Effect of acceptor heteroatoms on π-hydrogen bonding interactions: a study of indole···thiophene heterodimer in a supersonic jet.

Resonant two photon ionization (R2PI), IR-UV, and UV-UV double resonance spectroscopic techniques combined with quantum chemistry calculations have been used to determine the structure of indole···thiophene dimer observed in a supersonic jet. With the help of combined experimental and theoretical IR spectra it has been found that the observed dimer has a N-H···π hydrogen bonded slanted T-shaped structure. The present study demonstrates the effect of heteroatoms present in the acceptors on the strength of the π-hydrogen bonding interactions. It was concluded by Sherrill and co-workers from their theoretical study of benzene···pyridine dimer that aromatic rings containing heteroatoms are poorest π-hydrogen bond acceptors [E. G. Hohenstein and C. D. Sherrill, J. Phys. Chem. A 113, 878 (2009)]. But the current spectroscopic investigation exhibits that five membered aromatic heterocycles are favorable π-hydrogen bond acceptors. In this study, it has also been shown that thiophene is a better π-hydrogen bond acceptor than furan. The present work has immense biological significance as indole is the chromophore of tryptophan residue in the proteins and thiophene derivatives have potential therapeutic applications. Thus, understanding the binding motif between indole and thiophene in the heterodimer studied in this work may help in designing efficient drugs.