Published: August 11, 2011 r2011 American Chemical Society 10299 dx.doi.org/10.1021/jp205894q | J. Phys. Chem. A 2011, 115, 10299–10308 ARTICLE pubs.acs.org/JPCA Structure of 7-Azaindole 333 2-Fluoropyridine Dimer in a Supersonic Jet: Competition between NH 333 N and NH 333 F Interactions Sumit Kumar, Indu Kaul, Partha Biswas, † and Aloke Das* Department of Chemistry, Indian Institute of Science Education & Research (IISER), 900 NCL Innovation Park, Dr. Homi Bhabha Road, Pune-411008 Maharashtra, India b S Supporting Information 1. INTRODUCTION Hydrogen bonding interaction is ubiquitous due to its three unique properties namely directionality, speci ficity, and transience. 13 This noncovalent interaction has immense significance in pro- viding a specific shape of biomolecules, which is responsible for appropriate biological functions in living systems. 37 Hydrogen bonding interaction also plays a key role in molecular recognition, crystal structure, self-assembly, and supramolecular chemistry. 815 Consequently, there are extensive theoretical as well as experimen- tal studies in the gas phase to understand this intermolecular interac- tion in detail. 1619 A conventional hydrogen bond is defined as XH 333 Y, where both X and Y are electronegative atoms (i.e., O, N, and F). However, it has been found that covalently bound fluorine (F) is a poor hydrogen bond acceptor in spite of its highest electronegativity. 20 Very weak hydrogen bond formation ability of fluorine has been explained in terms of its low proton affinity. The existence of weak CF 333 HX (X = C, N, and O) hydrogen bonding interactions has first been reported by Shimoni et al. from their extensive analysis of the crystallographic data collected in the Cambridge Structural Database (CSD). 21 They have reported that though hydrogen bonding interactions through fluorine are quite weak, these are important in molecular packing in complexes and crystals. Later, Desiraju and co-workers have rigorously investigated the role of CH 333 F interactions in the crystal structures of partially fluorinated benzenes. 22 They have demonstrated that CH 333 F interactions are as important as CH 333 O and CH 333 N hydrogen bonding interactions in stabilizing crystal structures. On the other hand, hydrogen bonding interactions through the fluorine atom have been investigated in the gas phase in the complexes of H 2 O, CH 3 OH, and NH 3 with a series of partially fluorinated benzenes by experimental as well as com- putational methods. 2330 It has been found experimentally as well as theoretically that the most stable conformer of the com- plexes of both H 2 O and CH 3 OH with partially fluorinated benzenes has an in-plane six-membered cyclic structure with OH 333 F and CH 333 O double hydrogen bonding interac- tions. Tonge et al. have studied the 1:1 complex of fluorobenzene and ammonia by experimental as well as high level theoretical calculations. 30 They have confirmed that the most preferred con- formation of the complex has an in-plane six-membered cyclic structure with NH 333 F and CH 333 N double hydrogen bond- ing interactions, which are analogous to fluorobenzene 333 H 2 O and fluorobenzene 333 CH 3 OH structures. On the contrary, Vaupel et al. have reported a NH 333 π bound structure for fluorobenzene 333 NH 3 complex and σ-type NH 333 F hydro- gen bonded structure for difluorobenzene 333 NH 3 complex. 26 Recently, Singh et al. have performed theoretical investigation of Received: June 23, 2011 Revised: August 7, 2011 ABSTRACT: In the present work, we have investigated the structure of 7-azaindole 333 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 r 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 333 N and CH 333 N hydrogen bonding interactions. The less stable conformer with NH 333 F and CH 333 N interactions are not observed in the experiment. The competition between NH 333 N and NH 333 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 333 N) present in the dimer, although fluorine is believed to be very weak hydrogen bond acceptor.