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.

On the properties of S⋯O and S⋯π noncovalent interactions: the analysis of geometry, interaction energy and electron density

“…40 The geometries of model complexes were optimized at the MP2 41 /aug-cc-pVDZ level, which is regarded as a reliable approach for evaluating weak intermolecular interactions. [42][43][44][45][46][47][48][49][50][51][52][53] Its accuracy can be compared to that of CCSD with larger basis sets 54 and the energetic values are in good agreement with experimental data. 55 Harmonic vibrational frequencies were also calculated at the same level of theory on the optimized geometries to verify that the optimized model complexes were minimum on the potential energy surfaces.…”

Angular dependence of hydrogen bond energy in neutral and charged systems containing CH and NH proton donors

“…CF 3 H represents a simple molecule capable of forming CH· · ·D HBs by virtue of its three electron-withdrawing F atoms, and has been commonly used [28][29][30][31][32][33][34][35][36][37][38] as a model CH proton donor in the past. The optimized geometries for complexes combining CF 3 H with various anions, each involving a different proton-accepting atom, are displayed in Figure 1.…”

Tuning the Halogen/Hydrogen Bond Competition: A Spectroscopic and Conceptual DFT Study of Some Model Complexes Involving CHF2I

“…The second value is significantly larger than the experimental value of −43.6 cm −1 observed for the 2960 cm −1 band. However, taking into account the relative position of both bands and considering the fact that the predicted complexation shifts of CH ⋅⋅⋅ Y hydrogen‐bonded complexes often are largely overestimated due to changes in anharmonicity,10c, d, 23 this band can safely be assigned to the CH ⋅⋅⋅ N‐bonded complex.…”