Conformations and relative stabilities of the cis and trans isomers in a series of isolated N-phenylamides

The gas-phase conformations of a series of isolated N-phenylamides have been determined from vibrationally resolved electronic spectra obtained by resonant two-photon ionization in a supersonic jet expansion. Both the cis and trans isomers of formanilide were identified, with the cis isomer in 6.5% abundance. The spectral features displayed by this isomer are consistent with a nonplanar geometry which undergoes a large change in the phenyl torsional angle following electronic excitation. The more abundant trans isomer of formanilide adopts a planar structure and is stabilized by 2.5 kcal/mol with respect to the cis isomer. In the excited electronic state the relative stabilities of the two isomers are reversed. Acetanilide, in contrast, is found exclusively as the trans isomer, also having a planar structure. N-Methyl substitution causes a reversal of the relative isomer stabilities found in formanilide and leads to an isomer distribution consisting of approximately 90% E and 10% Z in N-methylformanilide. These experimental observations are compared to previous condensed phase structural determinations as well as to the relative energies and structures predicted from ab initio Hartree-Fock geometry optimizations.