Rotational spectroscopy of a mixture of thiirane and hydrogen bromide: detection and characterization of a short-lived complex (CH2)2SċHBr in a pulsed jet

Abstract

A hydrogen-bonded dimer formed by thiirane with hydrogen bromide has been detected and characterized in the gas phase by means of its ground-state rotational spectrum. The spectrum was detected within ca. 1 µs of the creation of the nascent dimer by using a fast mixing nozzle in conjunction with a pulsed-nozzle, FT microwave spectrometer. With the former device the reactive components thiirane and hydrogen bromide remain separate until they expand into the Fabry–Pérot cavity of the latter. Rotational constants A₀, B₀ and C₀, centrifugal distortion constants Δ_J, Δ_JK and δ_J, Br nuclear quadrupole coupling constants χ_aa, χ_bb and χ_ab, and Br spin–rotation coupling constants have been determined for each of the isotopomers (CH₂)₂SċH⁷⁹Br and (CH₂)₂Sċ;H⁸¹Br. Interpretation of the various spectroscopic constants leads to the conclusion that the dimer has C_s symmetry, with HBr lying in the plane perpendicular to the thiirane heavy-atom plane and with a pyramidal arrangement at S. There is some evidence of a bent hydrogen bond. The angle ϕ≈ 80° made by the SċH line with the thiirane plane is discussed in terms of some simple rules for predicting angular geometries of hydrogen-bonded dimers and a non-bonding electron-pair model for thiirane.