Blue-Shifted Hydrogen Bonding in the Gas Phase CH/D₃CN···HCCl₃ Complexes

Area of Science:

• Physical Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• Hydrogen bonding interactions are crucial in chemical processes.
• Previous studies indicated red-shifting H-bonded complexes between CHCl3 and CH/D3CN in Ar matrices.
• Matrix effects at low temperatures can significantly influence H-bond characteristics.

Purpose of the Study:

• To investigate the nature of hydrogen bonding between CHCl3 and CH/D3CN in the gas phase.
• To elucidate the reasons for discrepancies between gas-phase and matrix-isolated observations.
• To analyze the electronic and structural changes upon complex formation.

Main Methods:

• Fourier transform infrared (FTIR) spectroscopy in the gas phase at room temperature.
• Analysis of FTIR peak intensity variations with temperature and composition.
• Ab initio calculations, including molecular electrostatic potential and Natural Bond Orbital (NBO) analysis.
• Comparison with related complexes (e.g., F3CH···NCCH3, Cl3CH···NH3).

Main Results:

• Identification of blue-shifting H-bonded complexes between CHCl3 and CH3CN/CD3CN in the gas phase.
• Observed blue shifts of +8.7 cm-1 (CH3CN) and +8.6 cm-1 (CD3CN) for the C-H stretching frequency of CHCl3.
• Computational analysis revealed electron density shift from H to Cl atoms in CHCl3 upon complexation, causing C-H bond contraction and blue-shifting.
• Verified red-shifting in Ar matrix, attributed to matrix-induced increase in acetonitrile basicity.

Conclusions:

• Gas-phase H-bonding between CHCl3 and acetonitrile results in blue-shifting due to C-H bond contraction.
• Matrix isolation effects in Ar lead to red-shifting by enhancing the H-bond acceptor strength of acetonitrile.
• The observed frequency shifts are governed by the interplay of electronic structure changes and environmental factors.