Structure and Spectroscopic Signatures of Interstellar Sodium Isocyanate Isomers

Abstract

The investigation of metal-containing interstellar molecules stands as a prolific field for current astrochemical research. However, the search for many of these systems in the interstellar medium has remained inaccessible to date due to the lack of preliminary spectroscopic data. In this context, pioneering theoretical studies have inspired quantum chemists to study new appealing candidates to enable their subsequent search in space. The aim of this study is to provide high-level theoretical spectroscopic signatures of the tetratomic system [Na, N, C, O]. We have performed a thorough exploration of its potential energy surface employing different state-of-the-art quantum chemical methods and nine different species have been characterized. Moreover, we have evaluated the stability of the most stable isomers against dissociation and explored their main isomerization processes. We therefore suggest sodium isocyanate (NaNCO,1Σ) and sodium cyanate, (NaOCN, 1Σ) as the most relevant candidates for laboratory and interstellar detection. To aid in their eventual spectral search by means of rotational spectroscopy, we report a complete set of the required spectroscopic parameters including the nuclear quadrupole coupling constants, which are needed to interpret their complex hyperfine structure. NaNCO and NaOCN present exceptionally high values of the electric dipole moment (11.4 and 13.6 Debyes, respectively at the CCSD(T,rw)/aug-cc-pVTZ level), which strongly support to perform an eventual radio astronomical search. Furthermore, both isomers exhibit rather small vibrational frequencies, which indicates that these species are certainly floppy molecules.