- 00:04Visão Geral
- 00:52Crystals and X-Ray Diffraction
- 02:14Operation of an X-ray Diffractometer
- 03:25Single Crystal X-ray Diffraction of Mo2(ArNC(H)NAr)4 (where Ar = p-MeOC6H5)
- 04:45Powder X-ray Diffraction
- 05:50Representative Results
- 06:28Applications
- 07:57Summary
単結晶および粉末 x 線回折
English
COMPARTILHAR
Visão Geral
ソース: タマラ ・ m ・力、化学科テキサス A & M 大学
X 線結晶構造解析、分子の構造を研究する x 線を使用する手法です。X 線回折 (XRD) 実験は、単結晶または粉末サンプル実施定期的に。
単結晶 x 線回折:
単結晶 x 線回折は、絶対構造決定が可能です。単結晶 x 線回折データを正確な原子位置観察することができる、従って付着長さと角度を決定することができます。この手法は、材料のバルクを必ずしも表していない単結晶内の構造を提供します。したがって、id および化合物の純度を証明するために追加一括評価手法を利用する必要があります。
粉末 x 線回折:
単結晶 x 線回折とは異なり粉末 x 線回折、多結晶材料の大規模なサンプルを見て従って一括評価法と考えられます。粉末パターンの特定の材料の「指紋」である相 (多形) および材料の結晶性に関する情報を提供しています。通常、粉末 x 線回折は、鉱物、ゼオライト、有機金属フレームワーク (Mof)、他の拡張の固体の研究に使用されます。粉末 x 線回折は、バルク分子種の純度を確立するも使用できます。
以前は、我々 は x 線質結晶 (有機化学の必需品シリーズのビデオを参照してください) を成長する方法を見てきました。ここで x 線回折の原理を学習します。Mo2単結晶と粉末の両方のデータを収集しますし (ArNC(H)NAr)4、どこ Ar p– MeOC6H5を =。
Princípios
なぜ x 線?
距離を測定する場合、測定対象オブジェクトのスケールには、メジャーの単位を選択することが重要です。たとえば、鉛筆の長さを測定する 1 つたくない足のグラデーションはヤードの棒を使用します。同様に、車の長さを測定する場合は、ことがない適当 cm マークで 12 インチの定規を使用します。したがって、分子の結合を検討するために、これらの債券の長さに一致する光の波長を使用することが重要です。X 線は、典型的な結合距離 (1-3 Å) と完全に一致する Å の範囲で波長を持っています。
単位セル:
ペンの先端に分子のすべてを説明しようと想像してください。1 つは、6.02 × 10 の23の分子 (または 1 モル) で構成されていますが、近似場合、分子レベルでそのオブジェクトを記述するためほぼ不可能に思えます。オブジェクトの複雑さは、それが全体の構造を記述するための単位セルの内容を使用できます、結晶として存在する場合に簡略化されます。結晶の単位格子は、固体の繰り返しの単位を含む少なくともボリュームです。3 D の長さ a、b の「ボックス」として定義されていると c、および角度 α、β および γ (図 1)。単位セルでは、化学者の一部を使用して結晶または少数の原子または得たの内容を記述することができます。宇宙の単位セルを繰り返すことによって 1 つは固体の 3 D 表現を生成できます。
図 1.単位セル パラメーターを指定します。
実験のセットアップ:
単結晶および粉末 x 線回折と同様の計測器設定があります。単結晶 x 線回折用結晶がマウントされ、x 線ビームの中央に配置。粉末 x 線回折、結晶サンプルは微粉末に粉砕、プレートにマウントされています。サンプル (シングル – または多結晶) は x 線照射によるし、回折 x 線検出器を打ちます。データ収集中は、x 線源と検出器に対してサンプルを回転します。
二重スリットの実験:
光が両方の波および粒子状プロパティを思い出してください。とき単色光は、2 つのスリット、各スリットから球状のファッションで発せられる光に光の結果の波のようなプロパティを入力します。波が対話するとき彼らは一緒に追加できます (波は、同じ波長と位相を持っている) 場合お互いのキャンセル (波は同じ波長があるが、さまざまな段階がある) 場合、それぞれ建設と破壊的な干渉と呼ばれます。結果光のパターンは、一連の線、暗い部分が破壊的な干渉の結果、明るい領域が建設的な干渉を表す製です。
典型的な回折パターン: 単結晶と粉末:
X 線による結晶の照射、放射線を回折結晶内電子密度との相互作用に。水の建設と破壊的な干渉の結果古典的な二重スリット実験物理学、x 線回折からの波の相互作用と同様します。X 線回折、回折パターンは、原子と結晶内の結合のための電子密度を表します。(図 2) に単結晶の典型的な回折パターンを示します。回折パターンは二重スリット実験のような線ではなく点で構成されることに注意してください。実際には、これらの「点」は、3 次元球の 2D スライスが。Crystallographers では、コンピュータ プログラムを使用して、図形と回折 x 線の強度を決定するために結果のスポットを統合します。粉末試料の x 線は、ランダムな方向に多くの小さな結晶と対話します。したがって、代わりに観光スポットを見て、円形回折パターンに (図 3) が観察されます。回折円の強度は、粉末パターンとして既知の 2 次元プロットを与えることビーム軸 (印 2 θ) リング間の角度に対してプロットされます。
ここでは、Mo2単結晶および粉末 x 線回折データを収集します (ArNC(H)NAr)4 Ar = pMeOC-6H5モジュール”の準備と評価の Quadruply-金属で合成しました。「化合物を結合
図 2 。単結晶回折パターン。
図 3.粉末 x 線回折: 円形回折パターン。
Procedimento
Resultados
Figure 4. Single-crystal structure of Mo2(ArNC(H)NAr)4 where Ar = p-MeOC6H5.
Figure 5. Powder XRD pattern of Mo2(ArNC(H)NAr)4 where Ar = p-MeOC6H5.
Applications and Summary
In this video, we learned about the difference between single-crystal and powder XRD. We collected both single-crystal and powder data on Mo2(ArNC(H)NAr)4, where Ar = p-MeOC6H5.
Single-crystal XRD is a powerful characterization technique that can provide the absolute structure of a molecule. While structure determination is the most common reason chemists use XRD, there are a variety of special X-ray techniques, such as anomalous scattering and photocrystallography, which provide more information about a molecule.
Anomalous scattering can distinguish between atoms of similar molecular weights. This technique is particularly valuable for characterization of heteropolynuclear metal complexes (compounds that have more than one metal atom with different identities). Anomalous scattering has also been used in protein crystallography as a method to help resolve the phase of the diffracted beam, which is important for structure determination.
Photocrystallography involves single-crystal XRD coupled to photochemistry. By irradiating a sample with light in the solid state, we can observe small structural changes and monitor those changes by XRD. Examples of this technique include observing isomerization of a molecule by light as well as characterization of reactive intermediates.
Powder XRD is a non-destructive characterization method that can be used to gain information about the crystallinity of a sample. In addition, it is a useful technique to analyze mixtures of different materials. As previously mentioned, powder patterns are like fingerprints: the resulting pattern of a compound is dependent on how the atoms are arranged within the material. Therefore, an experimentally-determined powder pattern can be compared to a collection of known diffraction patterns of materials in the International Centre for Diffraction Data. This not only provides information about the identity of the product isolated, but also allows scientists to comment on the number of compounds present in the sample. While a majority of the diffraction patterns listed in the database are in the family of extended solids such as minerals and zeolites, examples of inorganic molecules can be found.
Transcrição
X-ray diffraction is a common analytical technique used in materials science and biochemistry to determine the structures of crystals.
It traces the paths of X-rays through crystals to probe the structure. There are two major techniques. Powder X-ray diffraction determines the phases and purity of a crystalline species. Single X-ray diffraction identifies the atoms in a crystal and their locations, as well as electron densities, bond lengths, and angles.
This video illustrates the operation of an X-ray diffractometer, procedures for both single-crystal and powder X-ray diffraction, and discusses a few applications.
We’ll start by examining the concept of crystal structure, and exploring how X-rays interact with crystals.
A crystal is a periodic configuration of atoms, that is, a geometric pattern of atoms that repeat at regular intervals. The smallest repeating element of a crystal is called a “unit cell.” It is described by its packing structure, dimensions and bond angles. “Miller indices” describe any fictitious planar cross sections of the unit cell.
X-rays are a form of electromagnetic waves whose wavelengths are similar to the atomic spacing in crystals. When a single X-ray strikes an individual atom, it is diffracted. When two coherent X-rays strike atoms in different planes, the diffracted X-rays interfere, resulting in constructive or destructive signals.
The diffraction pattern of a powder crystalline sample is comprised of intense spots, which form rings of constructive interference. The angles at which these spots occur correspond to the spacing of atoms in that plane. The spacing can be determined using Bragg’s Law.
Now that we have learned about crystals and X-ray diffraction patterns, let’s look at how an X-ray diffractometer works.
An X-ray diffractometer consists of three basic components: an X-ray source, a specimen, and a detector. All components are oriented in a coplanar, circular arrangement with the sample holder at the center. The source usually contains a copper target, that, when bombarded by electrons, emits a beam of collimated X-rays. The beam is directed at the sample, which refracts the X-rays. The sample and the detector are then rotated in opposite directions, until the angles of X-ray intensity are determined.
High X-ray intensity corresponds to constructive interference by a crystallographic plane in both single-crystal and powder X-ray diffraction. Powder X-ray diffraction reveals the crystal structure of the sample, while single-crystal X-ray diffraction additionally reveals the chemical content and locations of atoms.
Now, let us see a practical example of X-ray diffractometry.
Single-crystal X-ray diffraction requires high-quality crystals without impurities, grain boundaries, or other interfacial defects. Bring the crystals of the organo-molybdenum compound to the light microscope to analyze it.
Begin by adding a drop of paratone oil to a clean glass slide. Then add a small amount of paratone oil to a spatula, and scoop some crystals from the crystallization vial onto a slide.
Examine the crystals under the microscope, and select a crystal with uniform, well defined edges. Once an ideal crystal is chosen, use a Kapton loop to pick up the crystal, ensuring little oil sticks to the crystal.
Next, open the diffractometer doors to load the sample. Attach the Kapton loop to the gonoimeter head, centering the crystal with respect to the X-ray beam. Then close the doors.
Open the X-ray crystallography software and run a short data collection sequence that establishes the structure of the unit cell. Based on this data, select a data collection strategy and run full data collection. Once a full data set has been collected, work up the data using a suitable program and refine it.
In comparison to single crystal X-ray diffraction, powder X-ray diffraction is a bulk characterization technique that does not require single crystals.
Choose an appropriately-sized sample holder and a diffraction plate that will not affect the readings at the angles of interest.
Place a fine mesh sieve over the diffraction plate. Carefully add 20 mg of sample to the sieve, keeping the sample over the plate. Tap the sieve on the benchtop until a monolayer of powder forms.
Secure the diffraction plate in the sample holder. Open the diffractometer doors and mount the sample. If the sample mount has locking pins, ensure the pins are engaged and the sample holder is secure before closing the doors.
Using a suitable software, load a standard data collection method. Enter a range of scan angles suitable for the material. Then enter the scan time; a longer scan time allows for better resolution. Then press “start”.
Now, let’s compare the results obtained from the single crystal and powder X-ray diffraction of the organo-molybdenum complex.
From the single crystal X-ray data, a structural model of the electron density map is generated, which is used to obtain experimentally determined bond lengths and angles within the structure.
Furthermore, the powder XRD provides additional information about the compound. The flat baseline of the spectrum indicates, that the sample used is highly crystalline, whereas curved baselines are indicative of amorphous materials.
X-ray diffraction is a valuable characterization tool in virtually every field of material science, and therefore plays a role in diverse applications.
A major component of heritage art conservation includes understanding how works of art were produced and why they corrode. Recent developments in X-ray diffraction study corrosion by destructively testing less than 1 mg of sample. Since corrosion products are rarely monocrystalline, powder X-ray diffraction is required. Typical analyses occur at 2θ between 5-85º degrees over 20 hours. The locations of atoms within the crystal may be optimized algorithmically, providing insight into the location and nature of chemical attacks.
Films of material ranging from nanometers to micrometers in thickness have unique protective, electrical, and optical abilities that differ from those of bulk materials. X-ray diffraction provides information on film thickness, density, and surface texture. It is used to determine film stress, and the likelihood of film failure and breakage. It also helps characterize the optical behavior of films, since absorption largely depends on crystal structure. It is therefore used to characterize thin film light sensors and photovoltaic cells.
You’ve just watched JoVE’s introduction to single-crystal and powder X-ray diffraction. You should now be familiar with the principles of X-ray diffractometry, a procedure for obtaining diffraction patterns, and some applications. As always, thanks for watching!