An investigation of the gas-phase complex of water and iodine monochloride by microwave spectroscopy: geometry, binding strength and electron redistribution

Abstract

The ground-state rotational spectra of the four isotopomers H₂O···I³⁵Cl, H₂O···I ³⁷Cl, HDO···I³⁵Cl and D₂O···I ³⁵Cl of a complex formed by water and iodine monochloride were observed and analysed. The spectroscopic constants B₀, C₀, Δ_J, Δ_JK, χ_aa(X), {χ_bb(X)−χ_cc(X)} and M_bb(I) were determined for each of the first three species of this nearly prolate asymmetric rotor complex by fitting the spectra. Of the rotational and centrifugal distortion constants of HDO···I³⁵Cl, only ½(B₀+C₀) and Δ_J were determinable. The atoms were shown to lie in the order H₂O···ICl, with the weak bond formed by O and I. It was established that in the zero-point state the complex is effectively planar, with a very low potential energy barrier at the planar geometry (C_2v) separating two equivalent equilibrium conformers of C_s symmetry. By fitting the rotational constants of the four isotopomers, the distance r(O···Cl)=2.838(3) Å and the out-of-plane angle ϕ=46(2)° were determined as effective ground-state values. Ab initio calculations gave values of the equilibrium angle ϕ_e≈±45°. An interpretation of the halogen nuclear quadrupole coupling constants χ_aa(I) and χ_aa(Cl) based on the Townes–Dailey model indicated that when the complex is formed a fraction of an electron δ_i=0.010(4) is transferred from O to I while the polarisation of ICl by H₂O can be described by a transfer of a fraction δ_p(Cl) 0.065(1) of an electron from I to Cl. The intermolecular stretching force constant k_σ=15.6(3) Nm⁻¹ determined from the Δ_J values indicates that H₂O···ICl is about as strongly bound as H₂O···ClF. The opportunity is taken to compare several properties of the complexes H₂O···ICl, H₂O···HI, H₂O···ClF and H₂O···HCl and to identify similarities in these that reinforce an earlier invocation of a halogen bond analogue of the hydrogen bond.