Effect of aromatic ring fluorination on CH interactions: microwave spectrum and structure of the 1,2-difluorobenzeneacetylene dimer

Rotational spectra for the normal isotopic species and for six additional isotopologues of the 1,2-difluorobenzeneacetylene (C 6 H 4 F 2 HCCH) weakly bound dimer have been assigned in the 618 GHz region using chirped-pulse Fourier-transform microwave spectroscopy. This is the third complex in a series of fluorinated benzeneacetylene dimers. In 1,2-difluorobenzeneHCCH, the H distance (2.725(28) ) is longer by about 0.23 , 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 fluorobenzeneHCCH complex. In addition, in 1,2-difluorobenzeneacetylene, 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 C 2 axis of the monomer, while in the fluorobenzene and benzene complexes HCCH is perpendicular (benzeneHCCH) or nearly perpendicular (fluorobenzeneHCCH, 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 13 C substituted isotopologues. The H distance increases in o -C 6 H 4 F 2 HCCH, compared to C 6 H 5 FHCCH or C 6 H 6 HCCH, consistent with weaker interactions with increased ring fluorination.