Mass resolved IR spectroscopy of aniline–water aggregates

Abstract

Aniline is the simplest aromatic amine and therefore it is a prototypical system to study the microhydration and excited state dynamics of aromatic amines. However, to date a discrete spectrum of singly hydrated species has not been obtained. Here we present for the first time the IR/UV spectrum of, aniline₁(H₂O)₁, aniline₁(H₂O)₂, and aniline₂(H₂O)₁. The origin band of the monohydrated complex is shifted around 700 cm⁻¹ to the blue, as a result of large changes in geometry upon electronic excitation. In addition, massive fragmentation from higher-order clusters appears where the origin band was expected to be, helping the clusters to elude their detection for a long time. The addition of a second water or aniline molecule shifts the origin band back to the vicinity of that of aniline and fragmentation of these species is one of the sources of such fragmentation. The dramatic changes in the structure of monohydrated clusters that follow electronic excitation resemble the behavior of the solvent in other aromatic amine–water systems, reinforcing the idea of aniline–water as an interesting model system to study such behavior.