Research

Chirality is threaded through many areas of science and life. There is a desire to harness this property to drive chemical reactions, create new materials, and provide understanding for important life processes. Spectroscopy, specifically gas-phase spectroscopy, provides routes of control and intimate interrogation of these processes. However, chirality has always been a unique challenge for spectroscopy, as two enantiomers cannot be distinguished by their electronic structure alone and most chiral sensitive effects are weak, thus requiring more dense samples for observation, where individual interactions are more difficult to regulate and analyze. My group will utilize imaging photoelectron spectroscopy, and photoelectron circular dichroism to develop custom instruments capable of investigating chiral-sensitive interactions in the gas phase. Specific research directions include determining the role of chirality in dilute environments, and developing tools for enantio-sensitive analysis of chiral metabolites in low concentrations.

Spectroscopic Techniques

Anion Photoelectron Spectroscopy and Velocity Map Imaging

Figure 1. Diagram of the Operation of Velocity Map Imaging Optics in Anion Photoelectron Spectroscopy: From Detached Electrons to Photoelectron Spectrum.

My current and past research utilize anion photoelectron spectroscopy (aPES), in combination with velocity map imaging (VMI), to explore questions in physical chemistry. Anion photoelectron spectroscopy is a tunable action spectroscopy method, where laser light is used to photodetach an electron from the anion. The removed electron carries with it an unique kinetic energy (eKE), which is recorded. The recorded eKE is subtracted from the incident photon energy (hν) to determine the detachment energies of the different anion-to-neutral detachment channels.  

 


Over the past 2 decades, VMI of electrons has enabled higher resolution detection of PES, and provided a technique capable of recovering the angular information that is, also, carried with departing electrons. Electrons that pass through a VMI electrostatic lens form nested spheres, where the smallest, inner-most sphere corresponds to electrons with the smallest kinetic energies. Electrons are detected on a position-sensitive detector, where a photoelectron angular distribution is accumulated after numerous experimental cycles. The radial intensities of these electron images can be reconstructed into a DE spectrum, and additional information (such as orbital character) contained within the angular distribution can be extracted. VMI spectroscopy provides near 100% electron collection efficiency and is capable of sub-meV resolution. In combination, VMI-aPES is a flexible, well-resolved technique.

Photoelectron Circular Dichroism of Anions

Figure 2. Dichroic Effect of PECD. Irradiation of a Chiral Molecule by a Given Handedness of Light Leads to an Average Photoelectron Flux DIrection of the Outgoing Electron Flux. Switching Either the Handedness of Light or Handedness of the Molecule Leads to a Reversal of the Directionality.

The importance of chirality permeates many chemical and biological functions, but the study of chirality can be challenging for spectroscopic methods, due to the likeness of electronic structure in a pair of enantiomers. Photoelectron Circular Dichroism (PECD) spectroscopy is a method of chiral discrimination, which can aid in our fundamental understanding of electron dynamics and holds promise for future analytical techniques of chiral compounds. In PECD, emission of an electron from a non-racemic sample through irradiation by circularly polarized light, leads to a forward-backward asymmetry of the photoelectron angular distribution. In comparison to other optical CD methods, such as absorption CD, this technique has the advantage of bypassing the need for weak interactions with a molecule’s magnetic moment, in exchange for a sensitivity to the molecular chirality that can be measured within the electric-dipole approximation. This alternative leads to chiral signals that are orders of magnitude larger than the aforementioned method, which makes PECD spectroscopy a potentially robust tool for studying dilute, multi-component chiral samples, in the gas-phase.

Although there has been numerous studies of PECD in the photoionization of neutral gas-phase molecules, there are limited examples where this effect has been observed in a molecular beam of anions. The use of anions for this technique would allow for mass-selectivity and eliminate the need for X-ray based ionization sources, thus leading to a potentially robust analytical tool for chiral discrimination. In addition, as photodetachment of an electron from an anion leaves behind a neutral core, long range interactions between the departing electron and the parent molecule are expected to be minimal. With this in mind, it is possible to explore the effects of short-range interactions on the PECD signal.

J. Triptow, A. Fielicke, G. Meijer, and M. Green. “Imaging Photoelectron Circular Dichroism in the Detachment of Mass‐Selected Chiral Anions.” Angewandte Chemie International Edition (2022). DOI: 10.1002/anie.202212020

Figure 3. Energy-Resolved PECD Observation of Deprotonated Indanol Anion Reveals a Complex Composition, Even After Mass Selection.

Previous Work in Metallic Bonding

Hydrogen Bonding of Gold

Postdoctoral Project

Figure 4. Hydrogen Bonding Between Gold Anion and Four Organic Compounds (Fenchone, Menthone, 3-Hydroxy Tetrahydrofuran, Alaninol). This Figure has been Reproduced from the Publication to the Right.

Hydrogen bonds are conventionally considered to be bonds between hydrogen and the most electronegative atoms: oxygen, nitrogen, and fluorine. However, in recent history, attention has turned to transition metals as unconventional participants in hydrogen bonding. Specifically, the properties of gold make it an ideal candidate for engaging in non-conventional hydrogen bonding. We have used a combination of VMI-aPES of gold complexes and computational modeling to quantify the binding of Au- and identify similar markers to provide markers for characterizing Au-hydrogen bonding.

J. Triptow, G. Meijer, A. Fielicke, O. Dopfer, and M. Green. “Comparison of Conventional and Nonconventional Hydrogen Bond Donors in Au–Complexes.” The Journal of Physical Chemistry A (2022).  DOI: 10.1021/acs.jpca.2c02725

Abnormal Bonding in Beryllium Molecules

Graduate Thesis Work

Understanding abnormal bonding is paramount for furthering our command of chemistry. With the current exponential growth of computational chemical predictive methods, there is a need for comparable experimental data of chemical systems which have a history of defying our chemical intuition.As a material, beryllium’s utility is far-reaching, with applications in electronics, aircraft construction, nuclear reactor coatings and even on the recently launched James Webb Space telescope. However, due to this element’s toxicity and unexpected electronic structure, fundamental knowledge of this element is limited or ambiguous. As a graduate student, I used velocity map imaging spectroscopy and high-level ab initio methods to characterize many small beryllium-containing anions, including molecules that exhibited closed-shell bonding. 

Representative Publication:

M. L. Green, P. Jean, and M. C. Heaven. “Dative Bonding between Closed-Shell Atoms: The BeF–Anion.” The Journal of Physical Chemistry Letters 9, no. 8 (2018): 1999-2002. DOI: 10.1021/acs.jpclett.8b00784

Figure 5. Velocity Map Image and Spectrum of Closed-Shell Beryllium Fluoride Anion. Further Details of this Work are Described in the Publication to the Left.