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

The weakly bound trimer between two water molecules and one hydrogen chlori de molecule was identified from its rotational spectrum observed at conditi ons of supersonic expansion. The spectrum was measured with a cavity, Fouri er transform microwave spectrometer and the rotational, quartic, and chlori ne 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-1 6 ...(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 rotatio nal spectrum consists of transitions allowed by the mu(a) and the, much sma ller, mu(b) dipole moment components. The rotational transitions are furthe r split into components belonging to four low-lying vibration-rotation-tunn eling substrates. The trimer is bound strongly enough to attenuate the dyna mics of the water subunits to a level such that the differences in rotation al constants between the four states are below 1 MHz. The complex is near p lanar as evidenced by inertial defect of -0.5 u Angstrom(2). Analysis of ex perimental and theoretical results reveals that the three monomers are boun d in a triangular arrangement through O ... HO and O ... HCl hydrogen bonds and a primarily dispersive OH ... Cl bond. All atoms, with the exception o f 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 determina ble from the rotational spectrum are found to be in very good agreement wit h theoretical predictions. (C) 2000 American Institute of Physics. [S0021-9 606(00)02013-4].