Hole-burning spectroscopy and ab initio calculations for the aniline dimer

The mass-resolved spectrum indicates that only one conformation contributes to sharp peaks observed in the S1←S0 resonance two-photon ionization (R2PI) spectrum. However, geometry optimizations at the MP2/cc-pVDZ level suggest that two conformational isomers are stable: a head-to-head conformation with a single NH⋯N hydrogen bond and a head-to-tail conformation with double NH2⋯π hydrogen bonds. The calculations show that the head-to-tail conformation is more stable by 1.18 kcal mol−1.