Effect of aromatic ring fluorination on CH⋯π interactions: microwave spectrum and structure of the 1,2-difluorobenzene⋯acetylene dimer

Abstract

Rotational spectra for the normal isotopic species and for six additional isotopologues of the 1,2-difluorobenzene⋯acetylene (C₆H₄F₂⋯HCCH) weakly bound dimer have been assigned in the 6–18 GHz region using chirped-pulse Fourier-transform microwave spectroscopy. This is the third complex in a series of fluorinated benzene⋯acetylene dimers. In 1,2-difluorobenzene⋯HCCH, the H⋯π distance (2.725(28) Å) is longer by about 0.23 Å, and the estimated binding energy (E_B = 2.3(6) kJ mol⁻¹) is weaker by about 1.8 kJ mol⁻¹, than in the previously studied fluorobenzene⋯HCCH complex. In addition, in 1,2-difluorobenzene⋯acetylene, HCCH tips ∼46(3)° away from perpendicular to the aromatic ring, with the H nearest the ring moving away from the fluorine atoms along the C₂ axis of the monomer, while in the fluorobenzene and benzene complexes HCCH is perpendicular (benzene⋯HCCH) or nearly perpendicular (fluorobenzene⋯HCCH, ∼7° tilt) to the ring plane. Results from ab initio and DFT calculations will be compared to an experimental structure determined from rotational constants for the DCCD and five unique ¹³C substituted isotopologues.