Is there significant intermolecular charge transfer in the ground state of the HCN···ICI complex? An answer from rotational spectroscopy

Abstract

The ground-state rotational spectra of the four isotopomers HC14N···I35Cl, DC14N···I35Cl, HC15N···I35Cl and HC15N···I37Cl of a linear complex of hydrogen cyanide and iodine monochloride were detected by pulsed-jet, Fourier transform microwave spectroscopy and analysed to give rotational constants B0, centrifugal distortion constants DJ, nuclear quadrupole coupling constants χaa(I), χaa(Cl) and χaa(14N), and iodine spin-rotation coupling constants Mbb(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) A. The changes in the nuclear quadrupole coupling constants on complex formation lead to the conclusion that a fraction δi=0.021(4) of an electron is transferred from N to I while the polarisation of the ICl subunit by the HCN subunit, and viceversa, can be expressed in terms of transfers of fractions δp(Cl)=0.075(1) and δp(N)=0.07(2) of electrons within ICl and HCN to Cl and N, respectively. The intermolecular stretching force constant kσ has the value 14.5 N m-1. The properties of HCN···ICl are compared with those of other HCN···XY complexes, where XY=F2, Cl2, BrCl and ClF and some general conclusions are drawn. The intermolecular charge transfer in HCN···ICl is similar in magnitude to that in OC···ICl but 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 remove an electron from the n-pair orbitals in the series of Lewis bases HCN, CO, H2S and NH3.