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

Abstract

The ground-state rotational spectra of the four isotopomers H₃¹⁵N⋯⁷⁹Br³⁵Cl, H₃¹⁵N⋯⁸¹Br³⁵Cl, H₃¹⁵N⋯⁷⁹Br³⁷Cl and H₃¹⁵N⋯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 B_o, the centrifugal distortion constants D_J and D_JK, the halogen nuclear quadrupole coupling constants χ(Br) and χ(Cl), and the component M_bb(Br) of the bromine spin–rotation coupling tensor. The rotational constants allowed an r_o-type value of the distance N⋯Br of 2.627 Å to be established. An r_s-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 B_o for H₃¹⁴N⋯⁷⁹Br³⁵Cl obtained from unperturbed transition centres estimated without a full hyperfine structure analysis. A consideration of the intermolecular stretching force constant kσ, determined from D_J, 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.