Nonclassical double-bridged structure in silicon-containing molecules: Experimental evidence in Si₂H₂ from its submillimeter-wave spectrum

The first spectroscopic evidence of a double-bridged silicon molecule is reported. The submillimeter-wave rotational spectrum of the disilyne Si₂{H}₂$ has been observed in a low-power silane plasma cooled at liquid-nitrogen temperature, with argon as buffer gas. These measurements led to the determination of the ${r}₀$ structure, which is in good agreement with ab initio calculations. This nonclassical structure is confirmed by the observation of the $²⁹Si$ and $³⁰Si$ monosubstituted forms.