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...HCl)-Cl-35, ((CH2)2O...HCl)-Cl-37, and ((CH2)2O...DCl)-Cl-37 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 chi(aa), chi(bb), chi(cc), and chi(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 chi(xx), chi(yy), and chi(zz), (where z is the HCI direction in the dimer). The angle of rotation a is the angle between the HCl (z) direction and the a-axis direction in the equilibrium conformation of the dimer. It is larger by approximately 10-degrees than the angle-gamma 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-theta = approximately 16.5-degrees from the collinear arrangement O ... H-Cl (theta = 0). The angle 180-theta and the angle phi = 76.2-degrees 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.