Structure and properties of the (HCl)2H2O cluster observed by chirped-pulse Fourier transform microwave spectroscopy

Abstract

The rotational spectrum of the cyclic (HCl)₂H₂O cluster has been identified for the first time in the chirped pulse, Fourier transform microwave spectrum of a supersonically expanded HCl/H₂O/Ar mixture. The spectrum was measured at frequencies 6–18.5 GHz, and transitions in two inversion-tunneling states, at close to 1 : 3 relative intensity, have been assigned for the parent species. The two single ³⁷Cl isotopic species, and the double ³⁷Cl species have been assigned in the natural abundance sample, and the ¹⁸O and HDO species of the cluster were identified in isotopically enriched samples. The rich nuclear quadrupole hyperfine structure due to the presence of two chlorine nuclei has been satisfactorily fitted and provided useful information on the nonlinearity of intermolecular bonds in the cluster. The r_s heavy atom geometry of the cluster was determined and the strongest bond in the intermolecular cycle r(O⋯HCl)=3.126(3) Å, is found to be intermediate in length between the values in H₂O⋯HCl and (H₂O)₂HCl. The fitted spectroscopic constants and derived molecular properties are compared with ab initio predictions, and a discussion of complexation effects in these three clusters is made.