Agradecemos a Robert Berger (Philipps-Universität Marburg, Alemania) para inspirar los debates sobre la interpretación de nuestros datos y quiralidad molecular en general. Agradecemos a Julia Kiedrowski, Alexander Schießer y Michael Reggelin de TU Darmstadt (Alemania), así como Benjamin Spenger, Manuel Mazenauer y Jürgen Stohner de ZHAW Wädenswil (Suiza) para proporcionar la muestra.
El proyecto fue apoyado por la iniciativa del estado de Hessen para la ciencia y la excelencia económica bajo el enfoque ELCH (dinámica de sistemas quirales de electrones) y el Ministerio Federal de educación e investigación (BMBF). MS reconoce el apoyo financiero de la Fundación de Adolf Messer.

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H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Imaging+the+Absolute+Configuration+of+a+Chiral+Epoxide+in+the+Gas+Phase.">Imaging the Absolute Configuration of a Chiral Epoxide in the Gas Phase.</a> Science. 342 (6162), 1084-1186 (2013).</li><li>D&#246;rner, R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cold+Target+Recoil+Ion+Momentum+Spectroscopy:+a+'momentum+microscope'+to+view+atomic+collision+dynamics.">Cold Target Recoil Ion Momentum Spectroscopy: a 'momentum microscope' to view atomic collision dynamics.</a> Physics Reports. 330, 95-192 (2000).</li><li>Ullrich, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Recoil-ion+and+electron+momentum+spectroscopy:+reaction-microscopes.">Recoil-ion and electron momentum spectroscopy: reaction-microscopes.</a> Rep. 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A. 667, 11-15 (2012).</li><li>Kastirke, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Konstruktion und Aufbau einer UHV-tauglichen COLTRIMS-Kammer. , (2014).</li><li>Calegari, F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Charge+migration+induced+by+attosecond+pulses+in+bio-relevant+molecules.">Charge migration induced by attosecond pulses in bio-relevant molecules.</a> J. Phys. B.: At. Mol. Opt. Phys. 49 (14), 142001 (2016).</li><li>Gaie-Levrel, F., Garcia, G. 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Debido a la variedad de componentes, una instalación COLTRIMS requiere un nivel bastante alto de conocimientos técnicos, especialmente en las áreas de técnica de vacío, detección de partículas, análisis rápido de datos y electrónica. Antes de dedicarse a la investigación de especies complejas, se debe así ser examinado si la instalación funciona correctamente, por ejemplo, realizando y analizando una medición sobre una especie diatómica o triatómica.
Optimización de l...

Quiralidad es una característica de nuestra naturaleza que ha sido fascinante investigadores de más de 150 años. En el 19 demayo siglo, Pasteur, van ' t Hoff y otros descubrieron que las moléculas pueden ocurrir en dos estructuras de la imagen del espejo que no son imponibles en super - como izquierda y manos derechas. Esta propiedad se denomina quiral, de la palabra griega para 'mano'.
Hasta ahora, se ha encontrado ninguna diferencia en propiedades termodinámicas o niveles de energía de las formas de mano izquierda y derecha (los dos "enantiómeros"). Para analizar el uso de las manos de una determinada muestra y para separar los enantiómeros, interacción con otras moléculas quirales puede utilizarse, como por ejemplo se hace en diversos enfoques cromatográficas. 1 métodos de Chiroptical como dicroísmo circular (vibracional), CD (V) y la dispersión rotativa óptica, ORD, habitualmente se emplean para distinguir entre enantiómeros. 2
En cuanto a la determinación de la estructura microscópica, estas técnicas requieren información adicional, por ejemplo de los cálculos del quántum-producto químico. La única técnica que es ampliamente aceptada para determinar directamente la configuración absoluta es anómala difracción de rayos x. 3
Recientemente se ha demostrado que se puede determinar la configuración absoluta de especies quirales simple por proyección de imagen de explosión de Coulomb. 4 , 5 en este enfoque, las moléculas en la fase gaseosa se multiplican ionizado para que los restantes núcleos fuertemente repelen uno al otro. Esta repulsión lleva a la fragmentación rápida ('explosión') de las moléculas. La dirección y la magnitud de la correlación de ímpetus de fragmento a la estructura de la molécula-para moléculas pequeñas, las indicaciones de impulso corresponden asombrosamente a los ejes de enlace. Explosión de Coulomb para la determinación de la estructura molecular pionera utilizando haces de iones moleculares de un acelerador. 6 esta técnica de papel Haz recientemente ha sido aplicado para el reconocimiento quiral. 7
Contrariamente a la difracción anómala de rayos x, la muestra no debe ser cristalina pero en la fase de gas. Esto hace que el enfoque de la explosión de Coulomb ideal para especies volátiles y por lo tanto complementario de difracción de rayos x. En ciertos casos, incluso puede determinar el uso de las manos para las moléculas individuales.
En la práctica, la reconstrucción exacta de la estructura molecular se ha demostrado difícil incluso para los derivados del metano, por ejemplo, moléculas con un carbono central y diversos sustitutos. Esto se atribuye al hecho de que la interacción entre los fragmentos no es exactamente Coulombic y que no todos los bonos rompen al mismo tiempo. Para obtener información de stereochemical, especialmente para diferenciar los enantiómeros, esta reconstrucción no es afortunadamente necesaria. En cambio, los vectores del impulso de diferentes fragmentos pueden correlacionarse para producir una cantidad que es distinta para las moléculas de mano izquierda y derecha. Para obtener resultados confiables, por lo menos cuatro ímpetus de fragmento tienen a grabar.
Para medir esta información de impulso, los fragmentos de una – y sólo una – molecular ruptura tengan que ser detectados en el paso de una sola medición. Esta condición se conoce generalmente como 'detección coincidente'. Además, las direcciones de emisión debe ser analizado, que asciende en la práctica para registrar el tiempo y la posición del fragmento de impacto en un formato de datos de modo de lista.
En física atómica y molecular, se han desarrollado técnicas que aplicar este enfoque de medición empleando electrostáticas espectrómetros de masa separación y tiempo-posición-sensibles y detectores multi-hit. El ejemplo más prominente es la configuración COLTRIMS (frío destino retroceso Ion ímpetu espectroscopia) – también conocido como microscopio de la reacción. 8 , 9 un croquis para este tipo de experimento se dan en la figura 1. Contrario a una norma COLTRIMS que puede grabar electrones así, imágenes de explosión de Coulomb requiere sólo el detector de iones.
Detector y espectrómetro se montan en ultra alto vacío (&#60; 1 x 10-9 hPa) para evitar la creación de iones del gas residual. Las moléculas individuales de la muestra se proporcionan a través de un jet molecular libre gaseoso creado por expansión supersónica: en virtud de la presión de vapor, las moléculas de expansión a través de una boquilla pequeña (alrededor de 50 μm de diámetro) en el vacío. Esta parte del experimento, la cámara de origen, se separa de la región de interacción por etapas diferencialmente bombeadas y generalmente dos skimmers. Un adicional bombeada diferencialmente sección está situado detrás de la región de interacción para el jet del gas de la descarga y así evitar gas de fondo en la región de interacción.
La radiación ionizante se intersecta con el jet molecular menores de 90°. Mayoría de los laboratorios hoy en día utiliza pulsos de láser de femtosegundo, aunque la radiación sincrotrón, iones rápidos o impacto del electrón son posibles 'proyectiles' para inducir la explosión de Coulomb.
El siguiente protocolo hace la asunción que una configuración de ejecución para imágenes coincidentes de iones y un láser de femtosegundo son disponibles en el laboratorio. La intensidad de pico es necesaria para inducir la explosión de Coulomb en cuatro o incluso cinco fragmentos debe ser del orden de 6 x 1014 W/cm2. Para evitar medidas excesivamente largas, la tasa de repetición del láser debe ser 10 kHz o más. Esto es crucial porque, por un lado, detección coincidente puede sólo determinar si la probabilidad de fragmentación en el foco del láser está significativamente por debajo de 1 por cada pulso de láser (idealmente no más de 10%). La tasa de fragmentación total, por el contrario, no debe ser inferior a unos pocos kHz porque el porcentaje de las vías multifragmentation es generalmente menos de 10-4. Hecho tan alentadora, cabe mencionar que en principio ya es suficiente para identificar la configuración de una muestra de carbonil un evento de fragmentación solo, y que la detección de unos pocos cientos permite determinar la abundancia de los enantiómeros en un muestra de la composición enantiomérica desconocida.

<table><tbody><tr><td>CHBrCl2</td><td>SigmaAldrich</td><td>139181-10G</td><td>or other suitable sample</td></tr><tr><td>femtosecond laser system</td><td>KMLabs</td><td>Wyvern500</td><td></td></tr><tr><td>High-reflective mirrors</td><td>EKSMA</td><td>042-0800</td><td></td></tr><tr><td>mirror mounts</td><td>Newport</td><td>U100-A-LH-2K&amp;nbsp;&amp;nbsp;</td><td></td></tr><tr><td>focusing mirror (protected silver, f = 75 mm)</td><td>Thorlabs&amp;nbsp;</td><td>CM254-075-P01</td><td>(if available: f = 60 mm)</td></tr><tr><td>COLTRIMS spectrometer, including electronics and data acquisition system</td><td>RoentDek</td><td>custom</td><td>contrary to the standard COLTRIMS, only one detector is needed</td></tr></tbody></table>

coulomb explosion imaging as a tool to distinguish between stereoisomers

Este artículo muestra cómo el COLTRIMS (frío destino retroceso Ion ímpetu espectroscopia) o la técnica del "microscopio de la reacción" puede utilizarse para distinguir entre enantiómeros (estereoisómeros) de especies quirales simple en el nivel de las moléculas individuales. En este enfoque, un jet gaseoso molecular de la muestra se expande en una cámara de vacío y cruza con pulsos de láser de femtosegundo (fs). La alta intensidad de los impulsos conduce a rápida ionización múltiple, encendiendo una llamada explosión de Coulomb que produce varios fragmentos (cargados positivamente) catiónicos. Un campo electrostático guías de estos cationes en detectores sensibles a tiempo y posición. Similar a un espectrómetro de masas de tiempo de vuelo, la hora de llegada de cada ion aporta información sobre su masa. Como un superávit, el campo electrostático se ajusta de manera que la dirección de emisión y la energía cinética después de la fragmentación conducen a variaciones en el tiempo de vuelo y en la localización del impacto en el detector.
Cada impacto ion crea una señal electrónica en el detector; Esta señal es tratada por electrónica de alta frecuencia y grabada por evento por una computadora. Los datos registrados corresponden a las posiciones y tiempos de impacto. Con estos datos, se pueden calcular la energía y la dirección de emisión de cada fragmento. Estos valores están relacionados con las propiedades estructurales de la molécula bajo investigación, es decir, a las longitudes de enlace y las posiciones relativas de los átomos, lo que permite para determinar a la molécula por molécula el uso de las manos de simple especies quirales y otras características isoméricos.

En esta parte, mostramos los resultados obtenidos para halometanos. Estas especies son ideales para los experimentos de la prueba de principio debido a su simplicidad y alta presión de vapor. Mientras tanto, se ha investigado el halotano de especies más compleja usando solo suave radiografía fotones de una fuente del sincrotrón para inducir ionización múltiple. 14
CHBrClF
<p class="jove_content"...

Watch this Scientific Journal Video about Coulomb Explosion Imaging as a Tool to Distinguish Between Stereoisomers at JoVE.com

Coulomb Explosion Imaging as a Tool to Distinguish Between Stereoisomers

Imágenes de explosión de Coulomb como herramienta para distinguir entre los estereoisómeros

Para pequeñas especies quirales, imágenes de explosión de Coulomb proporciona un nuevo enfoque para determinar el uso de las manos de moléculas individuales.

This article shows how the COLTRIMS (Cold Target Recoil Ion Momentum Spectroscopy) or the "reaction microscope" technique can be used to distinguish ...

coulomb-explosion-imaging-as-tool-to-distinguish-between

Education

Research

JoVE Core

JoVE Journal

Organic Chemistry

Chemistry

Unit 1

Transition Metals and Coordination Complexes

Stereoisomerism

SCIENTISTS IN ACTION

9.0K vistas.  Goethe-Universität Frankfurt am Main. Para pequeñas especies quirales, imágenes de explosión de Coulomb proporciona un nuevo enfoque para determinar el uso de las manos de moléculas individuales.

Video: Coulomb Explosion Imaging as a Tool to Distinguish Between Stereoisomers

Spatial Separation of Molecular Conformers and Clusters

Gas-phase molecular physics and physical chemistry experiments commonly use supersonic expansions through pulsed valves for the production of cold molecular beams. However, these beams often contain multiple conformers and clusters, even at low rotational temperatures. We present an experimental methodology that allows the spatial separation of these constituent parts of a molecular beam expansion. Using an electric deflector the beam is separated by its mass-to-dipole moment ratio, analogous to a bender or an electric sector mass spectrometer spatially dispersing charged molecules on the basis of their mass-to-charge ratio. This deflector exploits the Stark effect in an inhomogeneous electric field and allows the separation of individual species of polar neutral molecules and clusters. It furthermore allows the selection of the coldest part of a molecular beam, as low-energy rotational quantum states generally experience the largest deflection. Different structural isomers (conformers) of a species can be separated due to the different arrangement of functional groups, which leads to distinct dipole moments. These are exploited by the electrostatic deflector for the production of a conformationally pure sample from a molecular beam. Similarly, specific cluster stoichiometries can be selected, as the mass and dipole moment of a given cluster depends on the degree of solvation around the parent molecule. This allows experiments on specific cluster sizes and structures, enabling the systematic study of solvation of neutral molecules.

We present a technique that allows the spatial separation of different conformers or clusters present in a molecular beam. An electrostatic deflector is used to separate species by their mass-to-dipole moment ratio, leading to the production of gas-phase ensembles of a single conformer or cluster stoichiometry.

Separación espacial de conformadores moleculares y cúmulos

Gas-phase molecular physics and physical chemistry experiments commonly use supersonic expansions through pulsed valves for the production of cold ...

Investigación

Ingeniería

Direct Imaging of Laser-driven Ultrafast Molecular Rotation

We present a method for visualizing laser-induced, ultrafast molecular rotational wave packet dynamics. We have developed a new 2-dimensional Coulomb explosion imaging setup in which a hitherto-impractical camera angle is realized. In our imaging technique, diatomic molecules are irradiated with a circularly polarized strong laser pulse. The ejected atomic ions are accelerated perpendicularly to the laser propagation. The ions lying in the laser polarization plane are selected through the use of a mechanical slit and imaged with a high-throughput, 2-dimensional detector installed parallel to the polarization plane. Because a circularly polarized (isotropic) Coulomb exploding pulse is used, the observed angular distribution of the ejected ions directly corresponds to the squared rotational wave function at the time of the pulse irradiation. To create a real-time movie of molecular rotation, the present imaging technique is combined with a femtosecond pump-probe optical setup in which the pump pulses create unidirectionally rotating molecular ensembles. Due to the high image throughput of our detection system, the pump-probe experimental condition can be easily optimized by monitoring a real-time snapshot. As a result, the quality of the observed movie is sufficiently high for visualizing the detailed wave nature of motion. We also note that the present technique can be implemented in existing standard ion imaging setups, offering a new camera angle or viewpoint for the molecular systems without the need for extensive modification.

We present a protocol for creating a real-time movie of a molecular rotational wave packet using a high-resolution Coulomb explosion imaging setup.

Direct Imaging del impulsada por el láser ultrarrápido Molecular Rotación

We present a method for visualizing laser-induced, ultrafast molecular rotational wave packet dynamics. We have developed a new 2-dimensional Coulomb ...

Química

Photoelectron Imaging of Anions Illustrated by 310 Nm Detachment of F&#8722;

Anion photoelectron imaging is a very efficient method for the study of energy states of bound negative ions, neutral species and interactions of unbound electrons with neutral molecules/atoms. State-of-the-art in vacuo anion generation techniques allow application to a broad range of atomic, molecular, and cluster anion systems. These are separated and selected using time-of-flight mass spectrometry. Electrons are removed by linearly polarized photons (photo detachment) using table-top laser sources which provide ready access to excitation energies from the infra-red to the near ultraviolet. Detecting the photoelectrons with a velocity mapped imaging lens and position sensitive detector means that, in principle, every photoelectron reaches the detector and the detection efficiency is uniform for all kinetic energies. Photoelectron spectra extracted from the images via mathematical reconstruction using an inverse Abel transformation reveal details of the anion internal energy state distribution and the resultant neutral energy states. At low electron kinetic energy, typical resolution is sufficient to reveal energy level differences on the order of a few millielectron-volts, i.e., different vibrational levels for molecular species or spin-orbit splitting in atoms. Photoelectron angular distributions extracted from the inverse Abel transformation represent the signatures of the bound electron orbital, allowing more detailed probing of electronic structure. The spectra and angular distributions also encode details of the interactions between the outgoing electron and the residual neutral species subsequent to excitation. The technique is illustrated by the application to an atomic anion (F&#8722;), but it can also be applied to the measurement of molecular anion spectroscopy, the study of low lying anion resonances (as an alternative to scattering experiments) and femtosecond (fs) time resolved studies of the dynamic evolution of anions.

Here, we present a protocol for photoelectron imaging of anionic species. Anions generated in vacuo and separated by mass spectrometry are probed using velocity mapped photoelectron imaging, providing details of anion and neutral energy levels, anion and neutral structure and the nature of the anion electronic state.

La proyección de imagen del fotoelectrón de aniones ilustrado por 310 Nm la separación de F−

Anion photoelectron imaging is a very efficient method for the study of energy states of bound negative ions, neutral species and interactions of unbound ...

Direct Comparison of Hyperspectral Stimulated Raman Scattering and Coherent Anti-Stokes Raman Scattering Microscopy for Chemical Imaging

Stimulated Raman scattering (SRS) and coherent anti-Stokes Raman scattering (CARS) microscopy are the most widely used coherent Raman scattering imaging technologies. Hyperspectral SRS and CARS imaging offer Raman spectral information at every pixel, which enables better separation of different chemical compositions. Although both techniques require two excitation lasers, their signal detection schemes and spectral properties are quite different. The goal of this protocol is to perform both hyperspectral SRS and CARS imaging on a single platform and compare the two microscopy techniques for imaging different biological samples. The spectral focusing method is employed to acquire spectral information using femtosecond lasers. By using standard chemical samples, the sensitivity, spatial resolution, and spectral resolution of SRS and CARS in the same excitation conditions (i.e., power at the sample, pixel dwell time, objective lens, pulse energy) are compared. The imaging contrasts of CARS and SRS for biological samples are juxtaposed and compared. The direct comparison of CARS and SRS performances would allow for optimal selection of the modality for chemical imaging.

This paper directly compares the resolution, sensitivity, and imaging contrasts of stimulated Raman scattering (SRS) and coherent anti-Stokes Raman scattering (CARS) integrated into the same microscope platform. The results show that CARS has a better spatial resolution, SRS gives better contrasts and spectral resolution, and both methods have similar sensitivity.

Comparación directa de la dispersión Raman estimulada hiperespectral y la microscopía de dispersión Raman antiespecial coherente para imágenes químicas

Stimulated Raman scattering (SRS) and coherent anti-Stokes Raman scattering (CARS) microscopy are the most widely used coherent Raman scattering imaging ...

Differential Imaging of Biological Structures with Doubly-resonant Coherent Anti-stokes Raman Scattering (CARS)

Coherent Raman imaging techniques have seen a dramatic increase in activity over the past decade due to their promise to enable label-free optical imaging with high molecular specificity 1. The sensitivity of these techniques, however, is many orders of magnitude weaker than fluorescence, requiring milli-molar molecular concentrations 1,2. Here, we describe a technique that can enable the detection of weak or low concentrations of Raman-active molecules by amplifying their signal with that obtained from strong or abundant Raman scatterers. The interaction of short pulsed lasers in a biological sample generates a variety of coherent Raman scattering signals, each of which carry unique chemical information about the sample. Typically, only one of these signals, e.g. Coherent Anti-stokes Raman scattering (CARS), is used to generate an image while the others are discarded. However, when these other signals, including 3-color CARS and four-wave mixing (FWM), are collected and compared to the CARS signal, otherwise difficult to detect information can be extracted 3. For example, doubly-resonant CARS (DR-CARS) is the result of the constructive interference between two resonant signals 4. We demonstrate how tuning of the three lasers required to produce DR-CARS signals to the 2845 cm-1 CH stretch vibration in lipids and the 2120 cm-1 CD stretching vibration of a deuterated molecule (e.g. deuterated sugars, fatty acids, etc.) can be utilized to probe both Raman resonances simultaneously. Under these conditions, in addition to CARS signals from each resonance, a combined DR-CARS signal probing both is also generated. We demonstrate how detecting the difference between the DR-CARS signal and the amplifying signal from an abundant molecule's vibration can be used to enhance the sensitivity for the weaker signal. We further demonstrate that this approach even extends to applications where both signals are generated from different molecules, such that e.g. using the strong Raman signal of a solvent can enhance the weak Raman signal of a dilute solute.

Differential Imaging of Biological Structures with Doubly-resonant Coherent Anti-stokes Raman Scattering CARS

A combination of three single wavelength short-pulsed lasers is used to generate coherent anti-Stokes Raman scattering (CARS) and doubly-resonant CARS (DR-CARS). The difference between these signals provides enhanced sensitivity for otherwise difficult to detect coherent Raman signals, enabling imaging of weak Raman scatterers.

Diferencial de imágenes de las estructuras biológicas con doblemente resonante coherente anti-Stokes Raman Scattering (CARS)

Coherent Raman imaging techniques have seen a dramatic increase in activity over the past decade due to their promise to enable label-free optical imaging ...

Biología

Uso de un microscopio VSFG para análisis químicos multimodal, rápido e hiperespectral

Multimodal Nonlinear Hyperspectral Chemical Imaging Using Line-Scanning Vibrational Sum-Frequency Generation Microscopy

Vibrational sum-frequency generation (VSFG), a second-order nonlinear optical signal, has traditionally been used to study molecules at interfaces as a spectroscopy technique with a spatial resolution of ~100 &#181;m. However, the spectroscopy is not sensitive to the heterogeneity of a sample. To study mesoscopically heterogeneous samples, we, along with others, pushed the resolution limit of VSFG spectroscopy down to ~1 &#181;m level and constructed the VSFG microscope. This imaging technique not only can resolve sample morphologies through imaging, but also record a broadband VSFG spectrum at every pixel of the images. Being a second-order nonlinear optical technique, its selection rule enables the visualization of non-centrosymmetric or chiral self-assembled structures commonly found in biology, materials science, and bioengineering, among others. In this article, the audience will be guided through an inverted transmission design that allows for imaging unfixed samples. This work also showcases that VSFG microscopy can resolve chemical-specific geometric information of individual self-assembled sheets by combining it with a neural network function solver. Lastly, the images obtained under brightfield, SHG, and VSFG configurations of various samples briefly discuss the unique information revealed by VSFG imaging.

Autor destacado: Desvelando el potencial de la microscopía VSFG en el estudio de estructuras autoensambladas mesoscópicamente heterogéneas 

A multimodal, rapid hyperspectral imaging framework was developed to obtain broadband vibrational sum-frequency generation (VSFG) images, along with brightfield, second harmonic generation (SHG) imaging modalities. Due to the infrared frequency being resonant with molecular vibrations, microscopic structural and mesoscopic morphology knowledge is revealed of symmetry-allowed samples.

Imágenes químicas hiperespectrales no lineales multimodales mediante microscopía de generación de suma de frecuencia vibracional de barrido lineal

Vibrational sum-frequency generation (VSFG), a second-order nonlinear optical signal, has traditionally been used to study molecules at interfaces as a ...

Stereoisomers

On the basis of mirror symmetry, stereoisomers of an organic molecule can be further classified into diastereomers and enantiomers. Diastereomers are stereoisomers that are not mirror images of each other. Substituted alkenes, such as the cis and trans isomers of 2-butene, are diastereomers, as these molecules exhibit different spatial orientations of their constituent atoms, are not mirror images of each other,&#160;and do not interconvert. Here, the interconversion is suppressed due to restricted rotation around the &#960; bond. Another class of diastereomers &#8212; those without &#960; bonds &#8212; are molecules that have non-superposable spatial orientations and exhibit different configurations of their substituent groups at some, but not all, stereocenters. For example, cis-1,2-dimethylcyclohexane and trans-1,2-dimethylcyclohexane are diastereomers, as these molecules are not superposable and have a different configuration of the methyl and hydrogen groups at only one out of their two stereocenters.
Enantiomers are stereoisomers that are mirror images of each other. As only chiral molecules can have non-superposable mirror images, enantiomers are chiral molecules. For example, the chiral molecule 2-butanol and its mirror image are enantiomers, as these molecules exhibit non-superposable spatial orientations of their constituent atoms and are mirror images of each other. A chiral molecule and its mirror image are collectively referred to as an enantiomeric pair, or a pair of enantiomers. The enantiomer of a chiral molecule can be drawn by taking its mirror image from any position or&#160;exchanging the positions of two of the substituents at each stereocenter of the molecule.

Stereoisomers: Diastereomers and Enantiomers

Estereoisomería

On the basis of mirror symmetry, stereoisomers of an organic molecule can be further classified into diastereomers and enantiomers. Diastereomers are ...

Educación

Química Orgánica

Unidad 1

TÉRMINOS Y CONCEPTOS CLAVES

<h3>Isomerism in Complexes</h3>
Isomers are different chemical species that have the same chemical formula.
Transition metal complexes often exist as geometric isomers, in which the same atoms are connected through the same types of bonds but with differences in their orientation in space. Coordination complexes with two different ligands in the cis and trans positions from a ligand of interest form isomers. For example, the octahedral [Co(NH3)4Cl2]+ ion has two isomers (Figure 1) In the cis configuration, the two chloride ligands are adjacent to each other. The other isomer, the trans configuration, has the two chloride ligands directly across from one another.
<img alt="Image1" src="/files/ftp_upload/11459/11459_Image1.svg" style="text-align: center; height: 200px;" />
Figure 1. The cis and trans isomers of [Co(H2O)4Cl2]+ contain the same ligands attached to the same metal ion, but the spatial arrangement causes these two compounds to have very different properties.
Different geometric isomers of a substance are different chemical compounds. They exhibit different properties, even though they have the same formula. For example, the two isomers of [Co(NH3)4Cl2]NO3 differ in color; the cis form is violet, and the trans form is green. Furthermore, these isomers have different dipole moments, solubilities, and reactivities. As an example of how the arrangement in space can influence the molecular properties, consider the polarity of the two [Co(NH3)4Cl2]NO3 isomers. Remember that the polarity of a molecule or ion is determined by the bond dipoles (which are due to the difference in electronegativity of the bonding atoms) and their arrangement in space. In one isomer, cis chloride ligands cause more electron density on one side of the molecule than on the other, making it polar. For the trans isomer, each ligand is directly across from an identical ligand, so the bond dipoles cancel out, and the molecule is nonpolar.
Another important type of isomers are optical isomers, or enantiomers, in which two objects are exact mirror images of each other but cannot be lined up so that all parts match. This means that optical isomers are non-superposable mirror images. A classic example of this is a pair of hands, in which the right and left hand are mirror images of one another but cannot be superposed. Optical isomers are very important in organic and biochemistry because living systems often incorporate one specific optical isomer and not the other. Unlike geometric isomers, pairs of optical isomers have nearly identical properties (boiling point, polarity, solubility, etc.). Optical isomers differ only in the way they affect polarized light and how they react with other optical isomers. For coordination complexes, many coordination compounds such as [M(en)3]n+ [in which Mn+ is a central metal ion such as iron(III) or cobalt(II)] form enantiomers, as shown in Figure 2. These two isomers will react differently with other optical isomers. For example, DNA helices are optical isomers, and the form that occurs in nature (right-handed DNA) will bind to only one isomer of [M(en)3]n+ and not the other.
<img alt="Image2" src="/files/ftp_upload/11459/11459_Image_2.svg" style="text-align: center; height: 200px;" />
Figure 2. The complex [M(en)3]n+ (Mn+ = a metal ion, en = ethylenediamine) has a non-superposable mirror image.
The [Co(en)2Cl2]+ ion exhibits geometric isomerism (cis/trans), and its cis isomer exists as a pair of optical isomers (Figure 3).
<img alt="Image3" src="/files/ftp_upload/11459/11459_Image_3.svg" style="text-align: center; height: 200px;" />
Figure 3. Three isomeric forms of [Co(en)2Cl2]+ exist. The trans isomer, formed when the chlorines are positioned at a 180&#176; angle, has very different properties from the cis isomers. The mirror images of the cis isomer form a pair of optical isomers, which have identical behavior except when reacting with other enantiomers.
This text is adapted from <a href="https://openstax.org/books/chemistry-2e/pages/19-2-coordination-chemistry-of-transition-metals">Openstax, Chemistry 2e, Chapter 19.2 Coordination Chemistry of Transition Metals.</a>

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Estereoisomerismo

Isomerism in Complexes
Isomers are different chemical species that have the same chemical formula.
Transition metal complexes often exist as geometric ...

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