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

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Abstract
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 χ aa , χ bb , χ cc , and χ ac of the coupling tensor were obtained. A detailed analysis of the rotational constants allows the conclusion that the dimer has C s 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 χ xx , χ yy , and χ zz (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 C 2 axis are interpreted in terms of a simple model of the hydrogen bond.

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