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