Laboratory rotational spectroscopy of the magnesium-carbon chains MgC[formula omitted]H and MgC[formula omitted]N.

Recent advances in circumstellar metal chemistry and laser-coolable molecules have spurred interest in the spectroscopy and electronic properties of alkaline earth metal-bearing polyatomic molecules. We report the microwave rotational spectra of two members of this important chemical family, the linear magnesium-carbon chains MgC 4 H and MgC 3 N, detected with cavity Fourier transform microwave spectroscopy of a laser ablation-electric discharge expansion. The rotation, fine, and hyperfine parameters have been derived from the precise laboratory rest frequencies. These experimental results, combined with a theoretical quantum chemical analysis, confirm the recent identification of MgC 4 H and MgC 3 N in the circumstellar envelope of the evolved carbon-rich star IRC+10216. The spectroscopic data also provide insight into the structural and electronic properties that influence the metal-based optical cycling center in this unique class of laser-coolable polyatomics. [Display omitted] • MgC 4 H and MgC 3 N were detected in the laboratory with microwave spectroscopy. • Confirmation of the astronomical identification in carbon-rich circumstellar envelopes. • Fine and hyperfine structure reveals a highly metal-localized unpaired electron. [ABSTRACT FROM AUTHOR]