, these substances make an
ordered one-dimensional (1D) dense packing with some 3D
ordering despite rotational and translational disorder. For
instance, the inelastic neutron scattering confirmed that toluene
inside p-tBC approaches the behavior of a quantum-
mechanically free rotor.1 The study also shows that the plane
of nitrobenzene makes an angle of 67.9° perpendicular to the z-
axis above p-tBC.2
2
2
shapes, H spin−lattice relaxation times, and H 2D NMR
exchange spectroscopy combined with the deuterium line shape
calculation to study the orientations of positions ortho-, meta-,
and para- of the pyridine 1-oxide (selectively and entirely
deuterated) guest in the host cavity of p-tBC. We are utilizing
these sensitive methods as a significant approach to the
Received: August 13, 2020
Revised: November 23, 2020
Published: December 8, 2020
2
1H, H, 13C, and 15N solid-state NMR measurements and
computational calculations3 have been deployed to study the
structure and dynamics of the guests and host systems, in
similarity to benzene, pyridine,4 nitrobenzene,5 and toluene6
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purpose of structure determination, 0.1 mg of the crystal was
1
dissolved in 10 mL of D2O, and the H NMR spectrum was
obtained and is shown in Figure 2a.
Figure 1. Chemical structures of 25PNO-x-dn/75NB/p-tBC. Guest
75% nitrobenzene (NB) (A) and 25% pyridine 1-oxide (PNO) fully
(B) and selectively (C and D) deuterated inclusion compounds in host
p-tert-butylcalix[4]arene (p-tBC) (E and F slightly in different
perspectives) and schematic drawings of the guest and host.
1
Figure 2. Fragment of 400 MHz H NMR spectra in CDCl3 of (a)
molecular dynamics behavior of pyridine 1-oxide as a driving
force, encapsulated inside the cavity of a host, where site
occupancy would display a potential role in the physical
properties of a material. The goal of the current study is to
understand the molecular dynamics reorientation mechanism
for the guest−host material, which could be important both for
future synthetic and crystal engineering works.
PNO-3,4,5-d3 and (b) PNO-2,6-d2. In spectrum (a), a resonance peak
of 8.22 ppm represents the protonated position of 2a and 6a of PNO-
3,4,5-d3. In spectrum (b), the resonances at 7.54 and 7.69 ppm are
assigned to protons of meta (positions 3 and 5) and para (position 4),
respectively. The doublet at 8.22 ppm shows tiny unexchangeable
protons of the ortho-position.
2.1.3. Pyridine 1-Oxide-2,6-d2 (PNO-2,6-d2). Selectively
deuterated PNO-2,6-d2 was obtained according to Pavlik and
Laohhasurayotin8 from a solution of PNO (2.0 g, 21 mmol) in
D2O (20 mL) containing Na2CO3 (3.0 g). This mixture was
heated at reflux at 100 °C for 9 h and cooled to room
temperature, and the solvent was removed under low pressure.
The concentrated dried product mixture with Na2CO3 was
sublimated under a vacuum at 90 °C, and the white solid of
2. RESULTS AND DISCUSSION
2.1. Materials and Synthesis. 2.1.1. Pyridine 1-Oxide-d5
(PNO-d5). Pyridine-d5 (99.5% deuterated) was purchased from
Sigma-Aldrich without further purification. In a round-bottom
flask fitted with a cold finger was added 2 mL of pyridine-d5
dissolved into 15 mL of glacial acetic acid and 3.2 mL of 30%
H2O2, magnetically stirred and heated for 24 h.7 The solvent was
removed by vacuum distillation techniques, and PNO-d5 was
dried with 3.0 g of anhydrous sodium sulfate (Na2SO4). The
mixture of PNO-d5 and Na2SO4 was loaded into the sublimation
apparatus and under a vacuum heated at 90 °C. The white solid
crystals of PNO-d5 were collected under an argon atmosphere.
2.1.2. Pyridine 1-Oxide-3,4,5-d3 (PNO-3,4,5-d3). Selectively
deuterated PNO-3,4,5-d3 was obtained from a solution of PNO-
d5 (2.0 g, 21 mmol) in protonated H2O (20 mL) containing
sodium carbonate (Na2SO4) (3.0 g). This was heated at reflux at
100 °C for 9 h and cooled to room temperature, and the solvent
was removed under low pressure. The concentrated product
sublimated under a vacuum at 90 °C, and the white solid of
PNO-3,4,5-d3 was collected under an argon atmosphere. For the
1
PNO-2,6-d2 was collected under an argon atmosphere. H
solution NMR shows that the peaks at 7.54 ppm belong to (m,
3H, 5H), 7.69 ppm to (p, 4H), and 8.22 ppm to the
unchangeable tiny residuals of (o, 2H, 6H) shown in Figure 2b.
2.1.4. Synthesis of Inclusion Compounds. The mixture of
solution of nitrobenzene (NB) (2.53 mmol), PNO-d5 (PNO-
3,4,5-d3 or PNO-2,6-d2) (8 mmol), and p-tBC (0.693 mmol)
were prepared at 100 °C5 and immersed into the water Dewar at
80 °C, slowly cooling itself to an ambient temperature after 48 h,
and the crystals were collected by filtration. The PNO/NB/p-
tBC formed an inclusion compound with a stoichiometric
mixture of 25% PNO and 75% NB. For solid-state NMR
measurements, all crystal samples were ground to powder. For
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the purpose of simplicity, 25% pyridine 1-oxide/75% nitro-
benzene/p-tBC hereafter abbreviates as 25PNO-x-dn/75NB/p-
tBC, where x indicates the number position in the ring and dn
shows the total number of deuterons.
2.2. XRD Powder Diffraction Pattern of 25PNO/75NB/
p-tBC. Figure 3 shows the X-ray powder diffraction patterns of
Table 1. Crystallographic Data for 25PNO/75NB/p-tBC at
−70 °C
Crystal data
chemical formula
Mr
C49H61NO5
743.99
crystal system, space group
temperature (K)
a, b, c (Å)
monoclinic, Pn
203
12.778 (2), 13.095 (2), 12.781 (2)
a, b, g (°)
90, 90.010 (3), 90
2138.6 (5)
2
V (Å3)
Z
radiation type
Mo Ka
0.07
m (mm−1
)
crystal size (mm)
0.4 × 0.3 × 0.1
Data collection
diffractometer
Kappa Apex II
no. of measured, independent, and observed [I > 2 s(I)] 13,943, 8350,
reflections 5070
Rint 0.018
Refinement
R[F2 > 2 s(F2)],
wR(F2), S
0.049, 0.162, 0.96
no. of reflections
no. of parameters
no. of restraints
H-atom treatment
8350
534
3
H atoms treated by a mixture of independent and
constrained refinement
Dnmax, Dnmin
̃ ̃
(e Å3) 0.45−0.31
crystallographic axis before mounting the crystal in the NMR
goniometer angle-rotation experiment (see Figure 7B).
2.2.2. 2H Single-Crystal NMR Rotation Study. For the single-
crystal deuterium rotation NMR study, a single crystal of
25PNO-d5/75NB/p-tBC or 25PNO-2,6-d2/75NB/p-tBC is
mounted in a home-made goniometer as shown in Figure 7B,
which fits inside the 10 mm coil of the NMR probe. The crystal
was mounted after defining the crystallographic axis according to
Table 2. The single-crystal rotations in an increment of 5° from 0
Figure 3. XRD powder diffraction patterns of 25PNO/75NB/p-tBC
inclusion compounds. Structure data collected on a Bruker AXS system
at ambient temperature (A) and powder X-ray diffraction patterns
computed with the Mercury program employing a single crystal (SC) of
25PNO/75NB/p-tBC inclusion compounds, X-ray data collected at
−70 °C (B). The result clarifies that a low temperature structure of the
crystal is preserved at ambient temperature.
Table 2. The Single Crystal of 25PNO-d5/75NB/p-tBC (0.4
× 0.3 × 0.1 mm, 43 mg) Is Mounted in a Goniometer in the
a(c)-Axis According to the Crystallographic Axis (c.f. Figure
7), Where φ Is Goniometer Angle Versus the B0 Magnetic
Field
PNO/NB/p-tBC inclusion compounds, which were recorded at
room temperature using a Bruker AXS model diffractometer.
XRD data were analyzed using an EVA program (DIFFRACplus
EVA, Bruker AXS), which indexes (h,k,l) all the intense lines.
The radiation of Cu Kα = 1.540 Å was used. The XRD
diffraction studies have shown that the crystals are in monoclinic
with a space group of Pn where a = 12.778(19) Å, b = 13.095(2)
Å, c = 12.780(18) Å, and β = 90.01°. These results are well-
suited with X-ray of single-crystal data (c.f. Figure 3B).
2.2.1. Single-Crystal X-ray Data of 25PNO/75NB/p-tBC. For
the location of PNO in the cavity of p-tBC, a single crystal of
25PNO/75NB/p-tBC from the same growth with dimensions
of 0.4 × 0.3 × 0.1 mm was selected, and data were collected at
−70 °C. The results are shown in Table 1. Diffraction studies
have shown that a single crystal is monoclinic with a space group
of Pn, where a = 12.778(19) Å, b = 13.095(2) Å, c = 12.780(18)
Å, and β = 90.01°. Figure 7A shows the single-crystal X-ray of
25PNO-d5/75NB/p-tBC mounted in an X-ray goniometer. The
X-ray data collected for the large single crystal defined the
goniometer φ-angle
a(c)-axis
b-axis
φ⊥B0 = 0°
φ||B0 = 90°
φ⊥B0 = 180°
⊥B0
⊥B0
⊥B0
||B0
⊥B0
||B0
to 180° were made, and for every angle position, sufficient
numbers of scans were acquired to obtain a good signal to noise
(S/N) ratio (c.f. Figures 7−10).
2
2
2.3. H Spin−Lattice Relaxation and H Line Shape
2
Solid-State NMR Studies. The H NMR static spectra were
recorded at 46.06 MHz corresponding to 7.04 T using an Apollo
Tecmag NMR spectrometer equipped with a single resonance
Chemagnetic static probe (10 mm solenoid coil). The solid-
echo quadrupolar sequence (π/2)x-τe-(π/2)y-τe was optimized
where D2O was used to calibrate the π/2 pulse of 2.4 μs and an
echo time τe = 45 μs. For optimization of a phase-cycling scheme
for suppressing the spectral artifacts, DMSO2-d6 was used.
Magnetization was expected up to 5T1. The spectra were Fourier
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Figure 4. 2H NMR line shapes (a) 25PNO-d5/75NB/p-tBC, (b)25PNO-3,4,5-d3/75NB/p-tBC, and (c) 25PNO-2,6-d2/75NB/p-tBC at different
temperatures. The PNO groups undergoing the two-fold reorientation motion C2 symmetry inside the host calculated for the proposed molecular
motion. In (a), D2(D6) = 40%, D3(D5) = 40%, D4 = 20% and in (b) D3(D5) = 66% and D4 = 33%, the sum of subspectra are superimposed by dashed
lines to produce the final spectrum, respectively. The vibration motions of host structure clearly seen at low temperatures (i.e., − 130 °C) hinders the
anisotropy rotation of the guest. At −80 °C the spectra remind an intermediate exchange regime.
transformed from a maximum quadrupole echo signal by left
shifting, baseline correction, and exponential multiplication
zero-filled to 1 K data points with 10 Hz line broadening. The
temperatures. Each of these experiments has a different
sensitivity to a particular frequency of molecular motion. For
2
2
instance, H line shapes, H spin−lattice relaxation times, and
2H 2D exchange spectroscopy are used to investigate the
molecular dynamic motions in the megahertz (104−108 Hz),
gigahertz (108−1011 Hz), and kilohertz (101−103 Hz) range,
respectively. The value of the quadrupole coupling constant
(QCC) has been averaged over deuterons of PNO, where
vibration dynamical motions are typical in a fast exchange limit
with a symmetry axis of C2 jump.
2
frequency independence H spin−lattice relaxation data were
obtained from the recovery of quadrupolar echo amplitude
following the π-τ-(π/2)x-τQ-(π/2)y sequence where a total of 15
different τ values were used.9 The sample temperatures were
controlled by a Chemagnetic gas flow probe stack and
monitored by the calibrated resistive thermal device (RTD)
sensor with an accuracy of 0.5 °C.
2.3.1. 2H 2D NMR Exchange Spectroscopy. Deuterium
exchange solid-state spectra were collected and proceed
according to Schmidt et al.10 The experiments with a pulse
sequence of (π/2)-τd-(π/3.29)-τm-(π/3.29)-τ-(π/2) where a π/
2 of 2.4 μs and a pulse of π/3.29 of 1.51 μs with τd equal to the
dwell time of two dimensions, with the lowest value of 0.25 μs,
with a mixing time of 5 ms were recorded. The cosine and sine
spectra were collected separately with data matrix 256 × 256
complex values in time t1 and t2 domains, respectively. The
cosine was extracted from the sine signal to generate a pure 2D
exchange NMR spectrum after two subsequent Fourier
transformations. The spectral width was 250 kHz in both
dimensions. The recycling delay was adjusted to 5T1.11
3.1. 2H NMR Powder Line Shapes. Variable temperature
2H NMR studies was performed on the three 25PNO-x-dn/
75NB/p-tBC inclusion compounds prepared with PNO (fully
and selectively) deuterated in the carbon−hydrogen covalent
2
bond position of the ring. Figure 4 shows the solid-state H
NMR line shapes of polycrystalline 25PNO-d5/75NB/p-tBC
from −130 to 20 °C. The guest PNO-d5 in the p-tBC cavity
2
exhibits C2 jump motion inside the host where H line shapes
registered an asymmetric shape at lower temperatures until 0 °C.
At 10 °C, the line shape became an axially symmetric Pake
pattern with the splitting range of Δνxx = 48 4 kHz, Δνyy = 76
4 kHz, and Δ□zz = 120 4 kHz (c.f. Figure 4a).
In the case of guest 25PNO-2,6-d2/75NB/p-tBC, the central
peaks were observed from −130 till 20 °C due to the unbounded
residual PNO-x-dn. The residual PNO assigned as a narrow
component at the center, which was trapped outside the host
cavity, makes the rapid random molecular tumbling as a single
peak at the Larmor resonance frequency, ν0, which is typical of
isotropic fluid positions. The clear Pake line of guests at 20 °C
3. RESULTS AND DISCUSSION
In this paper, we present H solid-state NMR line shapes, H
spin−lattice relaxation time, T1, and H 2D NMR exchange
spectroscopy to study the molecular dynamics of the system
2
2
2
25PNO-x-dn/75NB/p-tBC inclusion compounds at different
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for 25PNO-3,4,5-d3/75NB/p-tBC and for 25PNO-2,6-d2/
75NB/p-tBC with the splitting range Δνxx = 48 4 kHz, Δνyy
= 80 4 kHz, and Δνzz = 120 4 kHz were observed.
It is obvious that guest molecules are highly mobile
throughout the whole accessible temperature range, resulting
in a motional averaged Pake pattern. Even at a low temperature
It can be seen that at low temperatures (−130 °C, Δνzz = 140
4 kHz), the typical non-axially symmetric Pake patterns were
observed for all. The simulated spectra between −130 and 20 °C
are also shown in Figure 4. It is known that12−14 the quadrupolar
of −130 °C, Δνzz = 140 4 kHz, the average QCC, χ = 175
5
kHz, and ⟨η⟩ = 0.04−0.21, observed for 25PNO-d5/75NB/p-
tBC that are close in value with those of pyridine/p-tBC, χ = 174
kHz and η = 0.04, at −173.15 °C, and χ = 165 kHz and η = 0.1, at
−93.15 °C, were reported.17 The line shape of this doublet does
not change drastically as the temperature increased, as shown in
2
splitting of the H spectrum of a single crystal placed in an
external magnetic field Bo can be described by
2
Figure 4a. The theoretical H NMR line shape of 25PNO-d5/
3
Δν(θ, ϕ) = χ[(3cos2 θC−D − 1) − (ηsin2 θcos 2ϕ)]
75NB/p-tBC was superimposed using the two-fold symmetry
jump cone model. Due to the existence of three deuteron species
and their percentage contribution D2(≈D6) = 40%, D3(≈D5) =
40%, and D4 = 20%, the sum of three subspectra reproduced the
simulated line with equal site population. The lines were
superimposed to show the contribution among each of them in
the final spectrum. It can be seen that the populations of
deuterons D2(D6) and D3(D5) derived from analysis are almost
similar. The lines characterized by an average value are typical
for molecules undergoing motions in the fast exchange limit (1·
10−8 s−1), a jump angle of φ = 180°, and an opening angle of θ =
60° for D2(D6) and D3(D5) and 0° for D4. Schematic drawings
of the two-sided ring jump based on the cone model are shown
in Figure S1 in the Supporting Information. This model’s
assumption is sufficient to provide a quantitative reproduction of
the experimentally obtained Pake lines at a fixed pulse delay of
45 μs. Since the guest motion is within the fast exchange limit,
the jumping rate was fixed to 1·10−8 s−1 for all spectra calculated.
The three 2H NMR line shapes from particular deuterons of the
ring observed do not converge upon heating. At the lowest
temperature, the inclination axis of PNO inside p-tBC is not
perpendicular within the motional axis of a host (C4) where such
a position of molecules in motion is energetically favorable for
the fast rotation motion (see the X-ray picture or Figure 1). The
favored rotation on a cone model is in line with the presence of
stronger non-bonded van der Waals interactions between the
PNO guests confined inside the host p-tBC matrix, which arise
from the large dipole moment of PNO (μ = 4.25 D) compared
to pyridine (μ = 2.23 D) and nitrobenzene molecules (μ = 3.98
D). For instance, nitrobenzene’s large dipole moment in the ab
plane causes a net dipole moment in the non-centrosymmetric
α-tBC/NB crystal. Such a significant dipole moment in PNO
suggests that the resonance structure was derived due to
movement of electrons from oxygen to the ring C2(D2), C4(D4),
and C6(D6) carbon atom positions or when the direction of
polarization is reversed and the C2 and C4 carbon atoms of the
ring become positively charged. It is clear that such resonance
makes the ring atoms magnetically inequivalent.18 Upon
examining the location of the PNO dipole moment, methyl
groups of adjacent host (C−H) and guest (−ON+) distances
are in average d(N−O...H−C) = 2.27 Å and are relatively
shorter than the distances of d(H...N) = 2.58 Å of pyridine in the
host calixarene. Such a short distance suggests that nucleophilic
oxygen is pointed directly toward the methyl hydrogens of the
host tert-butyl group enabling formation of hydrogen bonding,
wherein the center of molecular mass of the PNO ring is tilted at
some angle (c.f. Figure 1). For example, a host−guest distance
determination studied for the pyridine/calixarene tert-butyl
group produced a weak C−H...N hydrogen bond interaction of
8 kJ/mol. Also, it was reported that the nitrobenzene nitro group
protrudes to one side in which the C−N bond of the
nitrobenzene points 67° with respect to the calixarene C4 axis.2
4
(1)
where Δν is the 2H Larmor frequency, χ is the QCC given by
e2qQ/h, expressed in frequency units, and η = (Δνyy − Δνxx)/
Δνzz is the asymmetry parameter of the electric field gradient
(EFG) tensor such that 0 < η < 1. The angles describe the
orientation of the magnetic moment with respect to B0 in the
principal axis system (PAS) of the quadrupole coupling tensor. If
the orientations of a C−D bond are equally distributed, then the
spectrum becomes a Pake pattern. Reorientations of the bonds
reduce the EFG and thus result in a change of the line shape,
depending on the nature and frequency of the motional
processes involved. In general, the powder spectrum is
characterized by three features, where the outer shoulders
(edges) are separated by Δνzz = (3/2)χ, inner shoulders by Δνyy
= (3/4)χ(1 + η), and a pair of singularities (peaks) by Δνxx = (3/
4)χ(1 − η). In the case when the rate of the molecular motion is
greater than the QCC, the spectral splitting becomes averaged
3
Δν = χ(3cos2 θC−D − 1)
(2)
4
where θC−D describes the orientation of the C−D vector with
respect to the motional axis. For aliphatic deuterons, the EFG
tensor is more or less cylindrically symmetric about C−D, and
the asymmetry parameter is close to zero in the polycrystalline
substance. The splitting between the singularities of the
spectrum reduces to a certain frequency, and therefore,
reduction of χ can be observed. For instance, the QCC for
pure nitrobenzene-d5 is 177.3 kHz and for nitrobenzene-d5/p-
tBC is 105.3 kHz.2,5 In the case of the intermediate exchange
regime (k = 104−107 Hz), the transverse spin−spin relaxation
time (T2) becomes comparable to the τQ value in the
quadrupolar echo experiment for certain orientations in
powdered samples. For these orientations, the second pulse
does not refocus the magnetization resulting in spectral intensity
loss. For other orientations, T2 is long when the motion does not
change the orientation of the C−D vector with respect to the
applied magnetic field B0. Thus, these orientations significantly
2
reduce the width of the prominent singularities in the H line
shape when a fast molecular motion (k > 107 Hz) occurs at a
timescale much shorter than 1·107 s−1. When motion is slow (k <
104 Hz), the exchange has a great impact on the observed line
shape.15 However, the observed non-axial symmetric Pake
pattern is typical for molecules undergoing motions in a fast
exchange limit with a symmetry axis of C2 jump. Further
observation of spectral fluctuation also depends on the
deuterated position of the guest species. In fact, if underlying
guest motion is too fast and time scales out of range for 2H NMR
line shape analysis, then the best solution for study is the spin−
lattice relaxation times, T1. Thus, the 2H NMR line shape can be
calculated by using the cone model as described by Macho et
al.16
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If we assume that deuteron lines overlap, then all C−D bonds
should become magnetically equivalent. However, Ripmeester
et al.4 found that the line shape of the para-deuteron of pyridine
was unaffected by two-fold molecular orientation since all tensor
components were aligned with the molecular axis, but the line
shape can drastically change only if the C−D bond is oriented
close to the magic angle regarding the four-fold axis of p-tBC.
Also, the authors determined that the angle between the two
motional axes C2 aligned with para-deuteron (D4) and C4 of the
host is 52.9°. Hence, we established that the splitting of D4
produced the experimental line shape with the following
averaged parameters: Δνxx = 10 kHz, Δνyy = 14 kHz, and Δνzz
= 24 kHz, (χ = 32 kHz and ⟨η⟩ = 0.25−0.58). It is clear that the
para-deuteron on-axis component never changes with motion
and the ortho- and meta-deuteron population changes with
motion; therefore, the line shape is a sum of contributions from
all deuterons.
The open angle (see Figure S1 in the Supporting
Information) of 0° for C2 with the same jump ratio 1·10−8 s−1
was established, assuming that any population of sites was
equally balanced during orientations. Such a small averaged
QCC, χ = 32 kHz, suggests that para-D4 is affected by the
thermal vibration motions of the host structure slightly
influenced by the symmetry axis of C2 of the PNO guest,
inclined at an angle of around 15° with respect to the C−D4
bond direction, as shown in Figure 1. Nevertheless, the powder
pattern in our case has a discrepancy due to the s/n ratio and
imperfect pulse width, where the QCC reduces to an uncertain
number. Therefore, we recalculate the QCC through an angle
dependence using the formula of Kawaguchi et al.17
and added up to reproduce the equivalent experimental splitting
as shown in Figure S1 in the Supporting Information.
In the case of 25PNO-2,6-d2/75NB/p-tBC, D2(D6) two
subspectra belonging to the same deuterons contributed to the
two-fold orientation. Taking into account the error of vibrational
motion, the spectrum was reproduced at 20°, assuming that the
experimental line shape also sustained from free unbound PNO
molecules. Upon the temperature change, the vibrational
motion of the guest, as well as the host molecules, clarified the
2H EFG splitting due to two-fold jumping of fast reorientation
motion, as well as the 2H line shape alternation that shows the
νxx-half line shape axially asymmetric for the PNO inclusion
compound. This molecular motion reduced the QCC, χ from
175 kHz ⟨η⟩ = 0.04 at −130 °C to 81 kHz ⟨η⟩ = 0.21 at 20 °C for
PNO-d5. The large averaged asymmetry parameters 0 < η < 1
indicate rapid C2 fold reorientation most reasonably about the
molecular two-fold axis, wherein at the low temperature (e.g., at
−130 °C), all spectra do not show additional sharp features in
the resonance line. The lack of other features suggests that the
two-fold reorientation is already in fast motional mode. Upon
increasing temperature clearly, the weak edge features can be
seen at −80 °C for PNO samples with a width of 120 kHz, which
characterizes the two-fold averaging reorientation. As such, the
gradual and steady development of weak features in lines is
characteristic of the onset of additional motion in an
intermediate exchange regime that reached the room temper-
ature lines as clearly shown in Figure 4 and distinctly separated
the individual motion as shown in Figure 5. The same weak line
features observed are for nitrobenzene.
We assume that the molecular rotations of the guest go
through the thermal disorder vibrational dynamics. It also is
known that asymmetry can be the result of the finite pulse width,
off-resonance effect, and characteristic of an electronics
detection system between time delays by filters and the phase
cycling in pulse program. Each time during the experiment, the
probe was carefully tuned and matched to eliminate the off-
resonance effect, which could cause an asymmetric line shape
observation. At low temperature (e.g., −150 °C, deuterium line
shape is not informative and not shown), the two-fold
reorientation of 25PNO-d5/75NB/p-tBC was not well-resolved.
In the case of pyridine/p-tBC, Ripmeester et al.4 observed that
the para-deuteron NMR line shape was drastically narrowed by
about 12.6 kHz when the C−D4 bond was oriented close to the
magic angle 54.74°, taking into account the C4-fold axis
symmetry of p-tBC. We observed the same C−D4 orientation
during the single-crystal rotation experiment, as discussed
below. It is difficult to observe the clear line shape with relatively
long π/2 pulse lengths (i.e., 2.4 μs) used in our experiments. A
close inspection of Figure 4b,c reveals that even at −130 °C, still,
the 2H line shape of selective deuterated PNO suffers from the
thermal vibrational structural motion. The fluctuation of EFG is
involved upon the fast 180° jump reorientation of deuterons of
the ring of PNO upon further temperature increases. The PNO
ring is tilted at an angle that the para-deuteron is deeper by
about 1.067 Å (from the X-ray diffraction picture) in the cavity
of a host. This tilted the angle of the C2 guest and a C4 symmetry
of the host by about 15° (90 − 75°) (c.f. Figure 1 and Figure S1
in the Supporting Information.) in dynamically averaging the
QCC for the para-deuteron. It became clear that the para-
deuteron experiences a small variation of EFG during the
resonance stabilization of PNO, spreading two π-electrons
across the ring.
QCCeff
QCC
1
2
=
[(3 − η)cos2 θ − (1 − η)]
(3)
where QCCeff for PNO averages to 172 2 kHz at an angle of
15°, where observed QCC = 174 kHz taken from pyridine
produced ηeff = (0.04−0.5) that indicates a fast-motional state.
The line shape for a confined molecule such as pyridine 1-oxide
is never static, where this molecule rotates freely with some
vibration motion. We are assuming that the 17 1.3° is due to
the vibration motion of the guest and is caused by the thermal
vibration motions of the host structure. Figure S2 in the
Supporting Information shows the jump angle θ versus the
temperature. The exponential growth function was used for
fitting the experimental points.
2
It was established experimentally by H NMR that the low-
symmetry α-tBC/NB-d5 has an angle of 60° between the C−D
bond of NB and C4 calixarene. Thus, the frequency of motion is
related to temperature by matching the experimental and
simulated spectra. It is known19 that for the theoretical 2H NMR
line shape calculation, it is essential to calculate the subspectra of
various magnetically non-equivalent deuterons. The weighted
subspectra were added to produce the overall close-to-
experimental 2H NMR shape spectrum. In our case, the
calculations were made with three spectral contributions,
referring to the magnetically non-equivalent deuterons, an
ideal splitting of Δνxx = 55 kHz D2(D6), 60 kHz D3(D5), and 10
kHz D4. Values of the ortho-, meta-, and para-deuterons splitting
as a function of temperature, for instance, for pyridine were
estimated at room temperature to be 53.1, 57, and 8.1 kHz.20
These three spectra were calculated separately with their
corresponding simulation parameters and taking into account
the weights of 40% for D2(D6), 40% for D3(D5), and 20% for D4
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where τc is the correlation time describing the dynamic process,
ω0 is the Larmor angular frequency, and χ is the quadrupole
coupling constant given by e2qQ/h. For thermally activated
diffusion, the correlation time, τc, is usually described in the
Arrhenius form
E
RT
i
y
a
j
j
j
z
z
z
τ = τ exp
0
(5)
k
{
where τ0 is the pre-exponential factor corresponding to the
rotational correlation time at the infinitive temperature, R is the
gas constant, T is the absolute temperature in (K), and Ea stands
for the activation energy (kJ mol−1) for the dynamic process.
2
Figure 5 shows H experimental spin−lattice T1 data, from
inversion recovery experiments of 25PNO-d5/75NB/p-tBC,
25PNO-3,4,5-d3/75NB/p-tBC, and 25PNO-2,6-d2/75NB/p-
tBC powder as a function of reciprocal temperature (β =
1000/T (K−1)). It can be seen that T1 curves display a shallow
minimum of 4.76 ms for the 25PNO-d5/75NB/p-tBC at −100
°C (i.e., β = 5.78 K−1) and 5.4 ms for the 25PNO-3,4,5-d3/
75NB/p-tBC at −69 °C (i.e., β = 4.9 K−1). 25PNO-2,6-d2/
75NB/p-tBC shows a minimum of 6.67 ms at −109.2 °C (i.e., β
= 6.1 K−1). The values of relaxation parameters for the PNO
included compound in host p-tBC are listed in Table 3.
Table 3. Values of Relaxation Parameters for Resolved
Molecular Motion
PNO-d5
QQC
τ(0) (s)
I-motion T (K) = 154−210 II-motion T (K) = 210−286
184
122
3e−13
10.9
3.7e−13
16.9
Ea (kJ/mol)
PNO-2,6-d2 I-motion T (K) = 142.8−190 II-motion T (K) = 190−286
QQC
τ(0) (s)
Ea (kJ/mol)
144
122
4.8e−12
6.6
3.7e−13
16.9
I-motion
II-motion
PNO-3,4,5-d3
T (K) = 167−235 T (K) = 235−286
QQC
τ(0) (s)
164
104
9.5e−14
14.7
6e−15
21.6
116
Ea (kJ/mol)
average QCC over all deuterons of
the PNO ring
170 16
8
average τ(0) (s)
5.19e−12
7.46e−13
average Ea (kJ/mol)
10.73
18.47
Figure 5. Arrhenius plot of 2H relaxation rates T1 of 25PNO-d5/75NB/
Consequently, activation energies were averaged for the full and
selective PNO compounds in 25PNO-x-dn/75NB/p-tBC
determined for the first molecular motion to be Ea = 10.73 kJ/
mol, τ0(s) = 5.19·10−12, and for the second molecular motion is
18.47 kJ/mol, τ0(s) = 7.46·10−13. The first motion in Figure 5 is
due to the slow hindered anisotropic reorientation motion of
PNO, which took place at −109 till −69 °C (i.e., β = 6.1−4.9
K−1). The second motion in Figure 5 is due to an anisotropic
motion where the frequency of jumping C2 rotation of PNO
elevates as result of the temperature increase. The PNO
molecule receives high energy, and the probability of jumping
increases over a high activation potential energy barrier. Hence,
the (N−O) dipole moment and van der Waals interactions lead
to inhibition of the anisotropy of the rotational motion of the
PNO molecule inside the p-tBC host. However, upon increasing
the temperature, the crystal lattice of the host (p-tCB) changes,
which decreases an activation potential energy barrier of the host
cavity by weakening the molecular van der Waals interaction.
p-tBC, 25PNO-3,4,5-d3/75NB/p-tBC, and 25PNO-2,6-d2/75NB/p-
2
2
tBC at 40.6 MHz H NMR. The H experimental data for particular
molecular motions (I and II) were fitted by using 4, as indicated by the
solid lines. The values of relaxation are shown in Table 3.
3.2. 2H Spin−Lattice Relaxation Times. The nuclei with
spin I ≥ 1 possesses an electric quadrupole moment (eQ), which
interacts with the electrical field gradient (EFG). The
fluctuation of the EFG is a major relaxation mechanism in the
case of quadrupolar nuclei. Similarly, the 2H spin−lattice
14
D
relaxation rate (1/T1 ) can be described by the BPP model
i
j
j
j
j
j
y
z
z
z
z
z
i
j
j
j
j
j
i
j
j
j
j
j
y
z
z
z
z
z
τc
1
T1
3
50
2I + 3
I (2I − I)
2
=
π2
χ
D
2
2 2
1 + ω0 τc
k
{
k
{
k
y
4τc
z
z
z
+
z
z
1 + 4ω02τc2
(4)
{
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Upon further increasing the temperature, clearly, the character-
istic experimental point distribution in the 2H relaxation curve
can be seen at −80 °C (β = 5.18−4.75 K−1) for PNO, which
characterizes the region of an intermediate exchange regime.
Yet, upon increasing the temperature at β(K−1) = 4.25 onward,
the onset of fast rotation motion for PNO became activated.
Each of the characteristic points of Figure 5 indicates some
dynamical mode (i.e., molecular tumbling motion) in such a
system, which is further characterized by the evolution of T1
relaxations versus temperature. Upon increasing temperature,
from β(K−1) = 7 to 6, the reorientation motion is found to be in
the slow regime (1/τc ≪ Q0). The slow reorientation motion
occurs where the discrepancy between the values of the
relaxation times T1 and T2 increases. Until the β(K−1) = 7, the
Pake pattern would cover ultraslow molecular orientation within
QCC = 184 kHz. Recalculate the fitting of T1 and separate the
state of molecular motion at 1/τc ≈ Q0 where averaged T1min
=
5.32 ms. The molecular motion is in the intermediate regime
within the line shape Pake pattern in an average of QCC = 164
kHz. The 1/τc ≫ Q0high-temperature part, an area where fast
molecular motion occurs, is characteristic of liquids, in which the
times T1 or T2 do not differ much from each other. Upon
gradually increasing temperature from β(K−1) = 5.5 to 3.5, the
second reorientation molecular motion established within the
line shape is characterized by a reduced Pake pattern within
averaged QCC = 116 kHz. We established the QCCeff by
analyzing the angle distribution related to the rigid lattice
between 171 and 164 kHz, where θ = 17 1.3 degrees and ηeff =
0.4−0.5. The qualitative comparison of the evolution of
relaxation with temperature for the isotropic and anisotropic
signals shows that the two species have indeed notably different
dynamics, which supports our assumption to view free isotropic
motion as negligible.
2
Figure 6. Partially relaxed H NMR spectra from inversion recovery
experiments at ambient temperature for (a) 25PNO-d5/75NB/p-tBC,
(b) 25PNO-3,4,5-d3/75NB/p-tBC, and (c) 25PNO-2,6-d2/75NB/p-
tBC. The sharp signals at zero frequency with the recovery time of (b) 5
ms and (c) 11 ms indicate free unbounded PNO in crystals. The black
arrows indicate the spectral positions where the spin−lattice relaxation
times are measured.
help25 for studying complex molecular motion of inclusion
compounds as discussed below.
3.3. 2H Line Shape of Single-Crystal 25PNO-d5/75NB/
p-tBC and 25PNO-2,6-d2/75NB/p-tBC. Figure 8 shows a
single crystal of 25PNO-d5/75NB/p-tBC resonance frequency
versus the rotation angle. For this experiment, the crystallo-
graphic axis was determined by X-ray, as shown in Figure 7A.
The crystal was mounted after defining the crystallographic axis
as shown in Table 2, and the rotation crystallographic unit
structure is shown in Figure 8. It is difficult to differentiate
between a or c-axes where the monoclinic crystal with a space
group Pn has a = 12.778(19) Å, b = 13.095(2) Å, and c =
12.780(18) Å. For the deuterium rotation NMR study, a single
crystal of 25PNO-d5/75NB/p-tBC is mounted in the home-
made goniometer, as shown in Figure 7B, which fits inside the 10
mm coil of the NMR probe. The single-crystal position in the
goniometer incremented in angle every 5 degrees from 0 to
180°; thus, for every angle position, sufficient numbers of scans
were acquired to obtain a good signal to noise (S/N) ratio.
However, under goniometer rotational angles versus the axis of
an external magnetic field B0, the QCC of particular deuterons
D2(D6), D3(D5), and D4 of a guest inside p-tBC are separated
with particular fingerprints. Figure 9 shows that D2(D6),
D3(D5), and D4 deuterons of PNO-d5 collapsed at angles of
55 and 125°. The splitting of Δνxx = 124 kHz belongs to D3(D5),
χ = 172 kHz, the splitting of Δνxx = 100 kHz belongs to D2(D6),
χ = 139 kHz, and splitting of Δνxx = 52 kHz belongs to the D4
deuteron producing QCC, χ = 72 kHz, a single crystalline at
ambient temperature (c.f. Figure 10).
The guest’s rotational motion activation energies can be
compared with those of the cyclohexane/thiourea21 and
benzene/cyclophosphazene22 inclusion compounds with values
of 10.5 and 16.7 kJ/mol. In summary, the small activation
energy, derived for 25PNO-x-dn/75NB/p-tBC, strongly sug-
gests that the interaction between the host cavity and the guest
2
species is rather weak. The partially relaxed H NMR spectra
obtained from IR experiment at room temperature are depicted
in Figure 6 for the samples of (a) 25PNO-d5/75NB/p-tBC, (b)
25PNO-3,4,5-d3/75NB/p-tBC, and (c)25PNO-2,6-d2/75NB/
p-tBC. In all spectra, the recovery of magnetization at room
temperature occurs at the same time (30 ms), except for the free
tumbling molecules of PNO-3,4,5-d3, which relaxed at 5 ms and
PNO-2,6-d2 at 11 ms. Differences in T1 can be explained due to
an isotropic molecular motion of unbounded PNO. Kolokolov
et al.23 reported that the temperature evolution of 2H NMR spin
relaxation and the mobility of 2-methylimidazolate in zeolitic
imidazolate frameworks (ZIF-8) depend on the geometry of the
motion. Wasyluk et al.24 was restricted by structural geometries
in the mobile crystalline material (MCM-41) and the control
porous silica (CPG); a different value of the spin−lattice
relaxation times and the low activation energy (Ea) were
observed. The same can be examined in Figure 6b,c.
During analysis, it was necessary to identify the rotation
patterns related to particular deuterons. We did by assuming that
the principal direction of the component of quadruple coupling
lies close to the corresponding C−D bond directions. Therefore,
using the crystal data applying both types of symmetries (crystal
and molecular), we performed an overall best-fit analysis to
determine the QCC of the deuterons. Since the C−D bonds of
the PNO ring are under a titled angle to the channel axis, the
Experimental difficulties must be emphasized here. Thus,
complications could arise from the distribution of correlation
frequencies, including suppression of slow and fast motions that
often leads to highly anisotropic motion. As a result, detailed
interpretation of NMR relaxation is very complex and could be
controversial. The 2H 2D exchange NMR experiment is of great
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Figure 7. (A) 25PNO-d5/75NB/p-tBC crystal (0.4 × 0.3 × 0.1 mm, 43 mg) mounted along its a-axis on UHU patafix (a form of blue Tak) and
crystallographic axis study. (B) Same single crystal mounted along its a-axis on the solid-state deuterium goniometer NMR (c.f. Table 2) where the
black lines indicate 5° increments of rotation.
Figure 8. Goniometer single-crystal orientation measurement (A) at room temperature of 25PNO-d5/75NB/p-tBC mounted in the a(c)-axis
according to Table 2 (size 0.4 × 0.3 × 0.1 mm, 43 mg of crystal). The black triangles in the experimental spectra (A) indicate the region splitting of
peaks of p-deuterons. The deuteron single-crystal angles versus split frequency (B) collapsing at the goniometer angle of 55° (125 8°) with maximum
splitting at 5° (175°) separated using the equation Δνxx = A(3cos2(φ) − 1). The deuterium line shape is a sum of contributions of the ortho- and meta-
deuterons varying with molecular motion, whereas the para-deuteron on-axis component is invariant with motion.
crystallographic a(c)-axis is perpendicular to the external
magnetic field B0, according to Table 2, as shown in Figure 7
and Figure 8. In such a case, we expected the spectra to show a
single narrow line due to the overlap of deuterons at the vicinity
of the magic angle of 54.7°. Comparing these results with the
powder crystalline splitting at 20 °C where the interactions and
vibrational motions are averaged out of the effective QCC for
D2(D6) and D3(D5) around χ = (172 + 139)/2 = 155.5 kHz, η =
0.04, as shown in Figure 4a−c, they are consistent with results of
single-crystal data.
3.4. 2D Exchange Spectroscopy of 2H NMR. Figure 11A
shows the contour plot spectrum of 25PNO-d5/75NB/p-tBC at
room temperature obtained with a mixing time of 5 ms. The
contour plot spectrum demonstrates that a discrete two jump
molecular motion occurs. It can be seen in a spectrum plot that
the exchange processes during the evolution and detection times
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Figure 9. Single crystal of 25PNO/75NB/p-tBC structure collected at
−70 °C (the data are shown in Table 1). For clarification of the solid-
state 2H NMR goniometer single-crystal rotation, as shown in Figure 7,
the single crystal was mounted based on Table 2 with the results
obtained in Figure 8.
2
Figure 11. H 2D exchange NMR spectrum of 25PNO-d5/75NB/p-
tBC recorded at 20 °C with the mixing time, τm, of 5 ms. For
comparison, 2H 2D NMR exchange (A) surface plot and (B)
absorption spectrum are shown. The two-fold (C2) guest distorted
inside a cavity of the host is observed. The spectrum corresponds to the
average distribution reorientation angle of 180° = (105° + 75°) (A).
The diffraction X-ray data of 25PNO/75NB/p-tBC structure collected
at −70 °C (c, d) are good to compare (see Figure S3 in the Supporting
Information).
2D spectrum of the 25PNO-d5/75NB/p-tBC guest shows a
prominent butterfly spectrum. The cross frequencies in a
butterfly spectrum are at the angle of 90 15° due to rotational
alteration. One interpretation is that because the motional axes
of the guest inside the confined space led to controlled molecular
dynamics motion. 2H 2D exchange NMR spectroscopy can only
define the average distribution of reorientation angles, which
indicates the fast-molecular motion but with fewer distribution
angles. Several attempts of long mixing time experiments were
conducted, but results showed hardly any trace of elliptical
ridges informing the lack of slow motions. Therefore, the 2H 2D
exchange NMR spectroscopy results at room temperature are
consistent with the results of the 2H NMR line shape recorded at
20 °C (c.f. Figure 4a) and the jump model proposed (see Figure
S1 in the Supporting Information.). The contour plot (a) and
absorption (b) spectra show that D2(D6) and D3(D5) have the
jump angle of C2 (180°) crossing with C4-fold of the host. The
result of 2D exchange spectroscopy of Figure 11A,B becomes
understandable with the help of the single-crystal X-ray
diffraction data. The two-fold jump distorted around a four-
fold of host motion would show the vibrational motion of guest
C2 angles. We explain that the average directional angle of N−O
with the tertiary carbons of tert-butyl groups produces 105°.
Thus, due to the thermal vibrational motion of guest C2 jump
rotation motion inside of a host (vibration motions of host
Figure 10. Goniometer single-crystal orientation measurement at room
temperature of 25PNO-d5/75NB/p-tBC mounted in the a(c)-axis
according to Table 2 (size 0.4 × 0.3 × 0.1 mm, 43 mg of crystal).
Deuteron single crystal at the goniometer angle with splitting at 165°.
The deuterium line shape separated the ortho (D2(D6)) and meta
(D3(D5)) deuterons varying with molecular motion, whereas the para
(D4) deuteron on the C2 axis is invariant with jumping motion but
oscillating with a molecule.
are fast enough, and therefore, any elliptical ridges due to the
slow jump molecular motions, around well-defined angles, are
not visible. If no exchange occurs during the mixing time, the
NMR frequency of a particular C−D bond remains the same,
and only one diagonal spectrum will be detected. Thus, the 2H
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structure), the jump angle bond of N−O produces a distribution
angle of 15° between 180° = (105° + 75°). Of course, there may
exist other models including a small or large angle vibration, i.e.,
conformational motion, diffusive process, and population ratio
that follow experimental results. According to our knowledge so
far, not many 2H 2D NMR exchange spectroscopy experiments
were conducted for the inclusion compounds, in which the
single-crystal X-ray and additional angle model interpretation
was reported.
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4. CONCLUSION
The variable temperature H NMR line shape and the spin−
2
lattice relaxation studies were performed on three samples with
fully and selectively deuterated guest molecules in 25PNO-d5/
75NB/p-tBC, 25PNO-3,4,5-d3/75NB/p-tBC, and 25PNO-2,5-
d2/75NB/p-tBC and support the two-site jump model, which
was also modulated by four-fold symmetry of the host. The
analysis of partially relaxed spectra yields the correlation times
for the guest motional process revealing the fast exchange limit,
where 2D exchange spectroscopy confirmed the guest molecular
motion. The obtained results follow an Arrhenius behavior from
which the activation energy is derived. It is found that pyridine 1-
oxide is highly mobile from −130 °C to ambient temperature.
The small activation energy strongly suggests that the
interaction between the guest species and the host is rather
weak. Due to the thermal vibration of guest C2 rotation inside a
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dimensional exchange NMR in solids. J. Magn. Reson. 1988, 79, 269−
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.jpca.0c07414.
290.
■
sı
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Dimensions; Oxford, Clarendon Press: United Kingdom, 1987.
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(Figures S1−S3) Schematic drawings of the proposed
ring motional cone model, oscillational angle around θ
versus temperature in (K), and single-crystal diffraction
X-ray data of PNO inside the host (PDF)
AUTHOR INFORMATION
Corresponding Author
(14) Abragam, A. The Principles of Nuclear Magnetism; Oxford,
Clarendon Press: United Kingdom, 1983.
■
(15) Facey, G. A.; Ratcliffe, C. I.; Ripmeester, J. A. Solid State NMR
Studies of the Molecular Motion of Pyridine in its Tri-o-thymotide
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An internet approach to NMR lineshape analysis. Appl. Magn. Reson.
2001, 20, 405−432.
John Nobleman − Department of Chemistry and Biochemistry,
The City College University of New York, New York, New York
10031, United States; orcid.org/0000-0002-0215-2521;
Phone: +1 (212)650-6105; Email: jnobleman@
ccny.cuny.edu
(17) Kawaguchi, T.; Ohki, H.; Yamada, K.; Okuda, T.; Haino, T.;
Fukazawa, Y. Structure and guest dynamics for calix[4]arene derivative
inclusion compounds. J. Mol. Struct. 2007, 833, 108−113.
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and diffraction studies of p-tert-butylcalix[4]arene·nitrobenzene·xe
non. Chem. Commun. 1997, 939−940.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.jpca.0c07414
Notes
The author declares no competing financial interest.
(19) Villanueva-Garibay, J. A.; Muller, K. Molecular dynamics and
̈
ACKNOWLEDGMENTS
ordering of pyridine in its cyclophosphazene inclusion compound as
evaluated by solid state 2H NMR spectroscopy. Phys. Chem. Chem.
Phys. 2006, 8, 1394−1403.
■
The author is grateful for the financial support during the
postdoctoral position in the years of 2010/2011 in the
Hollingsworth’s group at the Department of Chemistry, KSU,
and thankful to Earl Emery from Tecmag Company and Dr.
Roman Gaja from the Department of Chemistry, University of
Warsaw for help during the setup of single-crystal X-ray
experiments and Prof. Teobald Kupka from the Department
of Chemistry, University of Opole, Poland for discussion during
preparation of the manuscript.
(20) Brouwer, E. B.; Enright, G. D.; Ratcliffe, C. I.; Facey, G. A.;
Ripmeester, J. A. Weak Intermolecular Interactions and Molecular
Recognition: Structure and Dynamics of the Benzene and Pyridinep-
tert-Butylcalix[4]arene Inclusions†. J. Phys. Chem. B 1999, 103,
10604−10616.
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the thiourea- cyclohexane inclusion compound by deuterium NMR
spectroscopy. J. Phys. Chem. 1991, 95, 407−413.
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