Structure of 7-Azaindole···2-Fluoropyridine Dimer in a Supersonic Jet: Competition between N–H···N and N–H···F Interactions

Abstract

In the present work, we have investigated the structure of 7-azaindole···2-fluoropyridine dimer in a supersonic jet by employing resonant two photon ionization (R2PI), IR-UV, and UV–UV double resonance spectroscopic techniques combined with quantum chemistry calculations. The R2PI spectrum of the dimer is recorded by electronic excitation of the 7-azaindole moiety, and a few low frequency intermolecular vibrations of the dimer are clearly observed in the spectrum. The electronic origin band of the dimer is red-shifted by 1278 cm–1 from the S1 ← S0 origin band of 7-azaindole monomer. The presence of a single conformer of the dimer is confirmed by IR-UV and UV–UV hole-burning spectroscopic techniques. RIDIR (Resonant ion dip infrared) spectrum of the dimer shows a red-shift of 265 cm–1 in the N–H stretching frequency with respect to that of the 7-azaindole monomer. Two planar double hydrogen bonded cyclic structures of the dimer have been predicted from DFT calculations. Comparison of experimental and theoretical N–H stretching frequencies confirms that the observed dimer is stabilized by N–H···N and C–H···N hydrogen bonding interactions. The less stable conformer with N–H···F and C–H···N interactions are not observed in the experiment. The competition between N–H···N and N–H···F interactions in the two dimeric structures are discussed from natural bond orbital (NBO) analysis. The current results demonstrate that fluorine makes a hydrogen bond of intermediate strength through cooperative interaction of another hydrogen bond (C–H···N) present in the dimer, although fluorine is believed to be very weak hydrogen bond acceptor.