Structure and Spectroscopic Properties of [Mg,C,N,O] Isomers: Plausible Astronomical Molecules

Abstract

Theoretical methodologies have been employed to study [Mg,C,N,O] isomers, which are possible species of interstellar interest. It has been found that, at all levels of theory used, the most stable isomer is the magnesium isocyanate radical. The corresponding cyanate counterpart lies 15.8 kcal/mol, at the CCSD­(T) level of theory, above MgNCO. Other isomers, such as the magnesium fulminate radical, MgCNO, the magnesium isofulminate radical, MgONC, and the two compounds that arise from the insertion of the magnesium atom between either O–N or O–C bonds, namely, OMgNC and OMgCN, are located clearly higher in energy than MgNCO. The barrier for the MgOCN → MgNCO process has been calculated to be 4.6 kcal/mol, suggesting a slow rate for the isomerization reaction. To examine the bonding interactions in the different [Mg,C,N,O] isomers, a natural bond orbital analysis, together with a topological analysis of the electron density in the framework of Bader’s quantum theory of atoms in molecules, has been carried out. For the two lowest lying isomers, predictions for the thermodynamic stabilities and spectroscopic parameters, which could aid in the detection of these species, have been made. The infrared (IR) spectra of both MgNCO and MgOCN are dominated by intense bands associated with the σ-NCO asymmetric stretching mode, especially in the case of MgNCO. Both MgNCO and MgOCN have large dipole moments (4.84 and 7.59 D, respectively, at the CCSD/aug-cc-pVTZ level). These two factors will likely make observation of both systems easier with either space-based IR or ground-based sum-mm telescopes. The global analysis of the relative stabilities and spectroscopic parameters suggests two linear isomers, namely, MgNCO (2Σ+) and MgOCN (2Σ+), as possible candidates for laboratory and space detection.

Relative thermodynamic stability of the [C,N,O] linkages as an indication of the most abundant structures in the ISM

“…The other two linked to CNO are now very close in energy with a gap between M-NCO and M-ONC about six times that betwe-en M-NCO and M-ONC. A reason for that can be found in a bonding analysis carried out by Vega-Vega et al (2017).…”

MgC6H2 Isomers: Potential Candidates for Laboratory and Radioastronomical Studies

“…The latter molecule has also been identified in the laboratory using Fourier transform microwave spectroscopy (FTMW) by McCarthy and Thaddeus in 2002 . While theoretical calculations on tetradehydrobenzenes (dibenzynes) have been done by various research groups, ,,− attempts to trap these molecules in low-temperature matrices have not yielded success yet although derivatives such as 3,6-difluoro-1,4-benzdiyne and 3,6-bis­(trifluoromethyl)-1,4-benzdiyne have been observed. ,, Identification of MgNC 3 and its metastable isomer MgCN 4 in IRC+10216 has triggered several theoretical and experimental studies on magnesium-bearing molecules, in particular, on their cyanopolyynes, , polyynes, − and other species. − The pure rotational spectrum of magnesium monoacetylide radical, MgC 2 H ( 2 Σ + ) has been recorded by Ziuyrs and co-workers in the laboratory using millimeter (mm)/submillimeter direct absorption spectroscopy in 1995. , These authors speculated the presence of MgC 2 H in space considering the fact that it is isoelectronic with both MgNC and MgCN, and is likely to be produced from HCCH , or CCH, as both are abundant molecules in the interstellar medium (ISM). In 2014, tentative assignments for MgC 2 H in IRC+10216 were given and in 2019 it was confirmed by Cernicharo and co-workers …”

Rovibrational Considerations for the Monomers and Dimers of Magnesium Hydride and Magnesium Fluoride

“…Magnesium is strongly believed to be necessary in the nucleation of dust grains that ultimately lead to planets and has been detected in known interstellar molecules − with others speculated to exist . Recent work has even shown a seeming correlation between higher abundances of magnesium in the spectra of stars known to host planets and lower abundances in stars known not to host planets. , Hence, the presence of small molecules containing magnesium such as small clusters of the same stoichiometric ratio as the larger crystalline minerals may be an early indication of planet formation .…”