π-Hydrogen Bonding Wins over Conventional Hydrogen Bonding Interaction: A Jet-Cooled Study of Indole···Furan Heterodimer

Abstract

In this study, we have explored the conformational landscape of the indole···furan dimer in a supersonic jet by using resonant two-photon ionization (R2PI) and IR-UV double-resonance spectroscopic techniques combined with dispersion-corrected density functional theory (DFT) calculations. Only one conformer of the dimer has been observed in the experiment. DFT/B97-D level calculation shows that N–H···π hydrogen-bonded conformer (T′) is energetically more stable than the N–H···O hydrogen-bonded conformer (HB). Natural bond orbital (NBO) calculation also shows that the hydrogen-bonding interaction in the HB conformer is very weak. Finally, the structure of the observed dimer has been determined to be tilted T-shaped N–H···π hydrogen-bonded (T′) from very excellent agreement between experimental and theoretical N–H stretch frequency. The most significant finding of this study is the first-time observation of a N–H···π bound conformer of a dimer, which wins over a conventional hydrogen-bonded conformer of the dimer.