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 ΔJ, ΔJK and δJ, Br nuclear quadrupole coupling constants χaa, χbb and χab, 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 Cs 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.