In this talk I review our work on theoretically modeling of spectra in the frequency range of the fundamentals of high-frequency XH vibrations (where X = C, N, or O) of medium size molecules. These vibrations are often coupled to nearly degenerate overtone and combination bands, and this coupling complicates the interpretation of many spectral features. When a molecule contains multiple XH groups, assigning the spectrum is more difficult, especially when multiple conformers are present. I will present experimental/theoretical collaborative approaches appropriate for addressing these challenges. Our work focuses on molecules for which the densities of states is sufficiently high at the energies of the fundamentals that calculating eigenstate-resolved spectra is not useful due to long time state-mixing effects. Nonetheless, using ideas based on perturbation theory, local modes, effective Hamiltonians, the transferability of anharmonic couplings, and empirical scalings of vibrational frequencies we have developed approaches for modeling complex spectra with Hamiltonians that approach the simplicity of Hückel Hamiltonians. Several molecular systems will be presented to illustrate these ideas as well as one for which is fails entirely.

Advances in quantum manipulation of molecules bring unique opportunities, including the use of molecules to search for new physics, harnessing molecular resources for quantum engineering, and exploring chemical reactions in the ultra-low temperature regime. In this talk, I will focus on the latter two topics. First, I will introduce our effort on building single ultracold molecules with full internal and motional state control in optical tweezers for future quantum simulators and computers. This work allows us to go beyond the usual paradigm of chemical reactions that proceed via stochastic encounters between reactants, to a single, controlled reaction of exactly two atoms. Second, I will present our work giving a detailed microscopic picture of molecules transforming from one species to another. We develop full quantum state mapping of chemical reaction product-pairs from single events, which we use to precisely benchmark statistical theory.

Strong H-bonding interactions often manifest in extremely broad shared proton stretch vibrational transitions and exhibit ultrafast relaxation dynamics which have made the study of strongly H-bonded systems challenging both experimentally and computationally. Here, we report on the characterization of vibrational signatures and dynamics of strong, neutral intramolecular O-H H-bonds in several model systems by complementing frequency-resolved cryogenic ion vibrational spectroscopy on isolated gas-phase species with ultrafast solution-phase transient and 2D IR spectroscopies. The gas-phase experiments reveal the complex interplay between stretch-bend Fermi resonance interactions and coupling of the proton stretch to H-bond soft-mode vibrations. The nonlinear ultrafast experiments directly reveal the high degree of anharmonic mode mixing and coupling between the OH stretch, OH bend, fingerprint modes, and soft modes and show rapid intramolecular population relaxation dynamics. Significant isotopic dependence in polarization anisotropy dynamics suggest key differences in proton vs. deuteron transfer dynamics in the vibrationally excited systems. Time permitting, the initial steps towards combining ultrafast IR spectroscopies with cryogenic ion techniques for the acquisition of multidimensional and time-resolved spectra of isolated ion ensembles will be discussed.

The last four years have seen a massive explosion in the spectroscopic detection and characterization of large carbon-containing molecules in the interstellar medium, including the first detections of individual polycyclic aromatic hydrocarbon (PAH) molecules. The detections of PAHs and other carbon rings in the cold, dark starless cloud TMC-1 by the GOTHAM and QUIJOTE projects has opened new frontiers for the exploration of this massive reservoir of as much as 25% of interstellar carbon. In this talk, I will highlight the GOTHAM collaboration's pioneering work in laboratory (rotational) spectroscopy, radio-astronomical observational spectroscopy, astrochemical modeling, and machine learning all working together to unravel the chemistry and physics underlying these new discoveries.

The rotational spectrum of phenyl acetate, CH₃COOC₆H₅, was measured using a free jet absorption millimeterwave spectrometer in the range from 60 to 78 GHz and two pulsed jet Fourier transform microwave spectrometers covering a total frequency range from 2 to 26.5 GHz. The features of two coupled large amplitude motions, the methyl group internal rotation and the skeletal torsion tunneling of the CH₃CO group with respect to the phenyl ring C₆H₅ (tilted of about 70^°), characterize the spectrum. The vibrational ground state splits into four widely spaced sublevels, labeled as A0, E0, A1, and E1, each of them with its set of rotational transitions, and with additional interstate transitions. A global fit of the line frequencies of the four sublevels leads to the determination of 40 spectroscopic parameters, including the ∆E_A0/A1 and ∆E_E0/E1 vibrational splittings of about 36.4 GHz and 34.0 GHz, respectively. These parameters were used to deduce the V₃ barrier to methyl internal rotation (about 136 cm⁻¹) and the skeletal torsion B₂ barrier to orthogonality of the two planes (about 68 cm⁻¹).

Chirality is pervasive in Nature and describes the property of an object not to be superimposable on its mirror image. To differentiate between the two mirror images of a chiral molecule, called enantiomers, one must probe them with a probe that is itself chiral. The probe can be of chemical nature, for example another chiral molecule, or of physical nature, for example a chiral light. I will give examples of these two approaches. I will describe how laser spectroscopy at low temperature sheds light on the structural differences between the homochiral and heterochiral complexes of chiral biomolecules, such as amino acids or sugars.Hirata, K.; Mori, Y.; Ishiuchi, S. I.; Fujii, M.; Zehnacker, A. Physical Chemistry Chemical Physics 2020, 22 (43), 24887-24894Tamura, M.; Sekiguchi, T.; Ishiuchi, S.-I.; Zehnacker-Rentien, A.; Fujii, M. The Journal of Physical Chemistry Letters 2019, (10), 2470-2474 Then I will illustrate the sensitivity of chiroptical spectroscopy to conformational isomerism and molecular interactions on the example of 1-indanol studied by Vibrational Circular Dichroism (VCD) in the condensed phase Le Barbu-Debus, K.; Scherrer, A.; Bouchet, A.; Sebastiani, D.; Vuilleumier, R.; Zehnacker, A. Physical Chemistry Chemical Physics 2018, 20 (21), 14635-14646nd PhotoElectron Circular Dichroism (PECD) under jet-cooled conditions.

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Footnotes:

Hirata, K.; Mori, Y.; Ishiuchi, S. I.; Fujii, M.; Zehnacker, A. Physical Chemistry Chemical Physics 2020, 22 (43), 24887-24894

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Footnotes:

Le Barbu-Debus, K.; Scherrer, A.; Bouchet, A.; Sebastiani, D.; Vuilleumier, R.; Zehnacker, A. Physical Chemistry Chemical Physics 2018, 20 (21), 14635-14646a