The rotational spectrum of thiophene⋯HBr and a comparison of the geometries of the complexes B⋯HX, where B is benzene, furan or thiophene and X is F, Cl or Br

Abstract

The ground-state rotational spectra of three isotopomers C₄H₄S⋯H⁷⁹Br, C₄H₄S⋯H⁸¹Br and C₄H₄S⋯D⁷⁹Br of a weakly bound complex formed by thiophene and hydrogen bromide have been observed in the gas phase by means of a pulsed-jet, Fourier-transform instrument. Each spectrum was analysed and fitted to give rotational constants A₀, B₀ and C₀, centrifugal distortion constants Δ_J, Δ_JK, Δ_K, δ_J and δ_JK and the components χ_aa, χ_bb − χ_cc and χ_ab of the bromine nuclear quadrupole coupling tensor. A detailed analysis of the spectroscopic constants established that the geometry of the complex is of the face-on type. The Br atom of HBr is located close to the perpendicular drawn through the centre of mass of the thiophene ring and the H atom of HBr lies between the Br atom and the ring. The angle α_az made by the HBr internuclear axis z with the a-axis has the two possible values ±9.83°. The preferred structure is that generated when the positive value of the angle is chosen and has the HBr sub-unit pointing in the direction of the S atom of thiophene. The determined geometrical parameters are r(S⋯H) = 2.728(3) Å, ϕ = 116.0(2)° and θ = 7.08(4)°, where ϕ is the angle made by the S⋯H internuclear line with the local C₂ axis of thiophene and θ is the angular deviation of the S⋯H–Br nuclei from collinearity.