The application of rotational spectroscopy-based methods as tools to deliver accurate and precise chirality-sensitive information are still breaking ground, but their applicability in the challenging field of analytical chemistry is already clear. In a recent study, we explored the current capabilities and challenges of microwave 3-wave mixing (M3WM) and chiral tag rotational spectroscopy - two emerging techniques for chiral analysis based on rotational spectroscopy. In this presentation, we compare the performance of these two techniques with respect to solving the absolute configuration and the enantiomeric excess (ee) of a sample prepared by others and containing a mixture of enantiomers of styrene oxide with a composition unknown to us.

Propylene oxide (PO), a simple chiral cyclic ether, has served as a valuable prototype molecule for the theoretical development of chiroptical activities¹ and for rotational spectroscopic studies of chirality recognition. Although a study of the PO dimer was completed more than ten years ago where six homo- and heterochiral structures were identified experimentally², no similar studies of larger chiral aggregates such as a PO trimer have been reported so far. Aided with the recent development in chirped pulse Fourier transform microwave techniques³ and in meta-dynamics conformational search algorithms⁴, we have explored the conformational space of the PO trimer. Several hundred possible PO trimer structures were predicted the first two most stable homochiral trimers were assigned. The number of possible conformers of the heterochiral PO trimer predicted is about three times of the homochiral trimer. Even so, no match could be made for the heterochiral trimer conformers detected, despite additional exhausted conformational searches. By studying all monosubstituted 13C isotopologues of the most stable homochiral and heterochiral PO trimer in natural abundance, we experimentally determined their C atom backbone structures. Guided with this information, additional PO trimer structures were calculated which match the observed rotational constants and dipole moments. The study showcases the power of rotational spectroscopic experiments and highlights the necessity of intensive interplay between experiment and theory in dealing with large aggregates. 1.R. W. Kawiecki, F. J. Devlin, P. J. Stephens, R. D. Amos, J. Phys. Chem. 1991, 95, 9817. 2.Z. Su, N. Borho, Y. Xu. J. Am. Chem. Soc. 2006, 128, 51. 3.G. Barratt Park, and Robert W. Field, J. Chem. Phys. 2016, 144, 20090. 4.P. Pracht, F. Bohle and S. Grimme, Phys. Chem. Chem. Phys., 2020.

Chiral analysis, the determination of the absolute configuration of a chiral molecule and its enantiomeric excess, is a challenging analytical chemistry problem. Rotational spectroscopy can perform chiral analysis through either three-wave mixing spectroscopy, as introduced into the field by Patterson, Doyle, and Schnell, or through the chiral tag method. In chiral tagging, a small, chiral molecule serves as the chiral resolving agent and is attached to the analyte through noncovalent interactions. This work considers suitable tags to perform chiral analysis on carboxylic acids. 2-phenylpropionic acid was used to illustrate the proof-of-principle for chiral analysis of carboxylic acids using propylene oxide and butynol as the chiral resolving agent. Propylene oxide is found to have a functional group specific binding motif. Candidate chiral tag complex structures were evaluated using DFT calculations (B3LYP GD3BJ def2TZVP). The presence of these complexes in the pulsed jet sample were verified by comparison between experimental and theoretical rotational constants. Chiral tag measurements with propylene oxide pose a challenge because there are geometries for homochiral and heterochiral complexes with nearly identical rotational constants and dipole moment components. The close similarity makes it difficult to determine the absolute configuration to high confidence. However, the tag complexes of similar geometry have different relative energies for homochiral and heterochiral interactions. The lower energy conformers were more likely to be observed, indicating efficient cooling in the jet pulse. In this case, it appears that energetic considerations can be used to establish the absolute configuration. Methodology and analysis, as well as C¹³ structure fits and EE determination, will be further discussed.

During measurements designed to develop a chiral tag rotational spectroscopy methodology for establishing the absolute configuration of a known persistent fluorinated carboxylic acid pollutant (FRD-903), an interesting reaction chemistry was observed. In several cases, highly fluorinated carboxylic acids participate in ring opening of epoxides under ambient conditions. Reactions of three fluorinated carboxylic acids, difluoroacetic acid, trifluoroacetic acid, and pentafluoropropionic acid, with propylene oxide (PO) were explored. The reactions were performed externally to the spectrometer and internally in the gas phase by flowing PO over the fluorinated acid samples. Reactions were performed by adding 4:1 molar equivalents of PO to carboxylic acid, and each reaction was highly exothermic. The reaction mixtures were analyzed by rotational spectroscopy using a chirped-pulsed Fourier transform rotational spectrometer. These measurements showed that the reaction products are created through epoxide ring-opening at both epoxide ring carbons. This reaction chemistry, in principle, offers a way to perform chiral analysis through covalent tagging, but this application would require retention of configuration in the reaction. The reaction products were subsequently chiral tagged to determine the extent of configuration retention. These experiments show that the products, both the ring-opening at the least- and most-substituted carbon on the epoxide ring, are racemized. While these results are not useful for obtaining the absolute configuration, these results further our understanding of the chemistry involved in these reactions. Covalently modifying fluorinated carboxylic acids can be achieved, and establishing absolute configuration could be possible with a more controlled reaction to prevent racemization of the chiral center on the epoxide.

Carveol, a primary constituent in spearmint and peppermint oil, is a monocyclic monoterpenoid alcohol that is produced in nature by the photooxidation of limonene.¹ Structurally, carveol contains two chiral centers with three substituents (methyl group, hydroxyl group, and propylene group) attached to it’s cyclohexane ring. The combination of the three flexible substituents, ring puckering, and two chiral centers introduce a complex conformational topography that is non-trivial to elucidate. However, a thorough study of the topology of chiral carveol may provide valuable insights into complex conformational dynamics, as well as chirality recognitions studies. We believe chirped-pulse Fourier transform microwave spectroscopy (CP-FTMW) is an excellent tool for this investigation as it is able to unambiguously distinguish structural conformers within a complex mixture.² The broadband rotational spectrum of cis and trans (-)-carveol was recorded using a chirped pulse Fourier Transform microwave spectrometer in the 2-6 GHz region.³ To aid in experimental assignment, a theoretical conformational search was carried out using a combination of a two dimensional potential energy scan, which scans over the propylene group and hydroxyl group, and the Conformer–Rotamer Ensemble Sampling Tool (CREST).⁴ The theoretical results yielded a total of 23 conformers for the trans and 19 for the cis. Utilizing these results, a total of five experimental conformers were then assigned: two for the trans conformer and three for the cis conformer. For trans, it was determined both experimental conformers occupy the equatorial position. For the cis conformer, two equatorial and one axial conformer was experimentally assigned. The conformational cooling pathways and intermolecular interactions within each conformer were also analyzed to further understand the conformational landscape of cis and trans (-)-carveol. 1. J. Laothawornkitkul, J. E. Taylor, N. D. Paul and C. N. Hewitt, New Phytol., 2009, 183, 27–51.; 2. G. Barratt Park, and Robert W. Field, J. Chem. Phys., 2016, 144, 20090.; 3. N. A. Seifert, J. Thomas, W. Jäger and Y. Xu, Phys. Chem. Chem. Phys., 2018, 20, 27630–27637.; 4. P. Pracht, F. Bohle, S. Grimme, Phys. Chem. Chem. Phys. 2020., 22(14), 7169-7192

The rules of engagement for chiral molecules have evolved beyond the static "three-point" interaction picture. Models where weak intermolecular interactions compete and have a decisive act in establishing the molecular fit are the new travel guides to molecular recognition.A. Zehnacker, M. Suhm. Angew. Chem. Int. Ed. 47, 6970–6992 (2008).oreover, molecules with dominant aromatic substructure foresee more unconventional intermolecular contact schemes given their delocalized electron distribution. This notion is valid for planar and helical motifs, as is the case of the smallest polycyclic aromatic hydrocarbon with a screw sense, tetrahelicene.S.R. Domingos, K. Martin, N. Avarvari, M. Schnell. Angew. Chem. Int. Ed. 58, 11257–11261 (2019).iven their compatibility with theoretical calculations, studies of cluster formation in the gas phase are pertinent approaches to disclose the ruling intermolecular forces at play and their role mediating molecular recognition.S.R. Domingos, C. Pérez, N.M. Kreienborg, C. Merten, M. Schnell. Commun. Chem. accepted (2021).n this scope, broadband rotational spectroscopy experiments are ongoing, and we will discuss our latest insights from studies on planar and helical aromatic systems.

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Footnotes:

A. Zehnacker, M. Suhm. Angew. Chem. Int. Ed. 47, 6970–6992 (2008).M S.R. Domingos, K. Martin, N. Avarvari, M. Schnell. Angew. Chem. Int. Ed. 58, 11257–11261 (2019).G S.R. Domingos, C. Pérez, N.M. Kreienborg, C. Merten, M. Schnell. Commun. Chem. accepted (2021).I

In chiral tagging, a smaller chiral molecule of known stereochemistry is non-covalently bonded, or “tagged” to the analyte of interest. This enantiomeric conversion results in a diastereomeric mixture of complexes whose stereochemistry can be spectroscopically resolved. In this work, rotational spectroscopy coupled with the chiral tag methodology was used to elucidate the multiple isomers produced by the analyte and tag motif of methylphenyl oxirane (MPO). The chosen tagging agent, 1,1,1-trifluoro-2-propanol (TFIP), has an energy difference of about 2 kJ/mol between its two possible monomer conformations. In this study, the 10 kcal/mol ( 42 kJ/mol) of internal energy available during the tag complex formation stabilizes the MPO/TFIP tag through non-covalent interactions, making it possible to observe the low energy isomers of the chiral complexes of both TFIP monomer conformations. The cooling of the TFIP monomer in the pulsed jet allows the population of the lowest energy conformer to be larger by at least a 10:1 ratio. As a result, the complexation energy can be viable tool in the exploration of the system’s potential energy surface and conformational preference of the tag. Experimental data was collected on a 2-8 GHz CP-FTMW spectrometer, with additional 6-18 GHz measurements for the analysis of less abundant conformers. Additional tagging agents, such as propylene oxide (PO), were also screened. Candidate chiral tag complex structures were evaluated using DFT calculations (B3LYP GD3BJ / def2TZVP). From quantum chemical calculations, optimized geometry structures of the tag complexes can be assigned to the observed spectra. By combining these optimized geometries with known energy and dipole information, a high confident determination of the absolute configuration of MPO can be made. A survey of candidate structures as well as the experimental assessment of observed conformers will be discussed.

The two non-superimposable mirror images of a chiral molecule are referred to as enantiomers. Their structures cannot be transformed into each other by pure translation or rotation. Even though most physical properties of enantiomers are identical – which makes them intrinsically difficult to separate – their handedness often determines their functionality. Chiral analysis, particularly for complex samples, is still challenging, thus there is a need for fast and reliable methods that can differentiate and/or separate enantiomers. Recently, the enantiomer-specific state transfer method¹ was developed. This method provides the means to populate or depopulate a rotational state of a chosen enantiomer. We have designed, built, and characterized a compact spectrometer capable of performing chirped-pulse Fourier transform microwave spectroscopy, including chiral analysis, and electronic spectroscopy. By combining optical methods with microwave spectroscopy, we seek to maximize the state-specific enantiomeric enrichment. Our recent studies mainly focus on the chiral alcohol 1-indanol. We have acquired and understood its rotationally resolved electronic spectra,² as well as performed microwave-UV double resonance measurements. The gained structural insight serves as a foundation for future, more sensitive enantiomer-specific measurements. I will discuss recent experimental results as well as details of our new experimental setup. ¹Eibenberger, S. et al., Phys Rev Lett 118, 123002 (2017). ²Hernandez-Castillo, A. O. et al., Phys Chem Chem Phys, (2021).

Amino acids, often refered as the building blocks of proteins, are not only crutial in our body for all sorts of biological functions they support, but also are significantly important to chemistry and life sciences for their chirality properties. The mirror-imaged pair of a chiral amino acid share the same chemical formula but may respond very differently when interacting with a chiral acceptor site. In this presented work, the protonated dimers of asparagine with either serine or valine in their homochiral and heterochiral forms were investigated using an Infrared Multiple-Photon Dissociation (IRMPD) spectrometer. A three-tiered approach was developd to identify possible homochiral and heterochiral dimers. Harmonic frequency calculations were further carried out to interpret the observed IRMPD spectra. Analyses of these ensembles reveal a strong preference for the migration of the excess proton during the photodissociation process from serine and valine to asparagine. The structural and energetic relationships in the protonated homochiral and heterochiral serine-asparagine and valine-asparagine dimers will also be discussed in this talk. 1. M. Heger, J. Cheramy, F. Xie, Z. Chen and Y. Xu, J. Mol. Spectrosc., 2018, 352, 36–44.

Photoelectron Circular Dichroism (PECD) 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, ionization of a non-racemic sample by circularly polarized light results in a forward-backward asymmetry of the photoelectron angular distribution. This technique has significant advantages over other optical CD methods, such as absorption circular dichroism, as sensitivity to the molecular chirality can manifest within the electron-dipole approximation, bypassing the need for observation of weak interactions with a molecule’s magnetic moment. PECD as it pertains to neutral chiral species has flourished, evident by the many theoretical and experimental works now available. However, PECD of anions has yet to come to fruition. The use of anions for this technique would provide benefits such as mass-selectivity and eliminating the need for X-ray based ionization sources. Also, it is expected that anion PECD would provide insights into the specific role of long-range and short-range forces in the PECD effect. As such, this talk will detail the progress made towards achieving PECD for anions. An overview of the different experimental approaches attempted and considerations of the inherent differences between PECD of neutrals and anions will be included. This discussion will provide the necessary context to answer the question: Can anion PECD work?

Vibrational circular dichroism (VCD) spectroscopy is a powerful spectroscopic tool for determination of structural properties of chiral molecules directly in solution, including their absolute configurations and conformations. More recently, the matrix-isolation (MI) technique which allows substantial control over the sample(s)/carrier gas ratio(s) and deposition conditions has been combined with VCD. It can avoid the solute-solvent interactions and have the molecular target largely in its isolated monomeric form. The MI technique generally yields much narrower IR and VCD bandwidths than those obtained in solution, providing the opportunity to investigate possible conformations of the target molecules in considerable details. In the current study, the conformational landscapes of tetrahydro-2-fuoric acid (THFA) and its binary aggregates in a cold argon matrix have been investigated. MI-IR and MI-VCD spectra have been obtained at three different deposition temperatures of 10, 24, and 30 K. In addition, extensive searches have been carried out to find stable conformers of the THFA monomer and dimer in an argon matrix. The well-resolved experimental MI-IR and MI-VCD features at 10 K allows one to identify the dominating monomeric conformations. Interestingly, the inclusion of argon using the polarizable continuum model improves the agreement between the theoretical and experimental IR and VCD spectra. At the 24 and 30 K deposition temperatures, the experimental IR and VCD spectral features reveal appearance new species, most likely binary aggregates of THFA, although the IR and VCD features differ greatly from those observed in the previous solution study. Further analyses indicate that these new species are due to the much less stable conformers of the THFA dimer, not observed experimentally before. The structures and the formation mechanism of these unusual species will also be discussed. 1. C. Merten, Y. Xu, Angew. Chem. Int. Ed., 2013, 52, 2073-2076. 2. T. Kuppens, W. Herrebout, B. Veken, P. Bultinck, J. Phys. Chem. A, 2006, 110, 10191-10200.

Hydration modulates the structure and function of biomacromolecules. However, probing water structures in hydration shells remains difficult due to a large background signal from bulk water and the dynamic nature of the solvation layer. Chiral vibrational sum frequency generation (SFG) spectroscopy can probe the solvation shell around a chiral solute without interference from the bulk. Computational modeling can guide the interpretation of chiral SFG spectra of water structures in hydration shells. We develop and apply computational methods to predict chiral SFG responses of water around a model protein system, the antiparallel β-sheet LK₇β, at the air-water interface. The SFG response of chiral water superstructures around the protein is modeled with molecular dynamics simulations and an electric field mapping strategy. Our computed spectra agree qualitatively with experimental results and flip when the amino acids are replaced with their (D-) enantiomeric equivalents. We show that the spectroscopic response of the chiral water assemblies arises from both strong and weak hydrogen bonds between water and protein. Intra- and intermolecular vibrational couplings of water molecules are shown to be necessary for generating the chiral SFG response. These results will improve our understanding of macromolecular solvation, a key factor in protein folding, denaturation, and ligand recognition. This study establishes a computational framework to support further development of chiral SFG in revealing the structure of biomacromolecular solvation shells, complementing current structural biology methods.

The structure and function of biomacromolecules depend on water structures in the hydration shell. However, elucidating the architectures of hydration shells around biomacromolecules has proven challenging. We used heterodyne chiral vibrational sum frequency generation (SFG) spectroscopy to study the structure of the solvation shell around an antiparallel β-sheet protein. Heterodyne chiral SFG spectroscopy reveals that water molecules form chiral superstructures around the protein, and that these water superstructures are induced by the chirality of the protein. Enantiomeric (L-) or (D-) proteins give rise to enantiomeric water superstructures. Molecular dynamics simulations expose the chiral architecture of the water superstructures around the protein. Modeling the chiral SFG response of the hydration shell suggests that hydrogen bonding between water and protein is a driving force behind the formation of the chiral water superstructures. Our work demonstrates the promise of chiral SFG spectroscopy to reveal structures and dynamics of hydration shells around biomacromolecules. We speculate on the involvement of chiral water superstructures in biomolecular recognition, and the role of chiral water superstructures in the origins of biological homochirality. r0pt Figure

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