Marcin Gronowski

Computational Chemistry

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Posts Tagged ‘ISM molecules’

An ab initio study of structure, stability, and spectroscopic parameters of 5-atomic [C, C, H, N, S] isomers

This theoretical study is focused on predicting structures, energetics, and selected spectroscopic constants for a range of 5-atomic chemical species sharing the [C, C, H, N, S] stoichiometry, including thioformyl cyanide and iminoethenethione. An in-depth study carried out on the four most stable isomers have yielded (i) structures and energy for the lowest singlet and triplet excited electronic states; (ii) anharmonic vibrational frequencies and IR absorption intensities for the fundamental, overtone and combination modes; (iii) molecular parameters of interest to microwave spectroscopy: rotational constants, electric dipole moments, and quadrupole coupling constants.

Full text: Journal of Molecular Structure, Volume 1090, 15 June 2015, Pages 76–85

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

Ab Initio Studies of the Structure and Spectroscopy of CHNMg Stoichiometry Molecules and van der Waals Complexes

A high-level ab initio study was conducted over the range of tetraatomic molecules containing H, C, N, and Mg. Potential energy surfaces were analyzed, leading, for selected molecules, to the optimization of their geometry in the lowest singlet and triplet excited states. Reliable ground state rotational constants are given for the most stable species, namely, HMgNC and HMgCN, together with respective anharmonic vibrational frequencies of fundamental, overtone, and combination bands. In addition, potential energy surfaces describing the interaction of HCN or HNC with a single magnesium atom have been investigated.

Full text: J. Phys. Chem. A, 2013, 117 (21), pp 4455–4461

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

Isomers of cyanodiacetylene: Predictions for the rotational, infrared and Raman spectroscopy

Recent ab initio (coupled-clusters) and density functional theory studies on cyanodiacetylene isomers are extended here to yield the quantities of direct interest to future spectroscopic investigations. A bond lengths scaling procedure was developed to obtain the corrected molecular geometries. These, together with calculated vibration–rotation coupling constants, yielded the ground-level rotational constants for seven most stable isomeric species of the HC5N stoichiometry. Former calculations regarding the vibrational transitions of these molecules are complemented here with predictions on Raman scattering activities, and on isotopic (2H, 15N) effects in IR absorption and Raman spectra.
Full text: J. Mol. Struct. 834-836 (2007) 102

Isomers of cyanodiacetylene: Theoretical structures and IR spectra

Coupled-clusters CCSD(T)/cc-pVTZ calculations have been carried out on several lowest energy isomers of cyanodiacetylene H–CC–CC–CN, previously selected in an extensive DFT study. Apart from the izonitrile H–CC–CC–NC, branched species CC(H)CCCN and CCCC(H)CN are found to be of particular importance, less stable than cyanodiacetylene by 48.5 and 51.0 kcal/mol, respectively. Harmonic frequencies and IR absorption intensities of vibrational fundamentals were predicted at the B3LYP/aug-cc-pVTZ level.
Full text: Chem. Phys. Lett. 428 (2006) 245