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 measure the ground-state rotational spectra of eight isotopomers H2O···79Br35Cl, H2O···81Br35Cl, H2O···79Br37Cl, H2O···81Br37Cl, D2O···79Br35Cl, D2O···81Br35Cl, HDO···79Br35Cl and HDO···81Br35Cl of a complex formed by water and bromine monochloride. Fitting of measured frequencies yields the spectroscopic constants B0, C0, ΔJ, ΔJK, χaa(X), [χbb(X) − χcc(X)] and Mbb(X) for each of the first six species. For HDO containing isotopomers only K−1 = 0 transitions were observed and hence only (B0 + C0)/2, ΔJ, χaa(X) and Mbb(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 (C2v) with a low potential energy barrier to the motion that interconverts the two equivalent equilibrium conformers of Cs symmetry. The distance r(O···Br) is determined to be 2.7809(3) Å and the out-of-plane angle, ϕ = 48.0(2)° in the ground state. The Townes–Dailey model has been used to interpret the halogen nuclear quadrupole coupling constants. This indicates that a fraction of an electron, δi = 0.013, is transferred from O to Br and a fraction, δp = 0.043, is transferred from Br to Cl on complex formation. Comparison between the intermolecular force constants, kσ, of the complexes H2O···BrCl and H2O···HBr indicates that the interaction in the H2O···BrCl complex is of similar strength to that of the hydrogen bond in H2O···HBr.