Structure, Conformational Equilibria, and Weak Hydrogen Bonding in the CH₂ F₂ -CF₃ CH₂ F Dimer

Area of Science:

• Physical Chemistry
• Chemical Physics
• Molecular Spectroscopy

Background:

• Weakly bound intermolecular complexes are crucial in understanding non-covalent interactions.
• Fluorinated organic compounds exhibit unique intermolecular forces.
• Pulsed jet Fourier transform microwave spectroscopy is a powerful tool for characterizing molecular structures and interactions.

Purpose of the Study:

• To investigate the multiple configurations of the difluoromethane and 1,1,1,2-tetrafluoroethane dimer.
• To determine the stabilizing interactions and relative stabilities of different isomers.
• To precisely determine the structures of the observed isomers using isotopic substitution.

Main Methods:

• Pulsed jet Fourier transform microwave spectroscopy.
• Ab initio quantum chemical calculations.
• Quantum Theory of Atoms in Molecules (QTAIM) analysis.
• Symmetry-Adapted Perturbation Theory (SAPT) analysis.
• Spectroscopic measurements on 13C isotopologues.

Main Results:

• Three isomers of the difluoromethane and 1,1,1,2-tetrafluoroethane dimer were detected in a supersonic jet.
• All observed isomers are stabilized by a network of three C-H⋅⋅⋅F-C interactions.
• Interaction energies range from 12-13 kJ/mol, primarily driven by electrostatic and dispersion forces.
• Isomer II was identified as the global minimum.
• Relative isomer populations in helium were determined as approximately N_I/N_II/N_III ≈ 1/11/4.

Conclusions:

• The study successfully characterized the structural diversity and stability of the difluoromethane and 1,1,1,2-tetrafluoroethane dimer.
• C-H⋅⋅⋅F-C interactions play a significant role in stabilizing these fluorinated intermolecular complexes.
• The findings provide valuable insights into the nature of non-covalent interactions in fluorinated systems.