  In polyatomic molecules with \Pi\ electronic ground state the ro-vibrational
spectrum can be strongly modified by the Renner-Teller effect. The linear form
of C3H molecule has particularly strong Renner-Teller interaction and a very
low lying vibronic \Sigma+ 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.
