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

The ground-state rotational spectra of the three isotopomers (H3N)-N-14 ... (ICl)-Cl-35, (H3N)-N-14 ... (ICl)-Cl-37 and (H3N)-N-15 ... (ICl)-Cl-35 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 cons tants B-0, D-J, D-JK, chi(N-14), chi(I), chi(Cl), M-aa(I) and M-bb(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 a nd ICl. The equilibrium geometry is one of C-3v symmetry, with the nuclei i n the order H3N ... ICl and with the distance r(N ... I)=2.711(2) Angstrom. The intermolecular stretching force constant k(sigma)=30.4(3) N m(-1) is l arge and indicates that the complex is strongly bound. The changes in the h alogen nuclear quadrupole coupling constant chi(I) and chi(Cl) from their r espective free molecule values lead to the conclusion that the intermolecul ar electronic charge transfer, delta(1)e(-), from N to I on complex formati on is 0.078(8) e(-), while the polarization of the ICl subunit can be repre sented by a transfer, delta(2)e(-), 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), delta(2) and k(s igma) for the series of complexes H3N ... XY, where XY=Cl-2, Br-2, BrCl, IC l or ClF, is presented and some general conclusions are drawn.