The rotational spectrum of the complex 2,5-dihydrofuran center dot center dot center dot HBr and the non-linearity of the O center dot center dot center dot H-Br hydrogen bond

a-Type and c-type ground-state rotational transitions belonging to the two isotopomers C4H6O...(HBr)-Br-79 and C4H6O...(HBr)-Br-81 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 A(o), B-o, C-o, the centrifugal distortion constants Delta(J) Delta(JK)Delta(K), delta(J), the complete Br-nuclear quadrupole coupling tensor chi alpha beta (alpha,beta = 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 C, axis of 2,5-dihydrofuran takes the value phi = 122.5(3)degrees while the O...H-Br nuclei are found to deviate by theta = 10.2(1)degrees from collinearity. The distance r(O...H) = 1.799(5) Angstrom. The quantities r(O...H), phi and theta are compared for the series of complexes (CH2)(2)O...HX andC(4)H(6)O...HX (X = Cl or Br) and their variation rationalized on the basis of a simple model of the hydrogen bond.