On the weak O-H center dot center dot center dot halogen hydrogen bond: a rotational study of CH3CHClF center dot center dot center dot H2O

We measured the molecular beam Fourier transform microwave spectra of six isotopologues of the 1 : 1 adduct of CH3CHClF with water. Water prefers to form an O-H center dot center dot center dot F rather than an O-H center dot center dot center dot Cl hydrogen bond. This is just the contrary of what was observed in the chlorofluoromethane-water adduct, where an O-H center dot center dot center dot Cl link was formed (W. Caminati, S. Melandri, A. Maris and P. Ottaviani, Angew. Chem., Int. Ed., 2006, 45, 2438). The water molecule is linked with an O-H center dot center dot center dot F bridge to the fluorine atom, with r(F center dot center dot center dot H-w) = 2.14 angstrom, and with two C-H center dot center dot center dot O contacts to the alkyl hydrogens with r(C-1-H-1 center dot center dot center dot O-w) = 2.75 angstrom and r(C-2-H-2 center dot center dot center dot O-w) = 2.84 angstrom, respectively. Besides the rotational constants, the quadrupole coupling constants of the chlorine atom have been determined. In addition, information on the internal dynamics has been obtained.