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

Abstract

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)° (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.

Acknowledgements

We would like to thank the Dirección General de Investigación – Ministerio de Ciencia y Tecnología (grant BQU2003-03275) and the Junta de Castilla y León - Fondo Social Europeo (grant VA087/03) for financial support. Walther Caminati thanks the molecular Spectroscopy Group of the University of Valladolid for their kind hospitality, and the University of Valladolid for a grant for a position as Invited Professor.