Coulomb Explosion Imaging as a Tool to Distinguish Between Stereoisomers

Summary

For small chiral species, Coulomb Explosion Imaging provides a new approach to determine the handedness of individual molecules.

Abstract

This article shows how the COLTRIMS (Cold Target Recoil Ion Momentum Spectroscopy) or the "reaction microscope" technique can be used to distinguish between enantiomers (stereoisomers) of simple chiral species on the level of individual molecules. In this approach, a gaseous molecular jet of the sample expands into a vacuum chamber and intersects with femtosecond (fs) laser pulses. The high intensity of the pulses leads to fast multiple ionization, igniting a so-called Coulomb Explosion that produces several cationic (positively charged) fragments. An electrostatic field guides these cations onto time- and position-sensitive detectors. Similar to a time-of-flight mass spectrometer, the arrival time of each ion yields information on its mass. As a surplus, the electrostatic field is adjusted in a way that the emission direction and the kinetic energy after fragmentation lead to variations in the time-of-flight and in the impact position on the detector.

Each ion impact creates an electronic signal in the detector; this signal is treated by high-frequency electronics and recorded event by event by a computer. The registered data correspond to the impact times and positions. With these data, the energy and the emission direction of each fragment can be calculated. These values are related to structural properties of the molecule under investigation, i.e. to the bond lengths and relative positions of the atoms, allowing to determine molecule by molecule the handedness of simple chiral species and other isomeric features.

Introduction

Chirality is a feature of our nature that has been fascinating researchers for more than 150 years. In the 19^th century, Pasteur, van't Hoff and others discovered that molecules can occur in two mirror image structures that are not super-imposable - like our left and right hands. This property was termed 'chiral', from the Greek word for 'hand'.

So far, no difference in thermodynamic properties or in energy levels of left- and right-handed forms (the two 'enantiomers') has been found. In order to analyze the handedness of a given sample and to separate the enantiomers, interaction with other chiral mol....

Protocol

Caution: Make sure to be familiar with all possible hazards connected with the experiment and in the laboratory. The procedure below includes class-IV lasers, high voltage and vacuum. Consult the material safety data sheet (MSDS) for the species to be investigated.

1. Preparation

1. Preparatory considerations
   NOTE: Before the actual experiment starts, two main choices have to be made; the first regarding the possible species under investigations and the second the electric field in the spectrometer. It is assumed here that the setup has been used previously for other experiments and that the ....

Results

In this part, we show results obtained for halomethanes. These species are ideal for proof-of-principle experiments due to their simplicity and high vapor pressure. In the meantime, the more complex species halothane has been investigated using single soft-x-ray photons from a synchrotron source to induce multiple ionization.¹⁴

CHBrClF

Bromochlo.......

Discussion

Due to the variety of components, a COLTRIMS setup requires a rather high level of technical expertise, especially in the areas of vacuum technique, particle detection, fast electronics and data analysis. Before turning to the investigation of complex species, it should thus be thoroughly checked if the setup is running properly, e.g. by performing and analyzing a measurement on a diatomic or triatomic species.

Optimizing the intensity and duration of the laser pulses and the overlap .......

Materials

List of materials used in this article

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