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

The ground-state rotational spectra of the four isotopomers H2O ... (ICl)-C l-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), {c hi(bb)(X)-chi(cc)(X)} and M-bb(I) were determined for each of the first thr ee species of this nearly prolate asymmetric rotor complex by fitting the s pectra. 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 effective ly planar, with a very low potential energy barrier at the planar geometry separating two equivalent equilibrium conformers of symmetry. By fitting th e 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 determ ined as effective ground-state values. Ab initio calculations gave values o f the equilibrium angle An interpretation of the halogen nuclear quadrupole coupling constants and based on phi(e) approximate to 45 degrees. An inter pretation of the halogen nuclear quadrupole coupling constants chi(aa)(I) a nd chi(aa)(Cl) the Townes-Dailey model indicated that when the complex is f ormed a fraction of an electron delta(i) = 0.010(4) is transferred from O t o I while the polarisation of ICl by can be described by a transfer of a fr action 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 va lues indicates that is about as strongly bound as H2O ... ClF. The opportun ity is taken to compare several properties of the complexes H2O ... ICl, H2 O ... HI, H2O ... ClF and and to identify similarities in these that reinfo rce an earlier invocation of a halogen bond analogue of the hydrogen bond.