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Abstract A new interstellar molecule, FeC (X3Δi), has been identified in the circumstellar envelope of the carbon-rich asymptotic giant branch star IRC+10216. FeC is the second iron-bearing species conclusively observed in the interstellar medium, in addition to FeCN, also found in IRC+10216. TheJ= 4 → 3, 5 → 4, and 6 → 5 rotational transitions of this free radical near 160, 201, and 241 GHz, respectively, were detected in the lowest spin–orbit ladder, Ω = 3, using the Submillimeter Telescope of the Arizona Radio Observatory (ARO) for the 1 mm lines and the ARO 12 m at 2 mm. Because the ground state of FeC is inverted, these transitions are the lowest energy lines. The detected features exhibit slight U shapes with LSR velocities nearVLSR≈ −26 km s−1and linewidths of ΔV1/2≈ 30 km s−1, line parameters characteristic of IRC+10216. Radiative transfer modeling of FeC suggests that the molecule has a shell distribution with peak radius near 300R*(∼6″) extending out to ∼500R*(∼10″) and a fractional abundance, relative to H2, off∼ 6 × 10−11. The previous FeCN spectra were also modeled, yielding an abundance off∼ 8 × 10−11in a larger shell situated near 800R*. These distributions suggest that FeC may be the precursor species for FeCN. Unlike cyanides and carbon-chain molecules, diatomic carbides with a metallic element are rare in IRC+10216, with FeC being the first such detection.
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Thiophene‐Based Double Helices: Radical Cations with SOMO–HOMO Energy Level Inversion †
Abstract We report relatively persistent, open‐shell thiophene‐based double helices, radical cations 1•+‐TMS12and 2•+‐TMS8. Closed‐shell neutral double helices, 1‐TMS12and 2‐TMS8, have nearly identical first oxidation potentials,E+/0 ≈ +1.33 V, corresponding to reversible oxidation to their radical cations. The radical cations are generated, using tungsten hexachloride in dichloromethane (DCM) as an oxidant,E+/0 ≈ +1.56 V. EPR spectra consist of a relatively sharp singlet peak with an unusually lowg‐value of 2.001–2.002, thus suggesting exclusive delocalization of spin density over π‐conjugated system consisting of carbon atoms only. DFT computations confirm these findings, as only negligible fraction of spin density is found on sulfur and silicon atoms and the spin density is delocalized over a single tetrathiophene moiety. For radical cation, 1•+‐TMS12, energy level of the singly occupied molecular orbital (SOMO) lies below the four highest occupied molecular orbitals (HOMOs), thus indicating the SOMO–HOMO inversion (SHI) and therefore, violating the Aufbau principle. 1•+‐TMS12has a half‐life of the order of only 5 min at room temperature. EPR peak intensity of 2•+‐TMS8, which does not show SHI, is practically unchanged over at least 2 h.
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- PAR ID:
- 10446368
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Photochemistry and Photobiology
- Volume:
- 97
- Issue:
- 6
- ISSN:
- 0031-8655
- Page Range / eLocation ID:
- p. 1376-1390
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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