Section of the rotational spectrum of CPCA showing a-type R-branch rotational transitions for the ground state, and ν₂₇ and ν₁₆ vibrationally excited states of the trans conformer, and b-type Q-branch transitions for the ground state of the cis species. The negative spectral features are produced by the folding of the frequency-switched data and appear at ±50 MHz of each rotational transition.