Published: June 02, 2011 r2011 American Chemical Society 7461 dx.doi.org/10.1021/jp202658r | J. Phys. Chem. A 2011, 115, 7461–7472 ARTICLE pubs.acs.org/JPCA Competition between Hydrogen Bonding and Dispersion Interactions in the Indole 333 Pyridine Dimer and (Indole) 2 333 Pyridine Trimer Studied in a Supersonic Jet Sumit Kumar, Partha Biswas, Indu Kaul, and Aloke Das* Department of Chemistry, Indian Institute of Science Education and Research (IISER), 900, NCL Innovation Park, Dr. Homi Bhabha Road, Pune-411008, Maharashtra, India b S Supporting Information 1. INTRODUCTION Hydrogen bonding, as well as π 333 π stacking, and noncon- ventional hydrogen bonding such as NÀH 333 π and CÀH 333 π interactions play a significant role in governing the specific functional structures of important biomolecules such as proteins and nucleic acids. 1À3 Crystal engineering, supramolecular chem- istry, self-assembly, molecular recognition, and DNA intercala- tion also deal with subtle balance among these noncovalent interactions. 4À12 The origins of the hydrogen bonding and stacking or π-hydrogen bonding interactions are entirely differ- ent. The hydrogen bonding one arises from electrostatic inter- action, while dispersion forces dominate the π 333 π stacking and π-hydrogen bonding interactions. Though hydrogen bonding interaction could be determined quite precisely by both experi- ment and theory, accurate determination of dispersion interac- tion is extremely challenging. Gas phase laser spectroscopy in a supersonic jet offers an ideal means to study the intermolecular forces present in noncova- lently bonded aromatic dimers and higher clusters. Experimental results available in the isolated gas phase are also very much desirable to compare with the data available from very high level quantum chemistry calculations. Up to this date, the most extensively studied aromatic dimer using numerous experimental as well as theoretical investigations is the benzene dimer. 13À29 The binding energy difference between parallel-displaced π-stacked and T-shaped benzene dimer is very small (about 0.1 kcal/mol) according to the highly accurate quantum chem- istry calculation at CCSD(T)/CBS level. 27 But gas phase super- sonic jet studies show the presence of only the T-shaped dimer of benzene stabilized due to CÀH 333 π interaction. 13À18 These benchmark studies on benzene dimer demonstrate that CÀH 333 π interaction is slightly stronger than the π 333 π interaction. On the other hand, the benzene 333 NH 3 complex has been studied quite extensively as a model for NH 333 π interaction employing both experimental as well theoretical methods. 30,31 NH 333 π bound slanted T-shaped geometry has been confirmed in the case of the pyrrole dimer by a few groups. 32,33 Leutwyler and co- workers have reported quite strong NH 333 π interaction in the case of the 2-pyridone 333 benzene dimer where the NÀH stretching frequency is red-shifted by 56 cm À1 . 34 Indole 333 benzene dimer has been studied by mass analyzed threshold ion- ization experiment as well as quantum chemistry calculations. 35À37 It has been found from CCSD(T)/CBS level of calculation that NH 333 π bound T-shaped geometry of the dimer is more stable than the parallel-displaced π-stacked one by about 1.1 kcal/mol. 37 Thus, the experimentally observed dimer is assigned to have NH 333 π bound T-shaped structure. It can be concluded from these studies that NH 333 π interaction is also stronger than the Received: December 12, 2010 Revised: May 11, 2011 ABSTRACT: Structures of the indole 333 pyridine dimer and (indole) 2 333 pyridine trimer have been investigated in a supersonic jet using resonant two-photon ionization (R2PI) and IR-UV double resonance spectroscopic techniques combined with quantum chemistry calculations. R2PI spectra of the dimer and the trimer recorded by electronic excitation of the indole moiety show that the red-shift in the band origin of the dimer with respect to the 0 0 0 band of the monomer is larger compared to that of the trimer. The presence of only one conformer in the case of both the dimer and the trimer has been confirmed from IR-UV hole-burning spectroscopy. The structures of the dimer and the trimer have been determined from resonant ion dip infrared (RIDIR) spectra combined with ab initio as well as DFT/M05-2X and DFT/ M06-2X calculations. It has been found that the dimer, observed in the experiment, has a V-shaped geometry stabilized by NÀH 333 N and CÀH 333 N hydrogen bonding interactions, as well as CÀH 333 π and π 333 π dispersion interactions. The geometry of the trimer has been found to be a cyclic one stabilized by NÀH 333 N, NÀH 333 π,CÀH 333 π, and CÀH 333 N interactions. The most important finding of this current study is the observation of the mixed dimer and trimer, which are stabilized by hydrogen bonding as well as dispersion interactions.