Structure and properties of the weakly bound trimer (H2O)2HCl observed by rotational spectroscopy

The weakly bound trimer between two water molecules and one hydrogen chloride molecule was identified from its rotational spectrum observed at conditions of supersonic expansion. The spectrum was measured with a cavity, Fourier transform microwave spectrometer and the rotational, quartic, and chlorine hyperfine coupling constants were determined for six isotopomers of the complex: ((H2O)-O-16)(2)(HCl)-Cl-35, ((H2O)-O-16)(2)(HCl)-Cl-37, ((HOD)-O-16 ...(OH2)-O-16)(HCl)-Cl-35, ((HOH)-O-18 ...(OH2)-O-16)(HCl)-Cl-35, ((HOH)-O-16 ...(OH2)-O-18)(HCl)-Cl-35, and ((H2O)-O-18)(2)(HCl)-Cl-35. The rotational spectrum consists of transitions allowed by the mu(a) and the, much smaller, mu(b) dipole moment components. The rotational transitions are further split into components belonging to four low-lying vibration-rotation-tunneling substrates. The trimer is bound strongly enough to attenuate the dynamics of the water subunits to a level such that the differences in rotational constants between the four states are below 1 MHz. The complex is near planar as evidenced by inertial defect of -0.5 u Angstrom(2). Analysis of experimental and theoretical results reveals that the three monomers are bound in a triangular arrangement through O ... HO and O ... HCl hydrogen bonds and a primarily dispersive OH ... Cl bond. All atoms, with the exception of two nonbonded hydrogens, lie near a common plane. The r(s), r(0), and r* structural parameters have been evaluated and the two hydrogen bonds r*(OH ... O) = 2.8151(8), and r*(O ... HCl) = 3.0840(11) Angstrom are appreciably shorter than analogous bonds in (H2O)(2) and H2O ... HCl, whereas r*(OH ... Cl) = 3.4152(12) Angstrom is close to the sum of the van der Waals radii for O and Cl. The experimental results concerning all observables determinable from the rotational spectrum are found to be in very good agreement with theoretical predictions. (C) 2000 American Institute of Physics. [S0021-9606(00)02013-4].