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Keywords:

  • ab initio calculations;
  • 7-azaindole;
  • chloroform;
  • fluoroform;
  • hydrogen bonding

Abstract

The C[BOND]H⋅⋅⋅Y (Y=hydrogen-bond acceptor) interactions are somewhat unconventional in the context of hydrogen-bonding interactions. Typical C[BOND]H stretching frequency shifts in the hydrogen-bond donor C[BOND]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[BOND]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[BOND]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[BOND]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[BOND]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[BOND]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.