Competition between Hydrogen Bonding and Dispersion Interactions in the Indole ••• Pyridine Dimer and (Indole)2 ••• Pyridine Trimer Studied in a Supersonic Jet

Structures of the indole center dot center dot center dot pyridine dimer and (indole)(2) center dot center dot center dot pyridine trimer have been investigated in a supersonic jet using resonant two-photon ionization (R2PI) and LR-UV double resonance spectroscopic techniques combined with quantum chemistry calculations. R2PI spectra of the dimer and the trimer recorded by electronic excitation of the indole moiety show that the red-shift in the band origin of the dimer with respect to the 0(0)(0) band of the monomer is larger compared to that of the trimer. The presence of only one conformer in the case of both the dimer and the trimer has been confirmed from IR-UV hole-burning spectroscopy. The structures of the dimer and the trimer have been determined from resonant ion dip infrared (RIDIR) spectra combined with ab initio as well as DFT/M05-2X and DFT/M06-2X calculations. It has been found that the dimer, observed in the experiment, has a V-shaped geometry stabilized by N-H center dot center dot center dot N and C-H center dot center dot center dot N hydrogen bonding interactions, as well as C-H center dot center dot center dot pi and pi center dot center dot center dot pi dispersion interactions. The geometry of the trimer has been found to be a cyclic one stabilized by N-H center dot center dot center dot N, N-H center dot center dot center dot pi, C-H center dot center dot center dot pi, and C-H center dot center dot center dot N interactions. The most important finding of this current study is the observation of the mixed dimer and trimer, which are stabilized by hydrogen bonding as well as dispersion interactions.