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

The ground-state rotational spectra of the four isotopomers H2O ... (ICl)-Cl-35, H2O ... (ICl)-Cl-37, HDO ... (ICl)-Cl-35 and D2O ... (ICl)-Cl-35 of a complex formed by water and iodine monochloride were observed and analysed. The spectroscopic constants B-0, C-0, Delta(J), Delta(JK), chi(aa)(X), {chi(bb)(X)-chi(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 ... (ICl)-Cl-35, only 1/2(B0 + C0) and Delta 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 separating two equivalent equilibrium conformers of symmetry. By fitting the rotational constants of the four isotopomers, the distance r(O ... Cl) = 2.838(3) Angstrom and the out-of-plane angle phi = 46(2)degrees were determined as effective ground-state values. Ab initio calculations gave values of the equilibrium angle An interpretation of the halogen nuclear quadrupole coupling constants and based on phi(e) approximate to 45 degrees. An interpretation of the halogen nuclear quadrupole coupling constants chi(aa)(I) and chi(aa)(Cl) the Townes-Dailey model indicated that when the complex is formed a fraction of an electron delta(i) = 0.010(4) is transferred from O to I while the polarisation of ICl by can be described by a transfer of a fraction delta p(Cl) 0.065(1) of an electron from I to Cl. The intermolecular stretching force constant k(sigma) = 15.6(3) Nm(-1) determined from the values indicates that 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 and to identify similarities in these that reinforce an earlier invocation of a halogen bond analogue of the hydrogen bond.