Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session X26: The Chemical Physics of Molecules in Space IVFocus Live
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Sponsoring Units: DCP Chair: Jordy Bouwman; Leiden University Leah Dodson, University of Maryland, College Park |
Friday, March 19, 2021 8:00AM - 8:36AM Live |
X26.00001: Rovibrational Quartic Force Fields of Metal Dicarbides and Tricarbides Invited Speaker: Nathan DeYonker For gas phase organic molecules, ab initio composite methods exist that are able to predict accurate rovibrational spectra. Typically, fundamental vibrational frequencies can be obtained to within 1 cm-1 of known experimental values. However, the question remains as to whether a similar methodology can be developed for metal-bearing molecules due to issues like scalar relativistic considerations, static (or strong/multireference) correlation, inner/outer core correlation, and other considerations not typically considered in “black box” quartic force field (QFF) computations. Rovibrational spectroscopic properties have been computed for scandium and titanium dicarbide (ScC2 and TiC2) and magnesium tricarbide (MgC3). Overall, we find that complete basis set CCSD(T) QFFs may not always provide the same level of agreement with experiment for inorganic molecules that it does for organic molecules. While agreement between theory and experiment is exceptional for ScC2 and its isotopologues, the ground and low-lying excited electronic states of TiC2 exhibit vibronic coupling and experimental characterization of its pure rotational spectrum is incomplete. Extremely small relative energies between various singlet and triplet isomers of MgC3 dramatically increase the computational cost of the project. Overall, our findings will greatly enhance the knowledge of metal-carbon bonding for laboratory groups working in molecular astrochemistry and laser spectroscopy. |
Friday, March 19, 2021 8:36AM - 8:48AM Live |
X26.00002: Anharmonic Frequencies for the Detection of Large Molecules in Space Brent Westbrook, Ryan Fortenberry Quartic force fields (QFFs) offer highly accurate anharmonic rovibrational spectroscopic constants. The state-of-the-art composite approach is known as CcCR and often achieves accuracies within 1 cm-1 of gas-phase experiment for fundamental frequencies and 20 MHz for principle rotational constants. The performance of explicitly-correlated methods is nearly as good, offering accuracies of 5-7 cm-1 at a much lower computational cost. However, both of these QFF approaches typically rely on a complex internal coordinate system that can be difficult to derive for large or highly symmetric molecules. Extending this methodology to use generic Cartesian coordinates and the analytic derivatives found in many popular quantum chemistry packages will allow the elucidation of such spectral data for larger molecules. Preliminary work on ammonia borane and other even larger molecules demonstrates the efficacy of this new approach. |
Friday, March 19, 2021 8:48AM - 9:00AM Live |
X26.00003: Recent developments in Chirped Pulse in Uniform Flow techniques for the measurement of low temperature reaction product branching ratios Théo Guillaume, Divita Gupta, Brian M Hays, Ilsa R Cooke, Omar Abdelkader Khedaoui, Thomas Hearne, Ian R Sims The CRESU (Reaction Kinetics in Uniform Supersonic Flow) technique has been used with great success to study the kinetics of a large number of gas-phase reactions down to very low temperatures [1]. While overall reaction rates have been measured reliably for a long time, product channel branching ratios are, for the most part, unknown at low temperatures but are needed to improve the accuracy of astrochemical models. This requires a detection technique which is sufficiently sensitive, specific and able to measure multiple species at the same time; the latter severely limiting the use of current LIF (laser-induced fluorescence) methods. A new combination of chirped-pulse spectroscopy with CRESU flows has been recently developed [2], allowing reaction products to be identified by their unique rotational spectra. In Rennes, we have constructed Ka-band and E-band spectrometers and coupled them to continuous CRESU flows, enabling measurement of cold reaction products. Recent technical advances will be presented, along with results on the detection of products from astrochemically relevant reactions of the CN radical with small hydrocarbons down to 10 K. |
Friday, March 19, 2021 9:00AM - 9:36AM Live |
X26.00004: The chemical pathways of O(1D) insertion reactions with methylamine (CH3NH2) Invited Speaker: Susanna Widicus Weaver Many molecules predicted to form from radical-radical chemistry on the icy surfaces of interstellar dust grains are unstable, reactive species that cannot be purchased commercially. Methods to produce these molecules in sufficient quantity to enable their spectroscopic detection are therefore useful tools for laboratory astrochemistry. We have previously successfully employed insertion reactions of oxygen atoms in the (1D) state to make methanol (CH3OH) from methane (CH4) and vinyl alcohol (CH2CHOH) from ethylene (CH2CH2). We are currently using the O(1D) + CH3NH2 reaction to attempt to make aminomethanol, NH2CH2OH, the direct precursor to glycine in the interstellar medium. To do this, we mixed O3 (in O2), CH3NH2, and Ar in the throat of a supersonic expansion. The gas mixture passed through a quartz tube wherein we produced O(1D) from photolysis of O3 at 248 nm. The products of these reactions were then probed downstream in the supersonic expansion using millimeter/submillimeter rotational spectroscopy. A complex chemical network producing formaldehyde (H2CO), methanimine (CH2NH), formamide (NH2CHO), hydrogen cyanide (HCN), and at least two unknown products has been observed. Neither unknown product appears to be aminomethanol; work is underway to identify the molecular carriers. We have employed a chemical kinetics box model using the Framework for 0-D Atmospheric Modeling (F0AM) software to further examine the results that we observe. In this talk we will present the experimental design, the spectroscopic results, the details of the model, and the conclusions that can be drawn about the O(1D) + CH3NH2 reaction network. |
Friday, March 19, 2021 9:36AM - 9:48AM Live |
X26.00005: Watching the Water Dance: A New Way to Monitor Slow Reaction Kinetics on the Molecular Level with Temperature-controlled Nanodroplets Nan Yang, Sean Coleman Edington, Mark Albert Johnson Cryogenic ion spectroscopy has the unique capability of recording the vibrational spectra of specific isomers and isotopomers of mass selected ions at very low temperature. In an exciting recent development, we moved beyond these static pictures by introducing a temperature- and time-dependent ion spectroscopy technique that can follow the kinetics of chemical processes in dynamic equilibrium in a finite system. Leveraging on our previous understanding of the spectral behavior of H2O molecules in a finite hydrogen bonded network, we tracked the time dependent frequency of a single, isolated OH oscillator in a cage of 20 water molecules. The frequency changes are fascinating because, at the onset of spectral dynamics, the oscillator is observed to “blink” between two widely separated frequencies before undergoing more diffusive excursions with increasing temperature as the cluster melts. With this demonstration of the technique, we open a new and exciting chapter on how we can unravel solvent-mediated chemistry in a regime where every atom counts. |
Friday, March 19, 2021 9:48AM - 10:00AM Live |
X26.00006: Rotational quenching of organic molecules by molecular hydrogen Laurent Wiesenfeld, Malek Ben Khalifa, Emna Sahnoun In order to characterize the chemical composition, isotopic enrichments and possibly, history of the interstellar medium, it is necessary to get an accurate estimate of the molecular densities of the polar molecules observed. Since the intensity of the spectral lines depends on both photonic and molecular collisions, knowing the quenching/excitation rates is an essential ingredient, as soon as the spontaneous photon emission rates becomes comparable to the collisional quenching rate. Many complex molecules are observed, and especially so, in pre-stellar and proto-stellar environments. |
Friday, March 19, 2021 10:00AM - 10:12AM Live |
X26.00007: Measuring the differences in collisional interactions of isomers at low temperatures: HCN and HNC with He Brian M Hays, Théo Guillaume, Divita Gupta, Omar Abdelkader Khedaoui, François Lique, Franck Thibault, Ian R Sims HCN and HNC represent the most abundant isomeric system observed in the interstellar medium, even though HCN is much more thermodynamically stable than HNC. For many years, observations of cold interstellar environments have found an apparent overabundance of HNC, a non-intuitive result considering that most mechanisms invoked to create these two molecules will produce them in similar ratios. Theory predicts that HNC is more easily collisionally excited than HCN by H2 and He, which would explain the observational results. We validate these calculations through measuring the elastic and inelastic scattering of HCN and HNC with He at low temperatures in a new experiment. Continuous supersonic uniform flows of He are used to cool photolytically produced HCN and HNC to 10 K. Using a new chirped pulse Fourier transform millimeter wave spectrometer, we measure the absolute pressure broadening cross sections of the astrophysically important j=1-0 transition, and compare these to close coupling calculations on ab initio surfaces. The cross sections for HNC with He are dramatically different from those of HCN with He, largely driven by the difference in inelastic rates. The underlying reasons for this strong difference will be explored and the astrophysical implications discussed. |
Friday, March 19, 2021 10:12AM - 10:24AM Live |
X26.00008: New IR Spectra of Molecules with Extreme Rotation Prepared in an Optical Centrifuge: N2O with J ≤ 205 and CO2 with J ≤ 280 Tara Michael, Hannah M Ogden, Amy Mullin We report new IR spectra of N2O and CO2 in extreme rotational states made with an optical centrifuge. Spectroscopic signatures of high-J rotational states may be important in the characterization of exoplanet atmospheres, for which only a fraction of observed spectral lines have been identified. In our experiments, an optical centrifuge based on shaped ultrafast laser pulses is used to trap and spin molecules into high-J states with oriented angular momentum. High-resolution transient IR absorption spectroscopy of N2O with J ≤ 205 (Erot = 17700 cm-1) and CO2 with J ≤ 280 (Erot = 30700 cm-1) was measured near λ=4.3 μm using the output of a frequency-stabilized quantum cascade laser. The observed transition frequencies for N2O and CO2 agree well with predictions based on an extrapolation from low-J transitions, with the exception of two CO2 spectral perturbations that result from mixing with (0330) and (1110) vibrationally excited states. Transient Doppler-broadened line profile measurements confirm that rotation is excited selectively in the optical centrifuge, without broadening in the spectral lines. These experiments demonstrate how new IR transitions of high-J states can be measured with the challenges associated with thermal heating in a gas sample. |
Friday, March 19, 2021 10:24AM - 10:36AM Live |
X26.00009: The 1.66 μm Spectrum of the Ethynyl Radical, CCH Eisen Gross, Anh T. Le, Gregory E Hall, Trevor J Sears Microwave astronomy has identified the Ethynyl radical, CCH, as an important precursor to the creation of polyaromatic hydrocarbons particularly in star-formation regions in space. We report high resolution measurements in the near-IR, close to 1.66 μm by frequency-modulated diode laser transient absorption spectroscopy. Two parallel bands were observed in the recorded spectra. The first originating from the ground X 2Σ+ state to a 2Σ+ level at 6055.6 cm-1. The second originates from the first excited bending vibrational level of 2Π symmetry to a 2Π level at 6413.5 cm-1. Neither of these bands have been previously observed. Parameters from an effective Hamiltonian representing rotational and fine structure levels, useful to up to J = 37/2 and J 29/2 for each band respectively have been obtained. The bands may well be useful for future near-IR astonomy. |
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