A non-linear hydrogen bond F⋯H–Br in vinyl fluoride ⋯HBr characterised by rotational spectroscopy

Abstract

The ground-state rotational spectra of four isotopomers C2H3F⋯H79Br, C2H3F⋯H81Br, C2H3F⋯D79Br and C2H3F⋯D81Br of a complex formed between vinyl fluoride and hydrogen bromide have been observed with a pulsed-nozzle, Fourier-transform microwave spectrometer. Rotational constants (A0, B0, C0), all five quartic centrifugal distortion constants (ΔJ, ΔJK, ΔK, δJ, δK), the Br nuclear quadrupole coupling tensor χαβ(α,β = a,b,c) and the Br spin-rotation tensor M were determined in each case. A detailed interpretation of the χαβ tensor leads to the conclusion that the complex is planar, or nearly so. The direction cosine Φaz associated with the HBr axis z and the principal inertial axis a was determined together with the principal components χzz, χxx, χyy of the Br nuclear quadrupole coupling tensor. The geometry of the complex was obtained by fitting the rotational constants of its isotopomers under the constraint that the resulting geometry must reproduce Φaz. The HBr molecule was found to form a non-linear hydrogen bond F⋯H–Br [deviation from collinearity θ = 20.0(1)°] to the F atom of vinyl fluoride, with r(F⋯H) = 2.205(9) A and the angle C1F⋯H = 123.8(5)°, when the nuclei were assumed coplanar. A comparison of the geometry with that of vinyl fluoride ⋯HCl determined in the same way shows that the angular geometries of the HBr and HCl complexes of vinyl fluoride are isomorphous. This is the case even for the extent of the non-linearity θ of the hydrogen bond, which has the values θ = 20.0(1)° and 18.3(1)° in C2H3F⋯HBr and C2H3F⋯HCl, respectively. A small extent of non-planarity is also consistent with the observations for both complexes but the conclusions are not significantly affected by relaxing the assumption of coplanar nuclei.