Is there significant intermolecular charge transfer in the ground state ofthe HCN center dot center dot center dot ICI complex? An answer from rotational spectroscopy

The ground-state rotational spectra of the four isotopomers (HCN)-N-14 ... (ICl)-Cl-35, (DCN)-N-14 ... (ICl)-Cl-35, (HCN)-N-15 ... (ICl)-Cl-35 and (HC N)-N-15 ... (ICl)-Cl-37 of a linear complex of hydrogen cyanide and iodine monochloride were detected by pulsed-jet, Fourier transform microwave spect roscopy and analysed to give rotational constants B-0, centrifugal distorti on constants D-J, nuclear quadrupole coupling constants chi(aa)(I), chi(aa) (Cl) and chi(aa)(N-14), and iodine spin-rotation coupling constants M-bb(I) . Detailed interpretations of various spectroscopic constants on the basis of simple models yielded a range of properties of the complex. The order of the nuclei was found to be HCN ... ICl with r(N ... I)=2.850(1) Angstrom. The changes in the nuclear quadrupole coupling constants on complex formati on lead to the conclusion that a fraction delta(i)=0.021(4) of an electron is transferred from N to I while the polarisation of the ICl subunit by the HCN subunit, and vice versa, can be expressed in terms of transfers of fra ctions delta(p)(Cl)=0.075(1) and delta(p)(N)=0.07(2) of electrons within IC l and HCN to Cl and N, respectively. The intermolecular stretching force co nstant k(sigma) has the value 14.5 N m(-1). The properties of HCN ... ICl a re compared with those of other HCN ... XY complexes, where XY=F-2, Cl-2, B rCl and ClF and some general conclusions are drawn. The intermolecular char ge transfer in HCN ... ICl is similar in magnitude to that in OC ... ICl bu t smaller than in the other n-pair donor type complexes H2S ... ICl and H3N ... ICl, a result consistent with the order of the energies required to re move an electron from the n-pair orbitals in the series of Lewis bases HCN, CO, H2S and NH3.