We numerically simulated rotational wavepacket dynamics in asymmetric top molecules. The molecules are aligned impulsively using a short, intense linearly-polarized laser pulse. The alignment of molecules is important for capturing dynamics in the molecular frame and for molecular imaging. The alignment process is challenging to simulate because of the large number of initial rotational states involved. The alignment is simulated under the rigid-rotor assumption and field-free conditions. The evolution of the degree of alignment and angular distributions of 4-fluorobenzotrifluoride (FC6H4CF3) are calculated and the numerical results show good agreement with the experimental results.

The first excited singlet state of atomic oxygen (O(¹D)) has relevance in numerous disciplines of chemistry including combustion chemistry, astrochemistry, and atmospheric chemistry. O(¹D) is known to react via highly exothermic insertion mechanisms with X-H bonds, where X is H, C, N, or O. Our lab uses these reactions to generate unstable prebiotic molecules and study them in the gas phase using a supersonic expansion and rotational spectroscopy. This technique has been benchmarked by producing and detecting methanol from O(¹D) insertion into methane, and vinyl alcohol from O(¹D) insertion into ethylene. We now wish to apply this approach to the production of prebiotic molecules of astrochemical interest. We present here our efforts to investigate the reaction between formic acid (HCOOH) and O(¹D) to produce carbonic acid (H₂CO₃).

Accessing simple molecules is crucial to our understanding of chemical reactivity. Particularly, characterization of heavier homologues of molecules essential to life, e.g., carbonic acid and urea, can lead to new insights. Synthetic routes to such compounds, followed by exploration of their reactivity, could not only deepen our knowledge of synthetic chemistry, but also create the basis for novel applications and new research fields. It is known that the reactivity and properties of a compound are a function of its structure. Subsequently, exploring molecular structure in an isolated environment in the absence of lattice and solvent effects is key to our understanding of the structure vs properties relationship. Carbonyl diisothiocyanate (CDIT), C₃N₂O₂S₂, a highly unstable and reactive compound, was first synthesized in 1902.¹ It recently has been characterized in the condensed phase,² however, to the best of our knowledge, this is the first study addressing its properties in the gas phase. The rotational spectra of two isomers of CDIT, syn-syn and syn-anti, were recorded with Chirped Pulse Fourier Transform Microwave (CP-FTMW). Presence of two quadrupolar nuclei (N, I=1) results in hyperfine splitting of rotational transitions, which is analyzed and provides a direct insight into the electronic environment of the molecule. Detection of ¹³C, ³⁴S, ¹⁵N and ¹⁸O isotopologues in their natural isotopic abundances allowed for structure determination of the most abundant syn-syn isomer in the gas phase. The gas phase structure is compared to the previously determined crystal structure and demonstrates a high level of consistency. Spectral analysis is guided by quantum chemical calculations and CDIT, owing to the isothiocyanate substituents, makes an interesting case for theoretical benchmarking. ¹ Dixon, A. E., Proc. Chem. Soc., London, 1902, 18 (257), 235 ² Pfeiffer, J.; Trost, C.; Pachkovska, A.; and Tambornino, F.; Inorg. Chem, 2021, 60 (14) 10722–10728

The class of polycylic aromatic hydrocarbons (PAHs) presents an important motif in building the chemical environment of the interstellar medium. Around 10 to 25% of all interstellar carbon is thought to be locked up as PAHs. Recently benzonitrileB. A. McGuire et al., Science 359 (2018) 202–205.s well as 1- and 2- cyanonapthalene (C₁₀H₇CN), have been identified in radio observations of TMC-1B. A. McGuire et al., Science 371 (2021) 1265–1269.ased on their laboratory rotational spectra. The detection of these molecules raises possibilities for the presence/formation of other cyano-aromatic molecules in the interstellar medium.
In this work, we present chirped-pulse Fourier transform microwave (CP-FTMW) spectra of 2- and 4-cyanobiphenyl (CBP) [C₁₂H₉CN], that are cyano-aromatic molecules. They are measured in the frequency range from 11.5 to 27 GHz. The Cologne CP-FTMW instrument has been designed to achieve high stability and sensitivity, which made it possible to measure the ¹³C isotopologs in natural abundance. In a recent modification to the instrument, we are using state-of-the-art RF modulation and detection technology to directly generate and receive signals in this frequency range, thus abandoning the up- and down-mixing processes of our previous chirped-pulse microwave spectrometer setupM. Hermanns et al., Journal of Molecular Spectroscopy 358 (2019) 25-36. As a result, the tedious side-band separation is no longer needed. Moreover, the number of elements influencing the intensities is reduced to a minimum. The setup and the results of the CBP measurements will be presented.

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Footnotes:

B. A. McGuire et al., Science 359 (2018) 202–205.a B. A. McGuire et al., Science 371 (2021) 1265–1269.b M. Hermanns et al., Journal of Molecular Spectroscopy 358 (2019) 25-36..

The presence of a fluorine atom in fluoronaphthalenes (C₁₀H₇F) induces both mesomeric and inductive effects. The mesomeric effect results in electron redistribution, making particular carbon atoms susceptible to undergo nucleophilic attack at meta-position and electrophilic attack around ortho- and para- positions, while the inductive effect creates a polar bond that affects the polarity and the stability of the molecule and makes the para-position pre-dominant for the electrophilic attack. By studying the rotational structure in both the ground and excited vibrationally states, the higher order terms of rotational constants inform on molecular distorsion relatively to the equilibrium structure from a reference frame in the molecular rotation model. Furthermore, the Coriolis or rotation-vibration terms which involve both the time variation of molecular displacement and the angular velocity of the frame can be characterized to demonstrate the influence of fluorine group on the variation of molecular flexibility in vibrational states. Our present study reports the rotational spectroscopic measurements of 1- and 2-fluoronaphthalene in the millimeter-wave spectral region using both a chirped-pulse instrument (75-110 GHz) and a frequency multiplication chain-based spectrometer (140-220 GHz). By combining newly recorded transitions of fluoronaphthalenes with existing literature dataCarey, S. J., Sun, M., & Kukolich, S. G. (2014). Microwave spectra of 1-fluoronaphthalene and 2-fluoronaphthalene. Journal of Molecular Spectroscopy, 304, 25-27. a refined set of spectroscopic parameters is derived for each species both in the ground and excited vibrational states up to 300 cm⁻¹. This study could serve as a valuable database for future investigations about the role of halogen groups in aromaticity.

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Footnotes:

Carey, S. J., Sun, M., & Kukolich, S. G. (2014). Microwave spectra of 1-fluoronaphthalene and 2-fluoronaphthalene. Journal of Molecular Spectroscopy, 304, 25-27.,

The gas-phase rotational spectrum of 1-cyano-2-methylencyclopropane (C₁, C₅H₅N), an isomer of pyridine first generated by the photoisomerization of (cyanomethylene)cyclopropane, has been synthesized by dehydration of its corresponding amide. Its rotational spectrum has been obtained from 235 GHz to 500 GHz and over 3600 a-, b-, and c-type transitions for the ground vibrational-state have been least-squares fit to partial octic, A- and S-reduced Hamiltonians with low statistical uncertainty (σ_fit = 42 kHz). Transitions for the two lowest-energy fundamentals (ν₂₇ and ν₂₆) and the lowest energy overtone (2ν₂₇) have been similarly least-squares fit to single state Hamiltonians. Many additional vibrationally excited states have been observed, which form a complex polyad of interacting states. The spectroscopic constants presented here form the foundation of future searches for 1-cyano-2-methylencyclopropane in the interstellar medium.

The rotational spectrum of 2-formylthiophene (C_s, μ_a = 3.9 D, μ_b = 2.4 D) has been observed from 2 to 750 GHz and over 8500 transitions have been observed, measured, and least-squares fit for the ground vibrational state. The extensive frequency coverage allowed measurement of transitions up to J = 180 and K_a = 72. Spectroscopic constants have been obtained for a complete sextic distorted-rotor A- and S-reduced Hamiltonians, in the I^r representation. The first two vibrationally excited states of 2-formylthiophene are the torsional mode (ν₂₇, A^′′, 122 cm⁻¹) and the in-plane C-C-O bend (ν₁₉, A^′, 173 cm⁻¹) of the formyl group. These two vibrationally excited states exhibit rotational transitions with frequencies perturbed by a- and b-axis Coriolis coupling despite an energy gap of nearly 50 cm⁻¹. Rotational transitions for the first two vibrationally excited states have been assigned, measured, and least-squares fit to a two-state Hamiltonian, which will provide an accurate and precise energy gap and Coriolis-coupling constants for these two modes.

Pyrimidine is an aromatic heterocycle in which meta C-H units of benzene are replaced by nitrogen atoms. Its detection in the ISM has been sought for decades as it is not only a building block of many biologically relevant molecules, but it could serve as a valuable tracer molecule for other aromatic and polycyclic aromatic compounds. In this work, we collected the millimeter-wave spectrum of pyrimidine from 130-750 GHz. This extended range greatly increases the range of rotational transitions we can observe in the ground and its nine lowest-energy vibrationally excited states. Over 10,000 transitions for the vibrational ground state have been least-squares fit to a A- and S-reduced sextic distorted-rotor Hamiltonians. To supplement the millimeter-wave spectrum, high-resolution infrared spectra were obtained from the Canadian Light Source and provide highly accurate and precise band origins for the vibrationally excited states. The three lowest energy fundamentals (ν₁₆, ν₁₁, and ν₂₄) have been fit to single-state Hamiltonians. The remaining vibrationally excited states presented in this work have perturbed frequencies due to Coriolis, Darling-Dennison, and Fermi coupling. The current progress toward a satisfactory treatment of these interactions will be presented.

2-Furonitrile and 3-furonitrile are highly polar CN-substituted derivatives of furan. These molecules are attractive targets for interstellar search due to their strong dipole moments (μ_a = 4.3 D, μ_b = 0.7 D and μ_a = 4.0 D, μ_b = 0.4 D, respectively) and their ability to serve as tracer molecules for furan. In our recent work, we analyzed and assigned the millimeter-wave and high-resolution infrared (IR) spectra of 2- and 3-furonitrile for the first time. From 140-750 GHz and 140-500 GHz, respectively, the vibrational ground-state transitions of each molecule have been least-squares fit to partial octic, distorted-rotor Hamiltonians with each data set containing several thousand independent transitions. The two lowest-energy fundamental modes of both furonitriles are the Coriolis-coupled bending modes of the nitrile (ν₁₇ and ν₂₄). High-resolution infrared spectra were obtained from the Canadian Light Source and provided the precise band origins of these modes for both furonitriles, as well as of ν₂₃ for 2-furonitrile. We recently reported our work on 2-furonitrile [1] and will discuss our progress in the analysis of the ground state and first two fundamental modes of 3-furonitrile. [1] Millimeter-Wave and High-Resolution Infrared Spectroscopy of 2-Furonitrile - A Highly Polar Substituted Furan. J.   Phys.   Chem.   A, ASAP (2023)

Musk odorants are key compounds in perfumery due to their warm, sensual and animalistic scent, and their fixative properties. Natural musks are macrocycles typically containing ketone or lactone functional groups. However, despite their widespread use, the molecular features conductive to musk smell are not clear. No experimental information is available on the conformations of macrocyclic musks and the structural elements determining their binding to odorant receptors. Here we present the study of several prototypical macrocyclic musks using broadband microwave spectroscopy in combination with quantum chemistry calculations. Their conformations have been identified and compared to start understanding the molecular determinants that lead to musk odour.