Ringraziamo Robert Berger (Philipps-Universität Marburg, Germania) per ispirare le discussioni circa l'interpretazione dei nostri dati e chiralità molecolare in generale. Siamo grati a Julia Kiedrowski, Alexander Schießer e Michael Reggelin da TU Darmstadt (Germania), così come Benjamin Spenger, Manuel Mazenauer e Jürgen Stohner da ZHAW Wädenswil (Svizzera) per la fornitura del campione.
Il progetto è stato sostenuto dall'iniziativa di uno stato di Hessen per scientifica ed eccellenza economica sotto il fuoco ELCH (Electron dynamics dei sistemi chirali) e il Ministero federale dell'istruzione e ricerca (BMBF). MS riconosce il sostegno finanziario della Fondazione Adolf Messer.

<ol>
	<li>G&#252;bitz, G., Schmid, M. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chiral+Separation+by+Chromatographic+and+Electromigration+Techniques.+A+Review.">Chiral Separation by Chromatographic and Electromigration Techniques. A Review.</a> Biopharm. Drug Disposition. 22, 291-336 (2001).</li><li>Berova, N., Polaravapu, P. L., Nakanishi, K., Woody, R. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Comprehensive Chiroptical Spectroscopy. , (2012).</li><li>Bijvoet, J. M., Peerdeman, A. F., van Bommel, A. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Determination+of+the+Absolute+Configuration+of+Optically+Active+Compounds+by+means+of+X-rays.">Determination of the Absolute Configuration of Optically Active Compounds by means of X-rays.</a> Nature. 168 (4268), 271-272 (1951).</li><li>Pitzer, M., 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=Direct+Determination+of+Absolute+Molecular+Stereochemistry+in+Gas+Phase+by+Coulomb+Explosion+Imaging.">Direct Determination of Absolute Molecular Stereochemistry in Gas Phase by Coulomb Explosion Imaging.</a> Science. 341 (6150), 1096-1100 (2013).</li><li>Pitzer, M., 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=Absolute+Configuration+from+Different+Multifragmentation+Pathways+in+Light-Induced+Coulomb+Explosion+Imaging.">Absolute Configuration from Different Multifragmentation Pathways in Light-Induced Coulomb Explosion Imaging.</a> Chem Phys Chem. 17 (16), 2465-2472 (2016).</li><li>Vager, Z., Naaman, R., Kanter, E. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Coulomb+Explosion+Imaging+of+small+molecules.">Coulomb Explosion Imaging of small molecules.</a> Science. 244 (4903), 426-431 (1989).</li><li>Herwig, P. 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. Prog. Phys. 66 (9), 1463-1545 (2003).</li><li>Jagutzki, O., 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=Multiple+Hit+Readout+of+a+Microchannel+Plate+Detector+With+a+Three-Layer+Delay-Line+Anode.">Multiple Hit Readout of a Microchannel Plate Detector With a Three-Layer Delay-Line Anode.</a> IEEE Trans Nucl Sci. 49 (5), 2477-2483 (2002).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> RoentDek GmbH MCP Delay Line Detector Manual. , (2017).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> RoentDek GmbH The RoentDek Constant Fraction Discriminators CFD8c, CFD7x, CFD4c, CFD1c and CFD1x. , (2017).</li><li>Zeller, S., 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+He2+quantum+halo+state+using+a+free+electron+laser.">Imaging the He2 quantum halo state using a free electron laser.</a> PNAS. 113 (51), 14651-14655 (2016).</li><li>Pitzer, M., 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=Stereochemical+configuration+and+selective+excitation+of+the+chiral+molecule+halothane.">Stereochemical configuration and selective excitation of the chiral molecule halothane.</a> J. Phys. B.: At. Mol. Opt. Phys. 49 (23), 234001 (2016).</li><li>Hine, J., Dowell, A. M., Singley, J. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Carbon+Dihalides+as+Intermediates+in+the+Basic+Hydrolysis+of+Haloforms:+IV+Relative+Reactivities+of+Haloforms.">Carbon Dihalides as Intermediates in the Basic Hydrolysis of Haloforms: IV Relative Reactivities of Haloforms.</a> J. Am. Soc. Chem. 78, 479-482 (1956).</li><li>Wales, B., 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=A+coincidence+detection+algorithm+for+improving+detection+rates+in+coulomb+explosion+imaging.">A coincidence detection algorithm for improving detection rates in coulomb explosion imaging.</a> Nucl. Instr. Meth. in Phys. Res. 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. A., Schwell, M., Nahon, L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=VUV+state-selected+photoionization+of+thermally-desorbed+biomolecules+by+coupling+an+aerosol+source+to+an+imaging+photoelectron/photoion+coincidence+spectrometer:+case+of+the+amino+acids+tryptophan+and+phenylalanine.">VUV state-selected photoionization of thermally-desorbed biomolecules by coupling an aerosol source to an imaging photoelectron/photoion coincidence spectrometer: case of the amino acids tryptophan and phenylalanine.</a> Phys. Chem. Chem. Phys. 13, 7024-7036 (2010).</li><li>Ablikim, U., 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=Identification+of+absolute+geometries+of+cis+and+trans+molecular+isomers+by+Coulomb+Explosion+Imaging.">Identification of absolute geometries of cis and trans molecular isomers by Coulomb Explosion Imaging.</a> Sci. Rep. 6, 38202 (2016).</li></ol>

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A causa della varietà di componenti, un programma di installazione COLTRIMS richiede un livello piuttosto elevato di competenza tecnica, soprattutto nei settori della tecnica del vuoto, rivelazione di particelle, veloce elettronica e analisi dei dati. Prima di dedicarsi alla ricerca delle specie complesse, dovrebbe quindi essere accuratamente controllato se l'installazione è in esecuzione correttamente, ad esempio eseguendo e analizzando una misura su una specie di biatomica o triatomica.
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Chiralità è una caratteristica della nostra natura che è stato affascinanti ricercatori per più di 150 anni. Nella 19th century, Pasteur, Van ' t Hoff e altri scoperto che molecole possono accadere in due strutture di immagine speculare che non sono super-imposable - come la nostra mano destra e sinistra. Questa proprietà è stata definita 'chirale', dalla parola greca per 'mano'.
Finora, nessuna differenza in proprietà termodinamiche o nei livelli di energia di destri e forme (i due ' enantiomeri') è stata trovata. Al fine di analizzare la manualità di un dato campione e a separare gli enantiomeri, interazione con altre molecole chirali può essere utilizzato, come avviene ad esempio in vari metodi cromatografici. 1 chiroottiche metodi quali dicroismo circolare (vibrazionale), CD (V) e dispersione rotatoria ottica, ORD, ordinariamente sono impiegati per distinguere tra enantiomeri. 2
Quando si tratta della determinazione della struttura microscopica, queste tecniche richiedono informazioni aggiuntive, ad esempio da calcoli di chimica quantistica. L'unica tecnica che ampiamente è accettato per determinare direttamente la configurazione assoluta è anomala diffrazione di raggi x. 3
Recentemente è stato dimostrato che la configurazione assoluta di specie chirali semplice può essere determinata da Coulomb esplosione Imaging. 4 , 5 in questo approccio, molecole in fase gassosa sono moltiplicare ionizzato affinché i restanti nuclei fortemente respingono ogni altro. Questa repulsione conduce alla frammentazione veloce ('esplosione') delle molecole. La direzione e l'entità del frammento momenta correlato alla struttura della molecola – per piccole molecole, le direzioni di slancio corrispondono sorprendentemente bene per gli assi di legame. Esplosione di Coulomb per determinazione della struttura molecolare è stato sperimentato utilizzando fasci di ioni molecolari da un acceleratore. 6 questa tecnica di lamina fascio ha recentemente stato anche chiesto il riconoscimento chirale. 7
Contrariamente anomala diffrazione di raggi x, il campione non deve essere cristallina ma fornito in fase gassosa. Questo rende l'approccio di Coulomb esplosione ideale per specie volatili e così complementari per diffrazione dei raggi x. In alcuni casi, la manualità può essere determinata anche per singole molecole.
In pratica, la ricostruzione esatta della struttura molecolare ha dimostrato difficile anche per i derivati del metano, ad es. molecole con un carbonio centrale e diversi sostituenti. Ciò è attribuita al fatto che l'interazione tra i frammenti non è esattamente Coulombic e che non tutti i legami rompono simultaneamente. Al fine di ottenere informazioni stereochimiche, soprattutto per distinguere tra enantiomeri, questa ricostruzione non è fortunatamente necessaria. Invece, i vettori di slancio di diversi frammenti possono essere correlati per produrre una quantità che si è distinta per le molecole di mano destra e sinistra. Per ottenere risultati affidabili, momenta frammento almeno quattro deve essere registrate.
Per misurare queste informazioni di slancio, i frammenti da uno – e un solo – disgregazione molecolare devono essere rilevato in un passaggio di misurazione singola. Questa condizione è definita solitamente come 'coincidente rilevamento'. Inoltre, le indicazioni di emissione devono essere analizzati, che gli importi in pratica per registrare il tempo e la posizione del frammento impatto in un formato di dati modalità elenco.
In fisica atomica e molecolare, tecniche sono state sviluppate che implementano questo approccio di misurazione impiegando elettrostatiche spettrometri di massa per massa rivelatori multi hit separazione e tempo - e posizione-sensibili. L'esempio più prominente è l'installazione COLTRIMS (freddo Target rinculo Ion Momentum Spectroscopy) – noto anche come reazione microscopio. 8 , 9 uno schizzo per questo tipo di esperimento è dato in Figura 1. Contrariamente a un COLTRIMS standard che può registrare anche gli elettroni, Coulomb esplosione Imaging richiede solo il rivelatore di ioni.
Spettrometro e rilevatore sono montati sotto vuoto ultraelevato (&#60; 1 x 10-9 hPa) per evitare la creazione di ioni da gas residuo. Singole molecole del campione sono fornite tramite un getto molecolare gratis gassoso creato da espansione supersonica: in virtù della pressione del vapore, le molecole espandono attraverso un piccolo ugello (circa 50 micron di diametro) nel vuoto. Questa parte dell'esperimento, la camera di origine, è separata dalla regione di interazione di solito due skimmer e fasi differenzialmente pompati. Un ulteriore pompata differenzialmente sezione si trova dietro la regione di interazione per il getto del gas di discarica e quindi evitare gas di fondo della regione di interazione.
Le radiazioni ionizzanti si interseca con il getto molecolare inferiore ai 90°. Maggior parte dei laboratori oggi utilizzano impulsi laser a femtosecondi, anche se la radiazione di sincrotrone, ioni veloci o impatto elettronico sono possibili 'proiettili' per indurre esplosione di Coulomb.
Il seguente protocollo rende il presupposto che esegue l'installazione per l'imaging coincidente di ioni e un laser a femtosecondi sono disponibili in laboratorio. L'intensità di picco necessaria per indurre l'esplosione di Coulomb in quattro o persino cinque frammenti deve essere dell'ordine di 6 x 1014 W/cm2. Per evitare misurazioni estremamente lunghi, il tasso di ripetizione del laser dovrebbe essere 10 kHz o più. Questo è fondamentale perché, da un lato, coincidente rilevazione può essere accertato solo se la probabilità di frammentazione nel fuoco del laser è significativamente inferiore a 1 ogni impulso laser (idealmente non più di 10%). Il tasso di frammentazione totale, d'altra parte, non dovrebbe essere inferiore a qualche kHz perché la quota delle vie multifragmentation pertinenti è di solito meno di 10-4. Fatto come incoraggiante, va ricordato che in linea di principio un evento singolo frammentazione è già sufficiente per identificare la configurazione di un campione di enantiopuri, e che la rilevazione di poche centinaia permette di determinare l'abbondanza di enantiomeri in un esempio di composizione enantiomerica sconosciuta.

<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

Questo articolo viene illustrato come COLTRIMS (freddo Target rinculo Ion Momentum Spectroscopy) o la tecnica del "reazione microscopio" può essere utilizzata per distinguere tra enantiomeri (stereoisomeri) di specie chirali semplici a livello di singole molecole. In questo approccio, un getto molecolare gassoso del campione si espande in una camera a vuoto e si interseca con impulsi laser a femtosecondi (fs). L'alta intensità degli impulsi conduce a digiuno più ionizzazione, accendendo una cosiddetta esplosione di Coulomb che produce parecchi frammenti cationici (caricati positivamente). Un campo elettrostatico guide questi cationi su rivelatori sensibili tempo e posizione. Simile ad uno spettrometro di massa di tempo di volo, l'orario di arrivo di ogni ione si ottengono informazioni sulla sua massa. Come un surplus, il campo elettrostatico è regolato in modo che la direzione di emissione e l'energia cinetica dopo frammentazione portare a variazioni nel tempo-di-volo e in posizione di impatto sul rivelatore.
Ogni impatto di ioni crea un segnale elettronico nel rivelatore; Questo segnale è trattato dall'elettronica ad alta frequenza e registrato evento per evento da un computer. I dati registrati corrispondono ai tempi di effetto e posizioni. Con questi dati, l'energia e la direzione di emissione di ogni frammento può essere calcolati. Questi valori sono correlati alle proprietà strutturali della molecola sotto inchiesta, vale a dire le lunghezze di legame e le posizioni relative degli atomi, che permette di determinare la molecola per molecola la manualità delle specie chirali semplici e altre caratteristiche isomerica.

In questa parte, vi mostriamo i risultati ottenuti per halomethanes. Queste specie sono l'ideale per gli esperimenti di prova-de-principio grazie alla loro semplicità e alta pressione di vapore. Nel frattempo, l'alotano specie più complessa è stato studiato facendo uso di singoli fotoni di raggi-x-soft da una fonte di sincrotrone per indurre ionizzazione più. 14
CHBrClF
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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

Formazione immagine di esplosione Coulombiana come uno strumento per distinguere tra stereoisomeri

Per piccole specie chirali, Coulomb esplosione Imaging fornisce un nuovo approccio per determinare la manualità delle singole molecole.

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

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Organic Chemistry

Chemistry

Unit 1

Transition Metals and Coordination Complexes

Stereoisomerism

SCIENTISTS IN ACTION

9.0K Views.  Goethe-Universität Frankfurt am Main. Per piccole specie chirali, Coulomb esplosione Imaging fornisce un nuovo approccio per determinare la manualità delle singole molecole.

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.

Separazione spaziale di conformatori molecolari e cluster

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

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Ingegneria

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 di laser-driven ultraveloce rotazione molecolare

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

Chimica

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.

Imaging di fotoelettroni di anioni illustrato da 310 Nm distaccamento di 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 ...

Using a Cyclic Ion Mobility Spectrometer for Tandem Ion Mobility Experiments

Accurate characterization of chemical structures is important to understand their underlying biological mechanisms and functional properties. Mass spectrometry (MS) is a popular tool but is not always sufficient to completely unveil all structural features. For example, although carbohydrates are biologically relevant, their characterization is complicated by numerous levels of isomerism. Ion mobility spectrometry (IMS) is an interesting complement because it is sensitive to ion conformations and, thus, to isomerism. 
Furthermore, recent advances have significantly improved the technique: the last generation of Cyclic IMS instruments offers additional capabilities compared to linear IMS instruments, such as an increased resolving power or the possibility to perform tandem ion mobility (IMS/IMS) experiments. During IMS/IMS, an ion is selected based on its ion mobility, fragmented, and reanalyzed to obtain ion mobility information about its fragments. Recent work showed that the mobility profiles of the fragments contained in such IMS/IMS data can act as a fingerprint of a particular glycan and can be used in a molecular networking strategy to organize glycomics datasets in a structurally relevant way. 
The goal of this protocol is thus to describe how to generate IMS/IMS data, from sample preparation to the final Collision Cross Section (CCS) calibration of the ion mobility dimension that yields reproducible spectra. Taking the example of one representative glycan, this protocol will show how to build an IMS/IMS control sequence on a Cyclic IMS instrument, how to account for this control sequence to translate IMS arrival time into drift time (i.e., the effective separation time applied to the ions), and how to extract the relevant mobility information from the raw data. This protocol is designed to clearly explain the critical points of an IMS/IMS experiment and thus help new Cyclic IMS users perform straightforward and reproducible acquisitions.

Ion mobility spectrometry (IMS) is an interesting complement to mass spectrometry for the characterization of biomolecules, notably because it is sensitive to isomerism. This protocol describes a tandem IMS (IMS/IMS) experiment, which allows the isolation of a molecule and the generation of the mobility profiles of its fragments.

Utilizzo di uno spettrometro ciclico per la mobilità ionica per esperimenti di mobilità ionica tandem

Accurate characterization of chemical structures is important to understand their underlying biological mechanisms and functional properties. Mass ...

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.

Confronto diretto tra lo scattering Raman stimolato iperspettrale e la microscopia a dispersione raman coerente anti-Stokes per l'imaging chimico

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.

Imaging differenziale delle strutture biologiche con Doppiamente-risonanza coerente Anti-Stokes scattering Raman (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 ...

Biologia

Utilizzo di un microscopio VSFG per analisi chimiche multimodali, rapide e iperspettrali

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.

Autore in primo piano: Svelare il potenziale della microscopia VSFG nello studio di strutture autoassemblate mesososcopicamente eterogenee 

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.

Imaging chimico iperspettrale non lineare multimodale mediante microscopia a scansione lineare con generazione di somma-frequenza vibrazionale

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

Stereoisomeri

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

Formazione immagine di esplosione Coulombiana come uno strumento per distinguere tra stereoisomeri

Riepilogo

Esplora Altri Video

Capitoli in questo video

Separazione spaziale di conformatori molecolari e cluster

Direct Imaging di laser-driven ultraveloce rotazione molecolare

Imaging di fotoelettroni di anioni illustrato da 310 Nm distaccamento di F⁻

Utilizzo di uno spettrometro ciclico per la mobilità ionica per esperimenti di mobilità ionica tandem

Confronto diretto tra lo scattering Raman stimolato iperspettrale e la microscopia a dispersione raman coerente anti-Stokes per l'imaging chimico

Imaging differenziale delle strutture biologiche con Doppiamente-risonanza coerente Anti-Stokes scattering Raman (CARS)

Imaging chimico iperspettrale non lineare multimodale mediante microscopia a scansione lineare con generazione di somma-frequenza vibrazionale

Imaging chimico iperspettrale non lineare multimodale mediante microscopia a scansione lineare con generazione di somma-frequenza vibrazionale

Imaging chimico iperspettrale non lineare multimodale mediante microscopia a scansione lineare con generazione di somma-frequenza vibrazionale

Stereoisomeri