The molecule 2-butanone, or methyl ethyl ketone (MEK), CH3COCH2CH3CH3COCH2CH3, has been studied from 8 GHz to 1 THz using a combination of chirped-pulse Fourier transform microwave spectroscopy and direct absorption millimeter/submillimeter spectroscopy. This molecule is of interest for the field of astrochemistry because it has functional groups in common with several known interstellar molecules, and therefore could serve as a tracer of grain surface formation pathways for complex organics in interstellar clouds. The results of the spectral studies and the analysis of the ground vibrational state of methyl ethyl ketone up to 1 THz are presented here. The challenges of spectral analysis for an organic molecule with spectral complexity arising both from internal rotation and many low-lying vibrational states are discussed. The performances of several standard fitting packages are compared in terms of handling this challenging spectral analysis problem.Highlights•The rotational spectrum of 2-butanone (methyl ethyl ketone, CH3COCH2CH3) was studied.•Chirped-pulse FT microwave and direct absorption mm/submm spectroscopy was used.•The ground vibrational state has been analyzed using CALPGM, ERHAM, and RAM36.•A low V3 barrier to internal rotation complicates the spectrum.

The pure rotational spectrum of glycolaldehyde has been recorded from 6.5–20 GHz and 25–40 GHz in two pulsed-jet chirped pulse Fourier transform microwave spectrometers. The high phase stability of the spectrometers enables deep signal integration, allowing transitions from the 13C-substituted, 18O-substituted, and deuterium-substituted isotopologues to be observed in natural abundance. Transitions from HCOCH218OH are reported for the first time. Additional transitions from the 13C-substituted, deuterium-substituted, and HC18OCH2OH isotopologues, as well as previously unobserved weak lines from the main isotopologue, have been observed. Transitions from all isotopologues are used with previously reported transitions to refine the spectroscopic parameters for each isotopologue. A Kraitchman analysis was performed using the experimental rotational constants to determine the molecular structure of glycolaldehyde.Graphical abstractHighlights► The rotational spectrum of glycolaldehyde has been recorded up to 40 GHz. ► Transitions from the 13C, 18O, and D substituted isotopologues were observed. ► Transitions from HCOCH218OH are reported for the first time.► A Kraitchman analysis was performed to determine the glycolaldehyde structure.

We present here the instrument design and first experimental results from a multipass millimeter/submillimeter spectrometer designed to probe dissociative reaction dynamics. This work focuses on benchmarking the instrument performance through detection of the CH3O and H2CO products from methanol dissociation induced by a high-voltage plasma discharge. Multiple rotational lines from CH3O and H2CO were observed when this plasma discharge was applied to a sample of methanol vapor seeded in an argon supersonic expansion. The rotational temperature of the dissociation products and their abundance with respect to methanol were determined using a Boltzmann analysis. The minimum detectable absorption coefficient for this instrument was determined to be αmin ≤ 5 × 10–9 cm–1. We discuss these results in the context of future applications of this instrument to the study of photodissociation branching ratios for small organic molecules that are important in complex interstellar chemistry.

A computational study of O(1D) insertion reactions with methanol (CH3OH), dimethyl ether (CH3OCH3), and methyl amine (CH3NH2) was performed to guide laboratory investigations of the insertion product molecules methanediol (HOCH2OH), methoxymethanol (CH3OCH2OH), and aminomethanol (HOCH2NH2), respectively. The minimum energy and higher energy conformer geometries of the products were determined at the MP2/aug-cc-pVTZ level of theory, and CCSD(T)/aug-cc-pVTZ calculations were performed on the reactants, products, and transitions states to examine the insertion reaction energetics. Torsional barriers for internal motion in methanediol, methoxymethanol, and aminomethanol were also determined. It was found that O(1D) insertion into the C–H bond was the most energetically favored reaction pathway, proceeding through a direct and barrierless insertion mechanism. The pathways of O(1D) insertion into N–H or O–H bonds are also possible, though these reactions are less energetically favored, as they proceed through an association product intermediate before proceeding to the insertion products. Predictions are presented for the pure rotational spectra for the methanediol, methoxymethanol, and aminomethanol products based on the determined molecular parameters. These results provide an excellent starting point to guide laboratory spectral studies of the products.

High-level ab initio calculations predict a C2v equilibrium geometry and large permanent dipole for H5+, whereas rigorous Diffusion Monte Carlo calculations on a global potential surface show a completely symmetric zero-point averaged D2d structure for H5+, HD4+, and D5+, resulting in no permanent dipole moment. This dramatic departure from the conventional molecular structure is due to the highly fluxional nature of this cation. For the isotopologues H4D+, H3D2+, and H2D3+, we predict nonzero dipole moments in the ground vibrational state and present corresponding simulated rotational spectra up to 3 THz. These predictions can guide the laboratory studies necessary for observational searches.Keywords: astrochemistry; H5+; pure rotational spectrum; terahertz spectroscopy; zero-point averaged structure;