The rotational spectrum of the complex 2,5-dihydrofuran···HBr and the non-linearity of the O···HνBr hydrogen bond

Abstract

a-Type and c-type ground-state rotational transitions belonging to the two isotopomers C₄H₆O···H⁷⁹Br and C₄H₆O···H⁸¹Br of a hydrogen-bonded complex formed by 2,5-dihydrofuran with hydrogen bromide have been measured by using a pulsed-nozzle, Fourier-transform microwave spectrometer. Spectral analysis leads to the rotational constants A₀, B₀, C₀, the centrifugal distortion constants Δ_J, Δ_JK, Δ_K, δ_J, the complete Br-nuclear quadrupole coupling tensor χ_αβ (α, β = a, b, c), and the spin–rotation coupling constant 1/2(M_bb + M_cc) in each case. A detailed interpretation of the spectroscopic constants establishes that the complex has C_s symmetry, with the HBr subunit lying in the molecular symmetry plane and forming a hydrogen bond to the oxygen atom of 2,5-dihydrofuran. The angle between the O···H internuclear line and the C₂ axis of 2,5-dihydrofuran takes the value ϕ = 122.5(3)° while the O···H–Br nuclei are found to deviate by ϑ = 10.2(1)° from collinearity. The distance r(O···H) = 1.799(5) Å. The quantities r(O···H), ϕ and ϑ are compared for the series of complexes (CH₂)₂O···HX and C₄H₆O···HX (X = Cl or Br) and their variation rationalized on the basis of a simple model of the hydrogen bond.