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, ΔJ, ΔJK, χaa(X), {χbb(X)−χcc(X)} and Mbb(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 ΔJ 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 (C2v) separating two equivalent equilibrium conformers of Cs 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 ϕe≈±45°. An interpretation of the halogen nuclear quadrupole coupling constants χaa(I) and χaa(Cl) based on the Townes–Dailey model indicated that when the complex is formed a fraction of an electron δi=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 δp(Cl) 0.065(1) of an electron from I to Cl. The intermolecular stretching force constant kσ=15.6(3) Nm−1 determined from the ΔJ 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.