Halogen nuclear quadrupole coupling in the rotational spectrum of H3N···ICl as a probe of inter- and intramolecular charge transfer

The ground-state rotational spectra of the three isotopomers H314N···I35Cl, H314N···I37Cl and H315N···I35Cl of a symmetric-top complex formed between ammonia and iodine monochloride were detected and measured by using the pulsed-jet, Fourier-transform microwave technique in combination with a fast-mixing nozzle. The spectroscopic constants B0, DJ, DJK, χ(14N), χ(I), χ(Cl), Maa(I) and Mbb(I) were determined for each isotopomer. Various of these constants were interpreted with the aid of simple models to give the geometry, the binding strength, and the electric charge redistribution relative to the free molecules NH3 and ICl. The equilibrium geometry is one of C3v symmetry, with the nuclei in the order H3N···ICl and with the distance r(N···I)=2.711(2) Å. The intermolecular stretching force constant kσ=30.4(3) N m-1 is large and indicates that the complex is strongly bound. The changes in the halogen nuclear quadrupole coupling constant χ(I) and χ(Cl) from their respective free molecule values lead to the conclusion that the intermolecular electronic charge transfer, δ1e-, from N to I on complex formation is 0.078(8) e-, while the polarization of the ICl subunit can be represented by a transfer, δ2e-, of 0.150(3) e- from I to Cl. The net change at I is -0.073(5) e-. A comparison of r(N···X), δ2 and kσ for the series of complexes H3N···XY, where XY=Cl2, Br2, BrCl, ICl or ClF, is presented and some general conclusions are drawn.