We collected the rotational spectrum of chlorobenzene (C₆H₅Cl, μ = 1.7117(14) D) in the frequency range of 130 – 360 GHz. For the [³⁵Cl]- and [³⁷Cl]- isotopologues, the ground states and a total of at least 15 excited vibrational states have been analyzed and fit as distorted rotors to sextic, A-reduced Hamiltonians with low-errors (σ_fit < 50 kHz). This analysis allowed for precise determination of the vibration-rotation interaction constants for each vibrationally excited state and demonstrated these constants are in quite close agreement with their predicted (B3LYP/6-311+G(2d,p)) values. For the lowest-energy vibrational states, transitions with high K_a and low J include hyperfine-resolved transitions of sufficient intensity to determine the quadrupole coupling constants. These data provide an exhaustive analysis of all accessible vibrational states in our frequency region and expand upon previously measured microwave transitions of chlorobenzene.

Thiophene is a five-membered, aromatic heterocycle with C_2v symmetry and a moderate dipole (C₄H₄S, μ_a = 0.55 D). A number of fundamental states of thiophene had previously been investigated through ro-vibrational studies in the infrared, but pure rotational spectroscopy had not extended beyond the ground state. We have measured and assigned transitions for 19 excited states by rotational spectroscopy, three of which are the IR inactive, A₂ symmetry states, and several of which are overtones or combination states. The new mm-wave data (40 - 360 GHz) and the previously published high resolution IR data are combined in a global least-squares fit. The interactions between the states permit the determination of several precise fundamental frequencies beyond those previously directly measured by high-resolution IR. The vibrationally excited states treated here are connected by a Fermi interaction and complex set of multiple Coriolis couplings, many of which are determined by the global fit. Several of these states may be treated as isolated vibrational states, but other states are involved in a pentad, a triad, and at least three dyads.

2-Cyanopyrimidine and 2-cyanopyrazine are attractive molecules for interstellar detection via radioastronomy because they are the pyrimidine and pyrazine analogs of the detected interstellar molecule, benzonitrile, and have substantial dipole moments (μ _a = 6.47 D and μ _a = 4.22 D, respectively). In the present work, we have analyzed and assigned the rotational spectrum of 2-cyanopyrimidine (C₅H₃N₃) and 2-cyanopyrazine (C₅H₃N₃) for the first time. Using data in the 130 – 360 GHz frequency region, the vibrational ground states of 2-cyanopyrimidine and 2-cyanopyrazine have been least-squares fit to partial octic, distorted-rotor Hamiltonians (N_lines  ∼  4000, σ = 40 kHz, and N_lines  ∼  3900, σ = 39 kHz, respectively). The resulting spectroscopic constants are sufficient for spectral prediction outside of the studied frequency range and enable, for the first time, radioastronomical searches for these molecules. Work currently continues on the least-squares fit of the Coriolis-coupled dyad of the two lowest-energy vibrationally excited states for each molecule. The progression of each least-squares fit will be presented along with a comparison between the two isomers. The complete analysis of these states is expected to yield a highly precise energy separation between these vibrationally excited states, along with an interesting comparison to the analogous dyads of benzonitrile and the cyanopyridines.

Recent advances in microwave spectroscopy, allowing for the measurement and fitting of thousands of spectral lines for a given chemical system, have prompted a need for high accuracy predictions of spectroscopic constants. The quartic Centrifugal Distortion (CD) constants are derived at fourth-order in Rayleigh-Schrödinger Vibrational Perturbation Theory (VPT4). Analytical expressions are presented. The constants are implemented in the CFOUR software package in both an explicit sum-over-states form and the analytical (i.e., algebraic) form. The expression for VPT4 quartic CD can be broken into ten distinct contributions, involving products of force constants, Coriolis constants, and coefficients in the expansion of the inverse moment of inertia tensor. It is considerably more complicated than the VPT2 vibration-rotation interaction constants and the VPT4 sextic CD constants. The quartic CD constants first appear at VPT2. The VPT4 level of approximation introduces corrections that are linear in the vibrational quantum numbers. Approximately linear relationships have been identified in analyses of microwave spectra, which allow for direct comparison with the computed CD constants. The VPT4 quartic CD constants require a partial quartic force field, containing all force constants except those for which all indices are different (i.e., ϕ_ijkl). As this truncation of quartic force field is usually computed for VPT2 vibrational frequencies, it will be possible to obtain the CD constants alongside routine VPT2 frequencies with negligible added cost.

In the last year, a paper was published by our group describing a multi-antenna detection, chirped-pulse Fourier transform microwave (MAD-CP-FTMW) spectrometer. A talk will also be given at this conference regarding this instrument and its design. The major unique aspect of the spectrometer, though, is that it possesses antennae in the traditional (0 degrees), quadrature (two at 90 degrees), and broadcast (180 degrees from the traditional antenna) locations. It was discovered that signal could be detected on the antennae that were not in line with the excitation. Because signal was able to be collected from nontraditional locations, the determination of the phase differences in the free induction decays from differing detection locations has become very important. The methods used to determine the phase shift of collected data, as well as the methods of shifting the data will be discussed.

The coupling of the quadrupolar moment of a nucleus to the molecular rotation causes hyperfine splitting of the rotational transitions, which provides important information about electronic environment and therefore chemical properties. It occurs when the molecular rotation couples with the nuclear spin. Since the coupling is highly sensitive to the electric field gradient, it is useful for structure determination and cross-validation of predicted electric field gradients by quantum chemical calculations. The series of mono-halogenated benzaldehydes containing fluorine has already been studied W. Sun, I.B. Lozada, J. van Wijngaarden, J. Phys. Chem. Chem. A, 2018, 122, 2060.^,J. L. Alonso, R. M. Villamañán, J. Chem. Soc., Faraday Trans. 2, 1989, 85, 137−149. We expand on the series by replacing fluorine with a quadrupolar halogen atom (I(Cl)= J. L. Alonso, R. M. Villamañán, J. Chem. Soc., Faraday Trans. 2, 1989, 85, 137-149..S. Arnold, J. Garrett, G. Brown, 68. Symposium ISMS, RK15, 2013.a

High resolution infrared absorption spectra of cyclohexane (C₆H₁₂) have been recorded from 1100 to 4000 cm⁻¹ at room temperature and 241 K. Cyclohexane is an oblate symmetric top with D_3d symmetry. A rotational analysis was obtained for the ν₂₇ (e_u) and ν₁₄ (a_2u) CH₂ scissor modes at 1452.9 cm⁻¹ and 1456.4 cm⁻¹, respectively. Several combination modes were also assigned and rotationally analyzed. The C-H stretching modes are perturbed by overtone and combination modes of the CH₂ scissor vibrations, and an anharmonic local mode calculation was needed to interpret the spectra. The 4 main strong allowed C-H stretching modes appear as two e_u a_2u pairs near at 2862 cm⁻¹ and 2933 cm⁻¹. The Fermi-resonance local mode model coupling terms give physical insight into the effects that organize the cyclohexane vibrational energy levels. The unstrained cyclohexane molecule is a useful paradigm for six-membered rings in larger chemical and biological systems.

r0pt Figure The two isomers (E and Z) of the aromatic imine phenylmethanimine (PMI) have been investigated by means of a multidisciplinary approach involving organic synthesis, nuclear magnetic resonance and rotational spectroscopy, supported and guided by computational chemistry. The choice of this molecule has been driven by its twofold relevance in chemistry, in the fields of organic synthesis and astrochemistry. To face both aspects, an accessible synthetic approach to generate stable imine-intermediates in the gas phase and in solution has been introduced. Exploiting this formation pathway (which is based on the thermal decomposition of hydrobenzamide, HBA) and thanks to a state-of-the-art computational characterization of PMI, we laid the foundation for its first laboratory observation by means of rotational spectroscopy. At first, the 3-26 GHz range has been investigated for E- and Z-PMI using a Fourier-transform microwave spectrometer. Successively, the measurements have been extended in the 83-100 GHz range for E-PMI exploiting a frequency-modulation millimeter-wave spectrometer. Both isomers have been accurately characterized, thus providing a reliable basis to guide future astronomical observations. Furthermore, the feasibility of this synthetic approach in solution has been assessed by nuclear magnetic resonance spectroscopy. Finally, the temperature dependence as well as possible mechanisms of the thermolysis process have been examined exploiting both NMR and rotational spectroscopy techinques.

2-chloroethanol, the simplest chlorohydrin, is known to exist in 5 distinct conformers. The lowest energy conformer is gauche-gauche, designating the internal rotation around the C-C bond and the C-O bond, respectively. High resolution microwaveR. G. Azark, E. B. Wilson, J. Chem. Phys., 52, 5299 (1970)nd far-infraredR. M. Soliday, H. Bunn, I. Sumner, P. L. Raston, J. Phys. Chem., 123, 1208-1216 (2019)pectra of the gauche-gauche conformer have been reported in the literature. 2-choloroethanol may play a role in interstellar chemistry. Since HCl constitutes at least 90% of the chlorine in the interstellar medium and its reaction with other known interstellar constituents (i.e., oxirane or ethylene glycol) is predicted to form 2-chloroethanol. However, attempts at its detection towards Sgr B2(N) have been unsuccessful. It is not clear if the lack of detection arises from its lack of presence in this sightline, its weak spectrum, or whether the extrapolated spectral information for 2-cholorethanol is not of sufficient accuracy to guide astronomical searches. Therefore, we have measured the spectrum of 2-chloroethanol in the millimeter/submillimeter region to further improve the constants and provide spectral frequencies directly comparable to telescopes such as ALMA. In this talk we will report on the experimental results and the progress on analysis of the 2-chloroethanol spectrum.

---

Footnotes:

R. G. Azark, E. B. Wilson, J. Chem. Phys., 52, 5299 (1970)a R. M. Soliday, H. Bunn, I. Sumner, P. L. Raston, J. Phys. Chem., 123, 1208-1216 (2019)s

2-Cyclopropylideneacetonitrile (C₅H₅N, C_s, μ_a = 4.4 D, μ_b = 1.2 D) is a nitrile-containing structural isomer of pyridine. While pyridine has not been detected in the interstellar medium, its nitrile-containing isomers remain attractive targets for spectroscopists and radioastronomers. We have recently generated 2-cyclopropylideneacetonitrile via a two-step synthesis and obtained its rotational spectrum from 130 – 360 GHz. We have assigned and least-squares fit several thousand observed transitions to A- and S-reduced, octic Hamiltonians with low statistical uncertainty (σ less than 50 kHz). The work has yielded precise rotational constants that are in good agreement with their B3LYP and CCSD(T) values. The spectra of several vibrationally excited states have also been observed and assigned using their computed vibration-rotation interaction constants (A₀-A_v).

Despite the chemical richness of Orion KL, one of the ubiquitous aldehydes - acetaldehyde (CH₃CHO; AA) - has been only tentatively identified toward this region. We have investigated an extensive ALMA archive dataset of which frequency coverage spreads between 142 GHz - 355 GHz to search for AA and other unexplored aldehydes. Four emission components of AA are clearly identified toward Orion KL, showing its main emission peak towards hot core-SW (HC-SW). The column densities of AA and its kinetic temperatures is estimated toward this main emission region, assuming molecular excitation under local thermodynamic equilibrium conditions. The distribution of acetaldehyde is compared with the other aldehyde-like (CHO-bearing) species from the literatures such as methyl formate (CH₃OCHO;MF), glycolaldehyde (CH₂OHCHO;GA) and formic acid (HCOOH;FA). AA shows the abundance and the spatial distribution similar to that of FA, implying the chemical relation between them. The relative abundance ratios between the aldehyde-like species (hereafter, relative aldehydes ratios) towards HC-SW are investigated with a chemical model. The model shows that relatively longer collapsing timescale and the MF binding energy similar to water are needed to explain the observation. The relative aldehydes ratios estimated from the model sensitively depend on the assumed kinetic temperature. This explains the high spatial variability of the relative aldehydes ratios from the observations.

Isoprene also known as 2-methyl-1,3-butadiene (CH₂=C(CH₃)-CH=CH₂) is a biologically important unsaturated hydrocarbon produced by many plants and animals. For example, it is present in exhaled human breath at a few hundred part per billion level of volumetric dilution. It is also thought to be contributing to the infrared spectra of interstellar carbonaceous dust, and thus can be relevant for analysis of interstellar gas clouds. In this presentation we report on our recent analysis of the rotational spectra of isoprene in its vibrational ground state recorded in 210 – 500 GHz spectral range.

Diacetyl is a common food flavoring, used most commonly in microwave popcorn. In addition to being a molecule of culinary significance, diacetyl is also a molecule of atmospheric interest, as it is an important source of HO_x in the atmosphere. Of specific interest to astronomy, diacetyl was recently made in an interstellar ice analogue. Despite the number of studies on its vibrational characteristics, there have been no rotational studies done on this molecule. In this talk we will present the experimental results and preliminary assignment of the diacetyl rotational spectrum from 20-1000 GHz. This spectrum will allow for the potential detection of this molecule in the interstellar medium.

r0pt Figure Simple aromatic molecules may be precursors for polycyclic aromatic hydrocarbons in space, and some of the simplest ones are now detected in the earliest stages of star formation [1]. Evidence of benzonitrile (C₆H₅-CN) in the interstellar medium [2] questions the presence of related aromatic nitriles and their ring-chain derivatives. In light of previous work [3] on phenylpropiolonitrile (C₆H₅-C₃N), we are investigating, by laboratory high-resolution studies, its ethynylbenzonitrile (HCC-C₆H₄-CN) derivatives where a -CN and a -CCH groups lie in ortho (2-ETB), meta (3-ETB) or para (4-ETB) positions. The pure rotational spectrum of these compounds has been recorded at room temperature in the millimeter-wave domain using a chirped-pulse W-band (75-110 GHz) spectrometer. To facilitate spectral assignments, quantum chemical calculations have been performed using density functional theory at the ωB97X-D/cc-pVQZ level of theory (geometry optimization, harmonic frequencies). We will report a description of the experimental set-up and of our assignment procedure.

[1] A.Burkhardt et al., Nature Astronomy 5, 181-187 (2021) [2] McGuire et al., Science 359, 202-205 (2018) [3] Z.Buchanan et al., Journal of Molecular Spectroscopy, in press (2021)