Isomer selective infrared spectroscopy of supersonically cooled cis- and trans-N-phenylamides in the region from the amide band to NH stretching vibration.

We measured the infrared (IR) spectra of supersonically cooled N-phenylformamide (formanilide) and N-phenylacetamide (acetanilide) in the amide band and X-H stretch vibration regions by using IR-UV depletion spectroscopy combined with a newly developed mid-IR light source based on difference frequency generation in ZnGeP(2). The two rotational isomers, cis- and trans- of the amide group were separately monitored to record the IR spectra. Both of the conformers showed similar features in the amide I and II regions, while major differences of the isomers appeared in the amide III vibration region. The IR spectrum of trans-acetanilide closely resembles that of trans-formanilide, except for vibrations of the methyl group; that is, substitution of the formyl hydrogen to a methyl group has only a minor effect on the amide vibrations.