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Astroscientists are using various spectroscopic techniques to root out relatively complex molecules lurking in the interstellar medium. The complexity of naphthalene, discovered in space, and corannulene, could provide new evidence of a cosmic origin for the precursor molecules of life on Earth. Studying interstellar chemistry is difficult. The most obvious problem is that scientists cannot directly sample their target compound. Instead, they rely on remote techniques such as observing the emission or absorption spectroscopy of the interstellar medium and plucking from filtered starlight the characteristic signs of familiar organic molecules floating lightyears from Earth. Despite these intrinsic difficulties more than 130 interstellar molecules have been identified by comparing the observed microwave emission spectra with reference spectra measured in the laboratory. However, efforts to observe molecules with high symmetry and lacking a permanent electric dipole moment, such as the polycyclic aromatic hydrocarbons (PAHs), are stymied because these compounds are not active in microwave spectroscopy. The closest researchers have got so far is the observation of infrared (IR) emission bands that hint strongly at the presence of individual PAHs in space, but none have so far been identified definitively. Now, Gaël Rouillé, Cornelia Jäger, Mathias Steglich, and Friedrich Huisken, of the Laboratory Astrophysics Group of the Max Planck Institute for Astronomy, Institute of Solid State Physics, Friedrich-Schiller University Jena, working with Thomas Henning of the Max Planck Institute for Astronomy in Heidelberg, and Gabriele Theumer, Ingmar Bauer, and Hans-Joachim Knölker of the Department of Chemistry at the Technical University Dresden, Germany, have developed a new approach to spotting molecules among the stars. Rouillé and colleagues explain that corannulene (C20H10), essentially five benzene molecules fused along each edge to form a flat, ring-shaped hydrocarbon, can be characterized spectroscopically is carried out by several techniques. However, corannulene is no ordinary PAH and actually has a permanent dipole moment that should make it amenable to microwave emission spectroscopy across the depths of space. The team has synthesised high purity corannulene, confirmed by gas chromatography-mass spectrometry (GC-MS), with the aim of providing reference spectra with which astrochemists might plot the movements of corannulene between the stars. They recorded UV-Vis absorption of the highly pure corannulene during a high-performance liquid chromatography (HPLC) procedure and obtained infrared (IR) absorption spectrum from caesium iodide pellets. They also determined the Raman scattering spectrum for pure crystal grains of corannulene. The team adds that the room temperature measurements were augmented with spectra recorded in an argon matrix at 12 Kelvin. A comparison of the experimental spectra with theoretical Raman and IR spectra and with calculated electronic transitions based on density functional theory (DFT), either normal or time-dependent (TDDFT) suggest that the search for cosmic corannulene should now be viable. Meanwhile, researchers at the Canary Islands Institute of Astrophysics (IAC) have identified a chemical cousin of corannulene, naphthalene, one of the most complex molecules yet discovered in the interstellar medium. The detection of this molecule suggests that a large number of the key components in prebiotic terrestrial chemistry could have been present in the interstellar matter from which the Solar System was formed. Susana Iglesias Groth, Arturo Manchado and Aníbal García, together with Jonay González, of the Paris Observatory, and David Lambert of the University of Texas reveal details of their finding in Astrophysical Journal Letters. The team discovered the naphthalene, the main constituent of mothballs, in a star formation region in the constellation Perseus, in the direction of the star Cernis 52. "We have detected the presence of the naphthalene cation in a cloud of interstellar matter located 700 lightyears from the Earth", explains Iglesias Groth. The spectral bands found in this constellation coincide with laboratory measurements of the naphthalene cation. "We aim to investigate whether other, more complex, hydrocarbons exist in the same region, including amino acids," adds Iglesias Groth. Naphthalene has previously been found in meteorites. The compound reacts under ultraviolet light with other small molecules to product various amino acids and naphthaloquinones, which are thought to be the ingredients of a prebiotic chemistry. The discovery of naphthalene and follow-up work aided by the work of Rouillé and colleagues could help astrochemists explain the so-called "diffuse" spectroscopic bands associated with the interstellar medium. Complex carbon compounds, including the molecule that "fell to Earth" buckminsterfullerene, could be the cause of these anomalous bands. "Our results show that PAHs, such as naphthalene, are responsible for the diffuse bands and should be present throughout the interstellar medium", adds Iglesias Groth. Reference:
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