Non-reactive interaction of oxirane and hydrogen bromide: isolation of an oxirane–hydrogen bromide dimer in a supersonic jet and its characterisation by microwave spectroscopy

Abstract

The rotational spectra of the two isotopomers (CH₂)₂O⋯H⁷⁹Br and (CH₂)₂O⋯H⁸¹Br of a dimer formed between oxirane and hydrogen bromide have been detected with a pulsed-nozzle, Fourier-transform microwave spectrometer. A fast-mixing nozzle was used to form and then isolate the observed species before the rapid ring-opening reaction could occur. Rotational constants, centrifugal distortion constants, and Br nuclear hyperfine (quadrupole and spin-rotation) coupling constants were determined. The following constants in particular were used to diagnose the nature of the observed species: [graphic omitted] A detailed analysis of the observed rotational constants allows the conclusion that the dimer has C_s symmetry with a pyramidal arrangement at oxygen and an oxirane geometry essentially unperturbed on dimer formation. When H of HBr is assumed to lie between O and Br the distance r(O⋯Br)= 3.2623 (12)Å and the angle ϕ= 78.45 (9)° are determined. Diagonalization of the complete Br nuclear quadrupole coupling tensor leads to χ_zz=–202.457 MHz and χ_yy=–205.527 MHz, indicating that the presence of the oxirane molecule only slightly perturbs the cylindical symmetry of the HBr subunit. The angle of rotation of (α_za)_χ= 23.3° required to diagonalize the χ tensor is ca. 10° larger than the angle (α_za)_struct implied by the determined geometry. This suggests that the angle O⋯H—Br might be correspondingly less than 180°, i.e. the hydrogen bond might be slightly bent.