Exploring indole dimer structures at the band origin: A spectroscopic gas phase IR analysis

Non-covalent interactions such as hydrogen bonding and π–π stacking are central to molecular architecture. This study investigates the indole dimer near the band origin at 35238 cm⁻¹ using IR-UV double resonance spectroscopy and DFT calculations. The IDIR spectrum shows a red-shifted NH stretch at 3420 cm⁻¹, indicative of hydrogen bonding. Computational analyses with B97D and B3LYP-D3 functionals favor N–H‧‧‧π bonded structures. Among these, we assign a tilted geometry featuring N–H‧‧‧π interaction as the most consistent with experimental observations. The broad spectral profile also suggests the coexistence of multiple low-energy isomers.