C-H···N hydrogen-bonding interaction in 7-azaindole:CHX3 (X=F, Cl) complexes

Area of Science:

• Physical Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• C-H···Y interactions are unconventional hydrogen bonds with small, bidirectional frequency shifts.
• Understanding these interactions is crucial for molecular recognition and supramolecular chemistry.

Purpose of the Study:

• To investigate the C-H···N hydrogen bonding in gas-phase 7-azaindole complexes with CHCl3 and CHF3.
• To analyze the C-H stretching frequency shifts and compare experimental results with computational predictions.

Main Methods:

• Complexes were prepared in the gas phase using supersonic jet expansion.
• Fluorescence depletion by infra-red (FDIR) spectroscopy was employed to study C-H stretching frequencies.
• Computational methods (MP2/aug-cc-pVDZ) were used to calculate dissociation energies.

Main Results:

• The C-H···N interaction was found to be dominant in 7-azaindole complexes.
• A record redshift of 82 cm⁻¹ for the C-H stretch was observed in the CHCl3 complex.
• A blueshift of 4 cm⁻¹ was observed in the CHF3 complex, contradicting computational predictions.

Conclusions:

• The study provides the largest gas-phase redshift for C-H···N interactions in haloform complexes.
• The observed blueshift in the CHF3 complex highlights discrepancies with computational methods and suggests Fermi resonance.
• Computed dissociation energies indicate stronger binding in the CHCl3 complex (6.46 kcal/mol) compared to the CHF3 complex (5.06 kcal/mol).