Rotational spectroscopy of the gas phase complex of water and bromine monochloride in the microwave region: Geometry, binding strength and charge transfer

Pulsed-nozzle Fourier transform microwave spectroscopy has been used to mea sure the ground-state rotational spectra of eight isotopomers H2O . . . (BC l)-B-79, H2O . . . (BCl)-B-81, H2O . . . (BCl)-B-79, H2O . . . (BCl)-B-81, D2O . . . (BCl)-B-79, D2O . . . (BCl)-B-81, HDO . . . (BCl)-B-79 and HDO . . . (BCl)-B-81 of a complex formed by water and bromine monochloride. Fitti ng of measured frequencies yields the spectroscopic constants B-0, C-0, Del ta (J), Delta (JK), chi (aa)(X), [chi (bb)(X)-chi (cc)(X)] and M-bb(X) for each of the first six species. For HDO containing isotopomers only K-1=0 tr ansitions were observed and hence only (B-0+C-0)/2, Delta (J), chi (aa)(X) and M-bb(X) were determinable. The geometry of the complex is shown to have the arrangement O . . . Br-Cl of the heavy nuclei, with a weak bond formed between O and Br. Analysis shows that the zero-point state of the complex is effectively planar (C-2v) with a low potential energy barrier to the mot ion that interconverts the two equivalent equilibrium conformers of C-s sym metry. The distance r(O . . . Br) is determined to be 2.7809(3) Angstrom an d the out-of-plane angle, phi =48.0(2)degrees in the ground state. The Town es-Dailey model has been used to interpret the halogen nuclear quadrupole c oupling constants. This indicates that a fraction of an electron, delta (i) =0.013, is transferred from O to Br and a fraction, delta (p)=0.043, is tra nsferred from Br to Cl on complex formation. Comparison between the intermo lecular force constants, k(sigma), of the complexes H2O . . . BrCl and H2O . . . HBr indicates that the interaction in the H2O . . . BrCl complex is o f similar strength to that of the hydrogen bond in H2O . . . HBr.