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 (Δ_J, Δ_JK, Δ_K, δ_J, δ_K), the Br nuclear quadrupole coupling tensor χ_αβ(α,β = a,b,c) and the Br spin-rotation tensor M were determined in each case. A detailed interpretation of the χ_αβ tensor leads to the conclusion that the complex is planar, or nearly so. The direction cosine Φ_az associated with the HBr axis z and the principal inertial axis a was determined together with the principal components χ_zz, χ_xx, χ_yy 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 Φ_az. 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) Å 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.