- 00:04Overview
- 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
Difracción de rayos X de monocristal y de polvos
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Overview
Fuente: Tamara M. Powers, Departamento de química, Texas A & M University
Cristalografía de la radiografía es una técnica que utiliza rayos x para estudiar la estructura de las moléculas. Experimentos de difracción de rayos x (DRX) se llevan a cabo habitualmente con muestras de solo-cristal o en polvo.
DRX de la solo-cristal:
Solo-cristal XRD permite estructura absoluta determinación. Con los datos XRD del solo-cristal, las posiciones atómicas pueden ser observadas y así longitudes en enlace y ángulos pueden ser determinados. Esta técnica proporciona la estructura dentro de un solo cristal, que no necesariamente representa la mayor parte del material. Por lo tanto, los métodos de caracterización adicional a granel deben ser utilizados para probar la identidad y la pureza de un compuesto.
Polvo de DRX:
A diferencia de DRX de monocristal, polvo DRX mira una gran muestra de material policristalino y por lo tanto se considera una técnica de caracterización a granel. El patrón de polvo se considera una “huella digital” para un determinado material; proporciona información acerca de la fase (polimorfo) y cristalinidad del material. Normalmente, se utiliza polvo de DRX para estudiar minerales, zeolitas, metal-organic frameworks (MOF) y otros sólidos extendidos. DRX en polvo puede utilizarse también para establecer la pureza de la mayor parte de especies moleculares.
Anteriormente, hemos visto cómo hacer crecer cristales de calidad de rayos x (ver video en serie de fundamentos de química orgánica ). Aquí aprenderemos los principios detrás de DRX. Luego recogeremos datos polvo y monocristal en Mo2(ArNC(H)NAr)4, donde Ar = pMeOC –6H5.
Principles
¿Por qué rayos x?:
Medición de distancia, es importante seleccionar una unidad de medida que está en la escala del objeto medido. Por ejemplo, para medir la longitud de un lápiz, uno no querría usar un palillo de la yarda que sólo tiene gradaciones de pies. Del mismo modo, si uno quisiera medir la longitud de un coche, sería inadecuado usar una regla de 12 pulgadas con marcas de cm. Por lo tanto, para estudiar los enlaces en las moléculas, es importante utilizar una longitud de onda de la luz que coincide con la longitud de esos bonos. Rayos x tiene longitudes de onda en la gama Å, que coincide perfectamente con las distancias de enlace típicos (1-3 Å).
La celda unidad:
Imagínese tratando de describir todas las moléculas en la punta de un lápiz. Si uno se aproxima a que se compone de 6,02 x 1023 moléculas (o 1 mol), parece casi imposible describir ese objeto en el nivel molecular. Se simplifica la complejidad de un objeto cuando existe como un cristal, donde el contenido de una celda unidad se puede utilizar para describir toda la estructura. La celda unidad de un cristal es el menos volumen que contiene una unidad de repetición de un sólido. Se define como un 3D “caja” con longitudes a, b y c, los ángulos α, β y γ (figura 1). La celda unidad permite químicos describir el contenido de un cristal con una fracción de o un pequeño número de átomos o molécula (s). Mediante la repetición de la celda unidad en el espacio, uno puede generar una representación 3D del sólido.
Figura 1. Parámetros de celda unidad.
Disposición experimental:
Solo cristal y polvo DRX tienen configuraciones similares de instrumentación. Para DRX de la solo-cristal, un cristal es montado y centrado en el haz de rayos x. Polvo de DRX, una muestra policristalina es molida en un polvo fino y montada sobre una placa. La muestra (simple o policristalino) es irradiada con rayos x y difracción de rayos x golpea un detector. Durante la recolección de datos, la muestra se gira respecto a la fuente de rayos x y el detector.
Experimento de doble rendija:
Recordemos que la luz tiene ambas propiedades de forma de onda y partícula. Cuando la luz monocromática entra en dos ranuras, la propiedad de onda como de luz da luz que emana de forma esférica de cada hendidura. Cuando interactúan las ondas, pueden sumar (si las ondas tienen la misma longitud de onda y fase) o cancelar uno otro hacia fuera (si las ondas tienen la misma longitud de onda, pero tienen diferentes fases), que se llama interferencia constructiva y destructiva, respectivamente. El patrón de luz resultante se hace de una serie de líneas, donde las zonas claras representan interferencia constructiva mientras que las áreas oscuras son el resultado de interferencia destructiva.
Patrones de difracción típico: Solo-cristal Versus polvo:
Sobre la irradiación de un cristal por rayos x, la radiación es difractada en interacción con la densidad de electrones dentro del cristal. Al igual que agua las ondas en el clásico experimento de doble rendija de la física, los difracción rayos x interactúan, dando lugar a interferencias constructivas y destructivas. En el DRX, el patrón de difracción representa la densidad del electrón debido a átomos y enlaces en el cristal. (Figura 2) muestra un patrón de difracción típico para un solo cristal. Observe que el patrón de difracción se compone de puntos en lugar de líneas como en el experimento de doble rendija. De hecho, estos “puntos” son rebanadas 2D de esferas de 3 dimensiones. Cristalógrafos utilizan un programa de computadora para integrar los puntos resultantes para determinar la forma y la intensidad de los rayos x difractados. En una muestra de polvo, los rayos x interactúan con muchos cristales pequeños en orientaciones al azar. Por lo tanto, en lugar de ver manchas, un patrón de difracción circular se observa (figura 3). Las intensidades de los círculos de difracción se trazan entonces contra los ángulos entre el anillo el eje del haz (denota 2θ) para dar una trama dimensional 2 conocida como patrón de polvo.
Aquí recopilaremos datos de DRX de cristal y polvo único Mo2(ArNC(H)NAr)4 donde Ar = pMeOC –6H5, que se sintetizan en el módulo “preparación y caracterización de un Quadruply Metal – Metal En condiciones de servidumbre compuesto.”
Figura 2. Patrón de difracción de monocristal.
Figura 3. Polvo de DRX: Patrón de difracción Circular.
Procedure
Results
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.
Transcript
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!