Microwave Detection of a Metal Peroxide and Two New Metal Dicarbides: The Precise Semiexperimental Equilibrium Structures of CaO ₂ , SrC ₂ , and YbC ₂

We report the pure rotational spectra of three alkaline earth(-like) metal-bearing molecules: calcium peroxide (CaO₂), strontium dicarbide (SrC₂), and ytterbium dicarbide (YbC₂), produced in a laser ablation-electric discharge supersonic expansion source and detected by cavity Fourier transform microwave spectroscopy. The semiexperimental equilibrium structure of each molecule has been derived to ≲1 mÅ uncertainty by combining comprehensive isotopic measurements with highly accurate <i>ab initio</i> rovibrational corrections. These precise structures provide direct physical probes of not only the highly ionic metal-ligand bonding but also the electronic structure of the dianionic ligands themselves. Our detection of CaO₂, in particular, appears to represent the only high-resolution gas-phase spectroscopy of a metal peroxide (M²⁺O₂^2-) molecule, providing a unique intramolecular proxy of the unstable O₂^2- dianion. In addition to these fundamental insights, we discuss our observation of highly nonequilibrated vibrational population distributions in the expansion source and the relevance of our results to the chemistry of circumstellar envelopes studied by radio astronomy.