In polyatomic molecules with a $\Pi$ electronic ground state, the rovibrational spectrum can be strongly modified by the Renner-Teller effect (the coupling between bending vibrational mode and electrons in the degenerate $\Pi$ state). The linear form of the C${}_3$H molecule has particularly strong Renner-Teller interactions and a very-low-lying vibronic ${\Sigma }_{1/2}^{+}$ level, which corresponds to the excited bending vibrational mode. This leads to the increased sensitivities of the microwave and submillimeter transition frequencies to the possible variation of the fine structure constant $\alpha$ and electron-to-proton mass ratio $\mu$.

• Received 3 February 2013

DOI:https://doi.org/10.1103/PhysRevA.87.032104