# Microwave Detection of a Metal Peroxide and Two New Metal Dicarbides: The Precise Semiexperimental Equilibrium Structures of CaO2, SrC2, and YbC2

## TL;DR

This paper reports the microwave detection and structural analysis of three metal compounds, offering insights into their bonding and relevance to astrophysical chemistry.

## Contribution

The study presents the first high-resolution gas-phase spectroscopy of a metal peroxide and precise equilibrium structures of three metal compounds.

## Key findings

- The semiexperimental equilibrium structures of CaO2, SrC2, and YbC2 were determined with high precision.
- The detection of CaO2 provides a unique proxy for the O2^2- dianion in gas-phase studies.
- Highly nonequilibrated vibrational populations were observed in the expansion source.

## Abstract

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 ab initio 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.