title: 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 S(1) ← S(0) 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.
authors.affiliation: Department of Chemistry, Indian Institute of Science Education & Research (IISER), 900 NCL Innovation Park, Dr. Homi Bhabha Road, Pune-411008 Maharashtra, India.
authors.forename: Sumit
authors.initials: S
authors.lastname: Kumar
authors.forename: Indu
authors.initials: I
authors.lastname: Kaul
authors.forename: Partha
authors.initials: P
authors.lastname: Biswas
authors.forename: Aloke
authors.initials: A
authors.lastname: Das
mesh_terms: D019281:Dimerization; D007211:Indoles / Q000737:chemistry*; D015394:Molecular Structure; D017785:Photons; D011725:Pyridines / Q000737:chemistry*; D011789:Quantum Theory; D013055:Spectrophotometry, Infrared; D013056:Spectrophotometry, Ultraviolet
chemical_list: C521930:2-fluoropyridine; C023422:7-azaindole dimer; D007211:Indoles; D011725:Pyridines