Conformational equilibrium of formanilide:: detection of the pure rotational spectrum of the tunnelling cis conformer

Formanilide has been investigated by molecular beam Fourier transform microwave spectroscopy. The rotational spectra of two conformers, with the formyl hydrogen cis or trans with respect to the phenyl group have been measured. Their relative energy has been estimated to be 350 +/- 150 cm(-1), the trans form being more stable. In the cis species the plane of the pseudo-peptidic group forms an angle of +/- 34.7(5)degrees (equilibrium value) with the ring. The two equivalent configurations, corresponding to the + or - sign, are tunnelling through a barrier to planarity of 152(2) cm(-1).