title: C-H···N hydrogen-bonding interaction in 7-azaindole:CHX3 (X=F, Cl) complexes.
abstract: The C-H···Y (Y=hydrogen-bond acceptor) interactions are somewhat unconventional in the context of hydrogen-bonding interactions. Typical C-H stretching frequency shifts in the hydrogen-bond donor C-H group are not only small, that is, of the order of a few tens of cm(-1) , but also bidirectional, that is, they can be red or blue shifted depending on the hydrogen-bond acceptor. In this work we examine the C-H···N interaction in complexes of 7-azaindole with CHCl3 and CHF3 that are prepared in the gas phase through supersonic jet expansion using the fluorescence depletion by infra-red (FDIR) method. Although the hydrogen-bond acceptor, 7-azaindole, has multiple sites of interaction, it is found that the C-H···N hydrogen-bonding interaction prevails over the others. The electronic excitation spectra suggest that both complexes are more stabilized in the S1 state than in the S0 state. The C-H stretching frequency is found to be red shifted by 82 cm(-1) in the CHCl3 complex, which is the largest redshift reported so far in gas-phase investigations of 1:1 haloform complexes with various substrates. In the CHF3 complex the observed C-H frequency is blue shifted by 4 cm(-1). This is at variance with the frequency shifts that are predicted using several computational methods; these predict at best a redshift of 8.5 cm(-1). This discrepancy is analogous to that reported for the pyridine-CHF3 complex [W. A. Herrebout, S. M. Melikova, S. N. Delanoye, K. S. Rutkowski, D. N. Shchepkin, B. J. van der Veken, J. Phys. Chem. A- 2005, 109, 3038], in which the blueshift is termed a pseudo blueshift and is shown to be due to the shifting of levels caused by Fermi resonance between the overtones of the C-H bending and stretching modes. The dissociation energies, (D0), of the CHCl3 and CHF3 complexes are computed (MP2/aug-cc-pVDZ level) as 6.46 and 5.06 kcal mol(-1), respectively.
authors.affiliation: Department of Chemical Sciences, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005 (India), Fax: (+91) 22-2278-2106; Current address: Institute of Physical Chemistry, Georg-August University of Göttingen, 37077 Göttingen (Germany).
authors.forename: Pranav R
authors.initials: PR
authors.lastname: Shirhatti
authors.forename: Dilip K
authors.initials: DK
authors.lastname: Maity
authors.forename: Surjendu
authors.initials: S
authors.lastname: Bhattacharyya
authors.forename: Sanjay
authors.initials: S
authors.lastname: Wategaonkar
mesh_terms: D005740:Gases / Q000737:chemistry; D006859:Hydrogen / Q000737:chemistry*; D006860:Hydrogen Bonding; D007211:Indoles / Q000737:chemistry*
chemical_list: C023422:7-azaindole dimer; D005740:Gases; D007211:Indoles; D006859:Hydrogen
keywords: 7-azaindole; ab initio calculations; chloroform; fluoroform; hydrogen bonding