Currently investigated molecules

  • Acetone-13C
    One of the isotopolog species currently investigated at SFB 956 is singly 13C-substituted acetone, (CH3)2CO, a typical mid-size species first interstellarly detected in 1987. In a PhD thesis affiliated to sub-project B3, rotational absorption spectra of both isotopolog forms (Figs. 1 and 2) are measured in the laboratory below 1 THz and analyzed. Empirically-based predictions of rotational spectra, like that of acetone-2-13C depicted in Fig. 3, allow for the detection of a molecule in interstellar gas clouds by observing molecular emission. Another attractive aspect is to compare the results for acetone with those for similar molecules like dimethyl ether, a molecule which has previously been studied within this project [SET LINK HERE].


    Fig. 1: Acetone-1-13C                                           Fig. 2: Acetone-2-13C


    Fig. 3: Predicted rotational spectrum for the rotational-torsional ground state of acetone-2-13C. x-axis: Frequency in MHz; y-axis: Absorption in arbitrary units. The color scheme reflects different types of rotational transitions.

    Besides its weak rotational symmetry (asymmetry parameter < + 0.38), additional complexity in the acetone spectrum arises from the internal motion of both methyl groups, which may couple to overall rotation via four channels (five for the methyl isotopolog) discerned by symmetry. Moreover, the rotational spectra of two low-lying torsionally excited modes appear with comparable intensity at room temperature, resulting in a threefold line density compared to the ground state spectrum. The excited states cannot be neglected because the temperature in hot cores is of the same order of magnitude as room temperature.

    A collaboration has been established with Dr. Peter Groner (University of Missouri-Kansas City), a renowned specialist in molecules featuring internal rotation who did the first measurements and analysis in the microwave range. Deploying Groner's dedicated software ERHAM, an effective rotational-torsional Hamiltonian is fit to the measured spectrum, based on which the spectrum can be predicted.
     

  • Ethanol-d1
    In 2012, first spectra of anti-13C1 and 13C2 isotopologs of ethanol have been measured in collaboration with A. Bouchez, Toulouse. Now we want to deepen the understanding of ethanol by investigating the three isotopologs of singly deuterated ethanol (C2H5OD, CH3CHDOH and CH2DCH2OH). In the framework of his master thesis, Mirko Schäfer will observe new spectral lines of these conformers to calculate new molecular constants.
    This work builds upon the results of J. P. Culot, who published first spectral measurements of deuterated ethanol between 26 and 58 GHz in 1969. From these data, we predicted the spectra for higher frequencies with H. M. Pickett's programs SPFIT and SPCAT. Our new data enables us to successively refine predictions in close collaboration with A. Walters, Toulouse.
    Isotopically enriched samples needed for spectroscopy are commercially available in liquid state, the amounts of which being very small due to high price levels, especially for CH2DCH2OH. As of May 2013, absorption lines have been measured between 35 and 130 GHz for all isotopologs; for C2H5OD and CH3CHDOH also between 210 and 370 GHz with a backward wave oscillator spectrometer. Future measurements will cover ranges at 400 / 500 GHz to improve predictions also for higher quantum numbers.
    [ADD MISSING FIGURES!]
     
  • Methyl and ethyl mercaptane
    Methyl mercaptan (CH3SH) and ethyl mercaptan (C2H5SH) are similar to the better known alcohols methanol (CH3OH) and ethanol (C2H5OH), where the the oxygen is replaced by a sulfur atom.

    Terahertz and Fourier transform far-infrared (FTFIR) synchrotron spectra of methyl mercaptan have been investigated in order to provide new laboratory information for enhanced observations of this species in interstellar molecular clouds and star-forming regions. Like its methanol cousin, methyl mercaptan has particularly rich spectra associated with its large-amplitude internal rotation that extend throughout the THz and FIR regions. We have recorded new spectra for CH3SH from 1.1-1.5 and 1.790-1.808 THz at the University of Cologne as well as high-resolution FTFIR synchrotron spectra from 50-550 cm-1 at 0.001 cm-1 resolution on the far-IR beam-line at the Canadian Light Source. The THz and FTFIR measurements together with literature results have been combined in a global analysis of a dataset comprising a total of 1,725 microwave and THz frequencies together with ~18,000 FTFIR transitions, ranging up to vt = 2 and Jmax = 30 for MW/THz and 40 for FTFIR. The global fit employs 78 torsion-rotation parameters and has achieved a weighted standard deviation of ~1.1. Predictions have been generated from the model, giving essentially complete coverage of observable CH332SH transitions within the bandwidths of major new astronomical facilities such as HIFI (Heterodyne Instrument for the Far Infrared) on the Herschel Space Observatory, ALMA (Atacama Large Millimeter Array), SOFIA (Stratospheric Observatory For Infrared Astronomy) and APEX (Atacama Pathfinder Experiment) to close-to-spectroscopic accuracy.

    Results on the microwave spectrum of ethyl mercaptan were first published by R. E. Schmidt and C. R. Quade in 1975. Spectral assignments of ethyl mercaptan in the millimeter wave and THz range will be made by Juliane Gerke in the course of her PhD thesis.
    Publications:

    • Xu, Li-Hong; Lees, R. M.; Crabbe, G. T.; Myshrall, J. A.; Müller, H. S. P.; Endres, C. P.; Baum, O.; Lewen, F.; Schlemmer, S.; Menten, K. M.; and Billinghurst, B. E.: Terahertz and far-infrared synchrotron spectroscopy and global modeling of methyl mercaptan, CH332SH. The Journal of Chemical Physics, 137, 104313 (2012)
       
  • Spectroscopy of ions
    Ions and radicals play a very important role in interstellar chemistry because the physical conditions of the ISM (interstellar medium), i.e. low temperatures and densitites, result in reasonably long lifetimes of these terrestriarly unstable molecules. Even before ions were detected in dense interstellar clouds, it had been suggested that these ions play a prominent role in gas-phase chemistry. Unlike most neutral-neutral reactions, ion-neutral reactions have no activation energy barrier, which is especially true in the cold regions, so they play a crucial role despite the low fractional abundance in dense clouds. The production of these molecules in the laboratory is not trivial, one of the most successfull methods used in the last 20 years has been the DC glow discharge.
    1. Spectroscopy of CO+
      The abundance of CO+ becomes important in hot layers of photon-dominated regions (PDRs). There are many open questions related to the abundance of CO+ in PDRs. One example is the process of excitation upon formation, which leads to rotational excitation temperatures as low as 10 K. This problem has not been fully understood also because of the lack of laboratory spectroscopic data at high frequencies. CO+ has been studied in several isotopically substituted species from 300 GHz to 1.3 THz. The submillimeter-wave spectra of CO+ (v = 0 and v = 1), 13CO+ and C18O+ have been measured through a hollow cathode DC discharge in a cryogenic cell cooled to liquid nitrogen temperature between 300 and 900 GHz. In addition, some transitions of the main isotopolog have been measured between 1.1 and 1.3 THz. An updated isotopically invariant fit including the Born-Oppenheimer breakdown corrections has been made: The derived set of independent molecular parameters is valid for all isotopologs of the molecule and it allows to predict the rotational spectrum at high frequencies with an accuracy of 280 kHz at 2 THz.
      CO+ was produced in a hollow cathode DC discharge of pure CO, for the 13CO+ measurements a 99% enriched sample was employed, while C18O+ was measured in natural abundance. A cryogenic discharged cell was used: The cell walls were cooled to liquid nitrogen temperature for all measurements. The radiation sources employed in the 200-900 GHz range are phase-locked backward-wave oscillators (BWOs). At 1.1-1.3 THz the radiation is generated by a commercially available amplifier/multiplier chain (Virginia Diodes), which generates the 72nd harmonic of a 18 GHz synthesizer signal (Rohde & Schwarz). The measured and predicted high-N transitions of CO+ will be useful for future observations: Ideal test candidates would be planetary and proto-planetary nebulae.
    2. New discharge cell
      A new design has been developed for a cryogenic discharge cell which will be used for the production of ions and other transient molecules. Currently preliminary tests are made to test the new cell and the production of ions inside it. The new cell is a double-wall cell and it will ensure a better cooling of the system. It is easier to handle than the previous one and allows a better control of the gas flow.
    3. First Interstellar Detection of c-C3D2
      c-C3D2 had been detected toward the starless cores TMC-1C and L1544 using the IRAM 30m telescope. Three transitions in the 3-mm range have been observed in both sources with high signal to noise ratio, better than 7.5 sigma in TMC-1C and 9 sigma in L1544. The abundance of doubly deuterated cyclopropenylidene with respect to the normal species is found to be 0.4-0.8% in TMC-1C and 1.2-2.1% in L1544. The deuteration of this small hydrocarbon ring has been analysed with a comprehensive gas-grain model, the first including doubly deuterated species. The observed abundances of c-C3D2 can be explained solely by gas-phase processes, supporting the idea that c-C3H2 is a good indicator of gas-phase deuteration. Doubly deuterated cyclopropenylidene appears to be a very interesting probe for the earliest stage of star formation. Unlike most of the six known multiply deuterated species observed in the radio band (CHD2OH, NHD2, D2CO, D2S, and D2CS), c-C3H2 and its deuterated isotopologues are believed to form solely by gas phase reactions. The interplay between the gas phase and grain surface reactions in the deuteration of interstellar molecules is not clear so far, partially because there are not many probes available for testing the models: c-C3D2 is an ideal molecule for this purpose. Its formation mechanism puts important constraints on gas-phase deuteration models and suggests the possibility of using c-C3D2 as a chemical clock.
      Fig. 1: A CO+ spectral line observed in two different types of discharge
       
  • 1,2-propane diol
    Propane diol, C3H6(OH)2, is a good molecule where isomerism can be deployed to understand reaction mechanisms under interstellar conditions. With one of its two OH groups always residing on the leading carbon atom, the second one may be connected to either the second carbon atom (1,2-propane diol) or the trailing one (1,3-propane diol). Comparing interstellar abundances of these similar species allows astrochemists for probing differences in the reactions leading to carbon-oxygen molecules. An open question is whether the oxygen in an observed species stems mainly from the highly abundant CO or whether it is added later to an existing carbon-chain molecule. It is also interesting to check for differences in reaction routes depending on the particular formation conditions, especially in the interfaces between separated carbon- and oxygen-chemistry regions.
     
  • Carbonyl sulfide
    Carbonyl sulfide (OCS) is a molecule which, though firstly spectroscopically investigated by Russian scientists as early as 1979, is still very interesting for astrochemical research due to its various isotopologs, which may be observed comparatively easy thanks to its large dipole moment of 0.715 Debye. For laboratory measurements of OCS isotopologs one has to keep in mind that the abundances in a normal, i.e. unenriched sample are very low (Table 2). They can be derived from the natural isotopic abundances of carbon, oxygen, and sulfur (Table 1).
     
    12C 13C 16O 17O 18O 32S 33S 34S 36S
    98.9 1.1 99.762 0.038 0.200 95.02 0.75 4.21 0.02

    Table 1: Terrestrial abundances of the stable isotopes of carbon, oxygen and sulfur.
     

    Configuration % Configuration % Configuration % Configuration %
    12-16-32 93.75 12-16-33 7.400E-1 12-16-34 4.154 12-16-36 1.973E-2
    13-16-32 1.042 12-16-33 8.230E-3 13-16-34 4.620E-2 13-16-36 2.195E-4
    12-17-32 3.571E-2 12-16-33 2.819E-4 12-17-34 1.582E-3 12-17-36 7.516E-6
    13-17-32 3.972E-4 12-16-33 3.135E-6 13-17-34 1.760E-5 13-17-36 8.360E-8
    12-18-32 1.879E-1 12-16-33 1.484E-3 12-18-34 8.327E-3 12-18-36 3.956E-5
    13-18-32 2.090E-3 12-16-33 1.650E-5 13-18-34 9.262E-5 13-18-36 4.400E-7

    Table 2: Derived abundances of OCS isotopologs in a sample of natural isotopic composition.

    With our newly built absorption spectrometer, featuring a double-pass cell with a total absorption path of 10 meters and an all-solid-state source and detector design covering the 3-mm range, we are currently observing rotational spectral lines from many OCS isotopologs not only in natural abundance, but also for the first time with sub-Doppler resolution, the uncertainty of the observed Lamb-dip peaks being only 4 kHz. [INSERT FIGURES!!] Predictions of spectra derived from these measurements can be used to determine isotopic ratios of carbonyl sulfide in the interstellar medium, which will serve as a starting point for future research on the interplay of interstellar carbon, oxygen, and sulfur chemistry.