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

Abstract

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, Δ_J, Δ_JK, χ_aa(X), [χ_bb(X) − χ_cc(X)] and M_bb(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, Δ_J, χ_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 motion that interconverts the two equivalent equilibrium conformers of C_s 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, δ_i = 0.013, is transferred from O to Br and a fraction, δ_p = 0.043, is transferred from Br to Cl on complex formation. Comparison between the intermolecular force constants, k_σ, 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.