Isomerism of the Aniline Trimer

Weaker intermolecular forces expand the isomerization alternatives for molecular aggregation, as observed for the prototype models of the aniline trimer (An3) and the monohydrated aniline dimer (An2‐W) when compared to the phenol trimer. In this experiment the aniline clusters were generated in a jet‐cooled expansion and probed using broadband (chirped‐pulsed) microwave spectroscopy. Three isomers of the aniline trimer and two isomers of the hydrated dimer were detected and characterized in the rotational spectrum. In the homotrimer the weak N−H⋅⋅⋅N hydrogen bonds are assisted by subtle combinations of N−H⋅⋅⋅π and C−H⋅⋅⋅π interactions, producing several competing low‐lying ring species in the gas phase. One of the aniline trimers is a symmetric top, topologically equivalent to the only observed phenol trimer. Conversely, addition of a water molecule to the aniline dimer introduces a leading O−H⋅⋅⋅N interaction, making water to behave as dominant hydrogen‐bond pivot between the two aniline molecules. This combination of weak intermolecular interactions critically tests the performance of dispersion‐corrected or parametrized density‐functional methods. Evaluation of the B3LYP‐D3(BJ) and M06‐2X methods revealed deficiencies of the Truhlar functional to reproduce the experimental rotational data. Weaker hydrogen bonds favour competing isomerism in neutral molecular clusters, as observed in five isomers of the aniline trimer and the hydrated aniline dimer.