Ground-state rotational spectra of the seven isotopomers H2O⋯35Cl2, H2O⋯37Cl35Cl, H2O⋯35Cl37Cl, D2O⋯35Cl2, D2O⋯37Cl35Cl, HDO⋯35Cl2, and HDO⋯37Cl35Cl of the complex formed between water and dichlorine were observed by pulsed-jet, Fourier-transform microwave spectroscopy. Rotational constants B0 and C0, centrifugal distortion constants ΔJ and ΔJK, and Cl nuclear quadrupole coupling constants χaa(Clx) and {χbb(Clx)−χcc(Clx)}, where x=i (inner) or o (outer), were obtained via spectral analyses. The spectroscopic constants were interpreted, on the basis of models for the complex, to give the geometry, the binding strength, and the extent of inter- and intramolecular electron transfer on complex formation. The zero-point geometry is defined in terms of the effective values r(O⋯Cli)=2.8479(3) Å and φ=43.4(3)°, where φ is the angle made by the bisector of the HÔH angle with the O⋯Cl internuclear line. The intermolecular stretching force constant kσ=8.0(1) N m−1. Fractions δi=0.005(5) and δp=0.034(3) of an electron were estimated to be transferred from O to Cl and from Cli to Clo, respectively. The geometrical conclusions are supported by ab initio calculations at the aug-cc-pVDZ/MP2 level of theory, with good agreement for φ and r(O⋯Cli). A comparison of the properties of H2O⋯Cl2 with those of H2O⋯HCl provides evidence in support of a recent proposal of a halogen bond B⋯XY that is the analog of the hydrogen bond B⋯HX, where X and Y are halogen atoms.

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