Marcin Gronowski

Computational Chemistry

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Posts Tagged ‘ISM’

A Theoretical Study on the Interstellar Synthesis of H2NCS+ and HNCSH+ Cations

HNCS and NCSH molecules, recently discovered in the interstellar medium, are likely formed via the dissociative recombination of H2NCS+ or HNCSH+ isomeric ions. Interstellar synthesis of the latter is discussed on theoretical grounds. The analysis of relevant potential energy surfaces suggests a key role for chemical processes in which CSH+ or HCS+ cations (most likely formed in CS + H3+ collisions) react with NH2 or NH3. The astrochemical kinetic database (kida.uva.2011), appended with 7 sulfur-bearing molecules and 48 corresponding reactions, has been applied to model the evolution of HNCS, NCSH, and their cationic precursors in a quiescent molecular cloud. Based on the model and on spectroscopic predictions, for an object like TMC-1, we expect the total intensity of H2NCS+ microwave lines to be comparable to that observed for HSCN. Theoretically derived molecular parameters, of interest for radio spectroscopy, are given for the most stable cations sharing the H2NCS+ stoichiometry.

Full text:  ApJ 792 89 doi:10.1088/0004-637X/792/2/89

EVIDENCE FOR DIACETYLENE CATION AS THE CARRIER OF A DIFFUSE INTERSTELLAR BAND

High-quality spectra acquired at three different observatories point to the presence of a new diffuse interstellar band (DIB) at 5069 Å. The spectral profile of this DIB matches published laboratory measurements of the diacetylene cation A2Πu–X2Πg (0–0) low-temperature gas-phase optical absorption. HC4H+ is approximately 60–80 times less abundant than CH along the analyzed lines of sight. Only an upper limit could presently be inferred from the search for an analogous band of the triacetylene cation HC6H+, expected at 6001.1Å, which implies the HC6H+ to HC4H+ ratio of less than ∼1/3.
Full text: Astrophys. J. 714 (2010) L64

PROSPECTS FOR THE DETECTION OF INTERSTELLAR CYANOVINYLIDENE

Prospects for the presence and detection of interstellar cyanovinylidene, CC(H)CN, a Y-shaped isomer of cyanoacetylene, are discussed. It is proposed that CC(H)CN can arise in interstellar clouds as one of the HC3NH+ + e– dissociative recombination products, by rearrangements of the neutral chain radical HC3NH into branched species HCCC(H)N, CC(H)C(H)N, and/or HCC(H)CN, and by the subsequent elimination of a hydrogen atom. It is deduced that the abundance of cyanovinylidene in molecular clouds should be confined between the abundances of its chain isomers HNCCC and HCNCC. Quantum chemical predictions regarding cyanovinylidene geometry, ground-state rotational constants, centrifugal distortion constants, spin-orbit coupling, IR absorption spectroscopy, and electric dipole moment are given. The spectroscopically observed molecules formyl cyanide, NC2(H)O, and propynal, HC3(H)O, with structures qualitatively resembling cyanovinylidene, served to prove the adequacy of the calculational procedures employed.
Full text: Astrophys. J. 701 (2009) 488