‘Charge-transfer’ complexes of ammonia with halogens. Nature of the binding in H3N⋯BrCl from its rotational spectrum

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Abstract
The ground-state rotational spectra of the four isotopomers H3 15N⋯79Br35Cl, H3 15N⋯81Br35Cl, H3 15N⋯79Br37Cl and H3 15N⋯81Br37Cl of a complex formed by ammonia and bromine monochloride have been observed by pulsed-nozzle, FT microwave spectroscopy. A fast-mixing nozzle was used to prevent a chemical reaction between the components and allow the encounter complex to be isolated. The nature of the spectra establishes that the observed isotopomers are symmetric-top molecules. Assignment and analysis of the spectrum in each case lead to the rotational constant Bo, the centrifugal distortion constants DJ and DJK, the halogen nuclear quadrupole coupling constants χ(Br) and χ(Cl), and the component Mbb(Br) of the bromine spin–rotation coupling tensor. The rotational constants allowed an ro-type value of the distance N⋯Br of 2.627 Å to be established. An rs-type method led to the distances r(N⋯Br)= 2.59(1)Å and r(Br—Cl)= 2.186 Å, the former requiring a value of Bo for H3 14N⋯79Br35Cl obtained from unperturbed transition centres estimated without a full hyperfine structure analysis. A consideration of the intermolecular stretching force constant kσ, determined from DJ, provides evidence of a relatively strong interaction between the subunits. However, an analysis of the χ(X)(X = Br, Cl) values reveals that the molecular interaction is mainly electrostatic in origin, with probably only a small extent of intermolecular electric charge redistribution on complex formation.

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