Structure and properties of the weakly bound cyclic trimer (H_2O)_2 HBr observed by rotational spectroscopy

摘要

The weakly bound cyclic trimer (H_2O)_2 HBr was observed in supersonic expansion and its rotational spectrum was measured in the region 3.6-17.7 GHz by cavity Fourier-transform microwave spectroscopy. Rotational, centrifugal distortion constants, and intertial and principal hyperfine splitting constants were determined for seven isotopomers of (H_2O)_2 HBr. In addition to the large bromine hyperfine splitting each rotational transition exhibits a fine vibrational splitting into four components, at relative intensities consistent with expectations from the G_8 group classification of the vibration-rotation-tunneling motions in the trimer. The associated four low-lying states are either very close together or well below the inversion barriers, since the differences between their rotational constants are all below 0.02%. The experimental moments of inertial were used to determine r_s, r_o, r_z, and r_m~(IL) geometries, in all of which the heavy atom distances are considerably shorter than similar distances in H_2O-HBr and (H_2O)_2. An improved analysis of the measured electric dipole moment of (H_2O)_2HBr is reported, and all experimental results are confronted with predictions from ab intio calculations.