The microsolvated complexes of formanilide, generated in a supersonic expansion, have been observed by Fourier transform microwave spectroscopy. Three 1:1 and one 1:2 formanilide‐water adducts have been observed and their structure characterized by the measurement of isotopologue rotational spectra. In one of the monohydrated complexes and in the dihydrated complex formanilide adopts a cis‐configuration. In these species water closes sequential cycles with the cis amino and carbonyl groups through a network of N‐H∙∙∙O and O‐H∙∙∙O hydrogen bonds. Furthermore, in these complexes cis‐formanilide has almost the same non planar configuration observed in the monomer. In the two monohydrated complexes detected with trans‐formanilide, a planar skeleton is detected with water interacting solely with either the amino (N‐H∙∙O bond) or the carbonyl group (O‐H∙∙∙O=C bond). The observed tunnelling splittings show a rich intermolecular dynamics in those complexes. The results seem to indicate that complexation with water switches the configuration of formanilide from trans, more stable for the bare monomer, to cis, more stable for the hydrated complexes.
