Rotational spectrum of a short-lived dimer of oxirane and hydrogen chloride : evidence for a bent hydrogen bond

Ground state rotational spectra of the three isotopomers (CH2)2O...H35Cl, (CH2)2O...H37Cl, and (CH2)2O...D35Cl of a short-lived hydrogen-bonded dimer have been detected in the reactive mixture of oxirane and hydrogen chloride by using a fast-mixing nozzle in conjunction with a Balle–Flygare Fourier-transform microwave spectrometer. Rotational constants, centrifugal distortion constants and Cl-nuclear quadrupole coupling constants were determined for each isotopomer. In particular, all four components χaa, χbb, χcc, and χac of the coupling tensor were obtained. A detailed analysis of the rotational constants allows the conclusion that the dimer has Cs symmetry, with a steeply pyramidal arrangement completed at oxygen by the hydrogen bond with HCl. Diagonalization of the complete Cl-nuclear quadrupole coupling tensor leads to the principal axis components χxx, χyy, and χzz (where z is the HCl direction in the dimer). The angle of rotation α is the angle between the HCl (z) direction and the a-axis direction in the equilibrium conformation of the dimer. It is larger by ∼10° than the angle γ between the O...Cl internuclear line and the principal inertial axis a in each case and implies that the hydrogen bond is bent by 180-θ=∼16.5° from the collinear arrangement O...H–Cl (θ=0). The angle 180-θ and the angle φ=76.2° made by the O...Cl internuclear line with the extension of the oxirane local C2 axis are interpreted in terms of a simple model of the hydrogen bond.