A ZEKE photoelectron spectroscopy and ab initio study of the cis- and trans-isomers of formanilide: Characterizing the cationic amide bond?

The cis- and trans- conformational isomers of formanilide, an aromatic amide which represents a model peptide, have been studied using zero electron kinetic energy (ZEKE) spectroscopy to investigate the properties of cationic amides. Accurate ionization energies of 67 408 and 67 710 ± 5 cm-1 were measured for the trans- and cis-isomers respectively, allowing the trans-conformation to be assigned as the lower energy conformation in the D0 state. For trans-formanilide, the ZEKE spectra are dominated by excitation of the in-plane side-arm bend (211 cm-1) and the amide stretch (952 cm-1), while the cis-formanilide spectra are dominated by the out-of-plane side-arm bend (80 cm-1) and the C1-N torsion (126 cm-1). Ab initio (HF/MP2) and CASSCF calculations are presented to support the experimental observations.