Structure and properties of the weakly bound cyclic trimer (H[sub 2]O)[sub 2]HBr observed by rotational spectroscopy.

The weakly bound cyclic trimer (H[sub 2]O)[sub 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 inertial and principal hyperfine splitting constants were determined for seven isotopomers of (H[sub 2]O)[sub 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[sub 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 inertia were used to determine r[sub s], r[sub 0], r[sub z], and r[sub m][sup (1L)] geometries, in all of which the heavy atom distances are considerably shorter than similar distances in H[sub 2]O–HBr and (H[sub 2]O)[sub 2]. An improved analysis of the measured electric dipole moment of (H[sub 2]O)[sub 2]HBr is reported, and all experimental results are confronted with predictions from ab initio calculations. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]