M. Winnewisser

The millimeter- and submillimeter-wave spectrum of trans-ethyl alcohol

The rotational‐torsional spectrum of the trans rotational isomer of ethyl alcohol was investigated in the 65–350 GHz frequency region. A total of 481 ground state transitions over a range of J and Ka values up to 33 and 10, respectively, were measured and assigned. Doublets or triplets arising from the A and E torsional states of the v=0 torsional level of the three‐fold‐symmetric methyl internal rotation have been resolved in 168 of these transitions. Internal rotation theory predicts a significant number of c‐type E‐state transitions normally forbidden, but allowed when the rotational asymmetry operators mix E‐state rotational‐torsional levels. Over 40 of these transitions have been observed. The newly measured transitions, along with the results of many previous measurements, have been analyzed using an IAM internal rotation Hamiltonian and a Watson A‐reduced Hamiltonian to determine the rotational, centrifugal distortion, and torsional constants. The IAM analysis accounts for all of the analyzed trans...

A rigorous detection of interstellar CH3NCO: An important missing species in astrochemical networks

The recent analysis of the composition of the frozen surface of comet 67P/Churyumov-Gerasimenko has revealed a significant number of complex organic molecules. Methyl isocyanate (CH3NCO) is one of the more abundant species detected on the comet surface. In this work we report extensive characterization of its rotational spectrum resulting in a list of 1269 confidently assigned laboratory lines and its detection in space towards the Orion clouds where 399 lines of the molecule have been unambiguously identified. We find that the limited mm-wave laboratory data reported prior to our work require some revision. The abundance of CH3NCO in Orion is only a factor of ten below those of HNCO and CH3CN. Unlike the molecular abundances in the coma of comets, which correlate with those of warm molecular clouds, molecular abundances in the gas phase in Orion are only weakly correlated with those measured on the comet surface. We also compare our abundances with those derived recently for this molecule towards Sgr B2 (Halfen et al. 2015). A more accurate abundance of CH3NCO is provided for this cloud based on our extensive laboratory work.

THE MILLIMETER- AND SUBMILLIMETER-WAVE SPECTRUM OF METHYL CARBAMATE (CH3OC(:O)NH2)

The rotational-torsional spectrum of the syn conformer of methyl carbamate [CH3OC(:O)NH2], an isomer of the essential amino acid glycine [NH2CH2C(:O)OH], has been recorded at room temperature in the spectral region from 79 to 371 GHz. Methyl carbamate possesses a methyl group internal rotor, which gives rise to A and E torsional substates, and associated splittings in the rotational spectrum. Almost 6000 new rotational transitions arising from the vibrational ground state have been assigned, about half of them belonging to the E torsional substate. Along with some earlier data, the newly measured lines were assigned and analyzed efficiently by the integration of two program packages: CAAARS, a suite for visual, interactive mouse-assisted line assignment of asymmetric rotor spectra; and ERHAM, a program that solves the effective Hamiltonian for molecules with up to two periodic large-amplitude internal motions. This Hamiltonian was used to fit 28 spectroscopic parameters for the methyl carbamate ground vibrational state to the observed line positions with a standard deviation of 0.081 MHz. With the determined spectroscopic constants and the available dipole moment components, we are able to predict the transition frequencies and intensities of many additional lines through 400 GHz. Methyl carbamate can now be searched for over a wide frequency range in appropriate interstellar sources such as hot molecular cores.

On the high resolution infrared spectrum of HCNO

Abstract The ν 1 fundamental of HCNO has been measured with a resolution of 0.05 cm −1 . Although the molecule is unstable, conditions have been established making it possible to measure the high resolution infrared absorption spectrum. The ground state rotational constant B 0 obtained from the ν 1 band agrees with the value obtained from the microwave work within the accuracy of infrared data.

High-Frequency Rotational Spectrum of Thioformaldehyde, H2CS, in the Ground Vibrational State

Pure rotational transitions have been measured for the normal isotopologue of thioformaldehyde, -->H212C32S , in the ground vibrational state in the 110-370 GHz, 570-670 GHz, and 850-930 GHz frequency ranges. A global data set has been constructed consisting of prior microwave and millimeter-wave transitions, the submillimeter-wave and terahertz (THz) transitions reported here, and far-infrared data and ground state combination differences. The 783 different transition frequencies in the data set were fit to Watsons S-reduced Hamiltonian with 20 parameters, resulting in a dimensionless weighted root-mean-square deviation of 0.78. New frequency predictions have been made for astronomical observations based on the molecular parameters obtained in the present study.

The Millimeter- and Submillimeter-Wave Spectrum of the trans-trans Conformer of Diethyl Ether (C2H5OC2H5)

Since dimethyl ether is found to have a high abundance in hot cores, it is reasonable to search among such sources for detectable abundances of the more complex analogs ethyl methyl ether (C2H5OCH3) and diethyl ether (C2H5OC2H5). Indeed, preliminary detections of both complex ethers have been reported. Definitive interstellar searches do require laboratory spectra, however, but for many years only low-frequency data have been available for both of these species. Following our recent study of the millimeter-wave and submillimeter-wave spectrum of ethyl methyl ether, we report here the study of the millimeter- and submillimeter-wave spectrum of the lowest energy conformer of diethyl ether. With four different spectrometers, over 1000 new spectral lines have been measured and analyzed at frequencies up to 350 GHz. Fitting the data to a set of spectroscopic parameters from the Watson A-reduced form of the asymmetric-top Hamiltonian has allowed us to predict the frequencies and intensities of many more transitions through 400 GHz. A more precise determination of the electric dipole moment of diethyl ether was also carried out, resulting in μ = μb = 1.0976(9) D.

Far-infrared spectrum of S(CN)2 measured with synchrotron radiation: global analysis of the available high-resolution spectroscopic data.

The high resolution Fourier transform spectrum of the chemically challenging sulfur dicyanide, S(CN)2, molecule was recorded at the far-infrared beamline of the synchrotron at the Canadian Light Source. The spectrum covered 50-350 cm(-1), and transitions in three fundamentals, ν4, ν7, and ν8, as well as in the hot-band sequence (n + 1)ν4 - nν4, n = 1-4, have been assigned and measured. Global analysis of over 21,300 pure rotation and rotation vibration transitions allowed determination of precise energies for 12 of the lowest vibrationally excited states of S(CN)2, including the five lowest fundamentals. These results constitute an extensive set of benchmarks for ab initio anharmonic force field calculations and the observed and calculated vibration-rotation constants and anharmonic frequencies are compared. The semiexperimental equilibrium, r(e)(SE), geometry of S(CN)2 has also been evaluated. In the course of the measurements, new information concerning the physical chemistry of S(CN)2 has been obtained.

The Millimeter- and Submillimeter-Wave Spectrum of Cyanoformamide

The organic species cyanoformamide (NCCONH2) is the cyano (CN) derivative of formamide (NH2CHO), a known interstellar molecule with a role in the synthesis of nucleic acid precursors under prebiotic conditions. Until quite recently, the rotational spectrum of cyanoformamide had not been studied. Based partially on the results of a newly published microwave study and on the results of our ab initio calculations, we have measured and assigned 2691 transitions of the millimeter- and submillimeter-wave spectrum of this molecule through 360 GHz with the fast-scan submillimeter spectroscopic technique (FASSST). These transitions have been added to the previously measured 135 microwave transitions to produce a global data set of 2826 transitions that was analyzed to within experimental accuracy with a standard asymmetric-top fitting procedure. With the determined spectroscopic constants and dipole moment components, we are able to predict the frequencies and intensities of many additional lines through 400 GHz. Cyanoformamide can now be searched for over a wide frequency range in likely interstellar sources such as hot molecular cores.

The Infrared Spectrum of CH3OH: Evidence for State Mixings

In the framework of our systematic investigation project of the infrared and far infrared spectrum of CH3OH by high resolution Fourier transform spectroscopy, we have currently assigned more than 35000 absorption lines. Two listings of 12700 assigned lines below 200 cm-1 have already been published1,2, and a listing of almost 8700 lines between 200 and 350 cm-1 is in press.