Spectroscopy of hydrogen-bonded formanilide clusters in a supersonic jet: Solvation of a model trans amide

The gas-phase structures of trans-formanilide (FA) clusters containing varying numbers of water and ammonia molecules have been investigated by resonant two-photon ionization spectroscopy in a supersonic jet expansion. A single structure is found for the 1:1 cluster of FA with ammonia in which the amide NH group functions as a hydrogen bond donor to the ammonia nitrogen. In contrast, vibronically resolved spectra reveal two distinct structures for the 1:1 cluster with water in which either the amide NH group functions as a hydrogen bond donor or the carbonyl oxygen functions as a hydrogen bond acceptor. The 1:1 clusters with both ammonia and water exhibit characteristic spectral shifts that depend on which amide site participates in the hydrogen bond. Three distinct types of 1:2 clusters with water have been found. Two of these can be viewed as water dimers interacting through a single hydrogen bond with either the amide NH group or the carbonyl oxygen. The third structure involves a hydrogen bond at each amide site to a separate water molecule. Ternary FA clusters containing one ammonia and one water molecule have also been investigated and found to be present in two distinct structural forms. Although each structure contains a hydrogen bond between the amide NH and one of the solvents, the structures differ with regard to which solvent serves as the acceptor of this hydrogen bond as well as in the role of the second solvent. Finally, clusters containing four water molecules have been identified, although in this case only a single cluster structure has been observed. This species is assigned to a structure containing a hydrogen-bonded chain of four water molecules forming a bridge between the NH and carbonyl oxygen binding sites on opposite sides of the trans amide. These experimental observations and structural assignment are supported by ab initio Hartree-Fock calculations.