Molecular parity violation has been critically discussed in relation to biomolecular homochirality in the early evolution of life M. Quack, Angew. Chem. Intl. Ed. (Engl.) 2002, 41(24), 4618; Adv. Chem. Phys. 2014, 157, 249, www.ir.ETHz.CH. In this context molecules of potential importance for prebiotic chemistry like the small, chiral three-membered heterocyclic molecule aziridine-2-carbonitrile (2-cyanoaziridine) are of interest A. Eschenmoser and E. Loewenthal, Chem. Soc. Rev. 1992, 21, 1. Indeed, this molecule has been previously examined S. Drenkard, J. Ferris, and A. Eschenmoser, Helv. Chim. Acta 1990, 73, 1373.nd the parity violating energy difference between the enantiomers in their ground state has also been calculated R. Berger, M. Quack and G.S. Tschumper, Helv. Chim. Acta 2000, 83(8), 1919. Molecular parameters for the ground state of this molecule are available from earlier microwave studies R.D. Brown, P.D. Godfrey, and A.L. Ottrey, J. Mol. Spectrosc. 1980, 82, 73. and its conformations have been examined by ab  initio theory G.S. Tschumper, J. Chem. Phys. 2001, 114(1), 225. Here we report initial results of a high resolution spectroscopic study of cyanoaziridine, carried out at room temperature with an instrumental resolution of 0.0011 cm⁻¹ in the 800-1000 cm⁻¹ region using the Bruker IFS125 Zurich Prototype (ZP2001) Fourier transform spectrometer S. Albert, K. Albert and M. Quack, Trends  in  Optics  and  Photonics 2003, 84, 177; S. Albert, K. Keppler Albert, M. Quack, Ch. 26, Handbook of High-Resolution Spectroscopy, Vol. 2, p. 965-1019, M. Quack, F. Merkt, Eds., Wiley, Chichester (2011). Transitions in the ν₁₅ and ν₁₆ bands have been assigned, and molecular parameters have been determined using the Watson Hamiltonian. Simulations performed using these parameters reproduce the observed spectra well. The results are discussed in relation to astrophysical spectroscopy and recent efforts on parity violation in chiral molecules M. Quack and G. Seyfang,“Tunnelling and Parity Violation in Chiral and Achiral Molecules: Theory and High-Resolution Spectroscopy,” Chapter 6, Tunnelling in Molecules: Nuclear Quantum Effects from Bio to Physical Chemistry, p.192-244, J. Kästner, S. Kozuch, Eds., RSC, Cambridge (2020), ISBN: 978-1-78801-870-8; M. Quack, G. Seyfang, G. Wichmann, Adv. Quantum Chem. 2020, 81, 51-104. M. Quack, Angew. Chem. Intl. Ed. (Engl.) 2002, 41(24), 4618; Adv. Chem. Phys. 2014, 157, 249, www.ir.ETHz.CH.. A. Eschenmoser and E. Loewenthal, Chem. Soc. Rev. 1992, 21, 1.. S. Drenkard, J. Ferris, and A. Eschenmoser, Helv. Chim. Acta 1990, 73, 1373.a R. Berger, M. Quack and G.S. Tschumper, Helv. Chim. Acta 2000, 83(8), 1919.. R.D. Brown, P.D. Godfrey, and A.L. Ottrey, J. Mol. Spectrosc. 1980, 82, 73., G.S. Tschumper, J. Chem. Phys. 2001, 114(1), 225.. S. Albert, K. Albert and M. Quack, Trends  in  Optics  and  Photonics 2003, 84, 177; S. Albert, K. Keppler Albert, M. Quack, Ch. 26, Handbook of High-Resolution Spectroscopy, Vol. 2, p. 965-1019, M. Quack, F. Merkt, Eds., Wiley, Chichester (2011).. M. Quack and G. Seyfang,“Tunnelling and Parity Violation in Chiral and Achiral Molecules: Theory and High-Resolution Spectroscopy,” Chapter 6, Tunnelling in Molecules: Nuclear Quantum Effects from Bio to Physical Chemistry, p.192-244, J. Kästner, S. Kozuch, Eds., RSC, Cambridge (2020), ISBN: 978-1-78801-870-8; M. Quack, G. Seyfang, G. Wichmann, Adv. Quantum Chem. 2020, 81, 51-104..

p-Ethylphenol (PEP) and other volatile phenols appear in wines contaminated with Brettanomyces yeast giving undesirable off-aromas (“Brett” or phenolic character) which spoil wines even at very low concentrations. These phenols are produced by an enzymatic transformation of the hydroxycinnamic acids present in wines. In this work, we have analyzed the rotational spectrum of PEP and its microsolvated complexes using CP-FTMW spectroscopy in the 2-8 GHz region. The equilibrium configuration of PEP has the ethyl group carbon plane perpendicular to the phenyl ring while the OH group lies in the ring plane. The two possible orientations of the OH group originate two non-superposable enantiomeric forms, energetically equivalent, but with opposite signs for the μ_b electric dipole component. The interconversion of both enantiomers by the OH internal rotation leads to a situation of transient chirality. This motion is expected to have a two-fold periodic potential energy function with the torsional states appearing as doublets as happen in phenol. The rotational spectrum reflects this behavior. The μ_a spectrum consists of single lines resulting from the collapse of the individual torsional 0⁺ and 0⁻ spectra. The μ_b transitions, forbidden within each torsional state, are allowed as chiral 0⁺ ↔ 0⁻ transitions. Therefore, the μ_b-type spectrum consists of doublets spaced twice the energy difference between the 0⁺ and 0⁻ torsional states. We have observed the ¹³C and OD isotopologues and have determined the molecular structure of PEP along with the internal rotation potential energy profile. In addition, we have measured the spectra of the PEP-H₂O, PEP-Ne-H₂O, and PEP-(H₂O)₂ complexes. The PEP-H₂O and PEP- Ne-H₂O spectra show doublets with 1:3 intensities revealing the water rotation dynamics exchanging the H atoms. For these species the spectra of different isotopologues ¹³C, D, ¹⁸O, and ²²Ne have been also measured to determine their structures.

The three minima obtained upon rotation of the difluoromethyl group in 2,3,3-trifluoromethylpropene correspond to a higher energy, achiral rotamer that contains a plane of symmetry while the two minima that share a lower energy value characterize a chiral, enantiomeric pair. Four isotopologues of each form are observed in the microwave rotational spectrum obtained using a pulsed-jet, chirped pulse Fourier transform spectrometer and the spectra of all eight have been assigned and analyzed. Additionally, spectra for four isotopologues of the gas phase heterodimer formed between the chiral rotamer and an argon atom have been obtained and analyzed using a narrowband, Balle-Flygare cavity Fourier transform instrument. For the heterodimer of the achiral rotamer with argon, only the spectrum of the most abundant isotopologue has been observed.

A cryogenic buffer gas cell at a few degrees Kelvin provides a bright source of internally cold and slow moving molecules. I will be presenting recent results from our buffer gas spectrometers, including direct observation of dimer formation with conformationally selected reagents and high-resolution spectroscopy in slow, bright buffer-gas cooled molecular beams.

Straightforward identification of chiral molecules in multi-component mixtures of unknown composition is extremely challenging. Current spectrometric and chromatographic methods cannot unambiguously identify components while the state of the art spectroscopic methods are limited by the difficult and time-consuming task of spectral assignment. Here, we introduce a highly sensitive generalized version of microwave three-wave mixing that uses broad-spectrum fields to detect chiral molecules in enantiomeric excess without any prior chemical knowledge of the sample. This method does not require spectral assignment as a necessary step to extract information out of a spectrum. We demonstrate our method by recording three-wave mixing spectra of multi-component samples that provide direct evidence of enantiomeric excess. Our method opens up new capabilities in ultrasensitive phase-coherent spectroscopic detection that can be applied for chiral detection in real-life mixtures, raw products of chemical reactions and difficult to assign novel exotic species.

We are exploring how argon binding to substituted oxiranes, which have potential applications as chiral tags, is modulated by varying the identity of the substituents on the epoxy ring. Previously studied systems generally showed close contacts primarily to atoms contained in the ring. However, for complexes with cis-1,3,3,3-tetrafluoro-1,2-epoxypropane (cFTFO) multiple minima with similar energies are predicted by quantum chemistry calculations including some with significant interactions between the argon atom and substituents on the oxirane. Analysis of the rotational spectra obtained using chirped pulse Fourier transform microwave spectroscopy for four isotopologues of Ar-cFTFO reveals that the argon atom binds to the back of the ring; very different from Ar-3,3,3-trifluoro-1,2-epoxypropane (Ar-TFO) and Ar-3,3-difluoro-1,2-epoxypropane, but similar to Ar-trans-1,3,3,3-tetrafluoro-1,2-epoxypropane. The utility of cFTFO in chiral analysis is explored via quantum chemistry calculations on the TFO-cFTFO heterodimer and progress on the observation and analysis of the two diastereomeric forms of this species will be reported.

Connected to our efforts in characterizing substituted oxiranes for use as potential chiral tags for the conversion of enantiomeric molecules into spectroscopically distinct diastereomeric complexes for chiral analysis, we have obtained and analyzed the spectra of both the cis and trans isomers of 1,1,1-trifluoro-2,3-epoxybutane. Although the spectrum of the trans isomer and all four of its singly-substituted ¹³C isotopologues, obtained in natural abundance, could be satisfactorily analyzed as a centrifugally-distorting rigid rotor asymmetric top, the spectrum of the cis isomer showed the effects of methyl group internal rotation. Progress on assigning and analyzing the spectrum of this isomer will be reported.

Chirality is ubiquitous in nature since most biologically active molecules are chiral. The two mirror images of a chiral molecule, which are called enantiomers, have almost identical physical properties, however, their chemical and biochemical properties can differ tremendously. Thus, beyond the structural analysis, enantiomer differentiation and separation are essential for a deeper understanding of their functionality. Over the past decade, microwave three-wave mixing has emerged as a chiral-sensitive technique enabling the differentiation of enantiomers using a sequence of microwave pulses.^[1] This technique was further extended to achieve enantiomer-selective population transfer in chiral molecules, that is, the energetic separation of enantiomers in a specific rotational state of interest.^[2−4] The efficiency of the enantiomer-selective population transfer is mainly limited by two factors: the spatial degeneracy and the thermal population of the rotational levels. To deal with the latter issue, we applied a chirped microwave pulse within the rapid adiabatic passage (RAP) regime to depopulate the initial thermal population in the relevant rotational state. The effect of the RAP pulse on the enantiomer-selective enrichment will be presented, in combination with a theoretical simulation.

1. D. Patterson, M. Schnell, J. M. Doyle, Nature 2013, 497, 475–477.
2. S. Eibenberger, J. Doyle, D. Patterson, Phys. Rev. Lett. 2017, 118, 123002.
3. C. Pérez, A. L. Steber, S. R. Domingos, A. Krin, D. Schmitz, M. Schnell, Angew. Chem. Int. Ed. 2017, 56, 12512–12517.
4. J. Lee, J. Bischoff, A. O. Hernandez-Castillo, B. Sartakov, G. Meijer, S. Eibenberger-Arias, arXiv:2112.09058 [physics.chem-ph] 2021.

Resonance Raman optical activity (RROA) measures the small intensity difference between right circularly polarized light, I_R, versus left circularly polarized light, I_L, when a randomly polarized light is in resonance with a chiral molecule. Researchers have explored RROA as a mean to significantly enhance the weak ROA response for the past two decades, although the progress has been severely hampered by the lack of agreement between theoretical and experimental RROA spectra so far. After examining a series of light-matter events which can occur simultaneously under a typical RROA experimental condition, we discovered a new form of chiral Raman spectroscopy, eCP-Raman-a combination of electronic circular dichroism (ECD) and circularly polarized Raman (CP Raman).¹ Further analyses of the I_R-I_L spectra of three resonating chiral molecules revealed that all of the I_R-I_L spectra observed can be satisfactorily explained by the novel eCP Raman mechanism without any detectable contributions from natural RROA.² The discovery of eCP-Raman allows one to extract true RROA contribution from the I_R-I_L signal obtained under resonance to facilitate the current theoretical RROA development. Furthermore, eCP-Raman offers a new way for sensitive chirality detection of molecular systems in biology and chemistry. 1. G. Li, M. Alshalalfeh, J. Kapitán, P. Bouř, Y. Xu, Chem. Eur. J. 2022, doi.org/10.1002/chem. 202104302. 2. a) G. Li, M. Alshalalfeh, Y. Yang, J. R. Cheeseman, P. Bouř, Y. Xu, Angew. Chem. Int. Ed. 2021, 60, 22004-22009. b) T. Wu, G. Li, J. Kapitán, J. Kessler, Y. Xu, P. Bouř, Angew. Chem. Int. Ed. 2020, 59, 21895-21898.

Circular dichroism (CD) spectroscopy is one of the most powerful methods to investigate the structures and reactions of chiral molecules. The CD of molecules with fixed spatial distribution is called anisotropic CD (ACD). ACD spectroscopy has been extensively used to probe the orientation of macromolecules in anisotropic medium. Here, we have obtained the resonant two photon ionization CD (R2PICD) spectra of (-)PED using a dual laser beam method. It is found that the CD values of the P-, Q-, and R-branch transitions of the origin bands are different from each other. Furthermore, the CD values of the rotational transitions of conformers A and C do not exhibit mirror images between (+) and (-)PED. These results are explained by ACD phenomena of jet-cooled molecules undergoing the P-, Q-, and R-branch transitions.