Rotational spectrum of vinyl fluoride···ClF: Are the π-bonding or nonbonding electrons the most nucleophilic region of vinyl fluoride?

The ground-state rotational spectra of C2H3F···35ClF and C2H3F···37ClF have been observed with a pulsed-nozzle, Fourier-transform microwave spectrometer. Rotational constants (A0, B0, C0), centrifugal distortion constants (ΔJ, ΔJK, δJ, δK) and the Cl nuclear quadrupole coupling tensor χαβ (α, β=a, b, c) were determined in each case. A detailed interpretation of the χαβ tensor leads to the conclusion that the complex is nearly planar and that the interaction between the vinyl fluoride and ClF subunits is weak. The direction cosines Φαz (α=a, b or c) associated with the ClF z axis and the principal inertial axes were determined together with the principal components χzz, χxx, χyy of the Cl nuclear quadrupole coupling tensor. The geometry of the complex was obtained by fitting the rotational constants of the two isotopomers under the constraint that the resulting geometry must reproduce the Φαz. The ClF molecule was found to form a nearly linear ‘chlorine’ bond F···Cl–F (deviation from collinearity ϑ=0.6°) to the F atom of vinyl fluoride, with r(F···Cl)=2.719(5) Å and the angle C(1)F···Cl=125.7(3)°. A comparison with the geometry of vinyl fluoride···HCl, determined in the same way, revealed the angular geometries of the ClF and HCl complexes of vinyl fluoride to be isomorphous, except for a significant deviation (ϑ=18.3°) of the hydrogen bond in C2H3F···HCl from collinearity.