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 D_J, nuclear quadrupole coupling constants χ_aa(I), χ_aa(Cl) and χ_aa(¹⁴N), 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) Å. The changes in the nuclear quadrupole coupling constants on complex formation lead to the conclusion that a fraction δ_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 viceversa, can be expressed in terms of transfers of fractions δ_p(Cl)=0.075(1) and δ_p(N)=0.07(2) of electrons within ICl and HCN to Cl and N, respectively. The intermolecular stretching force constant k_σ 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.