1 X-Ray Birefringence Imaging (XBI): A New Technique …
23
[low-temperature (LT) phase]. The phase transition temperature for 1,8-DBrO/urea
is 157 K and for 1,10-DBrD/urea is 140 K. Several techniques have been applied
to study the dynamics of the guest molecules in α,ω-dibromoalkane/urea inclusion
compounds. Incoherent quasielastic neutron scattering (IQNS) has shown [32] that,
in the HT phase, the guest molecules undergo rapid reorientation about the tunnel
axis (τ ≈ 10
–12
− 10
–10 s; τ denotes the timescale of motion) and restricted translational diffusion along this axis. Solid-state
2 H NMR studies (both lineshape analysis and spin-lattice relaxation time measurements) of 1,10-DBrD/urea also indicate
[30] that rapid reorientation (τ < 10
−8 s) of the guest molecules occurs about the
tunnel axis in the HT phase. Polarized Raman spectroscopy [33] has shown that the
α,ω-dibromoalkane guest molecules adopt predominantly the all-trans conformation within the urea tunnel structure, with only a small proportion (ca. 7%) of gauche
end-groups. For the predominant (ca. 93%) conformation with trans end-groups, the
C–Br bonds form an angle ψ ≈ 35.3° with respect to the tunnel axis of the urea host
structure (Fig. 1.13b).
XBI data were recorded at the Br K-edge for single crystals of 1,8-DBrO/urea
and 1,10-DBrD/urea as a function of crystal orientation, specified by angles χ and
φ. In these measurements, the sample reference axis (z s -axis) is the long-needle axis
of the crystal morphology, which corresponds to the tunnel axis (c-axis) of the urea
host structure. This axis was maintained in the plane (XY-plane) perpendicular to
the propagation direction (Z-axis) of the incident X-ray beam. Variation of χ refers
to rotation of the z s -axis (c-axis of the crystal) around the laboratory Z-axis and
variation of φ refers to rotation of the crystal around the z s -axis. For χ = 0°, the
z s -axis is parallel to the direction of linear polarization of the incident X-ray beam
(X-axis; horizontal).
Figure 1.14 shows XBI data recorded at the Br K-edge for single crystals (in the HT
phase) of 1,8-DBrO/urea (at 280 K) and 1,10-DBrD/urea (at 170 K) as a function of χ.
For both 1,8-DBrO/urea and 1,10-DBrD/urea, the X-ray intensity exhibits sinusoidal
variation as a function of χ, with maximum brightness at χ ≈ 45° and minimum
brightness at χ ≈ 0° and χ ≈ 90°. The X-ray intensity is uniform across the entire
crystal, indicating that all regions of the crystal have the same orientational properties
of the C–Br bonds.
XBI data recorded as a function of φ in the HT phase for single crystals of 1,8DBrO/urea (at 280 K) and 1,10-DBrD/urea (at 170 K) are shown in Fig. 1.15 (for
these measurements, χ was fixed at an orientation close to the maximum intensity
in Fig. 1.14). No significant changes in X-ray intensity are observed as a function
of φ, indicating that the orientational distribution of the C–Br bonds relative to the
laboratory frame (X, Y, Z) is not altered by rotating the crystal around the c-axis
(tunnel axis).
The observed XBI behavior indicates that, for these materials, the effective X-ray
optic axis (i.e., the resultant C–Br bond orientation) is parallel to the tunnel axis of the
urea host structure. For an α,ω-dibromoalkane guest molecule with trans end-group
conformation inside the urea host tunnel, the C–Br bond forms an angle ψ ≈ 35.3°
with respect to the tunnel axis (Fig. 1.13b). The fact that the resultant C–Br bond
vector is parallel to the tunnel axis for 1,8-DBrO/urea and 1,10-DBrD/urea in the HT
23
[low-temperature (LT) phase]. The phase transition temperature for 1,8-DBrO/urea
is 157 K and for 1,10-DBrD/urea is 140 K. Several techniques have been applied
to study the dynamics of the guest molecules in α,ω-dibromoalkane/urea inclusion
compounds. Incoherent quasielastic neutron scattering (IQNS) has shown [32] that,
in the HT phase, the guest molecules undergo rapid reorientation about the tunnel
axis (τ ≈ 10
–12
− 10
–10 s; τ denotes the timescale of motion) and restricted translational diffusion along this axis. Solid-state
2 H NMR studies (both lineshape analysis and spin-lattice relaxation time measurements) of 1,10-DBrD/urea also indicate
[30] that rapid reorientation (τ < 10
−8 s) of the guest molecules occurs about the
tunnel axis in the HT phase. Polarized Raman spectroscopy [33] has shown that the
α,ω-dibromoalkane guest molecules adopt predominantly the all-trans conformation within the urea tunnel structure, with only a small proportion (ca. 7%) of gauche
end-groups. For the predominant (ca. 93%) conformation with trans end-groups, the
C–Br bonds form an angle ψ ≈ 35.3° with respect to the tunnel axis of the urea host
structure (Fig. 1.13b).
XBI data were recorded at the Br K-edge for single crystals of 1,8-DBrO/urea
and 1,10-DBrD/urea as a function of crystal orientation, specified by angles χ and
φ. In these measurements, the sample reference axis (z s -axis) is the long-needle axis
of the crystal morphology, which corresponds to the tunnel axis (c-axis) of the urea
host structure. This axis was maintained in the plane (XY-plane) perpendicular to
the propagation direction (Z-axis) of the incident X-ray beam. Variation of χ refers
to rotation of the z s -axis (c-axis of the crystal) around the laboratory Z-axis and
variation of φ refers to rotation of the crystal around the z s -axis. For χ = 0°, the
z s -axis is parallel to the direction of linear polarization of the incident X-ray beam
(X-axis; horizontal).
Figure 1.14 shows XBI data recorded at the Br K-edge for single crystals (in the HT
phase) of 1,8-DBrO/urea (at 280 K) and 1,10-DBrD/urea (at 170 K) as a function of χ.
For both 1,8-DBrO/urea and 1,10-DBrD/urea, the X-ray intensity exhibits sinusoidal
variation as a function of χ, with maximum brightness at χ ≈ 45° and minimum
brightness at χ ≈ 0° and χ ≈ 90°. The X-ray intensity is uniform across the entire
crystal, indicating that all regions of the crystal have the same orientational properties
of the C–Br bonds.
XBI data recorded as a function of φ in the HT phase for single crystals of 1,8DBrO/urea (at 280 K) and 1,10-DBrD/urea (at 170 K) are shown in Fig. 1.15 (for
these measurements, χ was fixed at an orientation close to the maximum intensity
in Fig. 1.14). No significant changes in X-ray intensity are observed as a function
of φ, indicating that the orientational distribution of the C–Br bonds relative to the
laboratory frame (X, Y, Z) is not altered by rotating the crystal around the c-axis
(tunnel axis).
The observed XBI behavior indicates that, for these materials, the effective X-ray
optic axis (i.e., the resultant C–Br bond orientation) is parallel to the tunnel axis of the
urea host structure. For an α,ω-dibromoalkane guest molecule with trans end-group
conformation inside the urea host tunnel, the C–Br bond forms an angle ψ ≈ 35.3°
with respect to the tunnel axis (Fig. 1.13b). The fact that the resultant C–Br bond
vector is parallel to the tunnel axis for 1,8-DBrO/urea and 1,10-DBrD/urea in the HT