Article pubs.acs.org/crystal
Single-Crystal X‑ray Diffraction Analysis of Microcrystalline Powders Using Magnetically Oriented Microcrystal Suspensions Chiaki Tsuboi, Fumiko Kimura, Tatsuya Tanaka, and Tsunehisa Kimura* Division of Forest and Biomaterials Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan ABSTRACT: A technique for collecting single-crystal X-ray diffraction data using a suspension of microcrystalline powder is reported. The technique developed is based on the three-dimensional alignment of microcrystals by the intermittent rotation of the suspension under static magnetic field, in combination with in situ X-ray measurements. The magnetic attachment required to perform these in situ measurements is significantly simplified because the shutter system equipped with the magnetic system in the previous reported attachment is not necessary in the current technique owing to the application of intermittent rotation. Using this technique, the measurement time is significantly decreased in comparison to that required in our previous procedure. The successful performance of this technique is demonstrated by the structural determination of L-alanine from its microcrystalline powder.
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INTRODUCTION Single-crystal X-ray diffraction (XRD) method is a powerful means for elucidating molecular structures. Single crystals larger than 20 μm are required for in-house measurements,1 while those of a few micrometers are acceptable at synchrotron facilities.2 Alternatively, powder diffraction method can be employed for molecular structure determination3−6 when single crystals of appropriate sizes are not available. The third approach previously proposed by our group involves the use of magnetically oriented microcrystals.7 The application of timevarying magnetic fields to dia- or paramagnetic microcrystals suspended in a liquid medium results in the biaxial alignment of microcrystals,8,9 which in turn produces XRD images equivalent to those obtained from the corresponding actual single crystal. XRD data can be acquired by two methods: in situ XRD measurements, which are performed on the oriented suspension, and ex situ measurements, which are performed after the orientation is consolidated. We refer to the oriented suspension as magnetically oriented microcrystal suspension (MOMS) and the consolidated suspension as magnetically oriented microcrystal array (MOMA). MOMAs can be treated as single crystals; thus, the conventional procedure for analyzing single crystals can be utilized.10 It is difficult to create time-varying magnetic fields in actual experiments. Hence, instead, a suspension is rotated in a static magnetic field in a time-varying manner, which is equivalent to the application of time-varying magnetic fields to a still suspension. Therefore, MOMS has a limitation in that the rotation of the suspension has to be maintained9 so as to ensure biaxial alignment. If the XRD measurement is performed in situ under this situation, the continuous rotation renders biaxial alignment to uniaxial alignment about the sample rotation axis, resulting in fiber XRD patterns. Previously, we have reported11 that an X-ray shutter is suitable for solving this problem. The shutter permits the X-ray beam to impinge on the rotating suspension only when the suspension takes a specific rotation angle with respect to the X-ray beam, which allows for the © XXXX American Chemical Society
single-crystal structure determination from the rotating suspension.11 In this study, we report an improvement to our previous procedure, that is, a shutter-less system, which is combined with intermittent sample rotation,12 serving as timevarying sample rotation. With this improved procedure, the measurement time is significantly decreased and the magnetic attachment is largely simplified without loss in the quality of XRD data in comparison to those utilized in the previous procedure.
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EXPERIMENTS
Preparation of Suspension. As-received L-alanine crystals (Wako) were pulverized and passed through 120, 75, 45, and 20 μm meshes. Powders that were passed through the 20 μm mesh were mixed with a suspending medium (XVL-14, Kyoritsu; viscosity, 12 Pa s) to obtain a suspension of 20 wt %. The individual particles were not polycrystals under optical microscope observation. This suspension poured into a glass capillary (2.4 mm i.d.) was set in the sample rotating holder. This size of the capillary was employed so that the magnetic field generated by a pair of permanent magnets was uniform throughout the diameter of the capillary. X-ray Measurements. Figure 1 shows the schematic of the experimental setup. A capillary containing the suspension was located in the center of magnetic field B (ca. 1 T; ∥y-axis) generated by a pair of neodymium magnets, and its rotation was controlled using a stepping motor. The rotation axis, which lay in the xz plane, made an angle Ω with respect to the z-axis. This magnetorotation unit was mounted on the ω-stage (oscillation axis ∥y-axis) of an in-house X-ray diffractometer (Rigaku), and ω-scan was performed at given angles of Ω. The X-ray beam was impinged from the x-axis. To achieve the biaxial alignment, the capillary was rotated in an intermittent manner. In Figure 2, the rotating frame is defined by three orthogonal coordinates r1, r2, and r3. The r3-axis was parallel to the rotation axis of the capillary, while r1- and r2-axes were perpendicular to the rotation axis. The angle between the magnetic field and the r1Received: January 25, 2016 Revised: April 4, 2016
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DOI: 10.1021/acs.cgd.6b00129 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
followed by the interruption of rotation for a period of Tint = 5 s. Next, α changed from 180° to 360° for Trot = 5 s, followed by the interruption of rotation for Tint = 5 s. This was one cycle of intermittent rotation. With this experimental setting, the magnetic susceptibility axes χ1, χ2, and χ3 of the microcrystals were oriented parallel to the r1-, r2-, and r3-axes, respectively. Here, the susceptibility values were defined as χ1 > χ2 > χ3. After the biaxial alignment was achieved, the interruption time Tmes, which was significantly longer than Tint, was occasionally taken for XRD measurements. Rigaku R-AXIS RAPID diffractometer (collimator size = 0.8 mm i.d.; graphite-monochromated Mo Kα radiation; crystal-to-detector distance = 127.40 mm; voltage = 60 kV; current = 90 mA) was used for the measurement. During measurements, the r1axis remained parallel to the magnetic field. The ω-scan with an oscillating angle of 3° was performed at an angle of Ω for 30 s, followed by a reading time of 90 s. Then, an increment of 3° was made for the angle Ω, and the same procedure was performed five times. As a result, the XRD images for Ω ∼ Ω + 15° (five images) were acquired, with image acquisition taking approximately 10 min. Because the suspension was not rotated during this period, fluctuations in orientation possibly occur. Hence, intermittent rotation was applied again for a period of Talg (=15 min) for recovering the alignment before subsequent image acquisition. Finally, 73 images were acquired, corresponding to the range of −41° < Ω < 163°. The obtained XRD images were indexed, integrated, and scaled using Rapid Auto. The space group and initial structure were determined by CrystalStructure and Sir2011,13 respectively. Refinement was performed using SHELXL2013.14
Figure 1. (a) Schematic of the experimental setup. A glass capillary (C) containing a microcrystalline suspension is rotated about the capillary axis in an intermittent manner by using a stepping motor (SM). The x and y axes represent the directions of the X-ray beam (X) and static magnetic field (B), respectively. (b) View of the measurement setting from the y-axis. The direction of the capillary is changed to set the angle Ω by rotation about the y-axis.
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RESULTS AND DISCUSSION Among 73 XRD images, those of nos. 31−68 were impaired by the shadow of the magnet. Indexing was performed using all images including impaired ones. Figure 4 shows some typical Figure 2. Schematic of the rotational coordinates r1, r2, and r3 embedded in the capillary. The r3-axis coincides with the axis of capillary rotation. The r1 and r2 axes are in a plane perpendicular to the r3-axis. The direction of the r1-axis with respect to the y-axis (parallel to B) is defined by an angle α. The r1-axis coincides with the laboratory y (or −y)-axis (α = 0° or 180°) when the rotation is interrupted. When 3D orientation is achieved, the magnetic susceptibility axes χ1, χ2, and χ3 align parallel to the r1, r2, and r3 axes, respectively. This indicates that these magnetic axes are rotating synchronously with the rotation of the capillary. axis was denoted by α. Figure 3 shows the scheme of the intermittent rotation in terms of angle α. During a rotation period of Trot = 5 s, α changed from 0° to 180° at a constant rotation speed of 36°/s,
Figure 4. X-ray diffraction images of an L-alanine MOMS recorded at oscillation angles of (a) −41° ≤ Ω ≤ −38° and (b) 46° ≤ Ω ≤ 49°. The white area in panel b is attributed to the shadow of the magnet.
Figure 3. Scheme of the intermittent rotation of the rotating axis r1 used to achieve biaxial alignment. The capillary is rotated during the rotation period Trot by the insertion of the interruption period Tint every 180°. The angle α represents the direction of the r1-axis with respect to the laboratory y-axis as defined in Figure 2.
XRD images. Before scaling, integration was performed for the images from no. 1 to no. 30. The hkl file thus obtained was analyzed, and the space group was determined as P212121. The initial structure was determined, followed by refinement. The crystallographic data (Table 1) and determined structure (Figure 5) were compared with those obtained from the single crystal.15 The R1 value was 9.97%. RMS was 0.048; the hydrogen atom positions were slightly deviated from those determined using the single crystal. Three principal values χ1, χ2, and χ3 of the magnetic susceptibility tensor χ of biaxial crystals (triclinic, monoclinic, and orthorhombic systems) are different (here, we previously defined them as χ1 > χ2 > χ3). The χ1- and χ3-axes are referred to as easy and hard magnetization axes, respectively. Under static magnetic fields, the χ1-axis is aligned parallel to the applied field, while under rotating magnetic fields, the χ3-axis is aligned parallel to the rotation axis. Under intermittent rotation employed here, the χ1-axis is aligned parallel to the r1-axis because the r1-axis is parallelly exposed to the magnetic field for B
DOI: 10.1021/acs.cgd.6b00129 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
1), ⟨ω22⟩ = 2−1(2Rmag + 1)−1(Rmag + 1), and ⟨ω32⟩ = 2−1(Rmag + 1)Rmag−1, where common factors are ignored. The anisotropy of the half-width of XRD spots is proportional to ⟨ω12⟩1/2, ⟨ω22⟩1/2, and ⟨ω32⟩1/2. These values are 1.4, 0.58, and 1.0 for Rmag = 1 that were used in this study. ⟨ω12⟩1/2 is greater than the other two. This value can be made smaller at the cost of the increase of the other two values: we obtain 1.2, 0.61, and 1.2 if Rmag = 0.5. Judging from the obtained diffraction spots, the choice of Rmag = 1 was acceptable. The total experimental time required for the present procedure is significantly decreased as compared with that required previously11 where the shutter was open only when the direction of the MOMS assumed a designated angle (an oscillation angle of 3°) with respect to the X-ray beam. Although various factors should be considered when comparing the performance of these two procedures, an estimation may be possible: for taking 60 diffraction images under the condition of an X-ray exposure time of 10 s/deg and an oscillation angle of 3°, the experimental time required for the previous procedure and for the present procedure are approximately 41 and 5 h, respectively. For the present procedure, the performance may be expressed by Texposure/(Talg + Texposure + Treading), where Texposure and Treading are the X-ray exposure time and the data reading time, respectively. The measurement time is defined by Tmes = Texposure + Treading. If Talg and Treadingare decreased and Texposure is increased, the performance can be increased. In the present case, Texposure = 150 s and Treading = 450 s. We estimate the increase of the directional fluctuation in the χ2- and χ3-axes in the r2r3 plane during Tmes. During Tmes, a confining force acts on the χ1-axis because the magnetic field is applied parallel to the r1-axis, while the χ2- and χ3-axes are free to undergo rotational diffusion on the xz plane (Figure 1). As a result, if Tmes is too long, the biaxial alignment already achieved during Talg deteriorates because of increased fluctuations in the orientation of the χ2- and χ3-axes. The distribution of the χ2and χ3-axes is described by a distribution function f(ω1), where ω1 is the angle between χ3 and the r3-axis as defined previously. If the initial distribution of ω1 is G(ω1) = δ(ω1), where δ(ω1) is the δ function, then the distribution function after time t is expressed by G(ω1,t) = (4πDrott)−1/2 exp {−(4Drott)−1ω12} by solving the rotational diffusion equation ∂G/∂t = Drot (∂2G/ ∂ω12), where Drot [rad2/s] is the rotational diffusion constant. If the initial distribution of the angle ω1 is the Gaussian distribution,
Table 1. Crystallographic Data Obtained from a MOMS and a Single Crystal sample
L-alanine
cryst syst space group temp (K) a (Å) b (Å) c (Å) V (Å3) Z 2θmax indep reflecns completeness Rint R1 [F2 > 2σ(F2)] Rw [all data] GOF CCDC no.
MOMS (
